Mon. Not. R. Astron. Soc. 000, 1–6 (2007) Printed 29 October 2018 (MN LATEX style file v2.2)
Exciting the Magnetosphere of the Magnetar
CXOU J164710.2-455216 in Westerlund 1
M. P. Muno,1 B. M. Gaensler,2,3 J. S. Clark,3,4 R. de Grijs,5 D. Pooley,6,7
I. R. Stevens,8 & S. F. Portegies Zwart9,10
1Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125; mmuno@srl.caltech.edu
2School of Physics A29, The University of Sydney, NSW 2006, Australia
3Harvard-Smithsonian Center for Astrophysics, 60 Garden St. Cambridge, MA 02138
4Department of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
5Department of Physics & Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, U.K.
6Chandra Fellow
7Astronomy Department, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720, USA
8School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
9Astronomical Institute ’Anton Pannekoek’ Kruislaan 403, 1098SJ Amsterdam, the Netherlands
10Section Computational Science Kruislaan 403, 1098SJ Amsterdam, the Netherlands
Accepted 2007 March 15. Received 2007 February 13; in original form 2006 November 25
ABSTRACT
We describe XMM-Newton observations taken 4.3 days prior to and 1.5 days subse-
quent to two remarkable events that were detected with Swift on 2006 September 21
from the candidate magnetar CXOU J164710.2-455216: (1) a 20 ms burst with an en-
ergy of 1037 erg (15–150 keV), and (2) a rapid spin-down (glitch) with ∆P/P ∼ −10−4.
We find that the luminosity of the pulsar increased by a factor of 100 in the interval
between observations, from 1×1033 to 1×1035 erg s−1 (0.5–8.0 keV), and that its spec-
trum hardened. The pulsed count rate increased by a factor of 10 (0.5–8.0 keV), but
the fractional rms amplitude of the pulses decreased from 65 to 11 per cent, and their
profile changed from being single-peaked to exhibiting three peaks. Similar changes
have been observed from other magnetars in response to outbursts, such as that of 1E
2259+586 in 2002 June. We suggest that a plastic deformation of the neutron star’s
crust induced a very slight twist in the external magnetic field, which in turn generated
currents in the magnetosphere that were the direct cause of the X-ray outburst.
Key words: stars: neutron — pulsar: individual (CXOU J164710.2-455216) — X-
rays: bursts — stars: magnetic fields
1 INTRODUCTION
Young, isolated neutron stars come in a variety of mani-
festations, including ordinary radio pulsars, compact cen-
tral objects in supernova remnants, soft gamma repeaters
(SGRs), and anomalous X-ray pulsars (AXPs). The latter
two classes of source share long rotational periods (P=5–10
s), rapid spin-down rates (Ṗ&10−12 s s−1), X-ray luminosi-
ties (LX&10
33 erg s−1) that exceed their spin-down power,
and the frequent production of second-long soft gamma-ray
bursts (Woods & Thompson 2006). These properties sug-
gest that they are magnetars, neutron stars powered by the
unwinding of extremely strong (B&1015 G) internal mag-
netic fields (Thompson & Duncan 1995, 1996).
The phenomenology associated with magnetars is
thought to be driven by how the unwinding internal fields
interact with the crusts of the neutron stars, which in turn
determines the geometries of the external magnetic fields
(Thompson & Duncan 1995, 1996; Thompson, Lyutikov,
& Kulkarni 2002). In some cases, the crusts respond to
the unwinding fields plastically, and the energy is gradu-
ally deposited into the magnetospheres. This causes tran-
sient ‘active periods,’ in which the persistent fluxes in-
crease on timescales of weeks to years (Woods et al. 2004;
Gotthelf et al. 2004). Fractures may also occur in the crust,
which generate waves in the external fields, and in turn pro-
duce sudden soft gamma-ray ‘bursts’ with energies up to
1041 erg (Göğüş et al. 2001; Gavriil, Kaspi, & Woods 2002).
In the most extreme cases, instabilities can rearrange the
entire external magnetic field, producing ‘giant flares’ with
energies of 1044 −1046 erg (Hurley et al. 1999; Palmer et al.
2005; Hurley et al. 2005). Finally, changes in the coupling
between the bulk of the crust and a superfluid component
appear to change the crust’s angular momentum, as is sug-
gested by both secular variations in the spin down rates
c© 2007 RAS
http://arxiv.org/abs/0704.1630v1
2 M. P. Muno et al.
Figure 1. Images of the counts received by the EPIC-pn in the
0.5–10 keV band on 2006 September 16 (left) and 22 (right).
The images are centred on the core of the star cluster Wester-
lund 1 (α, δ = 251.h76792 –45.◦84972 [J2000]). In addition to
the AXP CXOU J164710.2-455216, also visible in the images are
three bright OB/WR stars. The blank strips in the image are
gaps between the chips in the detector array.
on time-scales of weeks (Gavriil & Kaspi 2004; Woods et al.
2006) or sudden, day-long episodes of spin-up (‘glitches’)
or spin-down (Woods et al. 1999; Gavriil & Kaspi 2003;
Dall’Osso et al. 2003; Kaspi et al. 2003; Woods et al. 2004).
Unfortunately, the frequent, sensitive monitoring observa-
tions that are required to identify transient active periods,
to detect bursts, and to track the rotation of these pulsars
have not always been available. Therefore, in many cases
the causal connections between these phenomena have been
unclear (e.g., Gavriil & Kaspi 2003; Woods et al. 2005).
Here we report XMM-Newton observations of the
10.6 s X-ray pulsar, CXOU J164710.2-455216 (Muno et al.
2006), that bracketed a series of events that occurred near
2006 September 21. Near this time, Swift detected a soft
gamma-ray burst (Krimm et al. 2006) and a glitch with
∆P/P ∼ −10−4 (Israel et al. 2007). These events con-
firm our original hypothesis that this source is a magnetar
(Muno et al. 2006). We find that during the interval between
our two XMM-Newton observations, there were also dra-
matic changes in the luminosity, spectrum, and pulse profile
of CXOU J164710.2-455216. We compare these to changes
observed during active periods from other magnetars, and
discuss the implications for the interaction between the mag-
netic fields and crusts of the these neutron stars.
2 OBSERVATIONS
As part of the guest observer programme, XMM-Newton ob-
served CXOU J164710.2-455216 for 46 ks starting on 2005
September 16 at 18:59:38 (UTC). Fortuitously, 4.3 days
later, on 2006 September 21 at 01:34:53 (UTC), the Swift
Burst Alert Telescope (BAT) detected a 20 ms burst from
the direction of Westerlund 1 (Krimm et al. 2006), with an
energy of 3×1037 erg (15–150 keV; for a distance D=5 kpc;
Clark et al. 2005). In response, the director of XMM-Newton
carried out an observation lasting 30 ks beginning 1.5 days
later on 2006 September 22 at 12:40:27 (UTC). We analysed
the XMM-Newton observations in order to study changes in
the X-ray flux, spectrum, and pulse profile.
We analysed the data taken with the European Photon
Figure 2. Phase-averaged spectra of CXOU J164710.2-455216
taken on 2006 September 16 and 22 (top panels), in units of de-
tector counts. Models for the spectra are shown with a solid line:
a single absorbed blackbody on September 16, and two absorbed
blackbodies on September 22. For the latter spectrum, the cool
and hot blackbodies are indicated with the dotted and dashed
lines, respectively. The bottom panels display the difference be-
tween the data and the models, in units of the 1σ uncertainty on
the data. There are systematic residuals at low energies in the
September 22 spectrum, but these are not significant enough to
affect the overall model.
Imaging Camera (EPIC). For most of the timing and spec-
tral analysis, we used data taken with 73.4 ms time resolu-
tion using the pn array. The data from the MOS arrays were
taken with 2.4 s time resolution, which was inadequate for
studying the profile of this 10.6 s pulsar. Moreover, the data
suffered from pile-up during the second observation, when
the source was bright (see below). Therefore, we only used
the MOS data to generate spectra for the first observation.
We processed the observation data files using the stan-
dard tools (epchain and emchain) from the Science Anal-
ysis Software version 7.0. The events were filtered in the
standard manner, and we adjusted the arrival times of the
events to the Solar System barycentre. Images from the
EPIC-pn data are displayed in Figure 1. Comparing the
data from before and after the Swift burst, we find that
CXOU J164710.2-455216 increased in count rate by a factor
of 80 (0.5–8.0 keV).
Next, we extracted pulse-phase-averaged spectra from
within 15′′ of the location of CXOU J164710.2-455216 (α,
δ = 251.h79250, –45.◦87136 [J2000]). Estimates of the back-
ground were extracted from a 30′′ circular region that was
located 1.′5 west of the source region. We obtained the detec-
tor response and effective area using standard tools (rmfgen
and arfgen). The EPIC-pn spectra are displayed in Figure 2.
We modeled these spectra using XSPEC version 12.2.1.
We first assumed that the spectra could be described as
blackbody emission absorbed by interstellar gas and scat-
tered by dust. This model was acceptable for the observa-
tions before the burst on September 16 (χ2/ν = 59.4/67),
but was inconsistent with the data from September 22 (χ2/ν
= 2255/1136). For the later observation, we could model the
spectrum with two continuum components, either the sum
of two blackbodies, or a blackbody plus power law contin-
uum. We assumed that the interstellar absorption column
toward the source did not change between observations. The
spectral parameters, fluxes, and luminosities for the above
models are listed in Table 1. For completeness, we also list
c© 2007 RAS, MNRAS 000, 1–6
Exciting a Magnetar’s Fields 3
Table 1. Spectral Models for CXOU J164710.2-455216
2005 2006
May–Jun Sep 16 Sep 22
Two Blackbodies
NH (10
22 cm−2) 1.28 1.28 1.28(2)
kT1 (keV) 0.60(1) 0.54(1) 0.67(1)
Abb,1 (km
2) 0.09(1) 0.08(1) 3.62(2)
kT2 (keV) . . . . . . 1.7(1)
Abb,2 (km
2) . . . . . . 0.021(6)
FX (10
−13 erg cm−2 s−1) 2.3 1.5 215.7
LX (10
33 erg s−1) 1.4 1.0 109.7
Blackbody Plus Power Law
NH (10
22 cm−2) 1.44 1.44 1.44(1)
kT1 (keV) 0.58(2) 0.52(1) 0.68(1)
Abb,1 (km
2) 0.11(1) 0.11(1) 2.87(3)
Γ . . . . . . 2.07(4)
NΓ (10
−3 cm−2 s−1 keV−1) . . . . . . 3.7(9)
FX (10
−13 erg cm−2 s−1) 2.3 1.5 214.1
LX (10
33 erg s−1) 1.5 1.1 130.3
The reduced chi-squared for both joint fits were 1423/1298. The
interstellar absorption was assumed not to have changed over
the course of these observations. To compute the area of the
blackbody emission, we assumed D=5 kpc. NΓ is the photon
flux density of the power law at 1 keV. Uncertainties are 1σ, for
one degree of freedom. Fluxes are in the 0.5–8.0 keV band.
parameters from models of the spectra taken with Chandra
during 2005 May and June (Muno et al. 2006).
For both models, we found that the luminosity was a
factor of 100 higher (0.5–8.0 keV) 1.5 days after the burst
than it was 4.3 days before the burst. The increase in flux
was largely because the area of the ≈0.5 keV blackbody
increased from 0.1 km2 before the burst to ≈3 km2 after
the burst. It also resulted from the prominence of the hard
component after the burst. Modeled as a kT=1.7 keV black-
body, it produced 26 per cent of the observed flux on 2006
September 22 (18 per cent of the absorption-correction flux).
Modeled as a Γ=2.07 power law, it produced 50 per cent of
the observed flux (70 per cent of the intrinsic flux; 0.5-8.0
keV). If we add these components to our models for the
spectra taken on 2006 September 16, we find that their frac-
tional contribution to the observed flux was lower: <15 per
cent for the blackbody, and <35 per cent for the power law.
To identify pulsations from CXOU J164710.2-455216,
we computed Fourier periodograms using the Rayleigh
statistic. (A search for pulsations from other point sources
in the field revealed no other pulsars.) This provided an
initial estimate of the pulse period, which we then refined
by computing pulse profiles from non-overlapping 5000 s
intervals during each observation, measuring their phases
by cross-correlating them with the average pulse profile
from each observation, and modeling the differences be-
tween the assumed and measured phases using first-order
polynomials. The best-fitting periods were 10.61065(7) s
and 10.61064(8) s for 2006 September 16 and 22, respec-
tively. These values are within 1.5σ of the periods measured
in 2005 May and June, 10.6112(4) s and 10.6107(1) s, re-
spectively (Muno et al. 2006). The reference epochs of the
pulse maxima for the two observations in 2006 September
were 53994.786313(2) and 54000.526588(1) (MJD, Barycen-
tre Dynamical Time). Monitoring observations taken with
Figure 3. Pulse profiles of CXOU J164710.2-455216 taken on
2006 September 16 (top panels) and 2006 September 22 (bottom
panels), and in three energy bands: 0.5–2.0 keV (left panels), 2.0–
3.5 keV (middle panels), and 3.5–7.0 keV (right panels). Two
identical cycles are repeated in each panel. The dashed line in the
top panel represents the background count rate.
Swift reveal that a glitch with a fractional period change of
∆P/P ∼ −10−4 occurred between these two observations; a
discussion of this result is presented in (Israel et al. 2007).
We used these ephemerides to compute the pulse pro-
files in the full band of 0.5–8.0 keV, and three sub-bands:
0.5–2.0 keV, 2.0–3.5 keV, and 3.5–7.0 keV. The root-mean-
squared (rms) amplitudes of the pulsations in the full band
(0.5–8.0 keV) increased from 0.02 count s−1 before the burst,
to 0.29 count s−1 after the burst. At the same time, the frac-
tional rms amplitudes declined from 64 per cent before the
burst to 11 per cent after the burst. Moreover, the pulse pro-
file changed dramatically after the burst, as can be seen in
the profiles from the sub-bands displayed in Figure 3. Before
the burst, the pulse at all energies was single peaked, and
the differences in the pulse profile as a function of energy
are not very pronounced. After the burst, the pulse in the
full band displayed three distinct peaks, and a dependence
on energy developed. Specifically, in the 3.5–7.0 keV band,
the third peak was absent and the flux between the first two
peaks (phases 0.1–0.3) was larger, so that the overall profile
was more sinusoidal at high energies than at low.
We examined whether phase-resolved spectroscopy
could provide any insight into the origin of the pulses. Un-
fortunately, CXOU J164710.2-455216 was too faint on 2006
September 16 to generate spectra for all but the peak of
the pulse. We did examine phase-resolved spectra for 2006
September 22, but found no systematic trend relating the
spectral parameters with the intensity as a function of phase.
Finally, we searched for bursts by examining the time
series of events recorded by the EPIC-pn. We found no ev-
idence for bursts producing more than 4 counts within the
73.4 ms frame time, which placed an upper limit to their ob-
served fluence of 3×10−11 erg cm−2 (for a Γ=1.8 power law;
Krimm et al. 2006), or an energy of <2× 1035 erg (0.5–8.0
keV; D=5 kpc).
c© 2007 RAS, MNRAS 000, 1–6
4 M. P. Muno et al.
3 DISCUSSION
In the 5.8 days between our two XMM-Newton obser-
vations of CXOU J164710.2-455216, a remarkable set of
events occurred. First, the phase-averaged luminosity of
CXOU J164710.2-455216 increased by a factor of ∼100,
from LX = 1 × 10
33 to LX = 1 × 10
35 erg s−1 (0.5–
8.0 keV; Fig. 1; Campana & Israel 2006), and the spec-
trum hardened (Table 1). Energetically, this is the most
important feature of this active period. In the 1.5 days af-
ter the burst, if we conservatively assume the persistent
flux from CXOU J164710.2-455216 was constant, the to-
tal energy released was ∼1040 erg (0.5–8.0 keV). Second,
a 20 ms long burst with an energy of 3 × 1037 erg (15–
150 keV) was detected from this source with the BAT on
board Swift (Krimm et al. 2006). Third, a glitch was ob-
served in the spin period of the pulsar, with ∆P/P ∼ −10−4
(Israel et al. 2007). Fourth, the pulse profile changed from
having a simple, single-peaked structure, to exhibiting three
distinct peaks with pronounced energy dependence (Fig. 3).
Similar changes in the fluxes, spectra, and timing properties
of magnetars have been observed before, but the combina-
tion observed from CXOU J164710.2-455216 is unique.
It is common for the persistent luminosities of mag-
netars to vary on time scales of weeks to years. The per-
sistent luminosities from the SGRs 1900+14 (Woods et al.
2001) and 1806–20 (Woods et al. 2006) and the bright
AXPs 1E 1048.1–5937 (Gavriil & Kaspi 2004; Tiengo et al.
2005) and 1E 2259+586 (Woods et al. 2004) have been
observed to vary by factors of 2–3 around ∼1034 − 1035
erg s−1 (0.5–10 keV). The luminosities of SGR 1627–
41 (Kouveliotou et al. 2003) and the transient AXP XTE
J1810–597 (Ibrahim et al. 2004; Gotthelf et al. 2004) have
been observed to increase by factors of 100, from ∼1033
to ∼1035 erg s−1 (0.5–10 keV). The larger luminosities,
∼1035 erg s−1, appear to be a rough upper envelope for
the persistent 0.5–8.0 keV fluxes of magnetars (not count-
ing bursts and giant flares). Indeed, the active period from
CXOU J164710.2-455216 also had LX ≈ 10
35 erg s−1 (0.5–
8.0 keV). This persistent flux is generally assumed to be
produced because the unwinding internal fields induce grad-
ual, plastic deformations in the crust and external magnetic
fields, which in turn heats the surface or magnetosphere
(Thompson & Duncan 1995, 1996). Therefore, the increase
in the flux from CXOU J164710.2-455216 demonstrates that
either the unwinding of the internal fields, or the response of
the crust to that unwinding, is intermittent and can activate
in .5 days.
The active periods from magnetars are often accompa-
nied by second-long bursts. These bursts are the hallmarks
of SGRs, and during their active periods hundreds will oc-
cur over the course of a year with energies of up to 1041
erg (2–60 keV; Göğüş et al. 2001).The bursts detected from
AXPs have all been weaker, with peak energies of .1038
erg (2–60 keV). In the AXPs XTE J1810–597 (Woods et al.
2005) and 1E 1048.1–5937 (Gavriil, Kaspi, & Woods 2006),
the bursts that have been detected are infrequent and rela-
tively isolated. In 1E 2259+586 (Woods et al. 2004), a series
of bursts were detected during an 11 ks observation that oc-
curred within 7 days of the start of an active period in 2002
June. The burst detected from CXOU J164710.2-455216 re-
sembles those from 1E 2259+586, in that it occurred very
near the start of an active period. The energy of the burst
(3×1037 erg; 15–150 keV) is trivial compared to that released
as persistent flux (&1040 erg; 0.5–8.0 keV), so it is probably
not a trigger, but a symptom of the active period. Under the
magnetar model, the bursts that accompany the active pe-
riods are caused by fractures that occur in the crust. These
fractures inject into the magnetosphere currents that are
unstable to to wave motion, which quickly generates hot, X-
ray emitting plasma (Thompson & Duncan 1995, 1996). It
is reasonable to expect that such fractures would be stronger
and occur more frequently when the persistent flux is higher,
because the crust is already under stress.
Variations in the spin-down rates have been observed
from several luminous (LX & 10
34 erg s−1; 0.5–8.0 keV)
magnetars. Torque variations have been detected from
1E 1048.1–5937 (Gavriil & Kaspi 2004) and SGR 1806–20
(Woods et al. 2006), in association with their active periods.
Sudden period changes have been seen in three cases. Two
glitches have been detected from 1RXS J170849–400910
with ∆P/P ∼ −1 × 10−6 and −6 × 10−6 (Gavriil & Kaspi
2003; Dall’Osso et al. 2003). Neither were associated with
active periods, but the monitoring observations were sparse,
so one could have been missed (Dall’Osso et al. 2003). One
glitch accompanied the 2002 June active period of 1E
2259+586 in which the spin period decreased by ∆P/P ∼
−10−6 (Kaspi et al. 2003; Woods et al. 2004). Finally, a dra-
matic episode of spin-down occurred near the time of a
1044 erg (3–100 keV) giant flare from SGR 1900+14, with
∆P/P ∼ 10−4 (Woods et al. 1999). This is of comparable
magnitude to the glitch from CXOU J164710.2-455216, al-
beit of the opposite sign (Israel et al. 2007).
The glitch appears to have been a major energetic
component of the outburst from CXOU J164710.2-455216.
Glitches are ascribed to sudden changes in the moments of
inertia of the neutron stars that occur when crustal move-
ments change how superfluid in the interior is coupled to
the bulk of the crust (e.g., Dall’Osso et al. 2003; Kaspi et al.
2003). The change in rotational energy during the glitch, as-
suming most of the star rotates as a solid body, is on order
∆Erot ∼ IΩ∆Ω, where I∼10
45 g cm2 is the moment of in-
ertia of a neutron star with mass M=1.4 M⊙ and radius
R=1 km. For CXOU J164710.2-455216 Ω=0.6 rad s−1 and
∆Ω=6×10−5 rad s−1, so ∆Erot∼10
40 erg. However, a larger
input of energy into the stellar interior may be required to
unpin the superfluid vortices and initiate the glitch, ∼1042
erg (e.g., Link & Epstein 1996; Thompson et al. 2000). In
contrast, the radiative output of CXOU J164710.2-455216 in
the first week of this active period was only ∼1040 erg (0.5–
8.0 keV). Whereas for the giant flare from SGR 1900+14 and
the 2002 June active period from 1E 2259+586 it appeared
that most of the energy was radiated away from the mag-
netosphere (Thompson et al. 2000; Woods et al. 2004), for
CXOU J164710.2-455216 most of the energy was probably
input into the interior of the neutron star.
The change in the pulse profile of
CXOU J164710.2-455216 is also difficult to understand
from an energetic standpoint. Changes in the qualitative
shape of the pulse profiles (as opposed to changes in the
pulsed fraction) have only been seen previously from three
sources. For 1E 2259+586, the profile before the 2002 June
burst exhibited two distinct peaks, whereas after the burst
the phases between the peaks contained more flux, so that
c© 2007 RAS, MNRAS 000, 1–6
Exciting a Magnetar’s Fields 5
part of the profile resembled a single plateau of emission
(Woods et al. 2004). This change is minor compared to that
from CXOU J164710.2-455216 in Figure 3. Large changes in
the harmonic structure of the pulse profile have only been
observed in response to the giant flares from SGRs. For
SGR 1900+14 the profile had three peaks before the flare
in 1998, and a single peak during and after (Woods et al.
2001). For SGR 1806–20, the opposite change occurred in
2004: it shifted from having a simple, single-pulsed profile
to having multiple peaks (Woods et al. 2006).
For the SGRs, the changes in the pulse profiles are
thought to occur because the multipole structure of the ex-
ternal magnetic fields are rearranged. This is reasonable,
because the giant flares release a significant fraction of the
energy in the external fields. For a dipole, this would be
B2extR
∼ 1045 G, where we take Bext∼10
14 G, and
R∼10 km (Woods et al. 1999; Hurley et al. 2005). However,
for CXOU J164710.2-455216, and to a lesser degree for 1E
2259+586, it is unreasonable to suggest that active periods
releasing only ∼1040 erg of X-rays resulted from a significant
rearrangement of the exterior magnetic fields.
Instead, we suggest that a change occurred in the dis-
tribution of currents in the magnetosphere. We hypothesize
that the emission in quiescence is thermal emission from
the cooling neutron star, which emerges through a hot spot
where the opacity of the highly-magnetized atmosphere is
lowest (Heyl & Hernquist 1998). A single hot spot on the
surface could explain the single-peaked, fully modulated
(≈70 per cent rms) pulse in quiescence (Özel, Psaltiz, &
Kaspi 2001). We suggest that the active period was initi-
ated when a very small twist was imparted to the magnetic
field by plastic motions of the crust. Currents formed to
compensate for this twist, which heated the surface of the
star and resonantly scattered the emission from its surface
(Table 1). Both of these would contribute to creating the
complex pulse profile (Thompson et al. 2002). If our sce-
nario is correct, when this source returns to quiescence, the
pulse should regain its single-peaked profile.
4 CONCLUSIONS
We have examined the X-ray luminosity, spectrum, and
pulse profile of CXOU J164710.2-455216 before and after
an interval during which Swift detected a soft gamma-ray
burst and a timing glitch from the source. The energy radi-
ated from the exterior was too small to have resulted from
a significant rearrangement of the external magnetic fields
of CXOU J164710.2-455216. Instead, the dramatic change
in the pulse profile indicates that the underlying emission
mechanism changed. Before the burst, the X-ray emission
was probably powered by the thermal energy of the star,
whereas afterwards it was powered by currents in the mag-
netosphere. Moreover, the glitch required an energy at least
as large as the energy released as X-rays, &1040 erg, which
suggests that much of the energy of this event was input
into the interior of the neutron star. Future X-ray observa-
tions of this source will reveal the duration and duty cycle
of this active period, which would constrain the amount of
energy input into the interior. This could help answer why
the emission, which is thought to be produced as the inter-
nal fields of magnetars unwind, can remain inactive for years
and then suddenly turn on in a few days.
ACKNOWLEDGMENTS
We thank N. Schartel for providing the discretionary obser-
vation, G. Israel for sharing the results of the Swift observa-
tions, and the referee for helpful comments. MPM was sup-
ported by the NASA XMM Guest Observer Facility; BMG
by a Federation Fellowship from the Australian Research
Council and an Alfred P. Sloan Research Fellowship; and
SFPZ by the Royal Dutch Academy of Arts and Sciences.
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	Introduction
	Observations
	Discussion
	Conclusions

  We describe XMM-Newton observations taken 4.3 days prior to and 1.5 days
subsequent to two remarkable events that were detected with Swift on 2006
September 21 from the candidate magnetar CXOU J164710.2-455216: (1) a 20 ms
burst with an energy of 1e37 erg (15-150 keV), and (2) a rapid spin-down
(glitch) with a fractionap period change of 1e-4. We find that the luminosity
of the pulsar increased by a factor of 100 in the interval between
observations, from 1e33 to 1e35 erg/s (0.5-8.0 keV), and that its spectrum
hardened. The pulsed count rate increased by a factor of 10 (0.5-8.0 keV), but
the fractional rms amplitude of the pulses decreased from 65 to 11 per cent,
and their profile changed from being single-peaked to exhibiting three peaks.
Similar changes have been observed from other magnetars in response to
outbursts, such as that of 1E 2259+586 in 2002 June. We suggest that a plastic
deformation of the neutron star's crust induced a very slight twist in the
external magnetic field, which in turn generated currents in the magnetosphere
that were the direct cause of the X-ray outburst.

Introduction
	Observations
	Discussion
	Conclusions
