Quasiperiodic Erupters: A Stellar Mass-transfer Model for the Radiation

Abstract

Quasiperiodic erupters are a remarkable class of objects exhibiting very-large-amplitude quasiperiodic X-ray flares. Although numerous dynamical models have been proposed to explain them, relatively little attention has been given to using the properties of their radiation to constrain their dynamics. Here we show that the observed luminosity, spectrum, repetition period, duty cycle, and fluctuations in the latter two quantities point toward a model in which a main-sequence star on a moderately eccentric orbit around a supermassive black hole periodically transfers mass to the Roche lobe of the black hole; orbital dynamics lead to mildly relativistic shocks near the black hole; and thermal X-rays at the observed temperature are emitted by the gas as it flows away from the shock. Strong X-ray irradiation of the star by the flare itself augments the mass transfer, creates fluctuations in flare timing, and stirs turbulence in the stellar atmosphere that amplifies the magnetic field to a level at which magnetic stresses can accelerate infall of the transferred mass toward the black hole.