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Session 100 - Pulsars & X-ray Binaries.
Oral session, Friday, January 09
The origin of millisecond pulsars (MSPs) remains an outstanding problem despite the early and considerable evidence that they are the descendents of neutron stars spun up by accretion in low mass x-ray binaries (LMXBs). The route to MSPs from LMXBs may pass through the high luminosity Z-source LMXBs but is (severely) limited by the very limited population (and apparent birth rate) of Z-sources available. The more numerous x-ray bursters, the Atoll sources, are likely to (still) be short in numbers or birth rate but are now also found to be likely inefficient in the spin-up torques they can provide: the accretion in these relatively low accretion rate systems is likely dominated by an advection dominated flow in which matter accretes onto the NS via sub-Keplerian flows which then transfer correspondingly less angular momentum to the NS. We investigate the implications of the possible ADAF flows in low luminosity NS-LMXBs and find it is unlikely they can produce MSPs. The standard model can still be allowed if most NS-LMXBs are quiescent and undergo transient-like outbursts similar to the soft x-ray transients (which mostly contain black holes). However, apart from Cen X-4 and Aql X-1, few such systems have been found and the SXTs appear instead to be significantly deficient in NS systems. Direct production of MSPs by the accretion induced collapse (AIC) of white dwarfs has been previously suggested to solve the MSP vs. LMXB birth rate problem. We re-examine AIC models in light of the new constraints on direct LMXB production and the additional difficulty imposed by ADAF flows and constraints on SXT populations and derive constraints on the progenitor WD spin and magnetic fields.
Program listing for Friday