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The progenitor of Nova Cygni 2007 (=V2467 Cyg)

ATel #1031; D. Steeghs (CfA), J. Drew (Imperial College London), R. Greimel (ING), M. Barlow (UCL), B. Gaensicke (Warwick), J. Drake (CfA), A. Witham (Southampton) and the IPHAS collaboration
on 22 Mar 2007; 16:34 UT
Credential Certification: Danny Steeghs (dsteeghs@cfa.harvard.edu)

Subjects: Infra-Red, Optical, Binary, Cataclysmic Variable, Nova, Transient

We report on the detection of the likely progenitor to the currently active Nova Cygni 2007 = V2476 Cyg (IAUC # 8821 , CBET #894, 897, 900) using images from the INT Photometric H-Alpha Survey (IPHAS; http://www.iphas.org ).

The field containing the classical nova was observed on several occasions with the Isaac Newton Telescope at La Palma Observatory and its Wide Field Camera. The closest point source in the IPHAS database to the reported position of V2467 Cyg was found at R.A.(J200)=20:28:12.47, Dec(J2000)=+41:48:36.4. This is consistent with the outburst position reported in IAUC # 8821 , CBET #894 and less than 0.3" from the USNO-A2.0 object 1275-13944467.

Three sets of r', i' and narrow-band H-alpha measurements were obtained in good observing conditions on August 8 & 9 2004, from which we can derive reliable r', i' magnitudes and a measure of H-alpha excess (see Drew et al. 2005 for a discussion of the IPHAS survey and its point source photometry). We obtain r'=18.46+/-0.01, i'=17.49+/-0.01 and hence r'-i'=0.97+/-0.02. Comparing the IPHAS r'-i' and r'-Halpha colours with those of other field stars within the same INT/WFC pointing suggests a source consistent with either a lightly reddened K dwarf with no significant H-alpha emission or a bluer, more heavily reddened object with an r'-H-alpha excess of ~0.2 magnitudes. Given the absolute magnitude of K dwarfs (M~7), a distance of ~2kpc is implied in the first scenario. This seems hard to reconcile with a lightly reddened object, in particular when considering the considerable galactic extinction along the line of sight.

In order to further disentangle these two possibilities, we comment that a source consistent with our IPHAS position is present in the 2MASS catalog at K=14.74+/-0.12 and with J-H=0.60+/-0.10 and H-K=0.39+/-0.13. For the low reddening scenario, the 2MASS magnitudes imply a M5 dwarf, not consistent with our r',i' magnitudes and also not consistent with an unreddened cataclysmic variable (CV) that would be the nova progenitor. However, an E(B-V) of 1.0-1.5 would make the 2MASS colours more in line with that of a high mass transfer rate CV (e.g. Figures 4&5 in Hoard et al. 2002, ApJ, 565, 511). Applying this reddening to the IPHAS colours suggests a progenitor with an optical SED analogous to that of an A/F dwarf, and exhibiting a modest H-alpha excess.

For distances of more than ~1 kpc, the sight line of V2467 Cyg traverses molecular gas in the Cygnus-X star forming region and this amount of reddening would be unremarkable. Hence a relatively blue SED with some line emission offers a consistent fit to both optical and NIR photometry. An early A-type spectral type with weak H-alpha emission is consistent with the appearance of an optically thick accretion disc, as found in a number of high mass transfer rate cataclysmic variables (e.g. Aungwerojwit et al. 2005, A&A 443,1995).

We conclude that we have likely detected the progenitor to Nova Cygni 2007. It is a moderately reddened point source that showed H-alpha emission in 2004 and is consistent with a cataclysmic variable progenitor accreting at a high rate. Given the unreddened magnitude of r'~16 and a typical absolute magnitude of 3-5 for such CVs leads us to a distance estimate of 1.5-4kpc for V2467 Cyg. Finally, we remark that the outburst amplitude of ~12 magnitudes is very typical for a FeII-class galactic nova.