Swift detection of super soft X-ray emission in Nova V2468 Cygni

ATel #3754; G. J. Schwarz (AAS), J. P. Osborne, K. Page (U. Leicester), J.-U. Ness (ESAC/ESA), C. E. Woodward (U. Minnesota), S. Starrfield (Arizona State U.), M. F. Bode (Liverpool JMU)
on 10 Nov 2011; 15:15 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

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Swift observed V2468 Cyg on August 8, October 3, and October 21, 2011. The combined X-ray (0.3-10 keV) count rate, ~ 0.015 +/- 0.001 ct/s, was fainter to that observed in campaigns in June-October 2009 (ATel #2157) and May 2010, however, the X-ray spectrum has changed significantly. Previously, all the spectra had both hard and soft components of roughly equal intensity. The recent spectra are now dominated by soft, < 1 keV, photons indicating that V2468 Cyg is in its super soft source phase; the (1-10/0.3-1) keV count rate hardness ratio has changed from >1 to <0.01. The Swift UVOT was set to the "filter of the day" and thus data were obtained in the uvw1 (2600 Angstroms) and u (3465 Angstroms) filters. The mean uvw1 magnitude was 16.95 +/- 0.02 while the u band magnitude was 16.16 +/- 0.02. The combined 2011 X-ray spectrum (11.8 ks) can be fit with a blackbody (kT = 36^+5_-4 eV) plus an optically thin Mekal model (kT = 0.61^+0.62_-0.46 keV). The N_H was fixed at 4.6e+21 cm^-2 which is consistent with E(B-V) = 0.77 (IAUC # 8936 ) and was used in our earlier modeling (ATel #2157). To illustrate the changes in the spectrum in the last two years the table gives the observed flux (0.3-10 keV in 10^-14 erg/s/cm^2) from each model component and the total flux in the last column.

Year	BB observed flux	Mekal observed flux	Total observed flux
2011	34.8	2.92	37.7
2010	3.96	60.3	64.3
2009	9.87	70.2	80.0

We aim to continue Swift observations of V2468 Cyg in the spring of 2012 to determine the duration of the super soft X-ray emission.

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