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Session 111 - Pulsars.
Display session, Saturday, January 10
Exhibit Hall,
The recently discovered 5.3 ms pulsar J1012+5307 at a distance of 520 pc is in an area of the sky which is particularly deficient in absorbing gas. The column density along the line of sight is less than 7.5 \times 10^19 cm^-2, which facilitates soft X-ray observations. Halpern (1996, ApJ, 459, L9) reported a possible ROSAT\/ PSPC detection of the pulsar in a serendipitous, off-axis observation. We have now confirmed the X-ray emission of PSR J1012+5307 in a 23 ksec observation with the ROSAT\/ HRI. A point source is detected within 3^\prime\prime of the radio position. Its count rate of 1.6 \pm 0.3 \times 10^-3 s^-1 corresponds to an unbsorbed 0.1--2.4 keV flux of 6.4 \times 10^-14 ergs cm^-2 s^-1, similar to that reported previously. This counts-to-flux conversion is valid for N_H = 5 \times 10^19 cm^-2, and either a power-law spectrum of photon index 2.5 or a blackbody of kT = 0.1 keV. The implied X-ray luminosity of 2.0 \times 10^30 ergs s^-1 is 5 \times 10^-4 of the pulsar's spin-down power \dot E, and similar to that of the nearest millisecond pulsar J0437--4715, which is nearly a twin of J1012+5307 in P and \dot E. We subjected the 37 photons (and 13 background counts) within the source region to a pulsar search, but no evidence for pulsation was found. The pulsar apparently emits over a large fraction of its rotation cycle, and the absence of sharp modulation can be taken as evidence for surface thermal emission, as is favored for PSR J0437--4715 (Zavlin amp; Pavlov 1997, Aamp;A, in press), rather than magnetospheric X-ray emission which is apparent in the sharp pulses of the much more energetic millisecond pulsar B1821--24 (Saito et al. 1997, ApJ, 477, L37). A further test of this interpretation will be made with a longer ROSAT observation, which will increase the number of photons collected by a factor of 5, and permit a more sensitive examination of the light curve for modulation due to emission from heated polar caps. If found, such modulation will be further evidence that surface reheating by the impact of particles accelerated along open field lines operates in these \sim 10^9 yr old pulsars.
