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X-ray identification of the prenova of V2491 Cyg

ATel #1473; A. Ibarra (ESA/ESAC, Spain) & E. Kuulkers (ESA/ESAC, Spain)
on 12 Apr 2008; 20:54 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

Subjects: X-ray, Binary, Nova, Transient, Variables

Referred to by ATel #: 1475, 1478, 1480, 1504, 1514

We report on the possible X-ray counterpart to the prenova of the recently discovered bright nova, V2491 Cyg (IAUC # 8934 , CBET #1334).

The last Swift observation of the prenova field before the outburst was on UT 2008 Jan 2 for an exposure of 5.6 ksec. We find a source in the XRT field of view at a position of (J2000.0) RA = 19h43m02.1s, Dec = +32d19'12.8", with an uncertainty of 4.1", with a net count rate of about 0.021 cts/s. The source lies only 1.5" from the optical nova position. We identify this source as the X-ray counterpart of the prenova, and designate it Swift J1943.0+3219.

The 0.1-5 keV spectrum can be well described (Chi^2_red = 1.1 for 3 degrees of freedom) by an absorbed power-law. We fixed the absorption column, using the Leiden/Argentine/Bonn (LAB) Survey of Galactic HI, to a value closest to the source of N_H = 2.9e21 atoms cm^-2. We find a power-law index of 1.30+/-0.25 and an unabsorbed flux of about 1.1e-12 ergs/cm^2/s^1.

We also found a source in the XMM-Newton full Slew Survey EPIC Source Catalogue, XMMSL1 J194301.9+321911, during a slew performed on 2006 Nov 15, at the following coordinates: (J2000.0) RA = 19h43m02.03s, Dec = 32d19'10.5", with error radius of 6.1". This source lies only 3.7" from the nova position, and is consistent with that found for Swift J1943.0+3219, and we therefore they are one and the same X-ray source.

We further note that in the ROSAT All-Sky Survey Faint Source Catalog there is a source close to those quoted above, 1RXS J194259.9+321940. It lies about 38" away from the nova position with coordinates (J2000.0) RA = 19h42m59.90s, Dec = 32d19'40.5", with a positional uncertainty of about 28".

We encourage follow up of the source at various wavelengths.