Different manifestations of accreting neutron stars and black holes from millisecond variability to super-Eddington luminosities in X-rays
Mehmet Hakan Erkut
Abstract: Since the discovery of the first extrasolar cosmic X-ray source, Scorpius X-1, in 1962, neutron stars and stellar-mass black holes accreting matter from their normal companions in binaries have become the subject of a long-standing active area of research in high-energy astrophysics. Variability timescale of high-energy emission from neutron-star and black-hole low-mass X-ray binaries (LMXBs) extends from milliseconds to seconds, including burst oscillations, kilohertz (kHz) quasi-periodic oscillations (QPOs), and millisecond pulsations in neutron-star sources, and hectohertz QPOs in black-hole sources. The X-ray luminosities of neutron-star and black-hole X-ray binaries, including high-mass X-ray binaries (HMXBs), are usually sub-Eddington. The ever-growing population of ultra-luminous X-ray sources (ULXs) is an exception. ULXs are off-nuclear point sources in nearby galaxies with super-Eddington luminosities for stellar-mass objects. They also exhibit QPOs, however, with a frequency range from mHz to a few Hz. In the present talk, we introduce the model of a unified description of both the high-frequency QPOs from accreting neutron stars and black holes in LMXBs and the low-frequency QPOs from ULXs. Given the evidence based on the recent discovery of pulsations from several ULXs in favor of neutron stars rather than black holes, we present the results of our evolutionary scenario based on the theory of magnetosphere-disk interaction to account for the currently observed spin periods and X-ray luminosities of neutron-stars in ULXs. Finally, we summarize the implications of our results.