ILLUMINATING MASSIVE BLACK HOLES WITH WHITE DWARFS: ORBITAL DYNAMICS AND HIGH-ENERGY TRANSIENTS FROM TIDAL INTERACTIONS

Abstract

White dwarfs (WDs) can be tidally disrupted only by massive black holes\n(MBHs) with masses less than $\\sim10^5 M_\\odot$. These tidal interactions feed\nmaterial to the MBH well above its Eddington limit, with the potential to\nlaunch a relativistic jet. The corresponding beamed emission is a promising\nsignpost to an otherwise quiescent MBH of relatively low mass. We show that the\nmass transfer history, and thus the lightcurve, are quite different when the\ndisruptive orbit is parabolic, eccentric, or circular. The mass lost each orbit\nexponentiates in the eccentric-orbit case leading to the destruction of the WD\nafter several tens of orbits. We examine the stellar dynamics of clusters\nsurrounding MBHs to show that single-passage WD disruptions are substantially\nmore common than repeating encounters. The $10^{49}$ erg s$^{-1}$ peak\nluminosity of these events makes them visible to cosmological distances. They\nmay be detectible at rates of as many as tens per year by instruments like\nSwift. In fact, WD-disruption transients significantly outshine their\nmain-sequence star counterparts, and are the most likely tidal interaction to\nbe detected arising from MBHs with masses less than $10^5 M_\\odot$. The\ndetection or non-detection of such WD-disruption transients by Swift is,\ntherefore, a powerful tool to constrain lower end of the MBH mass function. The\nemerging class of ultra-long gamma ray bursts all have peak luminosities and\ndurations reminiscent of WD disruptions, offering a hint that WD-disruption\ntransients may already be present in existing datasets.\n