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Two new supersoft X-ray sources in M31 identified as classical novae

ATEL # 1672; T. Nelson (UW Madison), J.F. Liu (CfA), R. Di Stefano (CfA), M. Orio (INAF Padova and UW Madison), B. Patel (CfA), F. Primini (CfA) and A. W. Shafter (SDSU)
on 22 Aug 2008; 1:01 UT
Password Certification: Thomas Nelson (nelson@astro.wisc.edu)

Subjects: Optical, X-ray, Nova, Transients

We report the identification of two previously unpublished supersoft X-ray sources (SSS) in M31 as classical novae. The X-ray sources were discovered during a comprehensive search of all archival Chandra/ACIS images of M31 for so-called "Very Soft Sources" (VSS, see Di Stefano & Kong 2003 ApJ, 592, 884). The identification with classical novae was made as part of a larger campaign aimed at optical identification and classification of SSS in M31, and made use of the list of novae in M31 compiled by Pietsch et al. (2007 A&A, 465, 375).

The first source, located at

RA(J2000) = 00h 42m 46.11s
Dec(J2000) = +40d 53m 34.54s

is identified as the nova M31N 1999-08d. This nova was first discovered during the POINT-AGAPE microlensing survey of M31 (Darnley et al., 2004 MNRAS, 353, 571), and was classified as a slow nova with a maximum observed m_r = 18.4 on 1999 Aug 09 UT. The X-ray counterpart, lying 0.8" from the nova position, was first detected during a 46 ks Chandra ACIS-S exposure on 2001 Jul 24 UT (obs ID 2017) with a total of 23 +/- 5.8 counts below 1 keV, and was classified as supersoft using the algorithm described in Di Stefano and Kong (2003, astro-ph/0311374). The source was detected again 4 days later in a 16 ks ACIS-S exposure (obs ID 2494), with a total of 10 +/- 4.3 counts below 1 keV. The unabsorbed luminosities are in the range 2 x 10^36 to 2 x 10^38 erg/s (assuming blackbody emission with 40 < T(eV) < 80 and 10^21 < N(H)(cm^-2) < 4 x 10^21), typical of shell burning white dwarfs in post nova systems.

The source was not detected in a 5 ks ACIS-I exposure on 2001 Oct 05 UT (obs ID 1576). However, a soft X-ray source is identified at the same location (although with larger positional uncertainty) with XMM-Newton on 2002 Jan 12 UT in the XMM Serendipitous Source Survey catalog (Watson et al., 2008, astro-ph/0807.1067). No X-ray source is observed at this position in all subsequent Chandra observations.

We identify the second new X-ray source, located at

RA(J2000) = 00h 44m 32.58s
Dec(J2000) = 41d 25m 21.25s

as nova M31N 2001-10b. This nova was also discovered during the POINT-AGAPE survey, and was classified as moderately fast. The maximum observed brightness was m_r = 15.9 on 2001 Oct 03 UT. A supersoft source was detected during a 5 ks ACIS-S exposure on 2002 Nov 29 UT (obs ID 2900) 1.1" from the optical position. A total of 17 +/- 5.7 source counts were detected, again all below 1 keV. The unabsorbed luminosity derived for this source is at least 1.6 x 10^37 erg/s. No source is detected at the nova position in a recent 10 ks ACIS-I observation carried out on 2007 Jun 02 UT (obs ID 7067).


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