Swift detects a strong increase in the X-ray flux of V2468 Cyg

ATel #4286; K. L. Page (U. Leicester), G. J. Schwarz (AAS), J. P. Osborne (U. Leicester), M. J. Darnley (LJMU), J. J. Drake (CfA), J.-U. Ness (ESA/ESAC), S. N. Shore (U Pisa) and S. Starrfield (ASU)
on 30 Jul 2012; 15:45 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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Swift observations of V2468 Cyg in August and October 2011 (days 1266 - 1322 after outburst) revealed a super-soft X-ray spectrum (ATel #3754); at that time, however, there was no noticeable change in count rate from the previous spectral state which had shown both hard and soft spectral components since the first Swift observation in June 2009 (~460 days after outburst). A further monitoring campaign was begun at the start of June 2012 and observations between 01 June and 13 July (days 1547 - 1589) showed similar spectra to those obtained in autumn 2011. However, data collected on 27 July (day 1602.5) revealed that the count rate had increased by almost a factor of 5, to a level of 0.064 +/- 0.005 (compared to 0.014 +/- 0.004 on 13 July).

The X-ray spectrum on 27 July is still soft and can be parameterised with a blackbody of temperature kT = 47 +8/-10 eV (~5.5 x 10⁵ K), plus an optically thin component of kT > 0.5 keV, absorbed by a column of N_H= (3.0 +1.5/-1.1) x 10²¹ cm^-2. The spectrum extracted from the 01 June - 13 July data can be fitted with parameter values consistemt with these, with the increase in observed flux over the Swift-XRT band (0.3-10 keV), from (4.4 +0.2/-1.0) x 10^-13 erg cm^-2 s^-1 (combined data from 01 June to 13 July) to (1.8 +0.4/-0.3) x 10^-12 erg cm^-2 s^-1 (27 July), being the only significant change. There is no evidence to suggest that the absorption column has decreased significantly.

With E(B-V) = 0.77, a visual maximum of 7.4 mag and the t2 time of ~ 10 days (IAUC # 8936 ; Schwarz et al., 2011, ApJS, 197, 31), the Maximum Magnitude versus Rate of Decline (MMRD) relation (Della Valle & Livio 1995) can be used to derive a distance to V2468 Cyg of 5.5 +1.4/-1.1 kpc. This leads to an estimated observed luminosity of 6.4 x 10³³ erg s^-1 (3.9 x 10³⁵ erg s^-1 unabsorbed) over the 0.3-10 keV range. Extrapolating the model over 0.01-100 keV provides an estimate of the unabsorbed bolometric luminosity of around 2.4 x 10³⁶ erg s^-1 implying that the source may yet get brighter.

The duration of the supersoft source (SSS) phase in V2468 Cyg, still detected > 4.38 years after outburst, is unusually long considering its t2 value. In the Swift Galactic nova sample analysed by Schwarz et al. (2011), the median SSS duration was only 1.4 years. Continued monitoring at other wavelengths during this period of increased soft X-ray emission is strongly encouraged. A further Swift observation should take place towards the end of August.

We thank the Swift PI and mission operations team for their support.