Post common envelope binaries from the SDSS. VI. SDSS J120615.73+510047.0: a new low accretion rate magnetic binary

TL;DR

This paper reports the discovery of a new low accretion rate magnetic binary system, expanding understanding of pre-polars with extremely low mass transfer rates and magnetic fields.

Contribution

It presents the identification and detailed analysis of the ninth known pre-polar binary with a magnetic white dwarf and a late-type secondary, including spectral and evolutionary insights.

Findings

White dwarf accretes at ~10^-14 Msun/yr from wind

Binary period estimated between 200 and 270 minutes

Magnetic field strength of 108 MG and a white dwarf temperature of ~9000 K

Abstract

We report the discovery of the ninth pre-polar, consisting of a late-type ZAMS secondary and a magnetic white dwarf. The white dwarf accretes at extreme low rate, dot{M} ~ 10**-14 Msun/yr, from the wind of the companion donor star. The source was found in our systematic search for WD/MS binaries within SDSS/SEGUE. Based on seven Sloan-spectra we estimate a binary period of ~200, 230, or 270 min. The UV to IR spectral energy distribution was decomposed into a dM3-dM4 ZAMS secondary and a cool white dwarf, ~9000 K, which consistently imply a distance between 360 and 420 pc. The optical spectrum displays one pronounced cyclotron hump, likely originating from a low-temperature plasma, ~1 keV, in a field of 108 MG. We comment on the evolutionary link between polars and pre-polars.