Astronomy & Astrophysics manuscript no. grb050730 c© ESO 2018
November 1, 2018
The exceptionally extended flaring activity in the X–ray afterglow
of GRB 050730 observed with Swift and XMM-Newton
M. Perri1,2, D. Guetta2, L.A. Antonelli1,2, A. Cucchiara3, V. Mangano4, J. Reeves5, L. Angelini5, A.P. Beardmore6,
P. Boyd5, D.N. Burrows3, S. Campana7, M. Capalbi1,2, G. Chincarini7,8, G. Cusumano4, P. Giommi1,9, J.E. Hill5,
S.T. Holland5,10, V. La Parola4, T. Mineo4, A. Moretti7, J.A. Nousek3, J.P. Osborne6, C. Pagani3, P. Romano7,
P.W.A. Roming3, R.L.C. Starling6, G. Tagliaferri7, E. Troja4,6, L. Vetere1,3 and N. Gehrels5
1 ASI Science Data Center, Via Galileo Galilei, I-00044 Frascati, Italy
2 INAF – Astronomical Observatory of Rome, Via Frascati 33, I-00040 Monte Porzio Catone (Rome), Italy
3 Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
4 INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, Via La Malfa 153, I-90146 Palermo, Italy
5 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
6 Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
7 INAF – Astronomical Observatory of Brera, Via Bianchi 46, I-23807 Merate, Italy
8 Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, I-20126 Milano, Italy
9 Agenzia Spaziale Italiana, Unità Osservazione dell’Universo, Viale Liegi 26, I-00198 Roma, Italy
10 Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA
Received: 11 August 2006 / Accepted: 27 March 2007
ABSTRACT
Aims. We observed the high redshift (z = 3.969) GRB 050730 with Swift and XMM-Newton to study its prompt and afterglow emission.
Methods. We carried out a detailed spectral and temporal analysis of Swift and XMM-Newton observations.
Results. The X–ray afterglow of GRB 050730 was found to decline with time with superimposed intense flaring activity that extended
over more than two orders of magnitude in time. Seven distinct re-brightening events starting from 236 s up to 41.2 ks after the burst were
observed. The underlying decay of the afterglow was well described by a double broken power-law model with breaks at t1 = 237 ± 20 s and
t2 = 10.1
−2.2 ks. The temporal decay slopes before, between and after these breaks were α1 = 2.1 ± 0.3, α2 = 0.44
+0.14
−0.08 and α3 = 2.40
+0.09
−0.07,
respectively. The spectrum of the X–ray afterglow was well described by a photoelectrically absorbed power-law with an absorbing column
density NzH=(1.28
+0.26
−0.25) × 10
22 cm−2 in the host galaxy. Evidence of flaring activity in the early UVOT optical afterglow, simultaneous with that
observed in the X–ray band, was found. Strong X–ray spectral evolution during the flaring activity was present. The rise and decay power-law
slopes of the first three flares were in the range 0.8–1.8 using as zero times the beginning and the peak of the flares, respectively. In the majority
of the flares (6/7) the ratio ∆t/tp between the duration of the event and the time when the flare peaks was nearly constant and ∼ 0.6–0.7. We
showed that the observed spectral and temporal properties of the first three flares are consistent with being due both to high-latitude emission,
as expected if the flares were produced by late internal shocks, or to refreshed shocks, i.e. late time energy injections into the main afterglow
shock by slow moving shells ejected from the central engine during the prompt phase. The event fully satisfies the Ep–Eiso Amati relation
while is not consistent with the Ep vs. Ejet Ghirlanda relation.
Key words. gamma rays: bursts – X–rays: individual (GRB 050730)
1. Introduction
The successful launch on 2004 November 20 of the Swift
Gamma–ray Burst Explorer (Gehrels et al. 2004) has opened
a new era in the study of Gamma Ray Bursts (GRBs). The
autonomous and rapid slewing capabilities of Swift allow the
prompt (1–2 minutes) observation of GRBs, discovered and
localised by the wide-field gamma–ray (15–350 keV) Burst
Alert Telescope (BAT, Barthelmy et al. 2005a), with the two
Send offprint requests to: e-mail: perri@asdc.asi.it
co-aligned narrow-field instruments on-board the observatory:
the X–Ray Telescope (XRT, Burrows et al. 2005a), operating
in the 0.2–10 keV energy band, and the Ultraviolet/Optical
Telescope (UVOT, Roming et al. 2005), sensitive in the 1700–
6000 Å band. The Swift unique fast pointing capability is cru-
cial in the X–ray energy band, where the reaction times of other
satellites are limited to time scales of several hours. With Swift,
thanks also to the XRT high sensitivity, it is possible for the first
time to study in detail the evolution of the X–ray afterglows of
GRBs during their very early phases.
http://arxiv.org/abs/0704.1297v1
2 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
Indeed, one of the main results of Swift is the identification
of unexpected and complex features in the early X–ray
afterglows. In particular, three distinct phases are observed in
the majority of GRB light curves: an early (t<500 seconds
from the trigger) steep decline, with a power-law index of ∼ 3,
a second (t<10 ks) very shallow phase with a slope of ∼ 0.5, a
third phase characterized by a more conventional decay slope
of ∼ 1 (e.g. Tagliaferri et al. 2005; Cusumano et al. 2006a;
Nousek et al. 2006; O’Brien et al. 2006). In a few cases (GRB
050315, Vaughan et al. 2006; GRB 050318, Perri et al. 2005;
GRB 050505, Hurkett et al. 2006; GRB 050525A,
Blustin et al. 2006; GRB 060614, Mangano et al. 2007) a
further steepening with a decay slope of ∼ 2, consistent
with a jet break, is observed. Moreover, Swift had detected
in about one half of the bursts strong flaring activity in
the X–ray energy band superimposed on the afterglow
decay (e.g. Burrows et al. 2005b; Romano et al. 2006;
Falcone et al. 2006).
The understanding of the origin of these bright X–ray
flares is intensively discussed in the literature. A mech-
anism proposed as responsible for the flaring activity is
late internal shocks (Burrows et al. 2005b; Fan & Wei 2005;
Zhang et al. 2006; Liang et al. 2006). In this scenario the X–
ray flares are produced by the same internal dissipation pro-
cesses which cause the prompt emission, likely internal shocks
within the expanding fireball occurring before it is decel-
erated in the external medium (e.g. Rees & Mészáros 1994).
This model requires that the GRB central engine is still ac-
tive after the end of the prompt emission and various mech-
anisms providing such extended internal activity have been
put forward (e.g. King et al. 2005; Perna et al. 2006). An al-
ternative scenario has been recently considered by Guetta et
al. (2007) who, based on a detailed analysis of the X–ray
flaring activity observed in the afterglow of GRB 050713A,
interpreted the X–ray flares as due to refreshed shocks, i.e.
late time collisions with the main afterglow shock of slow-
moving shells ejected from the central engine during the
prompt phase (Rees & Mészáros 1998; Kumar & Piran 2000;
Sari & Mészáros 2000).
In this paper we present a detailed analysis of Swift and
XMM-Newton observations of GRB 050730, focusing on the
intense and extended X–ray flaring activity that characterizes
its afterglow. In Section 2 the observations and the data re-
duction are presented, in Section 3 we describe the tempo-
ral analysis and Section 4 is dedicated to the spectral anal-
ysis. The results are discussed in Section 5 and finally in
Section 6 we summarize our findings. Throughout this pa-
per errors are quoted at the 90% confidence level for one pa-
rameter of interest (∆χ2 = 2.71) unless otherwise specified.
We adopted the standard ΛCDM cosmological parameters of
Ωm = 0.27, ΩΛ = 0.73 and H0 = 70 km s
−1 Mpc−1. Times are
referenced from the BAT trigger T0 while temporal and spec-
tral indices are written following the notation F(t, ν) ∝ t−αν−β.
Results on the optical spectrum of the afterglow of this
GRB are reported by Starling et al. (2005), Chen et al. (2005a)
and Prochaska et al. (2006). Multi-wavelength observations of
the afterglow of GRB 050730 are presented by Pandey et
al. (2006).
−100 0 100 200 300
Time since BAT trigger (s)
Swift BAT
Fig. 1. BAT 20–150 keV background subtracted light curve of
the prompt emission of GRB 050730. Data are binned to 10
seconds resolution and errors are at the 1σ level. The horizontal
dashed line indicates the 0 level.
2. Observations and data reduction
2.1. Swift BAT
The BAT detected and located GRB 050730 at
T0=19:58:23 UT on 2005 July 30 (Holland et al. 2005).
On the basis of the refined ground analy-
sis (Markwardt et al. 2005), the BAT position is
RA(J2000)=212.◦063, Dec(J2000)=−3.◦740, with a 90%
containment radius of 3′. The BAT prompt emission light
curve (Fig. 1) is characterized by a duration of T90 = 155±20 s.
The emission starts ∼ 60 s before the trigger, peaks at ∼ 10 s
after the trigger and declines out to ∼ 180 s after the trigger.
The time averaged (over T90) spectrum in the 15–150 keV
energy band is well described by a power-law model with en-
ergy index βBAT = 0.5 ± 0.1 and χ
r = 0.71 (with 56 degrees
of freedom, d.o.f.). The total fluence in the 15–150 keV band
is (2.4 ± 0.3) × 10−6 erg cm−2. Assuming as redshift z = 3.969
(Chen et al. 2005b, see Sect. 2.2), the isotropic-equivalent radi-
ated energy in the BAT bandpass (74.5–745.4 keV in the burst
rest frame) is EBATiso = (8.0 ± 1.0) × 10
52 erg.
2.2. Ground-based Observatories
Following the identification of the optical counterpart by
UVOT (Holland et al. 2005), the field of GRB 050730 was ob-
served by numerous ground-based telescopes. The afterglow
detections in the R (Sota et al. 2005), r′ (Gomboc et al. 2005),
I and J (Cobb et al. 2005) bands were soon distributed via the
GRB Circular Network (GCN). Observations in the optical
band with the MIKE echelle spectrograph on Magellan II led to
the GRB redshift measurement z = 3.969 (Chen et al. 2005b)
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 3
based on the detection of a strong hydrogen Lyα absorption line
and of several absorption lines from other elements. The red-
shift was later confirmed with observations by the ISIS spectro-
graph on the William Herschel Telescope (Rol et al. 2005), the
IMACS imaging spectrograph on the Magellan Observatory
Baade Telescope (Holman et al. 2005a), FORS1 and UVES on
the Very Large Telescope (D’Elia et al. 2005).
The optical afterglow decay at later times was followed
by several optical telescopes. In particular, measurements
were made in the R band up to ∼4 days after the trig-
ger (Holman et al. 2005a, 2005b; Burenin et al. 2005;
Klotz et al. 2005; Damerdji et al. 2005; D’Elia et al. 2005;
Bhatt & Sahu 2005; Kannappan et al. 2005).
Finally, a radio afterglow with a flux density
Fr = 145 ± 28 µJy at 8.5 GHz was detected about 2 days
after the trigger using the Very Large Array (Cameron 2005).
2.3. Swift UVOT
Swift UVOT began to observe the field of GRB 050730
at 20:00:22 UT, 119 seconds after the BAT trigger. The first
100 seconds exposure in the V band led to the identifi-
cation of the optical afterglow at RA(J2000)=14h08m17.s09,
Dec(J2000)=-03◦46′18.′′9 (Holland et al. 2005).
We refined the preliminary photometric analysis
(Blustin et al. 2005) processing all UVOT data using the
standard UVOT software package (Swift software v. 2.1 in-
cluded in the HEAsoft package v. 6.0.2). The flux in all filters
was estimated by integrating over a 3.5′′ region. A background
region for subtraction of the sky contribution to the flux has
been selected in a relatively empty part of the field of view.
The results are listed in Table 1. No significant detection was
found in the U and UV filters, which is consistent with the
high redshift measured for this GRB. All the magnitudes
are corrected for Galactic extinction (E(B − V) = 0.049,
Schlegel et al. 1998).
2.4. Swift XRT
The XRT observations of the GRB 050730 field started at
20:00:28 UT, 125 seconds after the BAT trigger, with the in-
strument in Auto State. After a first exposure in Image Mode
(see Hill et al. 2004 for a description of readout modes) dur-
ing which no on-board centroid was determined, the instrument
switched into Windowed Timing (WT) mode for the entire first
Swift orbit from T0+133 s to T0+794 s. Starting from the sec-
ond orbit (T0+4001 s), the instrument was in Photon Counting
(PC) mode for 26 consecutive orbits until 11:49:22 UT on 2005
August 1 (T0+143.4 ks). The field of GRB 050730 was re-
observed with the XRT from August 3 starting at 15:28:11 UT
(T0+329.4 ks) until August 5 13:57:02 UT (T0+496.7 ks).
The XRT data were processed with the XRTDAS soft-
ware (v. 1.7.1) included in the HEAsoft package (v. 6.0.4).
Event files were calibrated and cleaned with standard filter-
ing criteria with the xrtpipeline task using the latest calibration
files available in the Swift CALDB distributed by HEASARC.
Events in the energy range 0.3–10 keV with grades 0–12 (PC
mode) and 0–2 (WT mode) were used in the analysis (see
Burrows et al. 2005a for a definition of XRT event grades).
After the screening, the total exposure time for the first XRT
observation was 649 seconds (WT) and 58480 seconds (PC),
while for the follow-up observation the PC exposure time was
34669 seconds.
In the 0.3–10 keV PC image of the field a previ-
ously uncatalogued X–ray source was visible within the
BAT error circle with coordinates RA(J2000)=14h08m17.s2,
Dec(J2000)=−03◦46′19′′. This position, derived using data
not affected by pile-up (orbits 5–26, see below), has
a 90% uncertainty of 3.5′′ using the latest XRT bore-
sight correction (Moretti et al. 2006) and is consistent with
the position of the optical counterpart (Holland et al. 2005;
Jacques & Pimentel 2005).
For the WT mode data, events for temporal and spectral
analysis were selected using a 40 pixel wide (1 pixel corre-
sponds to 2.36′′) rectangular region centered on the afterglow
and aligned along the WT one dimensional stream in sky co-
ordinates. Background events were extracted from a nearby
source-free rectangular region of 50 pixel width. For PC mode
data, the source count rate during orbits 2–4 was above ∼ 0.5
counts s−1 and data were significantly affected by pile-up in the
inner part of the Point Spread Function (PSF). After comparing
the observed PSF profile with the analytical model derived by
Moretti et al. (2005), we removed pile-up effects by excluding
events within a 5 pixel radius circle centered on the afterglow
position and used an outer radius of 30 pixels. From orbit 5
the afterglow brightness was below the pile-up limit and events
were extracted using a 10 pixel radius circle, which encloses
about 80% of the PSF at 1.5 keV, to maximize the signal to
noise ratio. The background for PC mode was estimated from
a nearby source-free circular region of 50 pixel radius. Source
count rates for temporal analysis were corrected for the fraction
of PSF falling outside the event extraction regions. Moreover,
the loss of effective area due to the presence of 2 CCD hot
columns within the extraction regions was properly taken into
account. The count rates were then converted into unabsorbed
0.3–10 keV fluxes using the conversion factor derived from the
spectral analysis (see Sect. 4).
For the spectral analysis, ancillary response files were gen-
erated with the xrtmkarf task applying corrections for the PSF
losses and CCD defects. The latest response matrices (v. 008)
available in the Swift CALDB were used and source spectra
were binned to ensure a minimum of 20 counts per bin in order
to utilize the χ2 minimization fitting technique.
2.5. XMM-Newton
XMM-Newton follow-up observations of GRB 050730
started 26.4 ks (for the two EPIC-MOS cameras) and 29.4 ks
(for the EPIC-PN) after the initial BAT Trigger. The XMM-
Newton ODF (Observation Data Files) data were pro-
cessed with the epproc and emproc pipeline scripts, using
the XMM-Newton SAS analysis package (v. 6.5). A bright
rapidly decaying source was detected near the aim-point of
4 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
Fig. 2. Swift XRT (filled circles) and XMM-Newton (filled squares) 0.3–10 keV light curve of the X–ray afterglow of GRB 050730.
The BAT 20–150 keV prompt emission light curve, extrapolated to the 0.3–10 keV band, is also shown as filled triangles. UVOT
optical data in the V and B bands, arbitrarily scaled for comparison with the X–ray band, are indicated with open squares and open
triangles, respectively. The solid line is the best fit model to the XRT and XMM-Newton light curve. The dashed line represent
the underlying double broken power-law decay (see Sect. 3.1). The dotted line is a power-law model with temporal decay index
αV = 0.3 normalized to fit data points in the V band. Errors are at the 1σ level.
all three EPIC detectors and the afterglow was localized at
RA(J2000)=14h08m17.s3, Dec(J2000)=-03◦46′18.′′5. The dura-
tion of the XMM-Newton follow-up observation was 33.7 ks
(MOS) and 30.4 ks (PN). After screening out times with high
background flaring, the dead-time corrected net exposures were
25.0 ks (MOS) and 17.9 ks (PN). All three EPIC cameras (PN
and 2 MOS) were used in Full Window Mode, with PN and
MOS2 using the “Thin” filter and MOS1 using the “Medium”
optical blocking filter.
Source spectra and light curves for all 3 EPIC cameras were
extracted from circular regions of 30′′ radius centered on the
afterglow. Background data were taken from a 60′′ radius cir-
cle on the same chip as the afterglow, but free of any back-
ground X–ray sources. The data were further screened by in-
cluding only good X–ray events (using the selection expres-
sion flag=0 in evselect), by including events with single and
double pixel events (pattern<=4) for the PN and by selecting
single to quadruple pixel events (pattern<=12) for the MOS.
Data below 300 eV and above 10 keV were also removed.
For the temporal analysis, we adopted the light curve from
the MOS data, primarily because the PN data was heavily af-
fected by background flares towards the end of the observation,
while the MOS covered a wider duration at the beginning of the
observation. The data from the two MOS detectors were com-
bined and the count rate to 0.3–10 keV unabsorbed flux conver-
sion factor was calculated from the best fit absorbed power-law
spectrum (see Sect. 4).
For the spectral analysis, ancillary and redistribution re-
sponse files for fitting were generated with the SAS tasks ar-
fgen and rmfgen, respectively. Moreover, source spectra were
binned to a minimum of 25 counts per bin.
3. Temporal analysis
The background subtracted 0.3–10 keV Swift XRT and
XMM-Newton light curves of the X–ray afterglow of GRB
050730 are shown in Fig. 2. The same figure also shows the
BAT 20–150 keV prompt emission light curve, converted in the
0.3–10 keV energy band using the BAT spectral best fit model
which is valid also in the XRT bandpass (see Sect. 4.1). The
UVOT optical light curves in the V and B bands, in arbitrary
units, are also plotted (see Sect. 2.3).
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 5
Fig. 3. Swift XRT 0.3-10 keV light curve of the GRB 050730
X–ray afterglow during the first orbit. The solid line is the best
fit model to the data obtained considering a linear rise exponen-
tial decay for the three flares (see Sect. 3.1). The dashed vertical
lines delimit the seven time intervals considered for the spectral
analysis. Data are binned to 8 seconds resolution and errors are
at the 1σ level.
3.1. X–ray afterglow
The X–ray afterglow of GRB 050730 is characterized by
a very complex structure. The first Swift orbit (from T0+133 s
to T0+794 s), after an initial steep decay phase that joins well
with the end of the BAT prompt emission, is dominated by three
bright X–ray flares peaking at about 235, 435 and 685 seconds
after the BAT trigger. Taking into account cosmological time
dilation these times correspond to about 47, 88 and 138 sec-
onds in the GRB rest frame. A flaring episode is also observed
in the second orbit peaking at about T0+4500 s. While the un-
derlying decay of the afterglow during the first two orbits is
shallow, starting from the third orbit (T0+10 ks) the afterglow
light curve shows a much steeper decline with superimposed
flaring activity.
We first modeled the X–ray light curve of the afterglow
with a double broken power-law model with slopes α1, α2, α3
and temporal breaks t1, t2, describing the underlying power-law
decay of the afterglow, plus seven Gaussian functions model-
ing the flaring episodes. We found for the first power-law a
decay index α1 = 2.1 ± 0.3 followed, after a first time break at
t1 = 237±20 s, by a shallower decay with index α2 = 0.44
+0.14
−0.08.
A second temporal break is found at t2 = 10.1
−2.2 ks after
which a steep decay with index α3 = 2.40
+0.09
−0.07 is observed.
This model did not provide a good fit (χ2r = 1.73, 143 d.o.f.),
mostly due to short time scale fluctuations and to deviations of
the three first bright flares from a symmetric Gaussian shape.
We thus considered a different functional form for the three
X–ray flares, namely a linear rise exponential decay: F(t) ∝
(t − t0)/(tp − t0) for times between the flare start time t0 and
Fig. 4. Swift XRT 0.3–1.5 keV (upper panel) and 1.5–10 keV
(middle panel) light curve of the GRB 050730 X–ray afterglow
during the first Swift orbit. In the lower panel the corresponding
hardness ratio is plotted. Data are binned to 12 seconds reso-
lution and error bars indicate statistical uncertainties at the 1σ
level.
peak time tp, F(t) ∝ exp[−(t − tp)/tc] for t > tp where tc is the
exponential decay time. The model improves significantly the
fit with χ2r = 1.43 (140 d.o.f.) and F-test chance probability of
1.3 × 10−6. The best fit parameters of the overall underlying
double broken power-law model were unchanged with respect
to the previous fit listed above. The linear rise exponential de-
cay best fit parameters for the first three flares are given in Table
2 while Table 3 reports the Gaussian best fits for the other four
flares. As Fig. 3 illustrates, the second brightest flare (referred
to as flare 2 in the following) has a flux variation of amplitude
∼ 3.6 and is characterized by a steep rise, lasting ∼ 90 seconds,
followed by a slower decay with duration ∼ 170 seconds. An
asymmetric shape, with a steep rise followed by a shallower
decay, was also observed for the other two flares (flare 1 and 3)
and for these episodes the flux variation was ∼ 1.7 and ∼ 2.6,
respectively.
The ratio between the duration (∆t) and the peak time (tp) of
the X–ray flares was calculated using the linear rise exponential
decay and Gaussian best fit parameters for the first three and
the last four flares, respectively. For both analytical models the
duration of a flare was defined as the time interval during which
the flare intensity was above 5% of its peak value. The results
are given in Tables 2 and 3.
The rise and decay portions of the first three flares were
also fit with single power-laws. For the estimation of power-
law indices, times were expressed from the onset of the flares
(t0) for the rise, from the peak time (tp) for the decay, and the
contribution of the underlying afterglow decay was taken into
account. The power-law temporal indices for the rising (αr) and
decaying (αd) portions of the three flares are reported in Table
6 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
The temporal behavior of the afterglow during the first three
X–ray flares was also studied in different energy bands. In
Fig. 4 we show the 0.3–1.5 keV (upper panel) and 1.5–10
keV (middle panel) light curves of the early X–ray afterglow
together with the corresponding hardness ratio (lower panel).
From the figure it is apparent that i) in the harder band the pro-
file of the flares is sharper and ii) a peak time shift, with flares
in the hard band peaking at earlier times, is observed. We note
that these temporal properties as a function of energy are the
same observed for the prompt emission pulses of GRBs.
As already mentioned, late flaring activity of the X–ray af-
terglow was observed at ∼ 4.5, 10.4, 18.7 and 41.2 ks after
the trigger. Due to the Swift orbital gaps the temporal cover-
age of flares 4, 5 and 6 is poor and these episodes could not
be well studied, but the last flare was entirely covered by the
XMM-Newton follow-up observation allowing a detailed tem-
poral analysis. In Fig. 5 the XMM-Newton (MOS1+MOS2)
light curve is shown together with the XRT curve covering the
same time interval. A very good agreement between the two
curves is found and the ∼ 5% higher normalization of the XRT
data points is of the same order of the uncertainties in the abso-
lute flux calibration of the instrument (Campana et al. 2006).
We first fit the XMM-Newton light curve with a single
power-law decay and obtained an extremely poor fit to the
light curve, with a fit statistic of χ2r = 8.81 (32 d.o.f.) and
a decay index of α = 2.10 ± 0.04. The light curve was then
parameterized with a long duration flare super-imposed on a
steep power-law decay. We obtained an underlying decay in-
dex α = 2.45 ± 0.15, while the flare could be adequately mod-
eled with a Gaussian function; the fit statistic was acceptable
(χ2r = 1.22, 29 d.o.f.). The long duration flare peaked at 41 ks
(or 8.2 ks in the GRB rest frame), with a σ of 6.8 ks (1.4 ks
rest frame). The total fluence of the flare when parameterized
this way was 8.3 × 1050 erg (1.5-50 keV band in the GRB rest
frame), which represents 20% of the integrated afterglow emis-
sion over the XMM-Newton observation. The rise and decay
phases of flare 7 were also fit with single power-laws express-
ing times from the onset and from the peak time of the flare,
respectively. The measured temporal indices αr and αd are also
reported in Table 4.
3.2. Optical band
The UVOT optical light curve of the afterglow of GRB
050730 in the V and B bands is also characterized by a complex
behavior, as is illustrated in Fig. 2.
The early (T0+100 s - T0+800 s) UVOT V light curve re-
veals flaring activity. A re-brightening is in fact observed at
∼ T0+500 s, almost simultaneously with the brightest X–ray
flare observed with XRT. Due to the relatively poor sampling of
the UVOT light curve, a detailed temporal analysis is not possi-
ble, however we note that the amplitude of the flux variation in
the V band (∼ 3) is of the same order of the one measured in the
X–ray energy band. The decay of the afterglow in the V band,
up to about T0+12 ks and excluding the re-brightening episode,
is shallow (αV ∼ 0.3) and in agreement with the one measured
in the X–ray band (αX = 0.44
+0.14
−0.08). Hints of a steepening of
Fig. 5. XMM-Newton (MOS1+MOS2, filled circles) and Swift
XRT (open circles) 0.3–10 keV light curve of the late (∼ T0+12
hours) X–ray afterglow of GRB 050730. The solid line is the
best fit model to the XMM-Newton light curve. The dashed line
represent the underlying power-law decay (see Sect. 3.1). Data
points errors are at the 1σ level.
the V light curve at ∼ T0+12 ks are also found, but the limited
temporal coverage and the large statistical uncertainties char-
acterizing the UVOT data points do not allow us to constraint
the late optical afterglow decay.
The UVOT light curve in the B band is characterized by a
flat power-law decay (αB ∼ 0.3) without re-brightening events.
However, it should be noted that i) the curve is very sparsely
sampled, with only three data points during the first Swift orbit,
and ii) in the B band there is a strong flux reduction due to the
Lyman break and thus large statistical uncertainties affect the
data points.
Accurate optical observations of the afterglow of GRB
050730 in the R and I bands are presented by Pandey et
al. (2006). The authors report an early time flux decay with
indices αR = 0.54± 0.05 and αI = 0.66± 0.11 followed, after a
temporal break at about 9 ks after the trigger, by a strong steep-
ening with decay indices αR = 1.75±0.05 and αI = 1.66±0.07.
4. Spectral analysis
For the spectral analysis of Swift XRT and XMM-Newton data
we used the XSPEC package (v. 11.3.2, Arnaud 1996) included
in the HEAsoft package (v. 6.0.4).
4.1. XRT
As a first step, the 0.3–10 keV XRT average spectrum
during the first Swift orbit (WT mode, from T0+133 s to
T0+794 s) was fit adopting a single power-law model with the
neutral hydrogen-equivalent absorption column density fixed
at the Galactic value in the direction of the GRB (NGH =
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 7
3.0 × 1020 cm−2, Dickey & Lockman 1990). The fit obtained
was poor (χ2r = 1.28, 311 d.o.f.). From the inspection of
residuals a strong deficit of counts at low energies, likely due
to the presence of an absorbing column density in excess to
the Galactic one, was found. Indeed, the addition of a col-
umn density NzH using Solar metallicity redshifted to the rest
frame of the GRB host (z = 3.969) free to vary in the spec-
tral fit (zwabs model in XSPEC) resulted in a more acceptable
fit with a column density of NzH = (1.28
+0.26
−0.25) × 10
22 cm−2 (see
Table 5). This value is consistent with the neutral hydrogen
column densities derived from the optical spectra reported by
Starling et al. (2005) and Chen et al. (2005a).
From Fig. 4 it is apparent that strong spectral evolution
takes place during the intense flaring activity observed in the
first Swift orbit. We thus split the WT observation in seven
time intervals to study the spectra during the rise and the de-
cay portions of each flare (see Fig. 3). The time-resolved spec-
tral best fits (see Table 5) clearly show evidence for spectral
variation during the flares and an overall softening of the spec-
tra with time associated with a decrease of the rest frame col-
umn density. The time-resolved spectral analysis was also per-
formed adopting a broken power-law model to investigate the
possible presence of spectral breaks (e.g. Falcone et al. 2006;
Guetta et al. 2007). Also in this case an additional absorption
column density NzH at the rest frame of the GRB host was con-
sidered. For all segments we did not find evidence for spectral
breaks within the XRT energy band.
The XRT late 0.3–10 keV spectrum (PC mode, from
T0+4.0 ks to T0+143.4 ks) was also fit using a single power-
law model with a fixed Galactic absorption column density and
an additional absorbing column at the burst rest frame. The ob-
servation was divided in two time intervals (4.0–18.1 ks and
21.3–143.4 ks from the trigger). The results are listed in Table 5
where we see that spectral softening between the two time in-
tervals was found.
4.2. XMM-Newton
The PN and MOS 0.3–10 keV spectra were fit jointly al-
lowing the cross normalization between the detectors, which is
consistent within < 5%, free to vary . The two MOS spectra
and responses were combined to maximize the signal to noise,
after first checking that they were consistent with each other.
As in the case of the XRT spectrum, the PN and MOS spec-
tra were first fit with a single power-law model with the neu-
tral hydrogen-equivalent absorption column fixed at the known
Galactic value. The fit obtained was not acceptable (χ2r = 1.40,
489 d.o.f.), while the energy index was β = 0.76 ± 0.02.
The addition of a neutral absorption column in the GRB host
galaxy frame at z = 3.969 resulted in more acceptable fit
(χ2r = 1.14, 489 d.o.f.) with an excess absorption column above
the Galactic column of (6.8 ± 1.0) × 1021 cm−2, while the con-
tinuum energy index was now β = 0.87 ± 0.02.
The XMM-Newton afterglow spectra were also sliced into
three segments of approximately 10 ks in duration, in order to
search for any spectral evolution within the XMM-Newton ob-
servation. A small change in the continuum parameters was
found, the spectrum evolved from hard to soft; the energy
index changed from β = 0.87 ± 0.03 in the first 10 ks, to
β = 0.99 ± 0.05 during the final segment. No evidence was
found for a change in the column density, which was subse-
quently fixed at NH = 6.8 × 10
21 cm−2 in all the segments. The
spectral best fit parameters are shown in Table 5.
We also searched for any evidence of emission lines, either
in the mean spectrum, or in the three segments. No statistically
significant lines were found, at the level of > 99% confidence.
As the redshift of the burst is known, then we can set an upper-
limit to the equivalent width of any emission lines. Over the
range of 0.4–8 keV and using the mean spectrum, we found a
< 100 eV upper limit to any emission lines. More specifically
we can set a limit on the iron Kα line (e.g. the H-like line at
6.97 keV rest frame, 1.40 keV observed frame) of < 30 eV.
5. Discussion
5.1. Early X–ray light curve
This GRB, with its rather high redshift, z = 3.969, gives
us the possibility to investigate the X–ray and optical light
curves soon after the trigger and thus to study in detail the
soft tail of the prompt emission and the very beginning of
the afterglow phase. Indeed, due to cosmological time dilation
the XRT and UVOT observations started, in the rest frame of
the burst, only 27 seconds after the trigger. Other examples of
such early observations of high redshift bursts are GRB 050904
(z = 6.29, Cusumano et al. 2006b) and GRB 060206 (z = 4.0,
Monfardini et al. 2006).
The XRT light curve shows at the very beginning (133–
205 s from the BAT trigger) a rapidly decaying emission that
joins quite nicely with the BAT flux when converted to the
XRT bandpass, a feature that has been observed in various
Swift bursts (Tagliaferri et al. 2005, Barthelmy et al. 2005b;
Nousek et al. 2006; O’Brien et al. 2006). For GRB 050730,
the steep temporal decay index (α1 = 2.1 ± 0.3, see
Sect. 3.1) characterizing the early X–ray light curve sug-
gests that the observed emission is likely associated with
the tail of the prompt emission rather than to the beginning
of the shocked inter-stellar medium afterglow phase. Indeed,
in the internal shock model scenario for the prompt γ–ray
emission (e.g. Rees & Mészáros 1994), the cessation of the
emissivity is characterized by a rapid decay due to the de-
layed arrival of the high angular latitude prompt emission
of the shocked surface (high-latitude or curvature emission,
Kumar & Panaitescu 2000; Dermer 2004). The high-latitude
emission predicts a relationship between the temporal decay in-
dex αd and the spectral index βd during the decay given by the
equation αd = 2+βd (Kumar & Panaitescu 2000) where the de-
cay slope is measured using as zero time (t0) the beginning of
the prompt emission (F(t, ν) ∝ (t − t0)
−αdν−βd ). We have tested
this prediction by fitting the early XRT light curve (segment
“0” in Fig. 3) with the above constraint using the spectral index
measured during the decay (βd = 0.42 ± 0.08, see Table 5) and
leaving the zero time as a free parameter (see Liang et al. 2006
for a description of the method). We found t0 = −31
−31 s, i.e.
8 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
the zero time is located at the rising segment of the BAT prompt
emission light curve (see Fig. 1), as expected in the framework
of the high-latitude emission. This result indicates that the steep
decay observed in the early XRT light curve is most likely the
soft tail of the BAT emission.
This hypothesis is also supported by the BAT and early
(segment “0” in Fig. 3) XRT spectra. We found very similar
spectral slopes (βBAT = 0.5 ± 0.1 and βXRT = 0.42 ± 0.08,
respectively), likely indicating that the early X–ray and γ–ray
emissions are produced by the same emission mechanism.
5.2. Flaring activity
The exceptionally extended flaring activity of the X–ray af-
terglow of GRB 050730 allows us to study this phenomenon
over more than two orders of magnitude in time. The XRT
early steep decay is followed by three bright X–ray flares peak-
ing at 236, 437 and 685 seconds after the BAT trigger (see
Fig. 3). These flares were under the sensitivity of the BAT
instrument and thus were not detected in the hard X–ray en-
ergy band. These flares show, as other strong flares observed
with Swift (e.g. Romano et al. 2006; Falcone et al. 2006;
Pagani et al. 2006; Godet et al. 2006; Chincarini et al. 2006), a
clear spectral evolution with the hardness ratio that mimics
the variation of the light curve. A phase lag, with the harder
(E > 1.5 keV) light curve peaking at earlier times with respect
to the softer energy band is also found (see Fig. 4).
So far, most of the X–ray flares observed in Swift X–
ray light curves have been interpreted as due to late in-
ternal shocks (e.g. Burrows et al. 2005b; Romano et al. 2006;
Falcone et al. 2006; Godet et al. 2006). In this scenario the
GRB central engine is active far beyond the end of the
prompt γ–ray emission phase requiring new mechanisms ca-
pable of powering new relativistic outflows at late-time (e.g.
King et al. 2005; Perna et al. 2006). A diagnostic to check if
the re-brightenings are due to late internal shocks has been re-
cently proposed by Liang et al. (2006). In the internal-origin
scenario for X–ray flares the decay emission of the flaring
episodes should be dominated by high-latitude emission with
the decay temporal index related to the decay spectral index as
αd = βd + 2 (Kumar & Panaitescu 2000). We have checked this
hypothesis for the first three bright X–ray flares observed in the
GRB 050730 afterglow. The spectral indices measured during
the decays were used (segments “1b”, “2b” and “3b”, see Fig. 3
and Table 5) and we found best fit zero times t0,1 =193
−11 s,
t0,2 =341
−42 s and t0,3 =592
−93 s, respectively. We can see that
for all three episodes the zero time values are located at the be-
ginning of the corresponding flare and thus the observed decay
slopes are consistent with being due to high-latitude emission
as predicted by the late internal shock scenario.
Another possibility is that the X–ray flares are pro-
duced by refreshed shocks (Rees & Mészáros 1998;
Kumar & Piran 2000; Sari & Mészáros 2000). In the standard
internal-external fireball model (e.g. Rees & Mészáros 1994;
Sari & Piran 1997) re-brightening episodes in the afterglow
light curve are explained as slow shells ejected from the
central engine during the prompt phase that catch up with the
main afterglow shock after it has decelerated in the external
inter-stellar medium. Indeed, Guetta et al. (2007) have recently
interpreted the X–ray flares of the afterglow of GRB 050713A
as due to refreshed shocks. Here we applied the diagnostic
proposed for GRB 050713A to the case of GRB 050730. First,
to estimate the rise and decay slopes of the early flares we
operationally selected as zero time the beginning and the peak
of the flare, respectively. This assumption implicitly means
that the flare is completely independent from the main event
generating the prompt emission and the forward shock light
curve. We found temporal indices in the 0.8–1.8 range (see
Table 4). Much steeper slopes (∼ 3–6) are obtained if instead
times are referenced from the BAT trigger, as usually done
for most of the Swift X–ray flares (e.g. Burrows et al. 2005b;
Romano et al. 2006; Falcone et al. 2006). Second, we checked
if the temporal and the spectral indices during the decay
phase are related as predicted by the standard afterglow model
(Sari, Piran & Narayan 1998). We restricted the analysis to
the episodes which have enough statistics to allow a detailed
temporal and spectral study, i.e. the first three flares and the last
one. For the decay phases we expect the following relations
to hold (Sari, Piran & Narayan 1998; Dai & Cheng 2001):
Fν ∝ (νm/νc)
−1/2(ν/νm)
−p/2Fν,max ∝ t
−3p/4+1/2 (for p > 2)
and Fν ∝ (νm/νc)
(p−1)/2(ν/νc)
−p/2Fν,max ∝ t
−(3p+10)/16 (for
1 < p < 2). From the best fit spectral energy indices during
the decay of the four flares (see Table 5) we derived the
predicted temporal slopes α1d = 0.93 ± 0.05, α
d = 0.89 ± 0.03,
α3d = 1.02 ± 0.15, and α
d = 1.40 ± 0.05, which are consistent
with the measured values of the decay indices listed in Table
4. We note that for the last flare, peaking at 41.2 ks after the
trigger, we used the afterglow model relations for the slow
cooling case (Fν ∝ (ν/νm)
−(p−1)/2Fν,max ∝ t
3(1−p)/4) which very
likely applies at these late times (Sari, Piran & Narayan 1998).
Important information on the mechanism producing the
flares can also be obtained from the comparison between the
observed variability timescale and the time at which the flare
is observed (Ioka et al. 2005). We calculated, for all the 7 re-
brightenings of the GRB 050730 afterglow, the ratio between
the duration of the flare (∆t) and the time when the flare peaks
(tp) (see Tables 2 and 3). We found ∆t/tp ∼ 0.3 for the first
flare and ∆t/tp ∼ 0.6 − 0.7 for the others, in agreement with
the ∆t/tp > 0.25 limit discussed by Ioka et al. (2005) for the
refreshed shock scenario. Remarkably, the ∆t/tp ratio is nearly
constant for all flares, with the exception of the first one which
occurred during the bright and steep tail of the prompt emis-
sion (see Fig. 3) and most likely we are observing only the tip
of the flare. A duration of the flare proportional to tp, as ob-
served in the afterglow of GRB 050730, is explained both in
the refreshed shock scenario (Kumar & Piran 2000) and in the
late internal shock model (Perna et al. 2006). We note that the
last four flares have ∆t/tp values in the range 0.7–0.9, while
for flares 2 and 3 we observe slightly lower ratios (0.5–0.6),
possibly indicating a moderate temporal evolution of ∆t/tp.
However, due to the relatively large error bars, the measured
values are also consistent with being constant and ∼ 0.6 − 0.7.
Flaring activity is also observed in the optical afterglow
of GRB 050730. Indeed, the UVOT V early light curve (see
Fig. 2) shows a re-brightening at ∼ T0+500 s, almost simul-
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 9
taneously with the brightest X–ray flare observed with the
XRT peaking at T0+437 s. Moreover, the optical re-brightening
amplitude (∼ 3) is of the same order of the flux variation
observed in the X–ray energy band. Strong indications of
correlated variability between the X–ray and optical energy
bands are also present in the NIR/Optical light curves re-
ported by Pandey et al. (2006): a significant re-brightening
at about T0+4 ks (R filter), close to the X–ray flare peaking
at T0+4.5 ks (see Fig. 2), is observed. Moreover, a bump in
the J, I, R and V light curves at about T0+10 ks is observed
(Pandey et al. 2006), again simultaneous with the X–ray flare
peaking at T0+10.4 ks. Although the X–ray flaring activity is
not uniformly covered by optical observations, we find several
indications of simultaneous re-brightening events in the X–ray
and optical bands, in agreement with the refreshed shock model
(e.g. Granot et al. 2003).
5.3. Evidence of a jet break
The X–ray afterglow light curve shows a clear temporal
break around 10 ks after the trigger (see Sect. 3.1). At about
the same time a strong steepening of the I and R afterglow
light curves, although with a shallower post-break slope, is also
observed (see Pandey et al. 2006 for a detailed analysis on the
discrepancy between the X–ray and optical slopes). Due to the
achromatic nature of the temporal break it is very likely that a
jet break is occurring when the bulk Lorentz factor γ of the col-
limated relativistic outflow becomes lower than the inverse of
the jet opening angle θjet (e.g., Rhoads 1997, Sari et al. 1999),
as also reported by Pandey et al. (2006). In this framework,
the jet opening angle can be determined through the equation
θjet = 0.161[tb/(1 + z)]
3/8(nη/Eiso)
1/8 (e.g., Bloom et al. 2003)
where θjet is in radians, the jet temporal break tb in days, the
total isotropic-equivalent energy Eiso in units 10
52 erg, the den-
sity n of the circumburst medium in cm−3 and η is the efficiency
of conversion of the outflow kinetic energy in electromagnetic
radiation.
An accurate estimation of the bolometric isotropic-
equivalent energy radiated by GRBs requires the knowl-
edge of their intrinsic spectrum over a broad energy band
(Bloom et al. 2001; Amati et al. 2002). For GRB 050730, the
spectrum is well fit by a single power-law with energy index
βBAT = 0.5 ± 0.1 up to the high energy limit of the BAT sen-
sitivity bandpass (150 keV) indicating that i) we are observing
the low energy tail of the Band model (Band et al. 1993) gen-
erally used to describe GRB spectra and ii) the νF(ν) spectrum
peak energy Ep is above ∼ 750 keV (i.e. (1 + z) × 150 keV) in
the burst rest frame. A lower limit to the bolometric isotropic-
equivalent radiated energy Eiso is given by the observed radi-
ated energy in the BAT bandpass (EBATiso = (8.0±1.0)×10
52 erg,
see Sect. 2.1). An upper limit to Eiso is obtained in the most
conservative case where the peak energy Ep is equal to 10
4 keV
(Amati et al. 2002). By integrating the best fit BAT power-law
spectrum in the whole 1–104 keV rest frame energy band we
thus derived an upper limit of 4.5 × 1053 erg to Eiso. Taking
the central value of the interval derived above we obtained for
GRB 050730 Eiso = (2.6 ± 1.9) × 10
53 erg.
With z = 3.969, tb = 10.1
−2.2 ks (see Sect. 3.1) and the Eiso
range above derived, we find θjet = 1.6
−0.2 deg, for η = 0.2
(Frail et al. 2001) and assuming the value of circumburst den-
sity n = 10 cm−3 discussed by Bloom et al. (2003). With
this value of the jet opening angle, the inferred collimation-
corrected bolometric radiated energy is Ejet = (1.0
−0.8) ×
1050 erg.
Taking into account the Eiso and Ejet values derived above
and the lower limit to the peak energy (Ep > 750 keV rest
frame), we find that GRB 050730 is consistent with the Ep
vs. Eiso relation found by Amati et al. (2002) and recently
updated in Amati (2006). We also find that GRB 050730 is
inconsistent, even taking into account the 3σ scatter around
the best fit correlation, with the Ep vs. Ejet relation found by
Ghirlanda et al. (2004) and with its updated version presented
by Nava et al. (2006). In order to make this GRB consistent
with the Ep-Ejet relation a much higher circumburst density
(n ∼ 105 cm−3) would be required. GRB 050730 is inconsis-
tent with the model-independent Eiso-Ep-tb correlation found
by Liang & Zhang (2005).
6. Summary and conclusions
We have presented a detailed temporal and spectral analy-
sis of the afterglow of GRB 050730 observed with Swift and
XMM-Newton. The most striking feature of this GRB is the in-
tense and exceptionally extended, over more than two order of
magnitude in time, X–ray flaring activity.
Superimposed to the afterglow decay we observed seven
distinct re-brightening events peaking at 236 s, 437 s, 685 s,
4.5 ks, 10.4 ks, 18.7 ks and 41.2 ks after the BAT trigger. The
underlying decline of the afterglow was well described with a
double broken power-law model with breaks at t1 = 237± 20 s
and t2 = 10.1
−2.2 ks. The temporal decay slopes before, be-
tween and after these breaks were α1 = 2.1±0.3,α2 = 0.44
+0.14
−0.08
and α3 = 2.40
+0.09
−0.07, respectively.
Strong spectral evolution during the flares was present to-
gether with an overall softening of the underlying afterglow
with the energy index varying from β = 0.42 ± 0.08 dur-
ing the early (133–205 s) steep decay to β = 0.99 ± 0.05
at much later (50-60 ks) times. An absorbing column density
NzH = (1.28
+0.26
−0.25) × 10
22 cm−2 in the host galaxy is observed
during the early (133–781 s) Swift observations while a lower
column density NzH = (0.68 ± 0.10) × 10
22 cm−2 is measured
during the late (29.4–50.8 ks) XMM-Newton follow-up obser-
vation, likely indicating photo-ionization of the surrounding
medium. Evidence of flaring activity in the early UVOT op-
tical afterglow, simultaneous with that observed in the X–ray
band, was found.
From the temporal analysis of the first three bright X–ray
flares we found that the rise and decay power-law slopes are
in the range 0.8–1.8 if the beginning and the peak of the flares
are used as zero time, respectively. We also found that, with the
exception of the first flare, for all episodes the ratio between the
duration of the flare (∆t) and the time when the flare peaks (tp)
is nearly constant and is ∆t/tp ∼ 0.6 − 0.7.
We showed that the observed properties of the first three
flares are consistent with being due to both high-latitude emis-
10 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
sion, as expected for flares produced by late internal shocks,
or to late time energy injection into the main afterglow shock
by slow moving shells (refreshed shocks). An analysis of a
larger sample of bursts would help in understanding what are
the physical mechanisms responsible for the X-ray flaring ac-
tivity.
We interpreted the X–ray temporal break at around 10 ks
as a jet break and derived a cone angle of ∼ 2 deg and a radi-
ated energy Ejet = (0.2 − 3.3) × 10
50 erg against an isotropic-
equivalent energy Eiso = (0.7 − 4.5) × 10
53 erg. GRB 050730
satisfies the Ep vs. Eiso Amati relation while is inconsistent with
the Ep vs. Ejet Ghirlanda relation.
Acknowledgements. We are grateful to the referee for his/her useful
comments and suggestions. We also thank C. Guidorzi for a very care-
ful reading of the paper and F. Tamburelli and B. Saija for their work
on the XRT data reduction software. This work is supported in Italy
from ASI on contract number I/R/039/04 and through funding of the
ASI Science Data Center, at Penn State by NASA contract NAS5-
00136 and at the University of Leicester by the Particle Physics and
Astronomy Research Council on grant numbers PPA/G/S/00524 and
PPA/Z/S/2003/00507.
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http://swift.gsfc.nasa.gov/docs/heasarc/caldb/swift/docs/xrt/SWIFT-XRT-CALDB-09.pdf
http://arxiv.org/abs/astro-ph/0511107
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 11
Table 1. UVOT detections of GRB 050730 in the V and B filters. Column (1) gives the image mid time in seconds since the BAT
trigger, column (2) the net exposure time, column (3) the filter used, and column (4) the afterglow magnitude with 1σ error.
T(mid) Exposure Filter Magnitude
(s) (s)
170 99 V 17.4 ± 0.2
270 10 B 18.4 ± 0.4
299 9.7 V 17.2 ± 0.5
383 9.7 V 17.3 ± 0.5
468 9.7 V 16.4 ± 0.3
524 30 B 19.0 ± 0.4
552 9.7 V 16.3 ± 0.3
637 9.7 V 17.0 ± 0.4
721 9.7 V 16.8 ± 0.4
734 20 B 19.2 ± 0.5
10164 900 B 20.2 ± 0.3
11947 837 V 18.6 ± 0.2
23519 835 V 19.5 ± 0.3
34990 843 V 19.9 ± 0.4
Table 2. Temporal best fit parameters of the first three bright X–ray flares of GRB 050730 using a linear rise exponential decay
model. The corresponding ∆t/tp values for the three flares are also indicated (see Sect. 3.1).
Flare t0 tp tc K × 10
−10 ∆t/tp
(s) (s) (s) ( erg cm−2 s−1)
1 207+5
−5 236
−4 17
−5 8 ± 1 0.3 ± 0.1
2 344+5
−5 437
−4 54
−6 12.2 ± 0.7 0.58 ± 0.04
3 614+ 9
−10 685
−6 87
−21 6.1 ± 0.7 0.5 ± 0.1
Table 3. Temporal best-fit parameters of the X–ray flares 4, 5, 6 and 7 of GRB 050730 using Gaussian functions. An asterisk
indicates a frozen parameter. The corresponding ∆t/tp values are also indicated (see Sect. 3.1). Best-fit parameters for flare 7
were derived from XMM-Newton data only.
Flare tp σ K × 10
−11 ∆t/tp
(s) (s) ( erg cm−2 s−1)
4 4484+124
−235 641
−149 11 ± 2 0.7 ± 0.2
5 10391+330
−315 1500
∗ 5.5 ± 1.0 0.71 ± 0.02
6 18714+952
−1156 3638
−789 1.5 ± 0.4 0.9 ± 0.2
7 41244+691
−744 6170
−718 0.12 ± 0.02 0.7 ± 0.1
Table 4. Best fit temporal indices of the rising (αr) and decaying (αd) portions of the X–ray flares 1, 2, 3 and 7 using a single
power-law model.
Flare αr αd
1 −1.56 ± 0.69 1.25 ± 0.32
2 −1.84 ± 0.30 1.31 ± 0.44
3 −1.46 ± 0.62 0.75 ± 0.35
7 −0.89 ± 0.34 1.61 ± 0.44
12 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
Table 5. Results of single power-law spectral fits to the 0.3–10 keV spectrum of the afterglow of GRB 050730. A local (z = 0)
absorption column fixed at the known Galactic value of NGH = 3.0 × 10
20 cm−2 (Dickey & Lockman 1990) was used in the fits.
An asterisk indicates a frozen parameter.
segment time interval NzH × 10
22 β χ2r (d.o.f.)
(s) (cm−2)
WT (all) 133–781 1.28+0.26
−0.25 0.70
+0.03
−0.03 1.01 (310)
WT (0) 133–205 1.8+0.9
−0.8 0.42
+0.08
−0.08 0.86 (76)
WT (1a) 205–233 1.6+2.5
−1.6 0.29
+0.16
−0.16 0.86 (24)
WT (1b) 233–313 3.1+1.3
−1.1 0.82
+0.12
−0.12 1.30 (51)
WT (2a) 313–433 2.1+0.8
−0.7 0.71
+0.08
−0.08 1.10 (93)
WT (2b) 433–601 0.9+0.5
−0.5 0.70
+0.07
−0.07 1.11 (111)
WT (3a) 601–681 0.7+0.8
−0.7 0.77
+0.12
−0.12 0.87 (46)
WT (3b) 681–781 1.0+0.6
−0.6 1.01
+0.10
−0.10 0.81 (65)
PC (1) 4001–18149 1.1+0.4
−0.4 0.61
+0.04
−0.04 0.95 (224)
PC (2) 21288–143438 1.0+0.7
−0.6 0.81
+0.08
−0.08 1.16 (86)
XMM (all) 29436–59811 0.68+0.10
−0.10 0.87
+0.02
−0.02 1.14 (489)
XMM (1) 29436–40000 0.68∗ 0.87+0.03
−0.03 0.88 (345)
XMM (2) 40000–50000 0.68∗ 0.93+0.03
−0.03 0.98 (243)
XMM (3) 50000–59811 0.68∗ 0.99+0.05
−0.05 0.77 (135)
	Introduction
	Observations and data reduction
	Swift BAT
	Ground-based Observatories
	Swift UVOT
	Swift XRT
	XMM-Newton
	Temporal analysis
	X–ray afterglow
	Optical band
	Spectral analysis
	XMM-Newton
	Discussion
	Early X–ray light curve
	Flaring activity
	Evidence of a jet break
	Summary and conclusions

  We present the results of a detailed spectral and temporal analysis of Swift
and XMM-Newton observations of the high redshift (z=3.969) GRB 050730. The
X-ray afterglow of GRB 050730 was found to decline with time with superimposed
intense flaring activity that extended over more than two orders of magnitude
in time. Seven distinct re-brightening events starting from 236 s up to 41.2 ks
after the burst were observed. The underlying decay of the afterglow was well
described by a double broken power-law model with breaks at t_1= 237 +/- 20 s
and t_2 = 10.1 (-2.2) (+4.6) ks. The temporal decay slopes before, between and
after these breaks were alpha_1 = 2.1 +/- 0.3, alpha_2 = 0.44 (-0.08) (+0.14)
and alpha_3 = 2.40 (+0.07) (-0.09), respectively. The spectrum of the X-ray
afterglow was well described by a photoelectrically absorbed power-law with an
absorbing column density N_H=(1.28 +/- 0.26) 10^22 cm^-2 in the host galaxy.
Strong X-ray spectral evolution during the flaring activity was present. In the
majority of the flares (6/7) the ratio Delta_t/t_p between the duration of the
event and the time when the flare peaks was nearly constant and about 0.6-0.7.
We showed that the observed spectral and temporal properties of the first three
flares are consistent with being due both to high-latitude emission, as
expected if the flares were produced by late internal shocks, or to refreshed
shocks, i.e. late time energy injections into the main afterglow shock by slow
moving shells ejected from the central engine during the prompt phase. The
event fully satisfies the E_p-E_iso Amati relation while is not consistent with
the E_p-E_jet Ghirlanda relation.

Introduction
The successful launch on 2004 November 20 of the Swift
Gamma–ray Burst Explorer (Gehrels et al. 2004) has opened
a new era in the study of Gamma Ray Bursts (GRBs). The
autonomous and rapid slewing capabilities of Swift allow the
prompt (1–2 minutes) observation of GRBs, discovered and
localised by the wide-field gamma–ray (15–350 keV) Burst
Alert Telescope (BAT, Barthelmy et al. 2005a), with the two
Send offprint requests to: e-mail: perri@asdc.asi.it
co-aligned narrow-field instruments on-board the observatory:
the X–Ray Telescope (XRT, Burrows et al. 2005a), operating
in the 0.2–10 keV energy band, and the Ultraviolet/Optical
Telescope (UVOT, Roming et al. 2005), sensitive in the 1700–
6000 Å band. The Swift unique fast pointing capability is cru-
cial in the X–ray energy band, where the reaction times of other
satellites are limited to time scales of several hours. With Swift,
thanks also to the XRT high sensitivity, it is possible for the first
time to study in detail the evolution of the X–ray afterglows of
GRBs during their very early phases.
http://arxiv.org/abs/0704.1297v1
2 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
Indeed, one of the main results of Swift is the identification
of unexpected and complex features in the early X–ray
afterglows. In particular, three distinct phases are observed in
the majority of GRB light curves: an early (t<500 seconds
from the trigger) steep decline, with a power-law index of ∼ 3,
a second (t<10 ks) very shallow phase with a slope of ∼ 0.5, a
third phase characterized by a more conventional decay slope
of ∼ 1 (e.g. Tagliaferri et al. 2005; Cusumano et al. 2006a;
Nousek et al. 2006; O’Brien et al. 2006). In a few cases (GRB
050315, Vaughan et al. 2006; GRB 050318, Perri et al. 2005;
GRB 050505, Hurkett et al. 2006; GRB 050525A,
Blustin et al. 2006; GRB 060614, Mangano et al. 2007) a
further steepening with a decay slope of ∼ 2, consistent
with a jet break, is observed. Moreover, Swift had detected
in about one half of the bursts strong flaring activity in
the X–ray energy band superimposed on the afterglow
decay (e.g. Burrows et al. 2005b; Romano et al. 2006;
Falcone et al. 2006).
The understanding of the origin of these bright X–ray
flares is intensively discussed in the literature. A mech-
anism proposed as responsible for the flaring activity is
late internal shocks (Burrows et al. 2005b; Fan & Wei 2005;
Zhang et al. 2006; Liang et al. 2006). In this scenario the X–
ray flares are produced by the same internal dissipation pro-
cesses which cause the prompt emission, likely internal shocks
within the expanding fireball occurring before it is decel-
erated in the external medium (e.g. Rees & Mészáros 1994).
This model requires that the GRB central engine is still ac-
tive after the end of the prompt emission and various mech-
anisms providing such extended internal activity have been
put forward (e.g. King et al. 2005; Perna et al. 2006). An al-
ternative scenario has been recently considered by Guetta et
al. (2007) who, based on a detailed analysis of the X–ray
flaring activity observed in the afterglow of GRB 050713A,
interpreted the X–ray flares as due to refreshed shocks, i.e.
late time collisions with the main afterglow shock of slow-
moving shells ejected from the central engine during the
prompt phase (Rees & Mészáros 1998; Kumar & Piran 2000;
Sari & Mészáros 2000).
In this paper we present a detailed analysis of Swift and
XMM-Newton observations of GRB 050730, focusing on the
intense and extended X–ray flaring activity that characterizes
its afterglow. In Section 2 the observations and the data re-
duction are presented, in Section 3 we describe the tempo-
ral analysis and Section 4 is dedicated to the spectral anal-
ysis. The results are discussed in Section 5 and finally in
Section 6 we summarize our findings. Throughout this pa-
per errors are quoted at the 90% confidence level for one pa-
rameter of interest (∆χ2 = 2.71) unless otherwise specified.
We adopted the standard ΛCDM cosmological parameters of
Ωm = 0.27, ΩΛ = 0.73 and H0 = 70 km s
−1 Mpc−1. Times are
referenced from the BAT trigger T0 while temporal and spec-
tral indices are written following the notation F(t, ν) ∝ t−αν−β.
Results on the optical spectrum of the afterglow of this
GRB are reported by Starling et al. (2005), Chen et al. (2005a)
and Prochaska et al. (2006). Multi-wavelength observations of
the afterglow of GRB 050730 are presented by Pandey et
al. (2006).
−100 0 100 200 300
Time since BAT trigger (s)
Swift BAT
Fig. 1. BAT 20–150 keV background subtracted light curve of
the prompt emission of GRB 050730. Data are binned to 10
seconds resolution and errors are at the 1σ level. The horizontal
dashed line indicates the 0 level.
2. Observations and data reduction
2.1. Swift BAT
The BAT detected and located GRB 050730 at
T0=19:58:23 UT on 2005 July 30 (Holland et al. 2005).
On the basis of the refined ground analy-
sis (Markwardt et al. 2005), the BAT position is
RA(J2000)=212.◦063, Dec(J2000)=−3.◦740, with a 90%
containment radius of 3′. The BAT prompt emission light
curve (Fig. 1) is characterized by a duration of T90 = 155±20 s.
The emission starts ∼ 60 s before the trigger, peaks at ∼ 10 s
after the trigger and declines out to ∼ 180 s after the trigger.
The time averaged (over T90) spectrum in the 15–150 keV
energy band is well described by a power-law model with en-
ergy index βBAT = 0.5 ± 0.1 and χ
r = 0.71 (with 56 degrees
of freedom, d.o.f.). The total fluence in the 15–150 keV band
is (2.4 ± 0.3) × 10−6 erg cm−2. Assuming as redshift z = 3.969
(Chen et al. 2005b, see Sect. 2.2), the isotropic-equivalent radi-
ated energy in the BAT bandpass (74.5–745.4 keV in the burst
rest frame) is EBATiso = (8.0 ± 1.0) × 10
52 erg.
2.2. Ground-based Observatories
Following the identification of the optical counterpart by
UVOT (Holland et al. 2005), the field of GRB 050730 was ob-
served by numerous ground-based telescopes. The afterglow
detections in the R (Sota et al. 2005), r′ (Gomboc et al. 2005),
I and J (Cobb et al. 2005) bands were soon distributed via the
GRB Circular Network (GCN). Observations in the optical
band with the MIKE echelle spectrograph on Magellan II led to
the GRB redshift measurement z = 3.969 (Chen et al. 2005b)
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 3
based on the detection of a strong hydrogen Lyα absorption line
and of several absorption lines from other elements. The red-
shift was later confirmed with observations by the ISIS spectro-
graph on the William Herschel Telescope (Rol et al. 2005), the
IMACS imaging spectrograph on the Magellan Observatory
Baade Telescope (Holman et al. 2005a), FORS1 and UVES on
the Very Large Telescope (D’Elia et al. 2005).
The optical afterglow decay at later times was followed
by several optical telescopes. In particular, measurements
were made in the R band up to ∼4 days after the trig-
ger (Holman et al. 2005a, 2005b; Burenin et al. 2005;
Klotz et al. 2005; Damerdji et al. 2005; D’Elia et al. 2005;
Bhatt & Sahu 2005; Kannappan et al. 2005).
Finally, a radio afterglow with a flux density
Fr = 145 ± 28 µJy at 8.5 GHz was detected about 2 days
after the trigger using the Very Large Array (Cameron 2005).
2.3. Swift UVOT
Swift UVOT began to observe the field of GRB 050730
at 20:00:22 UT, 119 seconds after the BAT trigger. The first
100 seconds exposure in the V band led to the identifi-
cation of the optical afterglow at RA(J2000)=14h08m17.s09,
Dec(J2000)=-03◦46′18.′′9 (Holland et al. 2005).
We refined the preliminary photometric analysis
(Blustin et al. 2005) processing all UVOT data using the
standard UVOT software package (Swift software v. 2.1 in-
cluded in the HEAsoft package v. 6.0.2). The flux in all filters
was estimated by integrating over a 3.5′′ region. A background
region for subtraction of the sky contribution to the flux has
been selected in a relatively empty part of the field of view.
The results are listed in Table 1. No significant detection was
found in the U and UV filters, which is consistent with the
high redshift measured for this GRB. All the magnitudes
are corrected for Galactic extinction (E(B − V) = 0.049,
Schlegel et al. 1998).
2.4. Swift XRT
The XRT observations of the GRB 050730 field started at
20:00:28 UT, 125 seconds after the BAT trigger, with the in-
strument in Auto State. After a first exposure in Image Mode
(see Hill et al. 2004 for a description of readout modes) dur-
ing which no on-board centroid was determined, the instrument
switched into Windowed Timing (WT) mode for the entire first
Swift orbit from T0+133 s to T0+794 s. Starting from the sec-
ond orbit (T0+4001 s), the instrument was in Photon Counting
(PC) mode for 26 consecutive orbits until 11:49:22 UT on 2005
August 1 (T0+143.4 ks). The field of GRB 050730 was re-
observed with the XRT from August 3 starting at 15:28:11 UT
(T0+329.4 ks) until August 5 13:57:02 UT (T0+496.7 ks).
The XRT data were processed with the XRTDAS soft-
ware (v. 1.7.1) included in the HEAsoft package (v. 6.0.4).
Event files were calibrated and cleaned with standard filter-
ing criteria with the xrtpipeline task using the latest calibration
files available in the Swift CALDB distributed by HEASARC.
Events in the energy range 0.3–10 keV with grades 0–12 (PC
mode) and 0–2 (WT mode) were used in the analysis (see
Burrows et al. 2005a for a definition of XRT event grades).
After the screening, the total exposure time for the first XRT
observation was 649 seconds (WT) and 58480 seconds (PC),
while for the follow-up observation the PC exposure time was
34669 seconds.
In the 0.3–10 keV PC image of the field a previ-
ously uncatalogued X–ray source was visible within the
BAT error circle with coordinates RA(J2000)=14h08m17.s2,
Dec(J2000)=−03◦46′19′′. This position, derived using data
not affected by pile-up (orbits 5–26, see below), has
a 90% uncertainty of 3.5′′ using the latest XRT bore-
sight correction (Moretti et al. 2006) and is consistent with
the position of the optical counterpart (Holland et al. 2005;
Jacques & Pimentel 2005).
For the WT mode data, events for temporal and spectral
analysis were selected using a 40 pixel wide (1 pixel corre-
sponds to 2.36′′) rectangular region centered on the afterglow
and aligned along the WT one dimensional stream in sky co-
ordinates. Background events were extracted from a nearby
source-free rectangular region of 50 pixel width. For PC mode
data, the source count rate during orbits 2–4 was above ∼ 0.5
counts s−1 and data were significantly affected by pile-up in the
inner part of the Point Spread Function (PSF). After comparing
the observed PSF profile with the analytical model derived by
Moretti et al. (2005), we removed pile-up effects by excluding
events within a 5 pixel radius circle centered on the afterglow
position and used an outer radius of 30 pixels. From orbit 5
the afterglow brightness was below the pile-up limit and events
were extracted using a 10 pixel radius circle, which encloses
about 80% of the PSF at 1.5 keV, to maximize the signal to
noise ratio. The background for PC mode was estimated from
a nearby source-free circular region of 50 pixel radius. Source
count rates for temporal analysis were corrected for the fraction
of PSF falling outside the event extraction regions. Moreover,
the loss of effective area due to the presence of 2 CCD hot
columns within the extraction regions was properly taken into
account. The count rates were then converted into unabsorbed
0.3–10 keV fluxes using the conversion factor derived from the
spectral analysis (see Sect. 4).
For the spectral analysis, ancillary response files were gen-
erated with the xrtmkarf task applying corrections for the PSF
losses and CCD defects. The latest response matrices (v. 008)
available in the Swift CALDB were used and source spectra
were binned to ensure a minimum of 20 counts per bin in order
to utilize the χ2 minimization fitting technique.
2.5. XMM-Newton
XMM-Newton follow-up observations of GRB 050730
started 26.4 ks (for the two EPIC-MOS cameras) and 29.4 ks
(for the EPIC-PN) after the initial BAT Trigger. The XMM-
Newton ODF (Observation Data Files) data were pro-
cessed with the epproc and emproc pipeline scripts, using
the XMM-Newton SAS analysis package (v. 6.5). A bright
rapidly decaying source was detected near the aim-point of
4 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
Fig. 2. Swift XRT (filled circles) and XMM-Newton (filled squares) 0.3–10 keV light curve of the X–ray afterglow of GRB 050730.
The BAT 20–150 keV prompt emission light curve, extrapolated to the 0.3–10 keV band, is also shown as filled triangles. UVOT
optical data in the V and B bands, arbitrarily scaled for comparison with the X–ray band, are indicated with open squares and open
triangles, respectively. The solid line is the best fit model to the XRT and XMM-Newton light curve. The dashed line represent
the underlying double broken power-law decay (see Sect. 3.1). The dotted line is a power-law model with temporal decay index
αV = 0.3 normalized to fit data points in the V band. Errors are at the 1σ level.
all three EPIC detectors and the afterglow was localized at
RA(J2000)=14h08m17.s3, Dec(J2000)=-03◦46′18.′′5. The dura-
tion of the XMM-Newton follow-up observation was 33.7 ks
(MOS) and 30.4 ks (PN). After screening out times with high
background flaring, the dead-time corrected net exposures were
25.0 ks (MOS) and 17.9 ks (PN). All three EPIC cameras (PN
and 2 MOS) were used in Full Window Mode, with PN and
MOS2 using the “Thin” filter and MOS1 using the “Medium”
optical blocking filter.
Source spectra and light curves for all 3 EPIC cameras were
extracted from circular regions of 30′′ radius centered on the
afterglow. Background data were taken from a 60′′ radius cir-
cle on the same chip as the afterglow, but free of any back-
ground X–ray sources. The data were further screened by in-
cluding only good X–ray events (using the selection expres-
sion flag=0 in evselect), by including events with single and
double pixel events (pattern<=4) for the PN and by selecting
single to quadruple pixel events (pattern<=12) for the MOS.
Data below 300 eV and above 10 keV were also removed.
For the temporal analysis, we adopted the light curve from
the MOS data, primarily because the PN data was heavily af-
fected by background flares towards the end of the observation,
while the MOS covered a wider duration at the beginning of the
observation. The data from the two MOS detectors were com-
bined and the count rate to 0.3–10 keV unabsorbed flux conver-
sion factor was calculated from the best fit absorbed power-law
spectrum (see Sect. 4).
For the spectral analysis, ancillary and redistribution re-
sponse files for fitting were generated with the SAS tasks ar-
fgen and rmfgen, respectively. Moreover, source spectra were
binned to a minimum of 25 counts per bin.
3. Temporal analysis
The background subtracted 0.3–10 keV Swift XRT and
XMM-Newton light curves of the X–ray afterglow of GRB
050730 are shown in Fig. 2. The same figure also shows the
BAT 20–150 keV prompt emission light curve, converted in the
0.3–10 keV energy band using the BAT spectral best fit model
which is valid also in the XRT bandpass (see Sect. 4.1). The
UVOT optical light curves in the V and B bands, in arbitrary
units, are also plotted (see Sect. 2.3).
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 5
Fig. 3. Swift XRT 0.3-10 keV light curve of the GRB 050730
X–ray afterglow during the first orbit. The solid line is the best
fit model to the data obtained considering a linear rise exponen-
tial decay for the three flares (see Sect. 3.1). The dashed vertical
lines delimit the seven time intervals considered for the spectral
analysis. Data are binned to 8 seconds resolution and errors are
at the 1σ level.
3.1. X–ray afterglow
The X–ray afterglow of GRB 050730 is characterized by
a very complex structure. The first Swift orbit (from T0+133 s
to T0+794 s), after an initial steep decay phase that joins well
with the end of the BAT prompt emission, is dominated by three
bright X–ray flares peaking at about 235, 435 and 685 seconds
after the BAT trigger. Taking into account cosmological time
dilation these times correspond to about 47, 88 and 138 sec-
onds in the GRB rest frame. A flaring episode is also observed
in the second orbit peaking at about T0+4500 s. While the un-
derlying decay of the afterglow during the first two orbits is
shallow, starting from the third orbit (T0+10 ks) the afterglow
light curve shows a much steeper decline with superimposed
flaring activity.
We first modeled the X–ray light curve of the afterglow
with a double broken power-law model with slopes α1, α2, α3
and temporal breaks t1, t2, describing the underlying power-law
decay of the afterglow, plus seven Gaussian functions model-
ing the flaring episodes. We found for the first power-law a
decay index α1 = 2.1 ± 0.3 followed, after a first time break at
t1 = 237±20 s, by a shallower decay with index α2 = 0.44
+0.14
−0.08.
A second temporal break is found at t2 = 10.1
−2.2 ks after
which a steep decay with index α3 = 2.40
+0.09
−0.07 is observed.
This model did not provide a good fit (χ2r = 1.73, 143 d.o.f.),
mostly due to short time scale fluctuations and to deviations of
the three first bright flares from a symmetric Gaussian shape.
We thus considered a different functional form for the three
X–ray flares, namely a linear rise exponential decay: F(t) ∝
(t − t0)/(tp − t0) for times between the flare start time t0 and
Fig. 4. Swift XRT 0.3–1.5 keV (upper panel) and 1.5–10 keV
(middle panel) light curve of the GRB 050730 X–ray afterglow
during the first Swift orbit. In the lower panel the corresponding
hardness ratio is plotted. Data are binned to 12 seconds reso-
lution and error bars indicate statistical uncertainties at the 1σ
level.
peak time tp, F(t) ∝ exp[−(t − tp)/tc] for t > tp where tc is the
exponential decay time. The model improves significantly the
fit with χ2r = 1.43 (140 d.o.f.) and F-test chance probability of
1.3 × 10−6. The best fit parameters of the overall underlying
double broken power-law model were unchanged with respect
to the previous fit listed above. The linear rise exponential de-
cay best fit parameters for the first three flares are given in Table
2 while Table 3 reports the Gaussian best fits for the other four
flares. As Fig. 3 illustrates, the second brightest flare (referred
to as flare 2 in the following) has a flux variation of amplitude
∼ 3.6 and is characterized by a steep rise, lasting ∼ 90 seconds,
followed by a slower decay with duration ∼ 170 seconds. An
asymmetric shape, with a steep rise followed by a shallower
decay, was also observed for the other two flares (flare 1 and 3)
and for these episodes the flux variation was ∼ 1.7 and ∼ 2.6,
respectively.
The ratio between the duration (∆t) and the peak time (tp) of
the X–ray flares was calculated using the linear rise exponential
decay and Gaussian best fit parameters for the first three and
the last four flares, respectively. For both analytical models the
duration of a flare was defined as the time interval during which
the flare intensity was above 5% of its peak value. The results
are given in Tables 2 and 3.
The rise and decay portions of the first three flares were
also fit with single power-laws. For the estimation of power-
law indices, times were expressed from the onset of the flares
(t0) for the rise, from the peak time (tp) for the decay, and the
contribution of the underlying afterglow decay was taken into
account. The power-law temporal indices for the rising (αr) and
decaying (αd) portions of the three flares are reported in Table
6 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
The temporal behavior of the afterglow during the first three
X–ray flares was also studied in different energy bands. In
Fig. 4 we show the 0.3–1.5 keV (upper panel) and 1.5–10
keV (middle panel) light curves of the early X–ray afterglow
together with the corresponding hardness ratio (lower panel).
From the figure it is apparent that i) in the harder band the pro-
file of the flares is sharper and ii) a peak time shift, with flares
in the hard band peaking at earlier times, is observed. We note
that these temporal properties as a function of energy are the
same observed for the prompt emission pulses of GRBs.
As already mentioned, late flaring activity of the X–ray af-
terglow was observed at ∼ 4.5, 10.4, 18.7 and 41.2 ks after
the trigger. Due to the Swift orbital gaps the temporal cover-
age of flares 4, 5 and 6 is poor and these episodes could not
be well studied, but the last flare was entirely covered by the
XMM-Newton follow-up observation allowing a detailed tem-
poral analysis. In Fig. 5 the XMM-Newton (MOS1+MOS2)
light curve is shown together with the XRT curve covering the
same time interval. A very good agreement between the two
curves is found and the ∼ 5% higher normalization of the XRT
data points is of the same order of the uncertainties in the abso-
lute flux calibration of the instrument (Campana et al. 2006).
We first fit the XMM-Newton light curve with a single
power-law decay and obtained an extremely poor fit to the
light curve, with a fit statistic of χ2r = 8.81 (32 d.o.f.) and
a decay index of α = 2.10 ± 0.04. The light curve was then
parameterized with a long duration flare super-imposed on a
steep power-law decay. We obtained an underlying decay in-
dex α = 2.45 ± 0.15, while the flare could be adequately mod-
eled with a Gaussian function; the fit statistic was acceptable
(χ2r = 1.22, 29 d.o.f.). The long duration flare peaked at 41 ks
(or 8.2 ks in the GRB rest frame), with a σ of 6.8 ks (1.4 ks
rest frame). The total fluence of the flare when parameterized
this way was 8.3 × 1050 erg (1.5-50 keV band in the GRB rest
frame), which represents 20% of the integrated afterglow emis-
sion over the XMM-Newton observation. The rise and decay
phases of flare 7 were also fit with single power-laws express-
ing times from the onset and from the peak time of the flare,
respectively. The measured temporal indices αr and αd are also
reported in Table 4.
3.2. Optical band
The UVOT optical light curve of the afterglow of GRB
050730 in the V and B bands is also characterized by a complex
behavior, as is illustrated in Fig. 2.
The early (T0+100 s - T0+800 s) UVOT V light curve re-
veals flaring activity. A re-brightening is in fact observed at
∼ T0+500 s, almost simultaneously with the brightest X–ray
flare observed with XRT. Due to the relatively poor sampling of
the UVOT light curve, a detailed temporal analysis is not possi-
ble, however we note that the amplitude of the flux variation in
the V band (∼ 3) is of the same order of the one measured in the
X–ray energy band. The decay of the afterglow in the V band,
up to about T0+12 ks and excluding the re-brightening episode,
is shallow (αV ∼ 0.3) and in agreement with the one measured
in the X–ray band (αX = 0.44
+0.14
−0.08). Hints of a steepening of
Fig. 5. XMM-Newton (MOS1+MOS2, filled circles) and Swift
XRT (open circles) 0.3–10 keV light curve of the late (∼ T0+12
hours) X–ray afterglow of GRB 050730. The solid line is the
best fit model to the XMM-Newton light curve. The dashed line
represent the underlying power-law decay (see Sect. 3.1). Data
points errors are at the 1σ level.
the V light curve at ∼ T0+12 ks are also found, but the limited
temporal coverage and the large statistical uncertainties char-
acterizing the UVOT data points do not allow us to constraint
the late optical afterglow decay.
The UVOT light curve in the B band is characterized by a
flat power-law decay (αB ∼ 0.3) without re-brightening events.
However, it should be noted that i) the curve is very sparsely
sampled, with only three data points during the first Swift orbit,
and ii) in the B band there is a strong flux reduction due to the
Lyman break and thus large statistical uncertainties affect the
data points.
Accurate optical observations of the afterglow of GRB
050730 in the R and I bands are presented by Pandey et
al. (2006). The authors report an early time flux decay with
indices αR = 0.54± 0.05 and αI = 0.66± 0.11 followed, after a
temporal break at about 9 ks after the trigger, by a strong steep-
ening with decay indices αR = 1.75±0.05 and αI = 1.66±0.07.
4. Spectral analysis
For the spectral analysis of Swift XRT and XMM-Newton data
we used the XSPEC package (v. 11.3.2, Arnaud 1996) included
in the HEAsoft package (v. 6.0.4).
4.1. XRT
As a first step, the 0.3–10 keV XRT average spectrum
during the first Swift orbit (WT mode, from T0+133 s to
T0+794 s) was fit adopting a single power-law model with the
neutral hydrogen-equivalent absorption column density fixed
at the Galactic value in the direction of the GRB (NGH =
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 7
3.0 × 1020 cm−2, Dickey & Lockman 1990). The fit obtained
was poor (χ2r = 1.28, 311 d.o.f.). From the inspection of
residuals a strong deficit of counts at low energies, likely due
to the presence of an absorbing column density in excess to
the Galactic one, was found. Indeed, the addition of a col-
umn density NzH using Solar metallicity redshifted to the rest
frame of the GRB host (z = 3.969) free to vary in the spec-
tral fit (zwabs model in XSPEC) resulted in a more acceptable
fit with a column density of NzH = (1.28
+0.26
−0.25) × 10
22 cm−2 (see
Table 5). This value is consistent with the neutral hydrogen
column densities derived from the optical spectra reported by
Starling et al. (2005) and Chen et al. (2005a).
From Fig. 4 it is apparent that strong spectral evolution
takes place during the intense flaring activity observed in the
first Swift orbit. We thus split the WT observation in seven
time intervals to study the spectra during the rise and the de-
cay portions of each flare (see Fig. 3). The time-resolved spec-
tral best fits (see Table 5) clearly show evidence for spectral
variation during the flares and an overall softening of the spec-
tra with time associated with a decrease of the rest frame col-
umn density. The time-resolved spectral analysis was also per-
formed adopting a broken power-law model to investigate the
possible presence of spectral breaks (e.g. Falcone et al. 2006;
Guetta et al. 2007). Also in this case an additional absorption
column density NzH at the rest frame of the GRB host was con-
sidered. For all segments we did not find evidence for spectral
breaks within the XRT energy band.
The XRT late 0.3–10 keV spectrum (PC mode, from
T0+4.0 ks to T0+143.4 ks) was also fit using a single power-
law model with a fixed Galactic absorption column density and
an additional absorbing column at the burst rest frame. The ob-
servation was divided in two time intervals (4.0–18.1 ks and
21.3–143.4 ks from the trigger). The results are listed in Table 5
where we see that spectral softening between the two time in-
tervals was found.
4.2. XMM-Newton
The PN and MOS 0.3–10 keV spectra were fit jointly al-
lowing the cross normalization between the detectors, which is
consistent within < 5%, free to vary . The two MOS spectra
and responses were combined to maximize the signal to noise,
after first checking that they were consistent with each other.
As in the case of the XRT spectrum, the PN and MOS spec-
tra were first fit with a single power-law model with the neu-
tral hydrogen-equivalent absorption column fixed at the known
Galactic value. The fit obtained was not acceptable (χ2r = 1.40,
489 d.o.f.), while the energy index was β = 0.76 ± 0.02.
The addition of a neutral absorption column in the GRB host
galaxy frame at z = 3.969 resulted in more acceptable fit
(χ2r = 1.14, 489 d.o.f.) with an excess absorption column above
the Galactic column of (6.8 ± 1.0) × 1021 cm−2, while the con-
tinuum energy index was now β = 0.87 ± 0.02.
The XMM-Newton afterglow spectra were also sliced into
three segments of approximately 10 ks in duration, in order to
search for any spectral evolution within the XMM-Newton ob-
servation. A small change in the continuum parameters was
found, the spectrum evolved from hard to soft; the energy
index changed from β = 0.87 ± 0.03 in the first 10 ks, to
β = 0.99 ± 0.05 during the final segment. No evidence was
found for a change in the column density, which was subse-
quently fixed at NH = 6.8 × 10
21 cm−2 in all the segments. The
spectral best fit parameters are shown in Table 5.
We also searched for any evidence of emission lines, either
in the mean spectrum, or in the three segments. No statistically
significant lines were found, at the level of > 99% confidence.
As the redshift of the burst is known, then we can set an upper-
limit to the equivalent width of any emission lines. Over the
range of 0.4–8 keV and using the mean spectrum, we found a
< 100 eV upper limit to any emission lines. More specifically
we can set a limit on the iron Kα line (e.g. the H-like line at
6.97 keV rest frame, 1.40 keV observed frame) of < 30 eV.
5. Discussion
5.1. Early X–ray light curve
This GRB, with its rather high redshift, z = 3.969, gives
us the possibility to investigate the X–ray and optical light
curves soon after the trigger and thus to study in detail the
soft tail of the prompt emission and the very beginning of
the afterglow phase. Indeed, due to cosmological time dilation
the XRT and UVOT observations started, in the rest frame of
the burst, only 27 seconds after the trigger. Other examples of
such early observations of high redshift bursts are GRB 050904
(z = 6.29, Cusumano et al. 2006b) and GRB 060206 (z = 4.0,
Monfardini et al. 2006).
The XRT light curve shows at the very beginning (133–
205 s from the BAT trigger) a rapidly decaying emission that
joins quite nicely with the BAT flux when converted to the
XRT bandpass, a feature that has been observed in various
Swift bursts (Tagliaferri et al. 2005, Barthelmy et al. 2005b;
Nousek et al. 2006; O’Brien et al. 2006). For GRB 050730,
the steep temporal decay index (α1 = 2.1 ± 0.3, see
Sect. 3.1) characterizing the early X–ray light curve sug-
gests that the observed emission is likely associated with
the tail of the prompt emission rather than to the beginning
of the shocked inter-stellar medium afterglow phase. Indeed,
in the internal shock model scenario for the prompt γ–ray
emission (e.g. Rees & Mészáros 1994), the cessation of the
emissivity is characterized by a rapid decay due to the de-
layed arrival of the high angular latitude prompt emission
of the shocked surface (high-latitude or curvature emission,
Kumar & Panaitescu 2000; Dermer 2004). The high-latitude
emission predicts a relationship between the temporal decay in-
dex αd and the spectral index βd during the decay given by the
equation αd = 2+βd (Kumar & Panaitescu 2000) where the de-
cay slope is measured using as zero time (t0) the beginning of
the prompt emission (F(t, ν) ∝ (t − t0)
−αdν−βd ). We have tested
this prediction by fitting the early XRT light curve (segment
“0” in Fig. 3) with the above constraint using the spectral index
measured during the decay (βd = 0.42 ± 0.08, see Table 5) and
leaving the zero time as a free parameter (see Liang et al. 2006
for a description of the method). We found t0 = −31
−31 s, i.e.
8 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
the zero time is located at the rising segment of the BAT prompt
emission light curve (see Fig. 1), as expected in the framework
of the high-latitude emission. This result indicates that the steep
decay observed in the early XRT light curve is most likely the
soft tail of the BAT emission.
This hypothesis is also supported by the BAT and early
(segment “0” in Fig. 3) XRT spectra. We found very similar
spectral slopes (βBAT = 0.5 ± 0.1 and βXRT = 0.42 ± 0.08,
respectively), likely indicating that the early X–ray and γ–ray
emissions are produced by the same emission mechanism.
5.2. Flaring activity
The exceptionally extended flaring activity of the X–ray af-
terglow of GRB 050730 allows us to study this phenomenon
over more than two orders of magnitude in time. The XRT
early steep decay is followed by three bright X–ray flares peak-
ing at 236, 437 and 685 seconds after the BAT trigger (see
Fig. 3). These flares were under the sensitivity of the BAT
instrument and thus were not detected in the hard X–ray en-
ergy band. These flares show, as other strong flares observed
with Swift (e.g. Romano et al. 2006; Falcone et al. 2006;
Pagani et al. 2006; Godet et al. 2006; Chincarini et al. 2006), a
clear spectral evolution with the hardness ratio that mimics
the variation of the light curve. A phase lag, with the harder
(E > 1.5 keV) light curve peaking at earlier times with respect
to the softer energy band is also found (see Fig. 4).
So far, most of the X–ray flares observed in Swift X–
ray light curves have been interpreted as due to late in-
ternal shocks (e.g. Burrows et al. 2005b; Romano et al. 2006;
Falcone et al. 2006; Godet et al. 2006). In this scenario the
GRB central engine is active far beyond the end of the
prompt γ–ray emission phase requiring new mechanisms ca-
pable of powering new relativistic outflows at late-time (e.g.
King et al. 2005; Perna et al. 2006). A diagnostic to check if
the re-brightenings are due to late internal shocks has been re-
cently proposed by Liang et al. (2006). In the internal-origin
scenario for X–ray flares the decay emission of the flaring
episodes should be dominated by high-latitude emission with
the decay temporal index related to the decay spectral index as
αd = βd + 2 (Kumar & Panaitescu 2000). We have checked this
hypothesis for the first three bright X–ray flares observed in the
GRB 050730 afterglow. The spectral indices measured during
the decays were used (segments “1b”, “2b” and “3b”, see Fig. 3
and Table 5) and we found best fit zero times t0,1 =193
−11 s,
t0,2 =341
−42 s and t0,3 =592
−93 s, respectively. We can see that
for all three episodes the zero time values are located at the be-
ginning of the corresponding flare and thus the observed decay
slopes are consistent with being due to high-latitude emission
as predicted by the late internal shock scenario.
Another possibility is that the X–ray flares are pro-
duced by refreshed shocks (Rees & Mészáros 1998;
Kumar & Piran 2000; Sari & Mészáros 2000). In the standard
internal-external fireball model (e.g. Rees & Mészáros 1994;
Sari & Piran 1997) re-brightening episodes in the afterglow
light curve are explained as slow shells ejected from the
central engine during the prompt phase that catch up with the
main afterglow shock after it has decelerated in the external
inter-stellar medium. Indeed, Guetta et al. (2007) have recently
interpreted the X–ray flares of the afterglow of GRB 050713A
as due to refreshed shocks. Here we applied the diagnostic
proposed for GRB 050713A to the case of GRB 050730. First,
to estimate the rise and decay slopes of the early flares we
operationally selected as zero time the beginning and the peak
of the flare, respectively. This assumption implicitly means
that the flare is completely independent from the main event
generating the prompt emission and the forward shock light
curve. We found temporal indices in the 0.8–1.8 range (see
Table 4). Much steeper slopes (∼ 3–6) are obtained if instead
times are referenced from the BAT trigger, as usually done
for most of the Swift X–ray flares (e.g. Burrows et al. 2005b;
Romano et al. 2006; Falcone et al. 2006). Second, we checked
if the temporal and the spectral indices during the decay
phase are related as predicted by the standard afterglow model
(Sari, Piran & Narayan 1998). We restricted the analysis to
the episodes which have enough statistics to allow a detailed
temporal and spectral study, i.e. the first three flares and the last
one. For the decay phases we expect the following relations
to hold (Sari, Piran & Narayan 1998; Dai & Cheng 2001):
Fν ∝ (νm/νc)
−1/2(ν/νm)
−p/2Fν,max ∝ t
−3p/4+1/2 (for p > 2)
and Fν ∝ (νm/νc)
(p−1)/2(ν/νc)
−p/2Fν,max ∝ t
−(3p+10)/16 (for
1 < p < 2). From the best fit spectral energy indices during
the decay of the four flares (see Table 5) we derived the
predicted temporal slopes α1d = 0.93 ± 0.05, α
d = 0.89 ± 0.03,
α3d = 1.02 ± 0.15, and α
d = 1.40 ± 0.05, which are consistent
with the measured values of the decay indices listed in Table
4. We note that for the last flare, peaking at 41.2 ks after the
trigger, we used the afterglow model relations for the slow
cooling case (Fν ∝ (ν/νm)
−(p−1)/2Fν,max ∝ t
3(1−p)/4) which very
likely applies at these late times (Sari, Piran & Narayan 1998).
Important information on the mechanism producing the
flares can also be obtained from the comparison between the
observed variability timescale and the time at which the flare
is observed (Ioka et al. 2005). We calculated, for all the 7 re-
brightenings of the GRB 050730 afterglow, the ratio between
the duration of the flare (∆t) and the time when the flare peaks
(tp) (see Tables 2 and 3). We found ∆t/tp ∼ 0.3 for the first
flare and ∆t/tp ∼ 0.6 − 0.7 for the others, in agreement with
the ∆t/tp > 0.25 limit discussed by Ioka et al. (2005) for the
refreshed shock scenario. Remarkably, the ∆t/tp ratio is nearly
constant for all flares, with the exception of the first one which
occurred during the bright and steep tail of the prompt emis-
sion (see Fig. 3) and most likely we are observing only the tip
of the flare. A duration of the flare proportional to tp, as ob-
served in the afterglow of GRB 050730, is explained both in
the refreshed shock scenario (Kumar & Piran 2000) and in the
late internal shock model (Perna et al. 2006). We note that the
last four flares have ∆t/tp values in the range 0.7–0.9, while
for flares 2 and 3 we observe slightly lower ratios (0.5–0.6),
possibly indicating a moderate temporal evolution of ∆t/tp.
However, due to the relatively large error bars, the measured
values are also consistent with being constant and ∼ 0.6 − 0.7.
Flaring activity is also observed in the optical afterglow
of GRB 050730. Indeed, the UVOT V early light curve (see
Fig. 2) shows a re-brightening at ∼ T0+500 s, almost simul-
M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 9
taneously with the brightest X–ray flare observed with the
XRT peaking at T0+437 s. Moreover, the optical re-brightening
amplitude (∼ 3) is of the same order of the flux variation
observed in the X–ray energy band. Strong indications of
correlated variability between the X–ray and optical energy
bands are also present in the NIR/Optical light curves re-
ported by Pandey et al. (2006): a significant re-brightening
at about T0+4 ks (R filter), close to the X–ray flare peaking
at T0+4.5 ks (see Fig. 2), is observed. Moreover, a bump in
the J, I, R and V light curves at about T0+10 ks is observed
(Pandey et al. 2006), again simultaneous with the X–ray flare
peaking at T0+10.4 ks. Although the X–ray flaring activity is
not uniformly covered by optical observations, we find several
indications of simultaneous re-brightening events in the X–ray
and optical bands, in agreement with the refreshed shock model
(e.g. Granot et al. 2003).
5.3. Evidence of a jet break
The X–ray afterglow light curve shows a clear temporal
break around 10 ks after the trigger (see Sect. 3.1). At about
the same time a strong steepening of the I and R afterglow
light curves, although with a shallower post-break slope, is also
observed (see Pandey et al. 2006 for a detailed analysis on the
discrepancy between the X–ray and optical slopes). Due to the
achromatic nature of the temporal break it is very likely that a
jet break is occurring when the bulk Lorentz factor γ of the col-
limated relativistic outflow becomes lower than the inverse of
the jet opening angle θjet (e.g., Rhoads 1997, Sari et al. 1999),
as also reported by Pandey et al. (2006). In this framework,
the jet opening angle can be determined through the equation
θjet = 0.161[tb/(1 + z)]
3/8(nη/Eiso)
1/8 (e.g., Bloom et al. 2003)
where θjet is in radians, the jet temporal break tb in days, the
total isotropic-equivalent energy Eiso in units 10
52 erg, the den-
sity n of the circumburst medium in cm−3 and η is the efficiency
of conversion of the outflow kinetic energy in electromagnetic
radiation.
An accurate estimation of the bolometric isotropic-
equivalent energy radiated by GRBs requires the knowl-
edge of their intrinsic spectrum over a broad energy band
(Bloom et al. 2001; Amati et al. 2002). For GRB 050730, the
spectrum is well fit by a single power-law with energy index
βBAT = 0.5 ± 0.1 up to the high energy limit of the BAT sen-
sitivity bandpass (150 keV) indicating that i) we are observing
the low energy tail of the Band model (Band et al. 1993) gen-
erally used to describe GRB spectra and ii) the νF(ν) spectrum
peak energy Ep is above ∼ 750 keV (i.e. (1 + z) × 150 keV) in
the burst rest frame. A lower limit to the bolometric isotropic-
equivalent radiated energy Eiso is given by the observed radi-
ated energy in the BAT bandpass (EBATiso = (8.0±1.0)×10
52 erg,
see Sect. 2.1). An upper limit to Eiso is obtained in the most
conservative case where the peak energy Ep is equal to 10
4 keV
(Amati et al. 2002). By integrating the best fit BAT power-law
spectrum in the whole 1–104 keV rest frame energy band we
thus derived an upper limit of 4.5 × 1053 erg to Eiso. Taking
the central value of the interval derived above we obtained for
GRB 050730 Eiso = (2.6 ± 1.9) × 10
53 erg.
With z = 3.969, tb = 10.1
−2.2 ks (see Sect. 3.1) and the Eiso
range above derived, we find θjet = 1.6
−0.2 deg, for η = 0.2
(Frail et al. 2001) and assuming the value of circumburst den-
sity n = 10 cm−3 discussed by Bloom et al. (2003). With
this value of the jet opening angle, the inferred collimation-
corrected bolometric radiated energy is Ejet = (1.0
−0.8) ×
1050 erg.
Taking into account the Eiso and Ejet values derived above
and the lower limit to the peak energy (Ep > 750 keV rest
frame), we find that GRB 050730 is consistent with the Ep
vs. Eiso relation found by Amati et al. (2002) and recently
updated in Amati (2006). We also find that GRB 050730 is
inconsistent, even taking into account the 3σ scatter around
the best fit correlation, with the Ep vs. Ejet relation found by
Ghirlanda et al. (2004) and with its updated version presented
by Nava et al. (2006). In order to make this GRB consistent
with the Ep-Ejet relation a much higher circumburst density
(n ∼ 105 cm−3) would be required. GRB 050730 is inconsis-
tent with the model-independent Eiso-Ep-tb correlation found
by Liang & Zhang (2005).
6. Summary and conclusions
We have presented a detailed temporal and spectral analy-
sis of the afterglow of GRB 050730 observed with Swift and
XMM-Newton. The most striking feature of this GRB is the in-
tense and exceptionally extended, over more than two order of
magnitude in time, X–ray flaring activity.
Superimposed to the afterglow decay we observed seven
distinct re-brightening events peaking at 236 s, 437 s, 685 s,
4.5 ks, 10.4 ks, 18.7 ks and 41.2 ks after the BAT trigger. The
underlying decline of the afterglow was well described with a
double broken power-law model with breaks at t1 = 237± 20 s
and t2 = 10.1
−2.2 ks. The temporal decay slopes before, be-
tween and after these breaks were α1 = 2.1±0.3,α2 = 0.44
+0.14
−0.08
and α3 = 2.40
+0.09
−0.07, respectively.
Strong spectral evolution during the flares was present to-
gether with an overall softening of the underlying afterglow
with the energy index varying from β = 0.42 ± 0.08 dur-
ing the early (133–205 s) steep decay to β = 0.99 ± 0.05
at much later (50-60 ks) times. An absorbing column density
NzH = (1.28
+0.26
−0.25) × 10
22 cm−2 in the host galaxy is observed
during the early (133–781 s) Swift observations while a lower
column density NzH = (0.68 ± 0.10) × 10
22 cm−2 is measured
during the late (29.4–50.8 ks) XMM-Newton follow-up obser-
vation, likely indicating photo-ionization of the surrounding
medium. Evidence of flaring activity in the early UVOT op-
tical afterglow, simultaneous with that observed in the X–ray
band, was found.
From the temporal analysis of the first three bright X–ray
flares we found that the rise and decay power-law slopes are
in the range 0.8–1.8 if the beginning and the peak of the flares
are used as zero time, respectively. We also found that, with the
exception of the first flare, for all episodes the ratio between the
duration of the flare (∆t) and the time when the flare peaks (tp)
is nearly constant and is ∆t/tp ∼ 0.6 − 0.7.
We showed that the observed properties of the first three
flares are consistent with being due to both high-latitude emis-
10 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
sion, as expected for flares produced by late internal shocks,
or to late time energy injection into the main afterglow shock
by slow moving shells (refreshed shocks). An analysis of a
larger sample of bursts would help in understanding what are
the physical mechanisms responsible for the X-ray flaring ac-
tivity.
We interpreted the X–ray temporal break at around 10 ks
as a jet break and derived a cone angle of ∼ 2 deg and a radi-
ated energy Ejet = (0.2 − 3.3) × 10
50 erg against an isotropic-
equivalent energy Eiso = (0.7 − 4.5) × 10
53 erg. GRB 050730
satisfies the Ep vs. Eiso Amati relation while is inconsistent with
the Ep vs. Ejet Ghirlanda relation.
Acknowledgements. We are grateful to the referee for his/her useful
comments and suggestions. We also thank C. Guidorzi for a very care-
ful reading of the paper and F. Tamburelli and B. Saija for their work
on the XRT data reduction software. This work is supported in Italy
from ASI on contract number I/R/039/04 and through funding of the
ASI Science Data Center, at Penn State by NASA contract NAS5-
00136 and at the University of Leicester by the Particle Physics and
Astronomy Research Council on grant numbers PPA/G/S/00524 and
PPA/Z/S/2003/00507.
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M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730 11
Table 1. UVOT detections of GRB 050730 in the V and B filters. Column (1) gives the image mid time in seconds since the BAT
trigger, column (2) the net exposure time, column (3) the filter used, and column (4) the afterglow magnitude with 1σ error.
T(mid) Exposure Filter Magnitude
(s) (s)
170 99 V 17.4 ± 0.2
270 10 B 18.4 ± 0.4
299 9.7 V 17.2 ± 0.5
383 9.7 V 17.3 ± 0.5
468 9.7 V 16.4 ± 0.3
524 30 B 19.0 ± 0.4
552 9.7 V 16.3 ± 0.3
637 9.7 V 17.0 ± 0.4
721 9.7 V 16.8 ± 0.4
734 20 B 19.2 ± 0.5
10164 900 B 20.2 ± 0.3
11947 837 V 18.6 ± 0.2
23519 835 V 19.5 ± 0.3
34990 843 V 19.9 ± 0.4
Table 2. Temporal best fit parameters of the first three bright X–ray flares of GRB 050730 using a linear rise exponential decay
model. The corresponding ∆t/tp values for the three flares are also indicated (see Sect. 3.1).
Flare t0 tp tc K × 10
−10 ∆t/tp
(s) (s) (s) ( erg cm−2 s−1)
1 207+5
−5 236
−4 17
−5 8 ± 1 0.3 ± 0.1
2 344+5
−5 437
−4 54
−6 12.2 ± 0.7 0.58 ± 0.04
3 614+ 9
−10 685
−6 87
−21 6.1 ± 0.7 0.5 ± 0.1
Table 3. Temporal best-fit parameters of the X–ray flares 4, 5, 6 and 7 of GRB 050730 using Gaussian functions. An asterisk
indicates a frozen parameter. The corresponding ∆t/tp values are also indicated (see Sect. 3.1). Best-fit parameters for flare 7
were derived from XMM-Newton data only.
Flare tp σ K × 10
−11 ∆t/tp
(s) (s) ( erg cm−2 s−1)
4 4484+124
−235 641
−149 11 ± 2 0.7 ± 0.2
5 10391+330
−315 1500
∗ 5.5 ± 1.0 0.71 ± 0.02
6 18714+952
−1156 3638
−789 1.5 ± 0.4 0.9 ± 0.2
7 41244+691
−744 6170
−718 0.12 ± 0.02 0.7 ± 0.1
Table 4. Best fit temporal indices of the rising (αr) and decaying (αd) portions of the X–ray flares 1, 2, 3 and 7 using a single
power-law model.
Flare αr αd
1 −1.56 ± 0.69 1.25 ± 0.32
2 −1.84 ± 0.30 1.31 ± 0.44
3 −1.46 ± 0.62 0.75 ± 0.35
7 −0.89 ± 0.34 1.61 ± 0.44
12 M. Perri et al.: The exceptionally extended flaring activity in the X–ray afterglow of GRB 050730
Table 5. Results of single power-law spectral fits to the 0.3–10 keV spectrum of the afterglow of GRB 050730. A local (z = 0)
absorption column fixed at the known Galactic value of NGH = 3.0 × 10
20 cm−2 (Dickey & Lockman 1990) was used in the fits.
An asterisk indicates a frozen parameter.
segment time interval NzH × 10
22 β χ2r (d.o.f.)
(s) (cm−2)
WT (all) 133–781 1.28+0.26
−0.25 0.70
+0.03
−0.03 1.01 (310)
WT (0) 133–205 1.8+0.9
−0.8 0.42
+0.08
−0.08 0.86 (76)
WT (1a) 205–233 1.6+2.5
−1.6 0.29
+0.16
−0.16 0.86 (24)
WT (1b) 233–313 3.1+1.3
−1.1 0.82
+0.12
−0.12 1.30 (51)
WT (2a) 313–433 2.1+0.8
−0.7 0.71
+0.08
−0.08 1.10 (93)
WT (2b) 433–601 0.9+0.5
−0.5 0.70
+0.07
−0.07 1.11 (111)
WT (3a) 601–681 0.7+0.8
−0.7 0.77
+0.12
−0.12 0.87 (46)
WT (3b) 681–781 1.0+0.6
−0.6 1.01
+0.10
−0.10 0.81 (65)
PC (1) 4001–18149 1.1+0.4
−0.4 0.61
+0.04
−0.04 0.95 (224)
PC (2) 21288–143438 1.0+0.7
−0.6 0.81
+0.08
−0.08 1.16 (86)
XMM (all) 29436–59811 0.68+0.10
−0.10 0.87
+0.02
−0.02 1.14 (489)
XMM (1) 29436–40000 0.68∗ 0.87+0.03
−0.03 0.88 (345)
XMM (2) 40000–50000 0.68∗ 0.93+0.03
−0.03 0.98 (243)
XMM (3) 50000–59811 0.68∗ 0.99+0.05
−0.05 0.77 (135)
	Introduction
	Observations and data reduction
	Swift BAT
	Ground-based Observatories
	Swift UVOT
	Swift XRT
	XMM-Newton
	Temporal analysis
	X–ray afterglow
	Optical band
	Spectral analysis
	XMM-Newton
	Discussion
	Early X–ray light curve
	Flaring activity
	Evidence of a jet break
	Summary and conclusions
