Mass Accreting Black Hole as A Power Generator in Ultraluminous X-Ray Sources
Kiki Vierdayanti, Institut Teknologi Bandung
Mass accreting black holes (hereafter, accreting black holes) have been known since 1960s as one of the most important powerhouses in the Universe. As the material is pulled in by the black hole, the efficiency of the conversion of gravitational potential energy to radiation can easily exceed the efficiency of nuclear reaction at the center of a normal star, including our very own Sun. Our Sun releases about 4x1033 erg/s of energy, while an accreting black hole of the same mass could release energy of about 105 times larger. The maximum limit of the released energy per unit time in accreting black hole, known as Eddington luminosity, is known to be proportional to the mass of the black hole. That is, the more massive the black hole, the greater its Eddington luminosity. Eddington luminosity is thus one of the important keys for estimating the mass of the accreting black hole. It is believed that our Galaxy hosts several dozens of such accreting black holes, one of which is located at its very center, with a mass of about 3 million solar mass, thus known as the super-massive black hole (SMBH). About twenty others, found in the spiral arms or the halo, have much smaller masses of about twenty (or less) solar mass, called the stellar-mass black holes (StMBHs). In the late 1970s, Ultraluminous X-ray sources (hereafter, ULXs) were discovered in nearby galaxies, with observed luminosities in the range of ~ 1039 – 1041 erg/s, placing them between the luminosity of StMBH (< 1038 erg/s) and those of SMBH (~ 1047 erg/s).
Further studies in the subsequent two decades indicate them to be accreting black holes with masses between StMBH and SMBH, thus introduce the new category of intermediate mass black hole (IMBH). The fact that the luminosities of ULXs is very much closer to those of StMBH raised several attempts to explain their luminosities
using StMBH model by assuming greater mass accretion rate which, however, violates the Eddington limit. Both IMBH and StMBH models are thought to be equally plausible, but if both models are correct then we need to consider ULXs as a heterogeneous class of object. In this talk, I will review the study of ULXs, with the assumption that they are accreting black holes. Some evidences supporting both plausible models for ULXs will be explained, as well as the consequences in adopting each plausible model, including the black hole mass estimations. We will also study the environments of the ULXs candidates, in particular those residing in interacting galaxies. All things considered, the study of ULXs will enrich our knowledge on the physics of accretion processes; which is
far from being complete.
Key words: accretion, accretion disk, black hole physics, X-rays: binaries