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Session 73 - White Dwarfs, Neutron Stars and Pulsars.
Display session, Thursday, June 13
A new class of X-ray pulsars, with four (possibly five) members, has been recently identified by Mereghetti amp; Stella (1995). These pulsars have low X-ray luminosities (\sim 10^35\,ergs/s), weak magnetic fields (\sim 10^11\,G), and pulse periods that lie within a very narrow range (5--9\,s), in contrast to the pulse periods of X-ray pulsars in high-mass X-ray binaries, which span a wide range of pulse periods of about four orders of magnitude. Of these five objects, only one is thought to have a low-mass binary companion, while only upper limits (P<3\,h) on the orbital periods exist for the others. We show that the characteristics of these sources are consistent with them being neutron stars with low-mass (<0.37\,M_ødot) Roche-lobe filling companions, from which mass transfer is driven by angular momentum losses due to gravitational radiation. We find that the properties of such systems are restricted within a very narrow range of orbital periods and mass transfer rates that are discontinuous from those of the other, non-pulsing low-mass X-ray binaries (LMXBs), which transfer mass either due to magnetic braking or nuclear evolution of the donors. Thus, the observed narrow range of their magnetic fields, luminosities, and pulse periods can be naturally explained. We perform population synthesis calculations and show that these low-mass companions have a very low survival rate through a common-envelope phase. Therefore the standard formation mechanisms for LMXBs cannot account for their observed number. On the other hand, the direct-supernova mechanism, recently proposed (Kalogera 1996) for LMXB formation, can account for the birth rate of these systems implied by the observations. Also, the direct-supernova mechanism leads to a galactic scale-height for the population of these X-ray pulsars that is smaller than the scale-height of the other, non-pulsing LMXBs, in agreement with the observations.
Program listing for Thursday