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ROSAT OBSERVATIONS OF PSR 0355+54 and PSR 1642--03 1
Patrick Slane
Harvard­Smithsonian Center for Astrophysics
60 Garden Street, Cambridge, MA 02138
USA
ABSTRACT
PSR 0355+54 and PSR 1642--03 are both radio pulsars whose X­ray behavior was
left unresolved by Einstein observations. 1;2 In each case, faint emission was detected near the
sources, but appeared to be concentrated 1--2 arcmin from the known pulsar positions. This
was interpreted as evidence of either extended emission or of the presence of a serendipitous
background object. We have carried out ROSAT PSPC observations of these two pulsars to
determine the nature of the emission; here we report on the results of preliminary analysis.
1. Introduction
PSR 0355+54 is a moderate age pulsar (T char = 10 5:75 y) with a fairly short period
(156 ms). The primary properties determined from radio observations 3 are summarized below:
P —
P=10 \Gamma15 log —
E log T DM D log B
PSR (s) ( s s \Gamma1 ) (ergs s \Gamma1 ) (years) (cm \Gamma3 pc) (kpc) (Gauss)
0355+54 0.156 4.4 34.66 5.75 57 2.07 11.92
1055--52 0.197 5.8 34.48 5.73 30 1.53 12.04
1642--03 0.388 1.8 33.08 6.54 36 2.90 11.93
Using the spin­down energy loss rate as an indicator, PSR 0355+54 is a good candidate for
observable X­ray emission; only 10 of the 119 radio pulsars closer than PSR 0355+54 have
larger values of —
E=D 2 , and 8 of these are known X­ray emitters. Observations carried out
with the Einstein Observatory were inconclusive, however. Weak emission from a position
¸ 1:7 arcmin from the radio position was detected, which was interpreted as possible evidence
of extended emission from an associated synchrotron nebula. 1 The similarity between this
pulsar and PSR 1055--52 (see Table above), which is a known X­ray source 4;5 and has been
recently detected as a fl­ray source 6 , makes further study particularly interesting.
Einstein studies of PSR 1642--03 provided results similar to those for PSR 0355+54;
faint emission concentrated ¸ 1 arcmin from the pulsar position was observed, and this was
interpreted as possible evidence for an extended synchrotron nebula. 1 The derived luminosity 2
L x = 8:5 \Theta 10 31 ergs s \Gamma1 is very high given the available spin­down power (see Table above).
Assuming the emission is actually associated with the pulsar, this suggests either a particularly
efficient mechanism for converting —
E into X­rays, or the presence of an additional energy source.
Cooling of the neutron star interior does not seem a viable interpretation, however, given the
age of the pulsar. Further study is thus of interest to determine whether the emission is
actually associated with the pulsar and what implications the large luminosity may have for
emission models as well as for detectability of other pulsars with moderate values of —
E.
2. ROSAT Observations
PSR 0355+54 was observed for 19,144 s using the ROSAT Position Sensitive Pro­
portional Counter (PSPC). After cleaning, 16,644 s of good data remain for analysis; here we
present a summary of the results from the full analysis. 7 A faint X­ray source is detected at the
position of the radio pulsar with a count rate R = 4:2(\Sigma1:3) \Theta 10 \Gamma3 s \Gamma1 . We have smoothed
1 From New Horizon of X­Ray Astronomy -- First Results from ASCA (Eds. F. Makino & T. Ohashi)
1

the image with a 32 arcsec Gaussian smoothing scale, and subtracted a scaled and similarly
smoothed point source for the PSPC to reveal weak residual emission concentrated ¸ 1:6 ar­
cmin from the pulsar, in the northeast. Inspection of the surrounding PSPC field reveals a
number of similar weak sources, suggesting that this emission is likely to be associated with
a background source rather than from an extended pulsar component. We have searched for
pulsations at the known radio period of 156 ms but find no evidence using all data (61 counts)
from a 30 arcsec circle centered on the point source. Given the small number of counts, this
is not surprising; the null detection permits us to establish a 3oe upper limit f ! 77%. Most
pulsars have pulsed fractions smaller than this. PSR 1055--52 has f ¸ 85% at higher energies,
but a corresponding effect would not be seen for PSR 0355+54 due to lack of photons.
With only ¸ 70 counts, there is not sufficient information to perform spectral model
fitting. Instead, we have considered two cases: 1)a Crab­like power law which might be asso­
ciated with magnetospheric emission; and 2)blackbody emission from the entire surface of the
neutron star (NS), which could result from cooling of the NS interior. By using hardness ra­
tios defined in the standard processing of PSPC spectral data, we have compared the observed
spectrum with that predicted by model spectra convolved through the telescope spectral re­
sponse. Assuming a column density NH = 3 \Theta 10 21 cm \Gamma2 (derived by scaling the Crab value
by the ratio of DM values) we find a surface temperature T = 1:33(\Sigma0:07) \Theta 10 6 K (corrected
for gravitational redshift) is required to reproduce the PSPC count rate. This temperature
is consistent with predictions from current cooling models. However, we find that models
for emission from the entire surface of a NS with R = 10 km are unable to reproduce the
observed hardness ratios, and are particularly poor representations for any reasonable val­
ues of NH . We conclude that the emission is not associated with cooling of the NS interior;
rather, it is derived from the pulsar spin­down. This does not rule out the possibility of
blackbody emission at a higher temperature, from smaller regions on the NS surface. Such
a scenario could result from bombardment of the polar caps by energetic particles acceler­
ated in the pulsar magnetosphere. A Crab­like spectrum power­law spectrum (ff = 2) yields
L x (0:1 \Gamma 2:4 keV) = 8:0 \Theta 10 31 ergs s \Gamma1 , consistent with observed luminosities for other pulsars.
In particular, it is very similar to that derived for the power­law component from PSR 1055--
52. 7 This similarity suggests that PSR 0355+54 is a good candidate for fl­ray observations.
Our scheduled 40 ks observation of PSR 1642­03 was cut short to ¸ 3 ks. Emission
from the vicinity of the pulsar is clearly detected, but as in the Einstein data the emission is off­
set from the pulsar position. The shortened observation yielded only ¸ 35 counts, insufficient
to assess whether the emission is associated with the pulsar or a background source. Using a
Crab­like power law spectrum with NH = 1:5 \Theta 10 21 cm \Gamma2 , scaled from the Crab value, the ob­
served count rate would indicate a pulsar luminosity L x (0:1\Gamma2:4 keV) ¸ 2\Theta10 32 ergs s \Gamma1 . This
is extremely large relative to other pulsars given the spin­down energy —
E = 1:2 \Theta 10 33 ergs s \Gamma1 .
If correct, the result has strong implications relative to the emission mechanisms. PSR 1642--
03 proper motion measurements indicate large velocity (¸ 660 km s \Gamma1 ) directed ¸ 30 ffi south
of east ­ in reasonable agreement with the direction of the emission. Models for synchrotron
emission from a ram­pressure confined wind 8 may explain the offset emission; luminosity values
appear problematic, however. Further analysis of this interpretation is in progress.
3. References
1. Helfand, D.J. 1983, Proc. IAU Symp. 101, ed. J. Danzinger and P. Gorenstein p.471
2. Seward, F. D., & Wang, Z­R 1988, ApJ, 332, 199
3. Taylor, J. H., Manchester, R. N., & Lyne, A. G. 1993, ApJ Supp., 88, 529
4. Cheng, A. F., & Helfand, D. J. 1983, ApJ, 217, 271
5. ¨
Ogelman, H., & Finley, J. P. 1993, ApJ, 413, L31
6. Fierro, J. M. et al. 1993, ApJ, 413, L27
7. Slane, P. 1994, ApJ, submitted.
8. Cheng, A. F. 1983, ApJ, 275, 790
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