Astronomy & Astrophysics manuscript no. 7480 October 24, 2018
(DOI: will be inserted by hand later)
Discovery of X-ray emission from the young radio pulsar
PSR J1357−6429
P. Esposito1,2, A. Tiengo2, A. De Luca2, and F. Mattana2,3
1 Università degli Studi di Pavia, Dipartimento di Fisica Nucleare e Teorica and INFN-Pavia, via Bassi 6, I-27100 Pavia, Italy
e-mail: paoloesp@iasf-milano.inaf.it
2 INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via Bassini 15, I-20133 Milano, Italy
3 Università degli Studi di Milano - Bicocca, Dipartimento di Fisica G. Occhialini, p.za della Scienza 3, I-20126 Milano, Italy
Received / Accepted
Abstract We present the first X-ray detection of the very young pulsar PSR J1357−6429 (characteristic age of 7.3 kyr) using
data from the XMM-Newton and Chandra satellites. We find that the spectrum is well described by a power-law plus black-
body model, with photon index Γ = 1.4 and blackbody temperature kBT = 160 eV. For the estimated distance of 2.5 kpc,
this corresponds to a 2–10 keV luminosity of ∼1.2 × 1032 erg s−1, thus the fraction of the spin-down energy channeled by
PSR J1357−6429 into X-ray emission is one of the lowest observed. The Chandra data confirm the positional coincidence with
the radio pulsar and allow to set an upper limit of 3× 1031 erg s−1 on the 2–10 keV luminosity of a compact pulsar wind nebula.
We do not detect any pulsed emission from the source and determine an upper limit of 30% for the modulation amplitude of
the X-ray emission at the radio frequency of the pulsar.
Key words. stars: individual (PSR J1357−6429) – stars: neutron – X-rays: stars
1. Introduction
X-ray observations of radio pulsars provide a powerful diag-
nostic of the energetics and emission mechanisms of rotation-
powered neutron stars. Due to the magnetic dipole braking, a
pulsar loses rotational kinetic energy at a rate Ė = 4π2IṖP−3,
where I is the moment of inertia of the neutron star, assumed to
be 1045 g cm2, and P is the rotation period. Though pulsars
have traditionally been mostly studied at radio wavelengths,
only a small fraction (10−7 to 10−5, e.g., Taylor et al. 1993)
of the “spin-down luminosity” Ė emerges as radio pulsations.
Rotation power can manifest itself in the X / γ-ray energy range
as pulsed emission, or as nebular radiation produced by a rela-
tivistic wind of particles emitted by the neutron star. Residual
heat of formation is also observed as soft X-ray emission from
young neutron stars. Such thermal radiation, however, can also
be produced as a result of reheating from internal or exter-
nal sources. The growing list of observable X-ray emitting
rotation-powered pulsars allows the study of the properties of
the population as a whole. Young pulsars constitute a particu-
larly interesting subset to investigate owing to their large spin-
down luminosities (&1036 erg s−1).
The discovery of PSR J1357−6429 during the Parkes multi-
beam survey of the Galactic plane (see Lorimer et al. 2006
and references therein) is reported in Camilo et al. (2004).
The pulsar is located near the supernova remnant candidate
G309.8−2.6 (Duncan et al. 1997) for which no distance or age
information is available. With a spin period of 166 ms, a pe-
riod derivative of 3.6 × 10−13 s s−1, and a characteristic age
τc = P/2Ṗ ≃ 7300 yr, this pulsar stands out as one of the ten
youngest Galactic radio pulsars known (see the ATNF Pulsar
Catalogue1, Manchester et al. 2005). The other main properties
of this source derived from the radio observations are the spin-
down luminosity of 3.1 × 1036 erg s−1 and the surface magnetic
field strength of 7.8 × 1012 G, inferred under the assumption of
pure magnetic dipole braking. Based on a dispersion measure
of ∼127 cm−3 pc (Camilo et al. 2004), a distance of ∼2.4 kpc
is estimated, according to the Cordes-Lazio NE2001 Galactic
Free Electron Density Model2.
Here we report the first detection of PSR J1357−6429 in
the X-ray range using the XMM-Newton observatory and we
present its spectral properties in the 0.5–10 keV energy band.
We also made use of two short Chandra observations to con-
firm the identification and to probe possible spatial extended
emissions, taking advantage of the superb angular resolution of
the Chandra telescope.
2. XMM-Newton observation and data analysis
In this section we present the results obtained with the EPIC in-
strument on board the XMM-Newton X-ray observatory. EPIC
consists of two MOS (Turner et al. 2001) and one pn CCD
1 See http://www.atnf.csiro.au/research/pulsar/psrcat .
2 See http://rsd-www.nrl.navy.mil/7213/lazio/ne model
and references therein.
http://arxiv.org/abs/0704.0205v2
2 P. Esposito et al.: X-ray observations of PSR J1357−6429
detectors (Strüder et al. 2001) sensitive to photons with ener-
gies between 0.1 and 15 keV. All the data reduction was per-
formed using the XMM-Newton Science Analysis Software3
(SAS version 7.0). The raw observation data files were pro-
cessed using standard pipeline tasks (epproc for pn, emproc
for MOS data). Response matrices and effective area files were
generated with the SAS tasks rmfgen and arfgen.
The observation was carried out on 2005 August 17 and
had a duration of 15 ks, yielding net exposure times of 11.7 ks
in the pn camera and 14.5 ks in the two MOSs. The pn
and the MOSs were operated in Full Frame mode (time res-
olution: 73.4 ms and 2.6 s, respectively) and mounted the
medium thickness filter. PSR J1357−6429 is clearly detected
in the pn and MOS images (see Figure 1) at the radio pul-
sar position (Right ascension = 13h 57m 02.4s, Declination
= −64◦ 29′ 30.2′′ (epoch J2000.0); Camilo et al. 2004). The
13:57:12.0 13:57:00.0 13:56:48.0
-64:27:35.8
-64:28:47.8
-64:29:59.8
-64:31:11.8
Right ascension
Figure 1. Field of PSR J1357−6429 as seen by the EPIC cam-
eras in the 0.5–10 keV energy range. The radio pulsar posi-
tion (Camilo et al. 2004) is marked with the white diamond
sign. The angular separation of the centroid of the X-ray source
(computed using the SAS task emldetect) from the radio pul-
sar position is (3.5 ± 0.6)′′ (1σ statistical error). Considering
the XMM-Newton absolute astrometric accuracy of 2′′ (r.m.s.),
the X-ray and radio positions are consistent.
source spectra were extracted from circular regions centered at
the position of PSR J1357−6429. The whole observation was
affected by a high particle background that led to the selec-
tion of a 20′′ radius circle in order to increase the signal-to-
noise ratio in the pn detector, particularly sensitive to particle
background, and a 40′′ radius for both the MOS cameras. The
background spectra were extracted from annular regions with
radii of 140′′ and 220′′ for the MOSs, and from two rectan-
gular regions with total area of ∼104 arcsec2 located on the
sides of the source for the pn. We carefully checked that the
choice of different background extraction regions does not af-
fect the spectral results. We selected events with pattern 0–4
3 See http://xmm.vilspa.esa.es/ .
Table 1. Summary of the XMM-Newton spectral results. Errors
are at the 90% confidence level for a single interesting param-
eter.
Parameter Value
PL PL +BB
NH (10
22 cm−2) 0.14+0.07
−0.06 0.4
Γ 1.8+0.3
−0.2 1.4 ± 0.5
kBT (keV) – 0.16
+0.09
−0.04
a (km) – 1.4+2.9
Fluxb (10−13erg cm−2 s−1) 2.3 3.6
Blackbody fluxb (10−13erg cm−2 s−1) – 1.3
χ2r / d.o.f. 1.00 / 72 0.85 / 70
a Radius at infinity assuming a distance of 2.5 kpc.
b Unabsorbed flux in the 0.5–10 keV energy range.
and pattern 0–12 for the pn and the MOS, respectively. The re-
sulting background subtracted count rates in the 0.5–10 keV
energy range were (4.2 ± 0.3) × 10−2 cts s−1 in the pn and
(1.9 ± 0.2) × 10−2 cts s−1 in the two MOS cameras, while the
background rate expected in the source extraction regions is
about 50% of these values. The spectra were rebinned to have
at least 20 counts in each energy bin. Spectral fits were per-
formed using the XSPEC version 12.3 software4.
The spectra from the three cameras were fitted together
in the 0.5–10 keV energy range with a power law and with
a power-law plus blackbody model (see Table 1). The latter
model provides a slightly better fit, with less structured resid-
uals (see Figure 2). Furthermore, considering the distance of
2.5 kpc, the interstellar absorption along the line of sight de-
rived with the power-law fit is too low if compared to the
typical column density of neutral absorbing gas in that direc-
tion of approximately 1022 cm−2 (Dickey & Lockman 1990).
The resulting best-fit parameters for the power-law plus black-
body model are photon index Γ = 1.4, blackbody temperature
kBT = 0.16 keV, and absorption NH = 4× 10
21 cm−2 with a re-
duced χ2 of 0.85 for 70 degrees of freedom. The corresponding
luminosity in the 0.5–10 keV band is 2.7 × 1032 erg s−1.
Young pulsars are often associated with pulsar wind neb-
ulae: complex structures that arise from the interaction be-
tween the particle wind powered by the pulsar and the
supernova ejecta or surrounding interstellar medium (see
Gaensler & Slane 2006 for a review). Inspecting the EPIC im-
ages in various energy bands, we find only a marginal (≈3σ)
evidence of diffuse emission, in the 2–4 keV energy band con-
sisting of a faint elongated (∼20 arcsec to the north-east, see
Figure 1) structure starting from PSR J1357−6429. We took
that excess as an upper limit for a diffuse emission: assuming
the same spectrum as the point source, it corresponds to a 2–10
keV luminosity of ≈6 × 1031 erg s−1.
For the timing analysis we applied the solar system
barycenter correction to the photon arrival times with the SAS
task barycen. We searched the data for pulsations around the
spin frequency at the epoch of the XMM-Newton observations,
predicted assuming the pulse period and the spin-down rate
measured with the Parkes radio telescope (Camilo et al. 2004).
4 See http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/ .
P. Esposito et al.: X-ray observations of PSR J1357−6429 3
1 2 5
Energy (keV)
Figure 2. EPIC pn spectrum of PSR J1357−6429. Top: Data
and best-fit power-law (dashed line) plus blackbody (dot-
dashed line) model. Middle: Residuals from the power-law
best-fit model in units of standard deviation. Bottom: Residuals
from the power-law plus blackbody best-fit model in units of
standard deviation.
As glitches and / or deviations from a linear spin-down may al-
ter the period evolution, we searched over a wide period range
centered at the value of ∼166 ms. We searched for significant
periodicities using two methods: a standard folding technique
and the Rayleigh statistic. No pulsation were detected near to
the predicted frequency with either method but, since the pn
timing resolution (73 ms) allows to only poorly sample the
166 ms pulsar period, a reliable upper limit on the pulsed frac-
tion cannot be set.
3. Chandra observations and data analysis
PSR J1357−6429 was observed by means of the Chandra
X-ray Observatory during two exposures of ∼17 ks duration
each on 2005 November 18 and 19. The observations were car-
ried out with the Spectroscopic array of the High Resolution
Camera (HRC-S; Murray et al. 2000) used without transmis-
sion gratings. The HRC is a multichannel plate detector sen-
sitive to X-ray over the 0.08–10 keV energy range, although
essentially no energy information on the detected photons is
available. The HRC-S time resolution is 16 µs.
We started from “level 1” event data calibrated and made
available through the Chandra X-ray Center5. The level 1 event
files contain all HRC triggers with the position information
corrected for instrumental (degap) and aspect (dither) effects.
After standard data processing with the Chandra Interactive
Analysis of Observations (CIAO ver. 3.3), a point-like source
has been clearly detected in both the observations at a position
consistent with that of PSR J1357−6429 (see Figure 3).
For the timing analysis we corrected the data to the solar
system barycenter with the CIAO task axbary and then we fol-
5 See http://cxc.harvard.edu/.
13:57:02.9 13:57:02.4 13:57:02.0
-64:29:27.4
-64:29:31.0
-64:29:34.6
Right ascension
Figure 3. Chandra 0.08–10 keV HRC-S image centered
on the radio pulsar position, marked with a diamond sign
(Camilo et al. 2004). The CIAO celldetect routine yields a
best-fit position for the X-ray source at an angular distance of
(0.9 ± 0.2)′′ (1σ statistical error) from the radio pulsar. This
value consistent with the Chandra pointing accuracy of 0.8′′
(99% confidence level).
lowed the same procedure described in Section 2, but we again
did not detect the source pulsation. By folding the light curve
of PSR J1357−6429 on the radio frequency and fitting it with a
sinusoid, we determine a 90% confidence level upper limit of
∼30% on the amplitude of a sinusoidal modulation. We stress
that this upper limit depends sensitively on data time binning
and on the assumed pulse shape.
We used the CIAO task merge all to generate a combined
image of the source. Our main purpose was to search for diffuse
structures on scales smaller than the XMM-Newton angular res-
olution. We compared the radial profile of the pulsar emission
with the Chandra High-Resolution Mirror Assembly point-
spread function at 1 keV generated using Chandra Ray Tracer
(ChaRT) and Model AXAF Response to X-rays (MARX).
We found that the emission we detect from PSR J1357−6429
(∼100 counts concentrated within a ∼0.5′′ radius circle) is con-
sistent with that from a point source.
We used the Chandra data and the PIMMS software6
to determine an upper limit on the luminosity of a possible
spatial extended emission. The 3σ upper limit on a pulsar
wind nebula brightness (in counts s−1) has been estimated as
3(bA)1/2τ−1, where b is the background surface brightness in
counts arcsec−2, A is the pulsar wind nebula area, and τ is the
exposure duration. Assuming the interstellar absorption value
from the XMM-Newton best-fit model (NH = 0.4 × 10
22 cm−2,
see Section 2) and typical parameters for a pulsar wind nebula
(radius of ∼2× 1017 cm, that corresponds to ∼5′′ for a distance
of 2.5 kpc, and power-law spectrum with photon index Γ = 1.6,
see, e.g., Gotthelf (2003)), this upper limit corresponds to a
2–10 keV luminosity of ≈3 × 1031 erg s−1 for a uniform dif-
fuse nebula. No significant diffuse excess was found even at
6 See http://heasarc.gsfc.nasa.gov/docs/tools.html .
4 P. Esposito et al.: X-ray observations of PSR J1357−6429
larger angular scale, but the corresponding upper limit for dif-
fuse emission is less constraining than that derived using the
XMM-Newton data.
4. Discussion
We have presented the results of the first X-ray observations of
PSR J1357−6429 by means of the XMM-Newton and Chandra
observatories. The source has been positively detected in all
the instruments although, probably due to the low statistics, we
could not detect the source pulsation. The high angular resolu-
tion Chandra observations favor the picture in which most of
the counts belong to a point source. We found that the spectrum
is well represented by either a power-law with photon index
Γ = 1.8+0.3
−0.2 or by a power-law plus blackbody model. In the
latter case the best-fit parameters are for the power-law com-
ponent a photon index Γ = 1.4 ± 0.5 and, for the blackbody
component, radius7 of ∼1.4+2.9
−0.2d2.5 km and temperature corre-
sponding to kBT = 0.16
+0.09
−0.04 keV.
It is generally believed that a combination of emission
mechanisms are responsible for the detected X-ray flux from
rotation-powered pulsars (see, e.g., Kaspi et al. 2006 for a re-
view). The acceleration of particles in the neutron star magne-
tosphere generates non thermal radiation by synchrotron and
curvature radiation and / or inverse Compton processes, while
soft thermal radiation could result by cooling of the surface
of the neutron star. A harder thermal component can arise from
polar-cap reheating, due to return currents from the outer gap or
from close to the polar-cap. The dominant emission mechanism
is likely related to the age of the pulsar. In pulsar younger than
≈104 yr the strong magnetospheric emission generally prevails
over the thermal radiation, making difficult to detect it.
As discussed in Section 2, we tend to prefer the power-law
plus blackbody spectral model for PSR J1357−6429. The re-
sulting blackbody size of ∼1.5d2.5 km may suggest that the soft
emission (.2 keV) is coming from hot spots on the surface due
to backflowing particles, rather than from the entire surface.
However this hint should be considered with caution, as the
surface temperature distribution of a neutron star is most likely
non uniform (since the heath conductivity of the crust is higher
along the magnetic field lines) and the small and hot blackbody
could result from a more complicated distribution of tempera-
ture. Moreover, currently we lack of reliable models of cooling
neutron star thermal emission and thus we cannot exclude that
the soft component is emitted from surface layers of the whole
neutron star.
To date, thermal emission has been detected in only a
few young radio pulsars. Among these, the properties of
PSR J1357−6429 are similar to those of the young pul-
sars Vela (PSR B0833−45; τc = 11 kyr, P = 89 ms,
Ė = 6.9 × 1036 erg s−1, and distance d ≃ 0.2 kpc; Pavlov et al.
2001) and PSR B1706–44 ( τc = 17.5 kyr, P = 102 ms,
Ė = 3.4 × 1036 erg s−1, and d ≃ 2.5 kpc; Gotthelf et al. 2002).
Notably, the efficiency in the conversion of the spin-down
energy loss into X-ray luminosity for PSR J1357−6429 is
L0.5−10 keV/Ė ≃ 8d
2.5×10
−5, significantly lower than the typical
7 We indicate with dN the distance in units of N kpc.
value of ≈10−3 (Becker & Truemper 1997), and similar to that
of PSR B1706–44 (∼10−4) and Vela (∼10−5).
Although a pulsar wind nebula would not came as a
surprise for this young and energetic source, we did not
find clear evidence of diffuse X-ray emission associated with
PSR J1357−6429. However, some known examples of wind
nebulae (see Gaensler & Slane 2006), rescaled to the distance
of PSR J1357−6429, would hide below the upper limits derived
from the XMM-Newton and Chandra data.
New deeper exposures using XMM-Newton or Chandra
would help determine if a thermal component is present in the
emission of PSR J1357−6429 as our spectral analysis suggests,
and possibly detect a pulsed emission. High sensitivity obser-
vations would also serve to address the issue of the presence
of a pulsar wind nebula. Although there is not any EGRET
γ-ray source coincident with PSR J1357−6429 (Hartman et al.
1999), young neutron stars and their nebulae are often bright
γ-ray sources and PSR J1357−6429 in particular, given its
high “spin-down flux” Ė/d2 and similarity with Vela and
PSR B1706–44, is likely to be a good target for the upcoming
AGILE and GLAST satellites and the ground based Cherenkov
air showers telescopes.
Acknowledgements. This work is based on data from observations
with XMM-Newton, an ESA science mission with instruments and
contributions directly funded by ESA member states and NASA. We
also used data from the Chandra X-ray Observatory Center, which is
operated by the Smithsonian Astrophysical Observatory Center on be-
half of NASA. The authors thank the anonymous referee for helpful
comments and acknowledge the support of the Italian Space Agency
and the Italian Ministry for University and Research.
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	Introduction
	XMM-Newton observation and data analysis
	Chandra observations and data analysis
	Discussion

  We present the first X-ray detection of the very young pulsar PSR J1357-6429
(characteristic age of 7.3 kyr) using data from the XMM-Newton and Chandra
satellites. We find that the spectrum is well described by a power-law plus
blackbody model, with photon index Gamma=1.4 and blackbody temperature kT=160
eV. For the estimated distance of 2.5 kpc, this corresponds to a 2-10 keV
luminosity of about 1.2E+32 erg/s, thus the fraction of the spin-down energy
channeled by PSR J1357-6429 into X-ray emission is one of the lowest observed.
The Chandra data confirm the positional coincidence with the radio pulsar and
allow to set an upper limit of 3E+31 erg/s on the 2-10 keV luminosity of a
compact pulsar wind nebula. We do not detect any pulsed emission from the
source and determine an upper limit of 30% for the modulation amplitude of the
X-ray emission at the radio frequency of the pulsar.

Introduction
X-ray observations of radio pulsars provide a powerful diag-
nostic of the energetics and emission mechanisms of rotation-
powered neutron stars. Due to the magnetic dipole braking, a
pulsar loses rotational kinetic energy at a rate Ė = 4π2IṖP−3,
where I is the moment of inertia of the neutron star, assumed to
be 1045 g cm2, and P is the rotation period. Though pulsars
have traditionally been mostly studied at radio wavelengths,
only a small fraction (10−7 to 10−5, e.g., Taylor et al. 1993)
of the “spin-down luminosity” Ė emerges as radio pulsations.
Rotation power can manifest itself in the X / γ-ray energy range
as pulsed emission, or as nebular radiation produced by a rela-
tivistic wind of particles emitted by the neutron star. Residual
heat of formation is also observed as soft X-ray emission from
young neutron stars. Such thermal radiation, however, can also
be produced as a result of reheating from internal or exter-
nal sources. The growing list of observable X-ray emitting
rotation-powered pulsars allows the study of the properties of
the population as a whole. Young pulsars constitute a particu-
larly interesting subset to investigate owing to their large spin-
down luminosities (&1036 erg s−1).
The discovery of PSR J1357−6429 during the Parkes multi-
beam survey of the Galactic plane (see Lorimer et al. 2006
and references therein) is reported in Camilo et al. (2004).
The pulsar is located near the supernova remnant candidate
G309.8−2.6 (Duncan et al. 1997) for which no distance or age
information is available. With a spin period of 166 ms, a pe-
riod derivative of 3.6 × 10−13 s s−1, and a characteristic age
τc = P/2Ṗ ≃ 7300 yr, this pulsar stands out as one of the ten
youngest Galactic radio pulsars known (see the ATNF Pulsar
Catalogue1, Manchester et al. 2005). The other main properties
of this source derived from the radio observations are the spin-
down luminosity of 3.1 × 1036 erg s−1 and the surface magnetic
field strength of 7.8 × 1012 G, inferred under the assumption of
pure magnetic dipole braking. Based on a dispersion measure
of ∼127 cm−3 pc (Camilo et al. 2004), a distance of ∼2.4 kpc
is estimated, according to the Cordes-Lazio NE2001 Galactic
Free Electron Density Model2.
Here we report the first detection of PSR J1357−6429 in
the X-ray range using the XMM-Newton observatory and we
present its spectral properties in the 0.5–10 keV energy band.
We also made use of two short Chandra observations to con-
firm the identification and to probe possible spatial extended
emissions, taking advantage of the superb angular resolution of
the Chandra telescope.
2. XMM-Newton observation and data analysis
In this section we present the results obtained with the EPIC in-
strument on board the XMM-Newton X-ray observatory. EPIC
consists of two MOS (Turner et al. 2001) and one pn CCD
1 See http://www.atnf.csiro.au/research/pulsar/psrcat .
2 See http://rsd-www.nrl.navy.mil/7213/lazio/ne model
and references therein.
http://arxiv.org/abs/0704.0205v2
2 P. Esposito et al.: X-ray observations of PSR J1357−6429
detectors (Strüder et al. 2001) sensitive to photons with ener-
gies between 0.1 and 15 keV. All the data reduction was per-
formed using the XMM-Newton Science Analysis Software3
(SAS version 7.0). The raw observation data files were pro-
cessed using standard pipeline tasks (epproc for pn, emproc
for MOS data). Response matrices and effective area files were
generated with the SAS tasks rmfgen and arfgen.
The observation was carried out on 2005 August 17 and
had a duration of 15 ks, yielding net exposure times of 11.7 ks
in the pn camera and 14.5 ks in the two MOSs. The pn
and the MOSs were operated in Full Frame mode (time res-
olution: 73.4 ms and 2.6 s, respectively) and mounted the
medium thickness filter. PSR J1357−6429 is clearly detected
in the pn and MOS images (see Figure 1) at the radio pul-
sar position (Right ascension = 13h 57m 02.4s, Declination
= −64◦ 29′ 30.2′′ (epoch J2000.0); Camilo et al. 2004). The
13:57:12.0 13:57:00.0 13:56:48.0
-64:27:35.8
-64:28:47.8
-64:29:59.8
-64:31:11.8
Right ascension
Figure 1. Field of PSR J1357−6429 as seen by the EPIC cam-
eras in the 0.5–10 keV energy range. The radio pulsar posi-
tion (Camilo et al. 2004) is marked with the white diamond
sign. The angular separation of the centroid of the X-ray source
(computed using the SAS task emldetect) from the radio pul-
sar position is (3.5 ± 0.6)′′ (1σ statistical error). Considering
the XMM-Newton absolute astrometric accuracy of 2′′ (r.m.s.),
the X-ray and radio positions are consistent.
source spectra were extracted from circular regions centered at
the position of PSR J1357−6429. The whole observation was
affected by a high particle background that led to the selec-
tion of a 20′′ radius circle in order to increase the signal-to-
noise ratio in the pn detector, particularly sensitive to particle
background, and a 40′′ radius for both the MOS cameras. The
background spectra were extracted from annular regions with
radii of 140′′ and 220′′ for the MOSs, and from two rectan-
gular regions with total area of ∼104 arcsec2 located on the
sides of the source for the pn. We carefully checked that the
choice of different background extraction regions does not af-
fect the spectral results. We selected events with pattern 0–4
3 See http://xmm.vilspa.esa.es/ .
Table 1. Summary of the XMM-Newton spectral results. Errors
are at the 90% confidence level for a single interesting param-
eter.
Parameter Value
PL PL +BB
NH (10
22 cm−2) 0.14+0.07
−0.06 0.4
Γ 1.8+0.3
−0.2 1.4 ± 0.5
kBT (keV) – 0.16
+0.09
−0.04
a (km) – 1.4+2.9
Fluxb (10−13erg cm−2 s−1) 2.3 3.6
Blackbody fluxb (10−13erg cm−2 s−1) – 1.3
χ2r / d.o.f. 1.00 / 72 0.85 / 70
a Radius at infinity assuming a distance of 2.5 kpc.
b Unabsorbed flux in the 0.5–10 keV energy range.
and pattern 0–12 for the pn and the MOS, respectively. The re-
sulting background subtracted count rates in the 0.5–10 keV
energy range were (4.2 ± 0.3) × 10−2 cts s−1 in the pn and
(1.9 ± 0.2) × 10−2 cts s−1 in the two MOS cameras, while the
background rate expected in the source extraction regions is
about 50% of these values. The spectra were rebinned to have
at least 20 counts in each energy bin. Spectral fits were per-
formed using the XSPEC version 12.3 software4.
The spectra from the three cameras were fitted together
in the 0.5–10 keV energy range with a power law and with
a power-law plus blackbody model (see Table 1). The latter
model provides a slightly better fit, with less structured resid-
uals (see Figure 2). Furthermore, considering the distance of
2.5 kpc, the interstellar absorption along the line of sight de-
rived with the power-law fit is too low if compared to the
typical column density of neutral absorbing gas in that direc-
tion of approximately 1022 cm−2 (Dickey & Lockman 1990).
The resulting best-fit parameters for the power-law plus black-
body model are photon index Γ = 1.4, blackbody temperature
kBT = 0.16 keV, and absorption NH = 4× 10
21 cm−2 with a re-
duced χ2 of 0.85 for 70 degrees of freedom. The corresponding
luminosity in the 0.5–10 keV band is 2.7 × 1032 erg s−1.
Young pulsars are often associated with pulsar wind neb-
ulae: complex structures that arise from the interaction be-
tween the particle wind powered by the pulsar and the
supernova ejecta or surrounding interstellar medium (see
Gaensler & Slane 2006 for a review). Inspecting the EPIC im-
ages in various energy bands, we find only a marginal (≈3σ)
evidence of diffuse emission, in the 2–4 keV energy band con-
sisting of a faint elongated (∼20 arcsec to the north-east, see
Figure 1) structure starting from PSR J1357−6429. We took
that excess as an upper limit for a diffuse emission: assuming
the same spectrum as the point source, it corresponds to a 2–10
keV luminosity of ≈6 × 1031 erg s−1.
For the timing analysis we applied the solar system
barycenter correction to the photon arrival times with the SAS
task barycen. We searched the data for pulsations around the
spin frequency at the epoch of the XMM-Newton observations,
predicted assuming the pulse period and the spin-down rate
measured with the Parkes radio telescope (Camilo et al. 2004).
4 See http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/ .
P. Esposito et al.: X-ray observations of PSR J1357−6429 3
1 2 5
Energy (keV)
Figure 2. EPIC pn spectrum of PSR J1357−6429. Top: Data
and best-fit power-law (dashed line) plus blackbody (dot-
dashed line) model. Middle: Residuals from the power-law
best-fit model in units of standard deviation. Bottom: Residuals
from the power-law plus blackbody best-fit model in units of
standard deviation.
As glitches and / or deviations from a linear spin-down may al-
ter the period evolution, we searched over a wide period range
centered at the value of ∼166 ms. We searched for significant
periodicities using two methods: a standard folding technique
and the Rayleigh statistic. No pulsation were detected near to
the predicted frequency with either method but, since the pn
timing resolution (73 ms) allows to only poorly sample the
166 ms pulsar period, a reliable upper limit on the pulsed frac-
tion cannot be set.
3. Chandra observations and data analysis
PSR J1357−6429 was observed by means of the Chandra
X-ray Observatory during two exposures of ∼17 ks duration
each on 2005 November 18 and 19. The observations were car-
ried out with the Spectroscopic array of the High Resolution
Camera (HRC-S; Murray et al. 2000) used without transmis-
sion gratings. The HRC is a multichannel plate detector sen-
sitive to X-ray over the 0.08–10 keV energy range, although
essentially no energy information on the detected photons is
available. The HRC-S time resolution is 16 µs.
We started from “level 1” event data calibrated and made
available through the Chandra X-ray Center5. The level 1 event
files contain all HRC triggers with the position information
corrected for instrumental (degap) and aspect (dither) effects.
After standard data processing with the Chandra Interactive
Analysis of Observations (CIAO ver. 3.3), a point-like source
has been clearly detected in both the observations at a position
consistent with that of PSR J1357−6429 (see Figure 3).
For the timing analysis we corrected the data to the solar
system barycenter with the CIAO task axbary and then we fol-
5 See http://cxc.harvard.edu/.
13:57:02.9 13:57:02.4 13:57:02.0
-64:29:27.4
-64:29:31.0
-64:29:34.6
Right ascension
Figure 3. Chandra 0.08–10 keV HRC-S image centered
on the radio pulsar position, marked with a diamond sign
(Camilo et al. 2004). The CIAO celldetect routine yields a
best-fit position for the X-ray source at an angular distance of
(0.9 ± 0.2)′′ (1σ statistical error) from the radio pulsar. This
value consistent with the Chandra pointing accuracy of 0.8′′
(99% confidence level).
lowed the same procedure described in Section 2, but we again
did not detect the source pulsation. By folding the light curve
of PSR J1357−6429 on the radio frequency and fitting it with a
sinusoid, we determine a 90% confidence level upper limit of
∼30% on the amplitude of a sinusoidal modulation. We stress
that this upper limit depends sensitively on data time binning
and on the assumed pulse shape.
We used the CIAO task merge all to generate a combined
image of the source. Our main purpose was to search for diffuse
structures on scales smaller than the XMM-Newton angular res-
olution. We compared the radial profile of the pulsar emission
with the Chandra High-Resolution Mirror Assembly point-
spread function at 1 keV generated using Chandra Ray Tracer
(ChaRT) and Model AXAF Response to X-rays (MARX).
We found that the emission we detect from PSR J1357−6429
(∼100 counts concentrated within a ∼0.5′′ radius circle) is con-
sistent with that from a point source.
We used the Chandra data and the PIMMS software6
to determine an upper limit on the luminosity of a possible
spatial extended emission. The 3σ upper limit on a pulsar
wind nebula brightness (in counts s−1) has been estimated as
3(bA)1/2τ−1, where b is the background surface brightness in
counts arcsec−2, A is the pulsar wind nebula area, and τ is the
exposure duration. Assuming the interstellar absorption value
from the XMM-Newton best-fit model (NH = 0.4 × 10
22 cm−2,
see Section 2) and typical parameters for a pulsar wind nebula
(radius of ∼2× 1017 cm, that corresponds to ∼5′′ for a distance
of 2.5 kpc, and power-law spectrum with photon index Γ = 1.6,
see, e.g., Gotthelf (2003)), this upper limit corresponds to a
2–10 keV luminosity of ≈3 × 1031 erg s−1 for a uniform dif-
fuse nebula. No significant diffuse excess was found even at
6 See http://heasarc.gsfc.nasa.gov/docs/tools.html .
4 P. Esposito et al.: X-ray observations of PSR J1357−6429
larger angular scale, but the corresponding upper limit for dif-
fuse emission is less constraining than that derived using the
XMM-Newton data.
4. Discussion
We have presented the results of the first X-ray observations of
PSR J1357−6429 by means of the XMM-Newton and Chandra
observatories. The source has been positively detected in all
the instruments although, probably due to the low statistics, we
could not detect the source pulsation. The high angular resolu-
tion Chandra observations favor the picture in which most of
the counts belong to a point source. We found that the spectrum
is well represented by either a power-law with photon index
Γ = 1.8+0.3
−0.2 or by a power-law plus blackbody model. In the
latter case the best-fit parameters are for the power-law com-
ponent a photon index Γ = 1.4 ± 0.5 and, for the blackbody
component, radius7 of ∼1.4+2.9
−0.2d2.5 km and temperature corre-
sponding to kBT = 0.16
+0.09
−0.04 keV.
It is generally believed that a combination of emission
mechanisms are responsible for the detected X-ray flux from
rotation-powered pulsars (see, e.g., Kaspi et al. 2006 for a re-
view). The acceleration of particles in the neutron star magne-
tosphere generates non thermal radiation by synchrotron and
curvature radiation and / or inverse Compton processes, while
soft thermal radiation could result by cooling of the surface
of the neutron star. A harder thermal component can arise from
polar-cap reheating, due to return currents from the outer gap or
from close to the polar-cap. The dominant emission mechanism
is likely related to the age of the pulsar. In pulsar younger than
≈104 yr the strong magnetospheric emission generally prevails
over the thermal radiation, making difficult to detect it.
As discussed in Section 2, we tend to prefer the power-law
plus blackbody spectral model for PSR J1357−6429. The re-
sulting blackbody size of ∼1.5d2.5 km may suggest that the soft
emission (.2 keV) is coming from hot spots on the surface due
to backflowing particles, rather than from the entire surface.
However this hint should be considered with caution, as the
surface temperature distribution of a neutron star is most likely
non uniform (since the heath conductivity of the crust is higher
along the magnetic field lines) and the small and hot blackbody
could result from a more complicated distribution of tempera-
ture. Moreover, currently we lack of reliable models of cooling
neutron star thermal emission and thus we cannot exclude that
the soft component is emitted from surface layers of the whole
neutron star.
To date, thermal emission has been detected in only a
few young radio pulsars. Among these, the properties of
PSR J1357−6429 are similar to those of the young pul-
sars Vela (PSR B0833−45; τc = 11 kyr, P = 89 ms,
Ė = 6.9 × 1036 erg s−1, and distance d ≃ 0.2 kpc; Pavlov et al.
2001) and PSR B1706–44 ( τc = 17.5 kyr, P = 102 ms,
Ė = 3.4 × 1036 erg s−1, and d ≃ 2.5 kpc; Gotthelf et al. 2002).
Notably, the efficiency in the conversion of the spin-down
energy loss into X-ray luminosity for PSR J1357−6429 is
L0.5−10 keV/Ė ≃ 8d
2.5×10
−5, significantly lower than the typical
7 We indicate with dN the distance in units of N kpc.
value of ≈10−3 (Becker & Truemper 1997), and similar to that
of PSR B1706–44 (∼10−4) and Vela (∼10−5).
Although a pulsar wind nebula would not came as a
surprise for this young and energetic source, we did not
find clear evidence of diffuse X-ray emission associated with
PSR J1357−6429. However, some known examples of wind
nebulae (see Gaensler & Slane 2006), rescaled to the distance
of PSR J1357−6429, would hide below the upper limits derived
from the XMM-Newton and Chandra data.
New deeper exposures using XMM-Newton or Chandra
would help determine if a thermal component is present in the
emission of PSR J1357−6429 as our spectral analysis suggests,
and possibly detect a pulsed emission. High sensitivity obser-
vations would also serve to address the issue of the presence
of a pulsar wind nebula. Although there is not any EGRET
γ-ray source coincident with PSR J1357−6429 (Hartman et al.
1999), young neutron stars and their nebulae are often bright
γ-ray sources and PSR J1357−6429 in particular, given its
high “spin-down flux” Ė/d2 and similarity with Vela and
PSR B1706–44, is likely to be a good target for the upcoming
AGILE and GLAST satellites and the ground based Cherenkov
air showers telescopes.
Acknowledgements. This work is based on data from observations
with XMM-Newton, an ESA science mission with instruments and
contributions directly funded by ESA member states and NASA. We
also used data from the Chandra X-ray Observatory Center, which is
operated by the Smithsonian Astrophysical Observatory Center on be-
half of NASA. The authors thank the anonymous referee for helpful
comments and acknowledge the support of the Italian Space Agency
and the Italian Ministry for University and Research.
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	Introduction
	XMM-Newton observation and data analysis
	Chandra observations and data analysis
	Discussion
