Tidal disruption of white dwarfs by intermediate mass black holes

Abstract

Modeling ultra-close encounters between a white dwarf and a spinning, intermediate mass black hole requires a full general relativistic treatment of gravity. This paper summarizes results from such a study. Our results show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. On the other hand, the late-time accretion onto the black hole follows the same decay, ˙ M ∝ t −5/3 , estimated from Newtonian gravity disruption studies. The spectrum of the fallback material peaks in the soft X-rays and sustains Eddington luminosity for 1-3yrs after the disruption. The orientation of the black hole spin has also a profound effect on how the outflowing debris obscures the central region. The disruption produces a burst of gravitational radiation with characteristic frequencies of ∼3.2Hz and strain amplitudes of ∼10 −18 for galactic intermediate mass black holes. In a tidal disruption event, the spin of black hole (BH) will most likely not be aligned with the orbital angular momentum of the incoming star. As a consequence, the tidal debris when it falls back will form a tilted disk (1). For thin disks, the Bardeen-Petterson effect will help align the inner part of the disk with the BH spin (2), but for the thick disks expected from tidal disruptions, the alignment may not happen (3-5), potentially changing our conventional picture of these explosive events. Even more remarkable will be ultra-close encounters, where an incoming star and associated tidal debris will experience general relativistic effects. Examples of strong curvature effects altering the dynamics of the disruption are frame dragging and the location of the innermost stable orbit, responsible for changing the traditional S-shape debris observed in Newtonian gravity simulations to a shell-like appearance, engulfing the BH. There are very few tidal disruption numerical simulations that account for general relativistic effects (6- 9). This paper summarizes the results from a study published recently (10). The study was primarily aimed at investigating observational signatures associated with strong gravity that may shed light on the presence of intermediate mass BHs. It included estimates of electromagnetic transient radiation using a slim disk model to compute the spectrum during the fallback phase. We found that the sources shine at Eddington luminosity ∼10 41 erg s −1 for about 1-3yrs, and then fade approximately as ∝ t −5/3 . Furthermore, there is a 50/50 chance that the inner disk will be obscured by outflowing debris for a fully misaligned BH spin. The study also included estimates of the gravitational wave burst signal produced during the encounters.