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Supersoft X-ray transient leads to the discovery of the second optical nova in a M 31 globular cluster

ATel #3019; M. Henze, W. Pietsch, V. Burwitz, J. Lloyd, (Max-Planck-Institut fuer extraterrestrische Physik), K. Hornoch (Astronomical Institute Ondrejov), K. Nishiyama, F. Kabashima (Miyaki-Argenteus Observatory), A. Kaur, D. H. Hartmann (Clemson University), P. Milne, G. Williams (University of Arizona)
on 9 Nov 2010; 16:49 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

Subjects: Optical, Request for Observations, Nova

Referred to by ATel #: 3074

Alerted by the detection of a transient supersoft X-ray source (SSS) in the M 31 globular cluster (GC) Bol 126 (see ATel #3013), we investigated the long-term optical light curve of Bol 126 based on our monitoring of M 31 with the robotic 60cm telescope with an E2V CCD (2kx2k, 13.5 micron sq. pixels) of the Livermore Optical Transient Imaging System (Super-LOTIS, located at Steward Observatory, Kitt Peak, Arizona, USA). This light curve shows Bol 126 at R = 16.7+-0.1 mag during the time from 2007 until 2010 Oct. 11 and from Oct. 25 onward. On Oct. 12 we observed a significant increase in the brightness of the GC by about one magnitude. The measured Super-LOTIS magnitudes are as follows (1 sigma errors are 0.1 mag): 2010 Oct. 11.10 UT: 16.6; 12.48: 15.7; 25.23: 16.5; 27.19: 16.7. The magnitudes were obtained from a photometric solution using R magnitudes of the Local Group Galaxies Survey (LGGS) M 31 catalogue (Massey et al. 2006, AJ, 131, 2478).

A search in archival observations taken with the 0.65m telescope at Ondrejov observatory confirmed the outburst with high significance and sampled the decay in several epochs: 2010 Oct. 6.932 UT: 16.72 (K. Hornoch and M. Wolf); 8.818: 16.69 (K. Hornoch); 10.806: 16.70 (K. Hornoch and P. Hornochova); 12.813: 15.90 (P. Kusnirak); 13.861: 16.19 (K. Hornoch and P. Zasche); 19.010, 16.54 (K. Hornoch and M. Wolf); 28.703: 16.68 (K. Hornoch and M. Wolf); 29.711: 16.70 (K. Hornoch). All magnitudes are taken in the R filter, the typical 1 sigma error is 0.02 mag, observers are listed.

Unfiltered images by K. Nishiyama and F. Kabashima (Miyaki-Argenteus observatory, Japan) taken with a Meade 200R 0.40m f/9.8 reflector (+SBIG STL1001E camera) were calibrated with R magnitudes from the LGGS (see above, 1 sigma errors about 0.1 mag): 2010 Oct. 10.63: 16.7; 15.58: 16.3; 16.65: 16.5; 17.57: 16.6; 18.51: 16.7; 26.55: 16.5; 27.65: 16.6. In the observation of Oct. 15.58 during the decaying part of the outburst, Bol 126 is clearly brighter than on average.

The appearance of the transient SSS (see ATel #3013) shortly after the increase in optical brightness of the GC is an expected signature of a nova outburst. Optical novae have been found to represent the major class of SSSs in the central region of M 31 (see Pietsch et al. 2005, A&A, 442, 879).

Assuming R = 16.7 mag as average brightness for Bol 126, the nova reached R = 16.2 mag in maximum at 2010-10-12.48 UT. From the last upper limit, we can determine the outburst with an accuracy of about 1.4 days. The brightness of the nova declined by 2 magnitudes in 4 days, which makes it a very fast nova. The SSS phase was detected about 22 days after the optical outburst, indicating a very massive white dwarf in the binary system. This nova is only the second found in a M 31 GC. The first nova in a M 31 GC, M31N 2007-06b (see Shafter & Quimby 2007, ApJ, 671, L121), was found as a bright SSS starting 141 days after the optical discovery (see Henze et al. 2009, A&A, 500, 769). Optical spectroscopy or H-alpha observations of Bol 126 might still lead to the optical confirmation of the presumed nova and are strongly needed.