Abstract
Aims: The VY Scl system (anti-dwarf nova) V751 Cyg is examined following a claim of a super-soft spectrum in the optical low state. Methods: A serendipitous XMM-Newton X-ray observation and, 21 months later, Swift X-ray and UV observations, have provided the best such data on this source so far. These optical high-state datasets are used to study the flux and spectral variability of V751 Cyg. Results: Both the XMM-Newton and Swift data show evidence for modulation of the X-rays for the first time at the known 3.467 hr orbital period of V751 Cyg. In two Swift observations, taken ten days apart, the mean X-ray flux remained unchanged, while the UV source brightened by half a magnitude. The X-ray spectrum was not super-soft during the optical high state, but rather due to multi-temperature optically thin emission, with significant (10^{21-22} cm^-2) absorption, which was higher in the observation by Swift than that of XMM-Newton. The X-ray flux is harder at orbital minimum, suggesting that the modulation is related to absorption, perhaps linked to the azimuthally asymmetric wind absorption seen previously in H-alpha.
Highlights
During the pre-release verification of the 3XMM-DR4 X-ray source catalogue (Rosen et al, in prep.), examination of a sample of bright point sources, variable on short timescales, revealed a serendipitous detection of the cataclysmic variable V751 Cyg
Position Sensitive Proportional Counter (PSPC) observations occurred in 1990 and 1992 when the optical source was in its usual high state, providing only upper limits for the source X-ray count rate, while the High Resolution Imager (HRI) data collected in June and December 1997 resulted in a detection of V751 Cyg
Greiner (1998) reports that the X-ray luminosity determined using the PSPC was below 2–6 × 1030 erg s−1 during the optical high state, while, during the optical low state, Greiner et al (1999) find that it had a super-soft blackbody-like spectrum with kT = 15+−1150 eV and a luminosity in the range 7 × 1033−36 erg s−1, despite the HRI having a variable gain and very modest spectral resolution (FWHM/E = 0.66–1.07, Fraser 19921); this led them to suggest an anti-correlation of X-ray and optical intensity, as was seen in RX J0513.9−6951 (e.g. Reinsch et al 1996)
Summary
During the pre-release verification of the 3XMM-DR4 X-ray source catalogue (Rosen et al, in prep.), examination of a sample of bright ( fX > 10−12 erg cm−2 s−1) point sources, variable on short timescales, revealed a serendipitous detection of the cataclysmic variable V751 Cyg. PSPC observations occurred in 1990 and 1992 when the optical source was in its usual high state, providing only upper limits for the source X-ray count rate, while the HRI data collected in June and December 1997 (during which time the optical emission was in the low state) resulted in a detection of V751 Cyg. Greiner (1998) reports that the X-ray luminosity determined using the PSPC was below 2–6 × 1030 erg s−1 during the optical high state, while, during the optical low state, Greiner et al (1999) find that it had a super-soft blackbody-like spectrum with kT = 15+−1150 eV and a luminosity in the range 7 × 1033−36 erg s−1 (for a distance of 500 pc), despite the HRI having a variable gain and very modest spectral resolution (FWHM/E = 0.66–1.07, Fraser 19921); this led them to suggest an anti-correlation of X-ray and optical intensity, as was seen in RX J0513.9−6951 We present serendipitious XMM-Newton observations, along with follow-up Swift data (all obtained when V751 Cyg was in its optical high state), investigating the timing (Sect. 3.1) and spectral (Sect. 3.2) properties of the source, using both X-ray and UV data
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