GSN 069 – A tidal disruption near miss

Abstract

ABSTRACT I suggest that the quasi-periodic ultrasoft X-ray eruptions recently observed from the galaxy GSN 069 may result from accretion from a low-mass white dwarf in a highly eccentric orbit about its central black hole. At $0.21\,\rm M_{\odot }$, this star was probably the core of a captured red giant. Such events should occur in significant numbers as less extreme outcomes of whatever process leads to tidal disruption events. I show that gravitational radiation losses can drive the observed mass-transfer rate, and that the precession of the white dwarf orbit may be detectable in X-rays as a superorbital quasi-period $P_{\rm super} \simeq 2\,{\rm d}$. The very short lifetime of the current event, and the likelihood that similar ones involving more massive stars would be less observable, together suggest that stars may transfer mass to the low-mass SMBH in this and similar galaxies at a total rate, potentially making a significant contribution to their masses. A similar or even much greater inflow rate would be unobservable in most galaxies. I discuss the implications for SMBH mass growth.