XMM-Newton and optical observations of the eclipsing polar CSS081231:071126+440405^⋆

Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: hworpel@aip.de

Received: 8 July 2015
Accepted: 27 August 2015

Abstract

Aims. We aim to study the temporal and spectral behaviour of the eclipsing polar CSS081231:071126+440405 from the infrared to the X-ray regimes.

Methods. We obtained phase-resolved XMM-Newton X-ray observations on two occasions in 2012 and 2013 in different states of accretion. In 2013 the XMM-Newton X-ray and UV data were complemented by optical photometric and spectroscopic observations.

Results. CSS081231 displays two-pole accretion in the high state. The magnetic fields of the two poles are 36 and 69 MG, indicating a non-dipolar field geometry. The X-ray spectrum of the main accreting pole with the lower field comprises a hot thermal component from the cooling accretion plasma, kT_plas of a few tens of keV, and a much less luminous blackbody-like component from the accretion area with kT_bb ~ 50–100 eV. The high-field pole, which was located opposite to the mass-donating star, accretes at a low rate and has a plasma temperature of about 4 keV. On both occasions the X-ray eclipse midpoint precedes the optical eclipse midpoint by 3.2 s. The centre of the X-ray bright phase shows accretion-rate-dependent longitudinal motion of ~20 deg.

Conclusions. CSS081231 is a bright polar that escaped detection in the RASS survey because it was in a low accretion state. Even in the high state it lacks the prominent soft component previously thought to be ubiquitous in polars. Such an excess may still be present in the unobserved extreme ultraviolet. All polars discovered in the XMM-Newton era lack the prominent soft component. The intrinsic spectral energy distribution of polars still awaits characterisation by future X-ray surveys such as eROSITA. The trajectory taken by material to reach the second pole is still uncertain.