Gravity seminar - Caroline D'Angelo and Alessandro Patruno Seminar

 

Accreting neutron stars present themselves as a surprisingly large diversity of astronomical objects, depending on their magnetic field strength and accretion rate. At the very highest accretion rates, neutron stars have been identified as ultraluminous X-ray sources, which accrete well above the Eddington limit, while high-field, fast-spinning newly-born neutron stars have been suggested to partly power core-collapse supernovae explosions. At much lower accretion rates, weak field accreting pulsars and non-pulsating accreting neutron stars are precursors to the large population of radio pulsar, and show accretion at much lower luminosities. However, the underlying physics of magnetospheric accretion remains constant across the wide range luminosity and magnetic field strength, so that studying one class can yield insights applicable to all accreting sources. In my talk I will present recent studies of several low-field, low-luminosity accreting neutron stars, and show how some surprising results of these investigations are relevant for understanding the much more extreme physics of high field neutron stars.

 

The orbital evolution of several neutron stars in low mass X-ray binaries (LMXBs) occurs on a timescale which is orders of magnitude shorter than expected from conservative binary evolution models. Something similar is also seen in several accreting white dwarfs and has also been recently observed in two black hole LMXBs. Finally, non-accreting binary pulsars known as black-widows and red-backs are also observed to evolve on (too) short timescales. Many processes have been invoked to explain these discrepancies, although most explanations are tailored to each specific system considered. Mechanisms proposed so far include spin-orbit coupling, massive mass ejection and ablation of the companion from pulsar winds, presence of a planetary third body and enhanced magnetic braking due to huge stellar magnetic fields. In this talk I will discuss recent findings on the orbital evolution of one specific neutron star LMXB and overview which mechanism(s) might be responsible for this peculiar and widespread phenomenon.