XMM-Newton Observations of the Extremely Low Accretion Rate Polars SDSS J155331.12+551614.5 and SDSS J132411.57+032050.5

Abstract

XMM-Newton observations of the polar SDSS J155331.12+551614.5 reveal that all the X-ray flux emerges at energies less than 2 keV. The best fit to the spectrum is with a thermal plasma with kT=0.8 keV plus a 20-90 eV black body, yielding a thermal X-ray luminosity of (8.0-9.5)×10²⁸ ergs s^-1. The low temperature and X-ray luminosity, together with the lack of variation of the X-ray flux during the observations, are all consistent with an extremely low accretion rate that puts the system in the bombardment regime of accretion, rather than accretion involving a standoff shock. It is likely that the observed X-rays originate from the M dwarf secondary star, thus providing a base activity level for late main-sequence stars in close binaries. SDSS J132411.57+032050.5 is detected by XMM-Newton at the faint EPIC pn count rate of 0.0012+/-0.0003 counts s^-1, giving an upper limit to the X-ray luminosity of ~7×10²⁸ ergs s^-1 for a distance of 300 pc, which is also consistent with the above scenario.