Astronomy & Astrophysics manuscript no. ms-revise October 31, 2018
(DOI: will be inserted by hand later)
Radio and X-ray nebulae associated with PSR J1509−5850
C. Y. Hui and W. Becker
Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85741 Garching bei München, Germany
Received 11 April 2007 / Accepted 10 May 2007
Abstract. We have discovered a long radio trail at 843 MHz which is apparently associated with middle age pulsar
PSR J1509−5850. The radio trail has a length of ∼ 7 arcmin. In X-rays, Chandra observation of PSR J1509−5850 reveals
an associated X-ray trail which extends in the same orientation as the radio trail. Moreover, two clumpy structures are observed
along the radio trail. The larger one is proposed to be the supernova remnant (SNR) candidate MSC 319.9-0.7. Faint X-ray
enhancement at the position of the SNR candidate is found in the Chandra data.
Key words. pulsars: individual (PSR J1509−5850)—stars: neutron—radio, X-rays: stars
1. Introduction
It is generally believed that a significant fraction of the rota-
tional energy of a pulsar leaves the magnetosphere in the form
of a magnetized pulsar wind consisting of electromagnetic ra-
diation and high energy particles. In view of this, it is energet-
ically important to study the physical properties of this wind.
When the relativistic wind particles interact with the shocked
interstellar medium, the charged particles will be accelerated in
the shock and hence synchrotron radiation from radio to X-ray
is generated. In order to obtain a better understanding of the
interaction nature, multiwavelength studies of the pulsar wind
nebulae are deeply needed. X-ray and radio observations have
recently revealed a number of pulsar wind nebulae. However,
there is only a handful of shocked emission detected in both
the X-ray and radio regimes (c.f. see Hui & Becker 2006 and
references therein).
PSR J1509−5850 was discovered by Manchester et
al. (2001) in the Parkes Multibeam Pulsar Survey. The pulsar
has a rotation period of P = 88.9 ms and a period derivative of
Ṗ = 9.17 × 10−15 s s−1. These spin parameters imply a charac-
teristic age of 1.54× 105 yrs, a dipole surface magnetic field of
B⊥ = 9.14 × 10
11 G and a spin-down luminosity of 5.1 × 1035
ergs s−1 (c.f. Table 1). The radio dispersion measure yields a
distance of about 3.81 kpc based on the galactic free electron
model of Taylor & Cordes (1993). Using the model of Cordes
& Lazio (2002) the dispersion measure based distance is es-
timated to be 2.56 kpc. The proper motion of this pulsar is
not yet known. Recently, a brief X-ray study of the field of
PSR J1509−5850 was presented by Kargaltsev et al. (2006).
The authors have reported that a trail-like pulsar wind nebula
associated with PSR J1509−5850 was observed in a Chandra
observation. The X-ray nebula is found to be extended in the
south-west direction.
Table 1. Pulsar parameters of PSR J1509−5850 (from
Manchester et al. 2005)
Right Ascension (J2000) 15h09m27.13s
Declination (J2000) −58◦ 50′ 56.1”
Pulsar Period, P (s) 0.088921760489
Period derivative Ṗ (10−15 s s−1) 9.1698
Age (105 yrs) 1.54
Surface dipole magnetic field (1012 G) 0.914
Epoch of Period (MJD) 51463
Dispersion Measure (pc cm−3) 137.7
Dispersion based distance (kpc) ∼ 2.6 − 3.8
Spin-down Luminosity (1035) ergs s−1 5.1
In this paper we report on the discovery of a possible radio
counterpart of the X-ray trail associated with PSR J1509−5850
and provide a detailed X-ray analysis of the trail. In §2 we de-
scribe the observations and the data analysis and in §3 we sum-
marize and discuss our results.
2. Observations and data analysis
PSR J1509−5850 was observed with Chandra in 2003 February
9−10 (Obs ID: 3513) with the Advanced CCD Imaging
Spectrometer (ACIS). The pulsar is located on the back-
illuminated (BI) ACIS-S3 chip which has a superior quantum
efficiency among the spectroscopic array. Standard processed
level-2 data were used. The effective exposure is about 40 ks.
Chandra observation has revealed an X-ray trail associated
with PSR J1509−5850. The signal-to-noise ratios for the pulsar
and the trail are found to be ∼ 19 and ∼ 3 in 0.5 − 8 keV
respectively. The X-ray image of the 4 × 4 arcmin field near to
PSR J1509−5850 is shown in Figure 1. The binning factor of
the image is 0.5 arcsec. Adaptive smoothing with a Gaussian
http://arxiv.org/abs/0704.1265v2
2 C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850
Fig. 1. Chandra’s 4×4 arcmin view of PSR J1509−5850 and its
X-ray trail in the energy band 0.3−8 keV. The pulsar position
is indicated by the black cross. The white circles indicate the
positions of field stars identified in the DSS image.
kernel of σ < 3 arcsec has been applied to the image. The trail
appears to have a length of ∼ 2 arcmin. From a Digitized Sky
Survey (DSS) image, 25 bright field stars are found in the field
of view of Figure 1. We subsequently identified the magnitudes
of these stars from the USNO-A2.0 catalog (Monet et al. 1998),
which are within the range of B ∼ 10 − 18. Their positions are
plotted as white circles in Figure 1.
For the spectral analysis, we extracted the spectrum of
PSR J1509−5850 from a circle of 4 arcsec radius (encircled
energy∼99%) centered on the pulsar position. To minimize the
possible contamination from the field stars, the spectrum from
the trail was extracted within a box of 25 × 95 arcsec, oriented
along the direction of the trail emission. Even with this con-
sideration, there are still two stars with magnitude B = 17 and
B = 16.4 located on the trail (cf. Fig. 1) and unavoidably lie
in the extraction region. Without the knowledge of the extinc-
tions, we are not able to estimate the possible contribution in
X-ray from these two stars. The background spectra were ex-
tracted from the low count regions nearby. After background
subtraction, there are ∼ 100 net counts and ∼ 270 net counts
extracted from the pulsar and the trail in 0.5 − 8 keV respec-
tively. Response files were computed by using the CIAO tools
MKRMF and MKARF. The spectra were dynamically binned
so as to have at least 10 counts per bin for the pulsar and 30
counts per bin for the trail. All the spectral fitting were per-
formed in the energy range of 0.5 − 8 keV by using XSPEC
11.3.1. The degradation of the ACIS quantum efficiency was
corrected by XSPEC model ACISABS. All the quoted errors
are 1 − σ and were computed for 1 parameter in interest.
For the X-ray emission from PSR J1509−5850, we found
that it can be modeled with an absorbed power-law fairly well
Fig. 2. Energy spectrum of the X-ray trail of PSR J1509−5850
as observed with the Chandra ACIS-S3 detector and fitted to
an absorbed power-law model (upper panel) and contribution
to the χ2 fit statistic (lower panel).
(χν=0.68 for 8 D.O.F.). This model yields a column density
of NH = 8.0
−2.1 × 10
21 cm−2, a photon index of Γ = 1.0+0.2
and a normalization at 1 keV of 5.1+1.3
−1.6 × 10
−6 photons keV−1
cm−2 s−1. The best-fitted model results in an unabsorbed flux
of fX = 5.9 × 10
−14 ergs cm−2 s−1 in the energy range of
0.5 − 8 keV. The dispersion based distance implies a luminos-
ity of LX = 4.8 × 10
31 and 1.0 × 1032 erg s−1 for d=2.6 and
3.8 kpc respectively. Although a blackbody model can give a
compatible goodness-of-fit (χν =0.82 for 8 D.O.F.), it infers a
rather high temperature (T ∼ 1.7 × 107 K) and a small pro-
jected blackbody radius (R ∼ 10 m). We hence regard this
model as not physically reasonable to describe the X-ray spec-
trum of PSR J1509−5850. We note that the characteristic age
indicates that PSR J1509−5850 belongs to the class of middle-
aged pulsars. Their spectra typically consist of a soft thermal
component, a harder thermal component from the heated po-
lar caps as well as contribution from the non-thermal emission
(cf. Becker & Aschenbach 2002). However, the small number
of collected photons and the high column density does not sup-
port any fitting with multicomponent models.
We have tested the hypothesis that the trail emission is orig-
inated from the interaction of pulsar wind and ISM by fitting an
absorbed power-law model to the trail spectrum. The model
yields an acceptable goodness-of-fit (χν=0.73 for 9 D.O.F.).
The best fitting spectral model is displayed in Figure 2. This
model yield a column density of NH = 8.2
−3.7 × 10
21 cm−2, a
photon index of Γ = 1.3+0.8
−0.4 and a normalization at 1 keV of
1.9+4.3
−1.9 × 10
−5 photons keV−1 cm−2 s−1. We note that the col-
umn density agrees with that inferred from the pulsar spectrum.
The unabsorbed flux deduced for the best-fitted model param-
eters are fX = 1.6 × 10
−13 erg s−1 cm−2 in the energy range of
0.5−8 keV. The dispersion based distance implies a luminosity
of LX = 1.3×10
32 and 2.7×1032 erg s−1 for d=2.6 and 3.8 kpc,
respectively.
We have searched for a possible radio counterpart for the
X-ray nebula with the Sydney University Molonglo Sky Survey
data (SUMSS) (Bock et al. 1999). We have discovered a long
C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850 3
Fig. 3. The 843 MHz SUMSS image of a field of 11×11 arcmin
around PSR J1509-5850. The pulsar position is indicated by the
black cross. The ∼ 7 arcmin long radio feature is found to have
the same orientation as the X-ray trail. The contour levels are
7, 23, 39, 54 and 70 mJy/beam. Two clumps are observed along
the trail. The larger clump, near to the center of this image, on
the trail is identified as the SNR candidate MSC 319.9-0.7.
radio trail apparently associated with PSR J1509−5850. The
radio image of the 11×11 arcmin field near to PSR J1509−5850
is displayed in Figure 3. The radio feature has a length of ∼ 7
arcmin. Radio contours were calculated at the levels of 7, 23,
39, 54 and 70 mJy/beam. These contours were overlaid on the
image of the Chandra ACIS-S3 chip in Figure 4. It is interesting
to note that the radio trail starts exactly from the position of
PSR J1509−5850 and has the same orientation as that of the X-
ray trail. These facts support the interpetation that this extended
radio feature is the radio counterpart of the X-ray trail and is
indeed physically associated with PSR J1509−5850.
There are two clumpy structures observed along the radio
trail (see Figure 3). The northern clump has its emission center
at RA=15h09m14.35s, Dec=−58◦ 54′ 50.7” (J2000) with a ra-
dius of ∼ 1.5 arcmin. The southern clump has its emission cen-
ter at RA=15h09m06.33s, Dec=−58◦ 58′ 34.7” (J2000) with a
radius of ∼ 1 arcmin. While the southern clump is unidentified
in SIMBAD and NED, the northern clump, which locates ∼ 4
arcmin away from PSR J1509−5850, has been proposed to be
a supernova remnant candidate MSC 319.9-0.7 (Whiteoak &
Green 1996). Comparing the X-ray and the radio data in Figure
4, we found that there is some faint X-ray emission near to the
location of MSC 319.9-0.7. The emission does not appear to be
the continuation of the trail associated with PSR J1509−5850.
It cannot be excluded that this faint emission is related to MSC
319.9-0.7. However, the limited photon statistics does not allow
any final conclusion.
Fig. 4. The X-ray image of the Chandra ACIS-S3 chip with the
radio contour lines from SUMSS data (cf. Figure 3) overlaid.
The X-ray image is binned with a pixel size of 2.5 arcsec and
adaptively smoothed with a Gaussian kernel of σ < 7.5 arcsec.
We note that there is a faint X-ray feature near to the location
of the SNR candidate MSC 319.9-0.7. Top is north and left is
east.
3. Discussion & Conclusion
In this paper, we report the detection of a possible radio coun-
terpart of the X-ray trail associated with PSR J1509−5850 and
present a first detailed X-ray study of the X-ray trail. Apart
from the radio trail, we have found that there are two clumpy
structures located on the trail. While the smaller one is still
unidentified, the larger one, which is located ∼ 4 arcmin away
from PSR J1509−5850, is identified as a SNR candidate MSC
319.9-0.7.
Despite the proximity of MSC 319.9-0.7, it seems unlikely
that it is the birth place of PSR J1509−5850. Assuming this
shell-like SNR candidate is in a Sedov phase, the radius of the
shocked shell emission can be estimated by (Culhane 1977):
Rs = 2.15 × 10
5 pc (1)
where t, E and n are the time after the explosion in units of
years, the released kinetic energy in units of ergs and the ISM
number density in units of cm−3 respectively. Taking the typical
values of E = 1051 ergs and n = 1 cm−3 and t to be the charac-
teristic age of PSR J1509−5850, we estimate that a SNR asso-
ciated with PSR J1509−5850 should have a radius of Rs ∼ 40
pc. However, MSC 319.9-0.7 only has a radius of ∼ 1.1 − 1.7
pc for d = 2.6−3.8 kpc. Thus, the discrepancy between the ex-
pected Rs and the observed value which with a factor of ∼ 30 is
not likely to be reconciled by the uncertainty of the dispersion
based distance. On the other hand, the characteristic age of the
pulsar can be older than its actual age if its inital spin period
was close to its current period. However, to reconcile such dis-
crepancy would require t to be smaller by a factor of ∼ 4000
4 C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850
which is not likely. Moreover, associating MSC 319.9-0.7 with
PSR J1509−5850 would leave the origin of the southern part of
the radio trail unexplained. Thus, with the current knowledge
of parameters it seems most reasonable for us to interpret MSC
319.9-0.7 as a background source.
Following the discussion in Hui & Becker (2006), we ap-
ply a simple one zone model (Chevalier 2000; Cheng, Tamm, &
Wang 2004) to model the X-ray emission properties of the pul-
sar wind nebula. Since the proper motion of PSR J1509−5850
is not yet known, we assume the pulsar is in supersonic motion
on the basis that the nebula resembles a bow-shock morphol-
ogy. For the supersonic motion, the termination shock radius
Rts is determined by the balance of the ram pressure between
the relativistic pulsar wind particles and the ISM at the head of
the shock (cf. Cheng et al. 2004):
Rts ≃
2πρIS Mv2pc
∼ 3 × 1016Ė1/234 n
−1/2v−1p,100cm (2)
where vp,100 is the velocity of the pulsar in units of 100 km s
Ė34 is the spin-down luminosity of the pulsar in units of 10
erg s−1, and n is the number density of the ISM in units of cm−3.
In all the following estimation, we assume PSR J1509−5850
has a transverse velocity comparable to the average velocity,
∼ 250 km s−1, of ordinary radio pulsars (Hobbs et al. 2005).
For a ISM density of 1 cm−3, equation (2) implies a termination
radius of Rts ∼ 8.6 × 10
16 cm.
The X-ray trail is found to be ∼ 2 arcmin long. For the
dispersion based distance in the range of ∼ 2.6 − 3.8 kpc, the
trail has a length of l ∼ (4.7 − 6.8) × 1018 cm. For the assumed
pulsar velocity of ∼ 250 km s−1, the timescale for the passage
of the pulsar over the length of its X-ray trail, tflow, is estimated
to be ∼ 6000 − 8600 years. The magnetic field in the shocked
region can be estimated by assuming tflow to be comparable to
the synchrotron cooling timescale of electrons:
τsyn =
6πmec
γσT B2
≃ 105
µG yrs (3)
where γ is the Lorentz factor of the wind, taken to be 106 (cf.
Cheng et al. 2004), σT is the Thompson cross section, and BµG
is the magnetic field in the shocked region in unit of micro
gauss. The inferred magnetic field in the shocked region is ∼
5 − 7 µG. For comparison, the magnetic field strength in the
ISM is estimated to be ∼ 2 − 6 µG (cf. Beck et al. 2003, and
references therein).
The X-ray luminosity and spectral index depend on the in-
equality between the characteristic observed frequency νobsX and
the electron synchrotron cooling frequency νc (see Chevalier
2000 and references therein):
18πemec
synB3
which is estimated to be νc = (1.3 − 1.8) × 10
17 Hz. Since
in general νobsX > νc, this suggests the X-ray emission is in
a fast cooling regime. Electrons with the energy distribution,
N (γ) ∝ γ−p, are able to radiate their energy in the trail with
photon index α = (p + 2)/2. The index p due to shock accel-
eration typically lies between 2 and 3 (cf. Cheng et al. 2004
and reference therein). This would result in a photon index
α ≃ 2.0− 2.5. In view of the large error of the observed photon
index Γ = 1.3+0.8
−0.4, we cannot firmly conclude the emission sce-
nario simply based on the photon index. We note that the pho-
ton index can still be possibly in the fast cooling regime within
the 1σ uncertainty. With this consideration and νobsX > νc, we
adopted the fast cooling scenario in the following discussion.
With the assumed value p = 2.2, the calculated photon index
α = 2.1 which is marginally within the 1σ uncertainty of the
observed value.
In a fast cooling regime, the luminosity per unit frequency
is given by (cf. Cheng et al. 2004):
p − 2
p − 1
)p−1 (
4π2mec3
ts Ė
2 (5)
Assuming the energy equipartion between the electron and
proton, we take the fractional energy density of electron ǫe to
be ∼ 0.5 and the fractional energy density of the magnetic field
ǫB to be ∼ 0.01. We integrate equation (5) from 0.5 keV to 8
keV and result in a calculated luminosity of ∼ 6 × 1032 ergs
s−1. With the reasonable choice of parameters stated above, the
luminosity estimated by this simple model is found to be the
same order as the observed value.
It is obvious that the radio nebula is significantly longer
than its X-ray counterpart (cf. Fig. 4). This is not unexpected.
Considering a scenario of constant injection of particles with a
finite synchrotron cooling time, the number of particles that can
reach at a further distance from the pulsar should decrease with
increasing frequency. This is because the synchrotron cooling
timescale decrease with frequency. This would result in a fact
that the synchrotron nebular size decreases with frequency.
To further constrain the physical properties of the pul-
sar wind nebula associated with PSR J1509−5850, multi-
wavelength observations are badly needed. Since SUMSS data
have a rather poor spatial resolution which has a typical beam
size of ∼ 45 arcsec, there might be details of the nebular emis-
sion remain unresolved. In particular, it is important to bet-
ter resolve the nebular emission from the contribution of the
SNR candidate MSC 319.9-0.7. In view of this, high resolu-
tion radio observations (e.g. ATCA) are required. In the X-
ray regime, although the Chandra observation has already pro-
vided us with a high resolution image of the nebula, the pho-
ton statistics is not sufficient to tightly constrain the spectral
properties. Owing to the superior collecting power, observa-
tions with XMM-Newton are expected to put a strict constraint
on the emission nature of the nebula as well as the pulsar itself.
Apart from the radio and X-ray observations, a complete
study of pulsar wind nebula should also include TeV observa-
tions (e.g. HESS). It is generally believed that the TeV pho-
tons are resulted from inverse Compton scattering of soft pho-
ton field by the relativistic particles in the nebulae. The seed
soft photons are possibly contributed by the cosmic microwave
background (Cui 2006). However, there is only a handful of
pulsar wind nebulae detected in TeV regime so far (see Cui
C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850 5
2006), a larger sample is needed for differentiating the afore-
mentioned interpetation from its competing scenario (e.g. neu-
tral pion decay).
From the above discussion, one should note that the pul-
sar’s transverse velocity is an important parameter in studying
the shock physics. And hence measuring the proper motion of
PSR J1509−5850 is badly needed. Moreover, although the ori-
entation of the trail suggests PSR J1509−5850 is likely moving
in the direction of northeast, it is not necessary for the trail
to be aligned with the pulsar velocity. PSRs J2124-3358 and
B2224+65 are the examples that the X-ray trails are misaligned
with the direction of the pulsars’ proper motion (Hui & Becker
2006, 2007).
References
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Supernova Remnants, eds. W.Becker, H.Lesch & J.Trümper,
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Beck, R., Shukurov, A., Sokoloff, D., & Wielebinski, R. 2003, A&A,
411, 99
Bock, D. C.-J., Large, M. I., & Sadler, Elaine M. 1999, AJ, 117, 1578
Cheng, K. S., Taam, R. E., & Wang, W., 2004, ApJ, 617, 480
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Cui, W. 2006, (astro-ph/0608042)
Culhane, J. L.: 1977, in D. N. Schram (ed.),Supernovae, D. Reidel
Publishing Co., Dordrecht, Holland, p. 29
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http://arxiv.org/abs/astro-ph/0208466
http://arxiv.org/abs/astro-ph/0207156
http://arxiv.org/abs/astro-ph/0608042
http://arxiv.org/abs/astro-ph/0610505
	Introduction
	Observations and data analysis
	Discussion & Conclusion

  We have discovered a long radio trail at 843 MHz which is apparently
associated with middle age pulsar PSR J1509-5850. The radio trail has a length
of ~7 arcmin. In X-rays, Chandra observations of PSR J1509-5850 reveal an
associated X-ray trail which extends in the same orientation as the radio
trail. Moreover, two clumpy structures are observed along the radio trail. The
larger one is proposed to be the supernova remnant (SNR) candidate MSC
319.9-0.7. Faint X-ray enhancement at the position of the SNR candidate is
found in the Chandra data.

Introduction
It is generally believed that a significant fraction of the rota-
tional energy of a pulsar leaves the magnetosphere in the form
of a magnetized pulsar wind consisting of electromagnetic ra-
diation and high energy particles. In view of this, it is energet-
ically important to study the physical properties of this wind.
When the relativistic wind particles interact with the shocked
interstellar medium, the charged particles will be accelerated in
the shock and hence synchrotron radiation from radio to X-ray
is generated. In order to obtain a better understanding of the
interaction nature, multiwavelength studies of the pulsar wind
nebulae are deeply needed. X-ray and radio observations have
recently revealed a number of pulsar wind nebulae. However,
there is only a handful of shocked emission detected in both
the X-ray and radio regimes (c.f. see Hui & Becker 2006 and
references therein).
PSR J1509−5850 was discovered by Manchester et
al. (2001) in the Parkes Multibeam Pulsar Survey. The pulsar
has a rotation period of P = 88.9 ms and a period derivative of
Ṗ = 9.17 × 10−15 s s−1. These spin parameters imply a charac-
teristic age of 1.54× 105 yrs, a dipole surface magnetic field of
B⊥ = 9.14 × 10
11 G and a spin-down luminosity of 5.1 × 1035
ergs s−1 (c.f. Table 1). The radio dispersion measure yields a
distance of about 3.81 kpc based on the galactic free electron
model of Taylor & Cordes (1993). Using the model of Cordes
& Lazio (2002) the dispersion measure based distance is es-
timated to be 2.56 kpc. The proper motion of this pulsar is
not yet known. Recently, a brief X-ray study of the field of
PSR J1509−5850 was presented by Kargaltsev et al. (2006).
The authors have reported that a trail-like pulsar wind nebula
associated with PSR J1509−5850 was observed in a Chandra
observation. The X-ray nebula is found to be extended in the
south-west direction.
Table 1. Pulsar parameters of PSR J1509−5850 (from
Manchester et al. 2005)
Right Ascension (J2000) 15h09m27.13s
Declination (J2000) −58◦ 50′ 56.1”
Pulsar Period, P (s) 0.088921760489
Period derivative Ṗ (10−15 s s−1) 9.1698
Age (105 yrs) 1.54
Surface dipole magnetic field (1012 G) 0.914
Epoch of Period (MJD) 51463
Dispersion Measure (pc cm−3) 137.7
Dispersion based distance (kpc) ∼ 2.6 − 3.8
Spin-down Luminosity (1035) ergs s−1 5.1
In this paper we report on the discovery of a possible radio
counterpart of the X-ray trail associated with PSR J1509−5850
and provide a detailed X-ray analysis of the trail. In §2 we de-
scribe the observations and the data analysis and in §3 we sum-
marize and discuss our results.
2. Observations and data analysis
PSR J1509−5850 was observed with Chandra in 2003 February
9−10 (Obs ID: 3513) with the Advanced CCD Imaging
Spectrometer (ACIS). The pulsar is located on the back-
illuminated (BI) ACIS-S3 chip which has a superior quantum
efficiency among the spectroscopic array. Standard processed
level-2 data were used. The effective exposure is about 40 ks.
Chandra observation has revealed an X-ray trail associated
with PSR J1509−5850. The signal-to-noise ratios for the pulsar
and the trail are found to be ∼ 19 and ∼ 3 in 0.5 − 8 keV
respectively. The X-ray image of the 4 × 4 arcmin field near to
PSR J1509−5850 is shown in Figure 1. The binning factor of
the image is 0.5 arcsec. Adaptive smoothing with a Gaussian
http://arxiv.org/abs/0704.1265v2
2 C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850
Fig. 1. Chandra’s 4×4 arcmin view of PSR J1509−5850 and its
X-ray trail in the energy band 0.3−8 keV. The pulsar position
is indicated by the black cross. The white circles indicate the
positions of field stars identified in the DSS image.
kernel of σ < 3 arcsec has been applied to the image. The trail
appears to have a length of ∼ 2 arcmin. From a Digitized Sky
Survey (DSS) image, 25 bright field stars are found in the field
of view of Figure 1. We subsequently identified the magnitudes
of these stars from the USNO-A2.0 catalog (Monet et al. 1998),
which are within the range of B ∼ 10 − 18. Their positions are
plotted as white circles in Figure 1.
For the spectral analysis, we extracted the spectrum of
PSR J1509−5850 from a circle of 4 arcsec radius (encircled
energy∼99%) centered on the pulsar position. To minimize the
possible contamination from the field stars, the spectrum from
the trail was extracted within a box of 25 × 95 arcsec, oriented
along the direction of the trail emission. Even with this con-
sideration, there are still two stars with magnitude B = 17 and
B = 16.4 located on the trail (cf. Fig. 1) and unavoidably lie
in the extraction region. Without the knowledge of the extinc-
tions, we are not able to estimate the possible contribution in
X-ray from these two stars. The background spectra were ex-
tracted from the low count regions nearby. After background
subtraction, there are ∼ 100 net counts and ∼ 270 net counts
extracted from the pulsar and the trail in 0.5 − 8 keV respec-
tively. Response files were computed by using the CIAO tools
MKRMF and MKARF. The spectra were dynamically binned
so as to have at least 10 counts per bin for the pulsar and 30
counts per bin for the trail. All the spectral fitting were per-
formed in the energy range of 0.5 − 8 keV by using XSPEC
11.3.1. The degradation of the ACIS quantum efficiency was
corrected by XSPEC model ACISABS. All the quoted errors
are 1 − σ and were computed for 1 parameter in interest.
For the X-ray emission from PSR J1509−5850, we found
that it can be modeled with an absorbed power-law fairly well
Fig. 2. Energy spectrum of the X-ray trail of PSR J1509−5850
as observed with the Chandra ACIS-S3 detector and fitted to
an absorbed power-law model (upper panel) and contribution
to the χ2 fit statistic (lower panel).
(χν=0.68 for 8 D.O.F.). This model yields a column density
of NH = 8.0
−2.1 × 10
21 cm−2, a photon index of Γ = 1.0+0.2
and a normalization at 1 keV of 5.1+1.3
−1.6 × 10
−6 photons keV−1
cm−2 s−1. The best-fitted model results in an unabsorbed flux
of fX = 5.9 × 10
−14 ergs cm−2 s−1 in the energy range of
0.5 − 8 keV. The dispersion based distance implies a luminos-
ity of LX = 4.8 × 10
31 and 1.0 × 1032 erg s−1 for d=2.6 and
3.8 kpc respectively. Although a blackbody model can give a
compatible goodness-of-fit (χν =0.82 for 8 D.O.F.), it infers a
rather high temperature (T ∼ 1.7 × 107 K) and a small pro-
jected blackbody radius (R ∼ 10 m). We hence regard this
model as not physically reasonable to describe the X-ray spec-
trum of PSR J1509−5850. We note that the characteristic age
indicates that PSR J1509−5850 belongs to the class of middle-
aged pulsars. Their spectra typically consist of a soft thermal
component, a harder thermal component from the heated po-
lar caps as well as contribution from the non-thermal emission
(cf. Becker & Aschenbach 2002). However, the small number
of collected photons and the high column density does not sup-
port any fitting with multicomponent models.
We have tested the hypothesis that the trail emission is orig-
inated from the interaction of pulsar wind and ISM by fitting an
absorbed power-law model to the trail spectrum. The model
yields an acceptable goodness-of-fit (χν=0.73 for 9 D.O.F.).
The best fitting spectral model is displayed in Figure 2. This
model yield a column density of NH = 8.2
−3.7 × 10
21 cm−2, a
photon index of Γ = 1.3+0.8
−0.4 and a normalization at 1 keV of
1.9+4.3
−1.9 × 10
−5 photons keV−1 cm−2 s−1. We note that the col-
umn density agrees with that inferred from the pulsar spectrum.
The unabsorbed flux deduced for the best-fitted model param-
eters are fX = 1.6 × 10
−13 erg s−1 cm−2 in the energy range of
0.5−8 keV. The dispersion based distance implies a luminosity
of LX = 1.3×10
32 and 2.7×1032 erg s−1 for d=2.6 and 3.8 kpc,
respectively.
We have searched for a possible radio counterpart for the
X-ray nebula with the Sydney University Molonglo Sky Survey
data (SUMSS) (Bock et al. 1999). We have discovered a long
C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850 3
Fig. 3. The 843 MHz SUMSS image of a field of 11×11 arcmin
around PSR J1509-5850. The pulsar position is indicated by the
black cross. The ∼ 7 arcmin long radio feature is found to have
the same orientation as the X-ray trail. The contour levels are
7, 23, 39, 54 and 70 mJy/beam. Two clumps are observed along
the trail. The larger clump, near to the center of this image, on
the trail is identified as the SNR candidate MSC 319.9-0.7.
radio trail apparently associated with PSR J1509−5850. The
radio image of the 11×11 arcmin field near to PSR J1509−5850
is displayed in Figure 3. The radio feature has a length of ∼ 7
arcmin. Radio contours were calculated at the levels of 7, 23,
39, 54 and 70 mJy/beam. These contours were overlaid on the
image of the Chandra ACIS-S3 chip in Figure 4. It is interesting
to note that the radio trail starts exactly from the position of
PSR J1509−5850 and has the same orientation as that of the X-
ray trail. These facts support the interpetation that this extended
radio feature is the radio counterpart of the X-ray trail and is
indeed physically associated with PSR J1509−5850.
There are two clumpy structures observed along the radio
trail (see Figure 3). The northern clump has its emission center
at RA=15h09m14.35s, Dec=−58◦ 54′ 50.7” (J2000) with a ra-
dius of ∼ 1.5 arcmin. The southern clump has its emission cen-
ter at RA=15h09m06.33s, Dec=−58◦ 58′ 34.7” (J2000) with a
radius of ∼ 1 arcmin. While the southern clump is unidentified
in SIMBAD and NED, the northern clump, which locates ∼ 4
arcmin away from PSR J1509−5850, has been proposed to be
a supernova remnant candidate MSC 319.9-0.7 (Whiteoak &
Green 1996). Comparing the X-ray and the radio data in Figure
4, we found that there is some faint X-ray emission near to the
location of MSC 319.9-0.7. The emission does not appear to be
the continuation of the trail associated with PSR J1509−5850.
It cannot be excluded that this faint emission is related to MSC
319.9-0.7. However, the limited photon statistics does not allow
any final conclusion.
Fig. 4. The X-ray image of the Chandra ACIS-S3 chip with the
radio contour lines from SUMSS data (cf. Figure 3) overlaid.
The X-ray image is binned with a pixel size of 2.5 arcsec and
adaptively smoothed with a Gaussian kernel of σ < 7.5 arcsec.
We note that there is a faint X-ray feature near to the location
of the SNR candidate MSC 319.9-0.7. Top is north and left is
east.
3. Discussion & Conclusion
In this paper, we report the detection of a possible radio coun-
terpart of the X-ray trail associated with PSR J1509−5850 and
present a first detailed X-ray study of the X-ray trail. Apart
from the radio trail, we have found that there are two clumpy
structures located on the trail. While the smaller one is still
unidentified, the larger one, which is located ∼ 4 arcmin away
from PSR J1509−5850, is identified as a SNR candidate MSC
319.9-0.7.
Despite the proximity of MSC 319.9-0.7, it seems unlikely
that it is the birth place of PSR J1509−5850. Assuming this
shell-like SNR candidate is in a Sedov phase, the radius of the
shocked shell emission can be estimated by (Culhane 1977):
Rs = 2.15 × 10
5 pc (1)
where t, E and n are the time after the explosion in units of
years, the released kinetic energy in units of ergs and the ISM
number density in units of cm−3 respectively. Taking the typical
values of E = 1051 ergs and n = 1 cm−3 and t to be the charac-
teristic age of PSR J1509−5850, we estimate that a SNR asso-
ciated with PSR J1509−5850 should have a radius of Rs ∼ 40
pc. However, MSC 319.9-0.7 only has a radius of ∼ 1.1 − 1.7
pc for d = 2.6−3.8 kpc. Thus, the discrepancy between the ex-
pected Rs and the observed value which with a factor of ∼ 30 is
not likely to be reconciled by the uncertainty of the dispersion
based distance. On the other hand, the characteristic age of the
pulsar can be older than its actual age if its inital spin period
was close to its current period. However, to reconcile such dis-
crepancy would require t to be smaller by a factor of ∼ 4000
4 C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850
which is not likely. Moreover, associating MSC 319.9-0.7 with
PSR J1509−5850 would leave the origin of the southern part of
the radio trail unexplained. Thus, with the current knowledge
of parameters it seems most reasonable for us to interpret MSC
319.9-0.7 as a background source.
Following the discussion in Hui & Becker (2006), we ap-
ply a simple one zone model (Chevalier 2000; Cheng, Tamm, &
Wang 2004) to model the X-ray emission properties of the pul-
sar wind nebula. Since the proper motion of PSR J1509−5850
is not yet known, we assume the pulsar is in supersonic motion
on the basis that the nebula resembles a bow-shock morphol-
ogy. For the supersonic motion, the termination shock radius
Rts is determined by the balance of the ram pressure between
the relativistic pulsar wind particles and the ISM at the head of
the shock (cf. Cheng et al. 2004):
Rts ≃
2πρIS Mv2pc
∼ 3 × 1016Ė1/234 n
−1/2v−1p,100cm (2)
where vp,100 is the velocity of the pulsar in units of 100 km s
Ė34 is the spin-down luminosity of the pulsar in units of 10
erg s−1, and n is the number density of the ISM in units of cm−3.
In all the following estimation, we assume PSR J1509−5850
has a transverse velocity comparable to the average velocity,
∼ 250 km s−1, of ordinary radio pulsars (Hobbs et al. 2005).
For a ISM density of 1 cm−3, equation (2) implies a termination
radius of Rts ∼ 8.6 × 10
16 cm.
The X-ray trail is found to be ∼ 2 arcmin long. For the
dispersion based distance in the range of ∼ 2.6 − 3.8 kpc, the
trail has a length of l ∼ (4.7 − 6.8) × 1018 cm. For the assumed
pulsar velocity of ∼ 250 km s−1, the timescale for the passage
of the pulsar over the length of its X-ray trail, tflow, is estimated
to be ∼ 6000 − 8600 years. The magnetic field in the shocked
region can be estimated by assuming tflow to be comparable to
the synchrotron cooling timescale of electrons:
τsyn =
6πmec
γσT B2
≃ 105
µG yrs (3)
where γ is the Lorentz factor of the wind, taken to be 106 (cf.
Cheng et al. 2004), σT is the Thompson cross section, and BµG
is the magnetic field in the shocked region in unit of micro
gauss. The inferred magnetic field in the shocked region is ∼
5 − 7 µG. For comparison, the magnetic field strength in the
ISM is estimated to be ∼ 2 − 6 µG (cf. Beck et al. 2003, and
references therein).
The X-ray luminosity and spectral index depend on the in-
equality between the characteristic observed frequency νobsX and
the electron synchrotron cooling frequency νc (see Chevalier
2000 and references therein):
18πemec
synB3
which is estimated to be νc = (1.3 − 1.8) × 10
17 Hz. Since
in general νobsX > νc, this suggests the X-ray emission is in
a fast cooling regime. Electrons with the energy distribution,
N (γ) ∝ γ−p, are able to radiate their energy in the trail with
photon index α = (p + 2)/2. The index p due to shock accel-
eration typically lies between 2 and 3 (cf. Cheng et al. 2004
and reference therein). This would result in a photon index
α ≃ 2.0− 2.5. In view of the large error of the observed photon
index Γ = 1.3+0.8
−0.4, we cannot firmly conclude the emission sce-
nario simply based on the photon index. We note that the pho-
ton index can still be possibly in the fast cooling regime within
the 1σ uncertainty. With this consideration and νobsX > νc, we
adopted the fast cooling scenario in the following discussion.
With the assumed value p = 2.2, the calculated photon index
α = 2.1 which is marginally within the 1σ uncertainty of the
observed value.
In a fast cooling regime, the luminosity per unit frequency
is given by (cf. Cheng et al. 2004):
p − 2
p − 1
)p−1 (
4π2mec3
ts Ė
2 (5)
Assuming the energy equipartion between the electron and
proton, we take the fractional energy density of electron ǫe to
be ∼ 0.5 and the fractional energy density of the magnetic field
ǫB to be ∼ 0.01. We integrate equation (5) from 0.5 keV to 8
keV and result in a calculated luminosity of ∼ 6 × 1032 ergs
s−1. With the reasonable choice of parameters stated above, the
luminosity estimated by this simple model is found to be the
same order as the observed value.
It is obvious that the radio nebula is significantly longer
than its X-ray counterpart (cf. Fig. 4). This is not unexpected.
Considering a scenario of constant injection of particles with a
finite synchrotron cooling time, the number of particles that can
reach at a further distance from the pulsar should decrease with
increasing frequency. This is because the synchrotron cooling
timescale decrease with frequency. This would result in a fact
that the synchrotron nebular size decreases with frequency.
To further constrain the physical properties of the pul-
sar wind nebula associated with PSR J1509−5850, multi-
wavelength observations are badly needed. Since SUMSS data
have a rather poor spatial resolution which has a typical beam
size of ∼ 45 arcsec, there might be details of the nebular emis-
sion remain unresolved. In particular, it is important to bet-
ter resolve the nebular emission from the contribution of the
SNR candidate MSC 319.9-0.7. In view of this, high resolu-
tion radio observations (e.g. ATCA) are required. In the X-
ray regime, although the Chandra observation has already pro-
vided us with a high resolution image of the nebula, the pho-
ton statistics is not sufficient to tightly constrain the spectral
properties. Owing to the superior collecting power, observa-
tions with XMM-Newton are expected to put a strict constraint
on the emission nature of the nebula as well as the pulsar itself.
Apart from the radio and X-ray observations, a complete
study of pulsar wind nebula should also include TeV observa-
tions (e.g. HESS). It is generally believed that the TeV pho-
tons are resulted from inverse Compton scattering of soft pho-
ton field by the relativistic particles in the nebulae. The seed
soft photons are possibly contributed by the cosmic microwave
background (Cui 2006). However, there is only a handful of
pulsar wind nebulae detected in TeV regime so far (see Cui
C. Y. Hui and W. Becker: The pulsar wind nebula associated with PSR J1509−5850 5
2006), a larger sample is needed for differentiating the afore-
mentioned interpetation from its competing scenario (e.g. neu-
tral pion decay).
From the above discussion, one should note that the pul-
sar’s transverse velocity is an important parameter in studying
the shock physics. And hence measuring the proper motion of
PSR J1509−5850 is badly needed. Moreover, although the ori-
entation of the trail suggests PSR J1509−5850 is likely moving
in the direction of northeast, it is not necessary for the trail
to be aligned with the pulsar velocity. PSRs J2124-3358 and
B2224+65 are the examples that the X-ray trails are misaligned
with the direction of the pulsars’ proper motion (Hui & Becker
2006, 2007).
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http://arxiv.org/abs/astro-ph/0208466
http://arxiv.org/abs/astro-ph/0207156
http://arxiv.org/abs/astro-ph/0608042
http://arxiv.org/abs/astro-ph/0610505
	Introduction
	Observations and data analysis
	Discussion & Conclusion
