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Swift/XRT and UVOT detection of the optical nova M31N 2006-11a

ATEL # 1116; W. Pietsch (MPE), J. Greiner (MPE), F. Haberl (MPE), G. Sala (MPE)
on 20 Jun 2007; 9:58 UT
Password Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: Optical, Ultra-Violet, X-ray, Nova

We report the detection of the optical nova M31N 2006-11a with the XRT and UVOT instruments in a 19.4 ks Swift observation starting at June 1, 2007 at 16:40 UT, about 190 d after the nova outburst. The optical nova M31N 2006-11a was detected by K. Itagaki on November 24.67, 2006 and reached 17.3 mag at maximum on unfiltered CCD images (see http://cfa-www.harvard.edu/iau/CBAT_M31.html#2006-11a).

During the Swift observation five U and 14 UVW2 exposures were collected with about 1 ks integration time each. During one of the U and six of the UVW2 exposures the nova was outside the Field of View (FOV) leaving a total exposure for the nova area of 4.2 ks in U and 7.7 ks in UVW2. The nova was clearly detected in both filters at magnitudes of 20.15+/-0.19 and 21.12+/-0.16, respectively, i.e. about 3 mag below outburst brightness. The emission in UVW2 may originate in the ejected shell, excited by the soft X-ray emission. The observed flux in the UVW2 band (180+/-20 nm) may well result from the CIII] 190.9 nm, NIII] 175.0 nm and/or OIII] 166.3 nm lines, typical for the spectra of novae in the nebular phase.

A soft X-ray source with about 26 cts is detected at the nova position, just inside the boarder of the XRT, with all photons below 530 eV. No X-ray source has been reported previously near this position in XMM-Newton and Chandra observations covering the area. The temperature of a blackbody fit to the source spectrum can be constrained to 40 eV < kT < 90 eV (90% confidence) using the Galactic foreground absorption (6.6^20 cm^-2) as lower bound and constraining the luminosity to below 3.5^38 erg/s (using the Eddington luminosity of a white dwarf at the Chandrasekhar mass limit, and assuming a He-rich atmosphere for a conservative upper luminosity limit). Column density - temperature confidence contours inferred from the Swift/XRT spectrum of M31N 2006-11a can be seen from the figure at http://www.mpe.mpg.de/~m31novae/xray/swift/index.php .

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


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