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Supersoft X-ray Emission from the Optical Nova M31N 2007-12b in M31

ATEL # 1360; A.K.H. Kong (NTHU), R. Di Stefano (CfA)
on 17 Jan 2008; 19:28 UT
Password Certification: Albert Kong (

Subjects: X-ray, Nova, Transients

We report a discovery of supersoft X-ray emission from the classical nova M31N 2007-12b (ATel #1324, #1332) with Swift. On 2008 Jan 13 17:49UT, Swift performed a 4ks follow-up observation of the supersoft X-ray source (SSS) in the M31 globular cluster Bol 194 (ATel #1334). While the SSS in Bol 194 was not detected with a 95% upper limit of 2.8e36 erg/s (0.3-1 keV), a new X-ray source was found in the field. The new source is located at R.A.=00:43:20.2, Decl.=+41:13:48 (J2000) with a 90% error radius of 4.5 arcsec. The position is consistent with the optical nova M31N 2007-12b discovered on 2007 Dec 10 (ATel #1324).

All the 48 X-ray photons of the new source are from < 0.8 keV indicating a supersoft spectrum. Assuming a 60 eV blackbody spectrum and N_H=7e20 cm^-2, the 0.3-1 keV unabsorbed luminosity is 6.3e37 erg/s (d=780 kpc). We note that the same field was also observed with Swift on 2007 Dec 16 and Dec 30 for about 4 ks each but the source was not seen. Supersoft X-ray spectra are common for X-ray counterparts of optical novae and our observations indicate that the X-ray emission was detected 34 days after the nova outburst. We therefore propose that the supersoft transient is associated with the nova M31N 2007-12b.

We would like to thank the Swift team for the scheduling of the observations.

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