XMM-Newtonand optical observations of the eclipsing polar CSS081231:071126+440405

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.

X-ray and optical observations of four polars

“…Finally, we present a previously unpublished Swift observation of the eclipsing polar V808 Aurigae in an X-ray faint state. The system was previously studied at high and intermediate luminosities Worpel & Schwope 2015). This work completes the characterisation of its X-ray and ultraviolet emission in all accretion states.…”

“…The companion star is probably an M4.6 red dwarf, as suggested by the period-secondary relations given in Knigge (2006). It has optical magnitudes m V ≈ 21 and m B ≈ 22.4 ( Thorne et al 2010), consistent with this identification, from which its distance was estimated to be 390 pc (Worpel & Schwope 2015). The variable star designation V808 Aur was assigned in December 2015 (Kazarovets et al 2015).…”

“…We found an accretion dip for X-rays and visual light for J0328, but it was not present in UV. It is the second source, after V808 Aur, to exhibit this phenomenon (Worpel & Schwope 2015). A strong wavelength dependence of the transparency of the accretion stream may provide clues regarding its structure and physical properties.…”

“…It is conceivable that a large soft excess might be present in the extreme UV/very soft X-ray regime. We estimated the largest unobserved blackbody in a manner very similar to the approach in Worpel & Schwope (2015). As reasoned in that paper, the unobserved soft excess can come from a region no larger than the white dwarf surface (which we assume to be R bb < 8000 km), its temperature can be no lower than that of the coolest known polar primary (0.64 eV; Schmidt et al 2005;Ferrario et al 2015), its presence should not affect the observed X-ray spectrum, and it should not overpredict the UV flux arising from the accretion region in either the UVW1 or UVM2 filters.…”

“…The source light curves show bright and faint phases in both optical and X-rays, indicative of an accreting pole moving in and out of view as the white dwarf primary rotates. The source was studied at high and intermediate accretion states by Worpel & Schwope (2015), who found that at high accretion rates the other magnetic pole accretes visibly in X-rays and optical. There is also a light curve dip, caused by the accretion stream passing in front of the main emitting spot, seen at optical wavelengths (Katysheva & Shugarov 2012) and in X-rays, but surprisingly not in the ultraviolet (Worpel & Schwope 2015).…”