X-ray and optical observations of four polars

Abstract

Aims. To investigate the temporal and spectral behaviour of four polar cataclysmic variables from the infrared to X-ray regimes, refine our knowledge of the physical parameters of these systems at different accretion rates, and to search for a possible excess of soft X-ray photons. Methods. We obtained and analysed four XMM-Newton X-ray observations of three of the sources, two of them discovered with the SDSS, one in the RASS. The X-ray data were complemented by optical photometric and spectroscopic observations and, for two sources, archival Swift observations. Results. SDSSJ032855.00+052254.2 was X-ray bright in two XMM-Newton and two Swift observations, and shows transitions from high and low accretion states on a timescale of a few months. The source shows no significant soft excess. We measured the magnetic field strength at the main accreting pole to be 39 MG, the inclination to be 45 • ≤ i ≤ 77 • , and we have refined the long-term ephemeris. SDSSJ133309.20+143706.9 was X-ray faint. We measured a faint phase X-ray flux and plasma temperature for this source, which seems to spend almost all of its time accreting at a low level. Its inclination is less than about 76 • . 1RXSJ173006.4+033813 was X-ray bright in the XMM-Newton observation. Its spectrum contained a modest soft blackbody component, not luminous enough to be considered a significant soft excess. We inferred a magnetic field strength at the main accreting pole of 20 to 25 MG, and that the inclination is less than 77 • and probably less than 63 • . V808 Aur, also known as CSS081231:J071126+440405, was X-ray faint in the Swift observation but there is nonetheless strong evidence for bright and faint phases in X-rays and perhaps in UV. Residual X-ray flux from the faint phase is difficult to explain by thermal emission from the white dwarf surface, or by accretion onto the second pole. We present a revised distance estimate of 250 pc. Conclusions. The three systems we could study in detail appear to be normal polars, with luminosities and magnetic field strengths typical for this class of accreting binary. None of the four systems studied shows the strong soft excess thought commonplace in polars prior to the XMM-Newton era.

The NuSTAR Hard X-Ray Survey of the Norma Arm Region

“…The temporal and spectral properties of NNR19 and 25 most closely resemble those of polars, CVs with magnetic fields so strong ( > B 10 7 G) that the WD magnetosphere inhibits the formation of an accretion disk. Thus, compared to other CVs, polar X-ray emission is very sensitive to changes in the mass transfer rate, and polars exhibit flux variations of factors 4 as they transition between low and high accretion states on ∼month-year timescales (Ramsay et al 2004;Worpel et al 2016), very similar to the behavior of NNR19 and 25. No IR counterparts in the VVV survey are found for NNR19 or 25 within the 90% positional uncertainty determined from Chandra.…”

Hard X-ray cataclysmic variables

“…Additionally, an optically thick soft (∼20-60 eV) component, arising from the heated polar cap, was believed to be the characterising spectral feature of polars (Beuermann, 1999), but not of IPs, except a handful of "soft" IP systems (Haberl & Motch, 1995;Haberl et al, 2002;de Martino et al, 2004). XMM-Newton has remarkably shown an increasing number of polars without a distinct soft X-ray excess (Ramsay & Cropper, 2004;Ramsay et al, 2004cRamsay et al, , 2009Bernardini et al, 2014;Worpel et al, 2016;Bernardini et al, 2017), suggesting that if a reprocessed component exist, this has a low temperature shifting the emission towards the EUV/UV ranges. This also implies that polars cannot be characterised as soft X-ray emitting sources anymore.…”

X-ray and optical observations of polars

“…We have one X-ray observation of this source, taken by XMM-Newton in 2012, but the source was in a low accretion state and little information could be obtained from the light curves. The phase-averaged X-ray spectrum was consistent with a cool plasma, with a temperature of 4.3 keV, but the data were not of sufficient quality to look for a soft excess [6].…”

“…The XMM-Newton observation of the eclipsing polar V808 Aurigae is the subject of [5]. We studied XMM-Newton and Swift observations of the other three sources, and a Swift observation of V808 Aur, in [6].…”

“…We here present the results of two papers [5,6] detailing XMM-Newton, Swift, and optical observations of the four polars (V808 Aur, SDSS J032855.00+052254.2, SDSS J133309.20+143706.9, and 1RXS J173006.4+033813) discovered after the year 2000. The first of these is an eclipsing source, and is approximately one optical magnitude brighter that the other three.…”