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Register a new userXMM-Newton observations of polars in low accretion states
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Authors
Gavin Ramsay
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We have made a series of snap-shot observations of 37 polars using XMM-Newton. We found that 16 of these systems were in a low, or much reduced, accretion state. Of those, 6 were not detected in X-rays. This suggests that in any survey of polars, around half will be in a low accretion state. We tested if there was a bias towards certain orbital periods: this is not the case. Of the 10 systems which were detected at low, but significant rates in X-rays, 8 showed significant variability in their X-ray light curves. This implies that non-uniform accretion still takes place during low accretion epochs. The bolometric luminosity of these systems is ~10^30 ergs s, two orders of magnitude less than for systems in a high accretion state. The X-ray spectra show no evidence of a distinct soft X-ray component. However, the X-ray and UV data imply that such a low temperature component exists: its temperature is low enough for its flux distribution to move outside the bandpass of the X-ray instruments.
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XMM-Newton observations of the polar SDSSJ155331.12+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 8-9.5E28 ergs/s. 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. SDSSJ132411.57+032050.5 is detected by XMM-Newton at the faint EPIC pn count rate of 0.0012 c/s, giving an upper limit to the X-ray luminosity of 7E28 ergs/s for a distance of 300 pc, which is also consistent with the above scenario.
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