X-ray orbital modulation of a white dwarf accreting from an L dwarf

The system SDSS J121209.31+013627.7

¹ INAF−Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: stelzer@astropa.inaf.it
² INAF−Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
³ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

Received: 10 November 2016
Accepted: 5 January 2017

Abstract

In an XMM-Newton observation of the binary SDSS J121209.31+013627.7, consisting of a white dwarf and an L dwarf, we detect X-ray orbital modulation as proof of accretion from the substellar companion onto the magnetic white dwarf. We constrain the system geometry (inclination as well as magnetic and pole-cap angle) through modelling of the X-ray light curve, and we derive a mass accretion rate of 3.2 × 10^-14M_⊙/ yr from the X-ray luminosity (~3 × 10²⁹ erg/s). From X-ray studies of L dwarfs, a possible wind driven from a hypothesized corona on the substellar donor is orders of magnitude too weak to explain the observed accretion rate, while the radius of the L dwarf is comparable to its Roche lobe (0.1 R_⊙), making Roche-lobe overflow the likely accretion mechanism in this system.