Context. Intermediate-mass black holes (IMBHs) are thought to be the seeds of early supermassive black holes (SMBHs). While &100 IMBH and small SMBH candidates have been identified in recent years, few have been robustly confirmed to date, leaving their number density in considerable doubt. Placing firmer constraints both on the methods used to identify and confirm IMBHs/SMBHs, as well as chara...

We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the observed AGN–radioloudness bimodality can be explained by the morphology related bimodality of black-hole spin distribution in the centers of galaxies: central black holes in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disc galaxies. We study how accretion from a warped dis...

The Galactic black hole X-ray binary XTE J1650–500 entered a quiescent regime following the decline from the 2001–2002 outburst that led to its discovery. Here we report on the first detection of its quiescent counterpart in a 36 ks observation taken in 2007 July with the Chandra X-ray Observatory. The inferred 0.5–10 keV unabsorbed flux is in the range 2.5–5.0 × 10 erg s cm. Notwithstanding la...

We calculate the nuclear composition of matter in accretion disks surrounding stellar mass black holes as are thought to accompany gamma-ray bursts (GRBs). We follow a mass element in the accretion disk starting at the point of nuclear dissociation and calculate the evolution of the electron fraction due to electron, positron, electron neutrino and electron antineutrino captures. We find that t...

Adopting the advection dominated accretion flow (ADAF) model, we have re-examined the radiation emitted by isolated stellar-mass black holes accreting from the interstellar medium of our Galaxy. Two distinct types of black holes are given consideration, 1) remnants of conventional massive star evolution (RBHs, mass ∼ 10M⊙), and 2) low-mass objects which possibly constitutes the dark halo and ca...