XMM-Newton observes flaring in the polar UZ For during a low state

During an XMM-Newton observation, the eclipsing polar UZ For was found in a peculiar state with an extremely low X-ray luminosity and occasional X-ray and UV flaring. For most of the observation, UZ For was only barely detected in X-rays and ∼ 800 times fainter than during a high state previously observed with ROSAT. A transient event, which lasted ∼ 900 s, was detected simultaneously by the X-ray instruments and, in the UV, by the Optical Monitor. The transient was likely caused by the impact of 10–10 g of gas on the main accretion region of the white dwarf. The X-ray spectrum of the transient is consistent with ∼7keV thermal bremsstrahlung from the shock-heated gas in the accretion column. A soft blackbody component due to reprocessing of X-rays in the white dwarf atmosphere is not seen. The increase in the UV flux during the transient was likely caused by cyclotron radiation from the shockheated gas. Two more flaring events were detected by the Optical Monitor while the X-ray instruments were not operating. We conclude from our analysis that the unusual flaring behavior during the low state of UZ For was caused by intermittent increases of the mass transfer rate due to stellar activity on the secondary. In addition to the transient events, the Optical Monitor detected a roughly constant UV flux consistent with 11000–K blackbody radiation from the photosphere of the white dwarf. We find a small orbital modulation of the UV flux caused by a large, heated pole cap around the main accretion region.