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M31N 1997-11k - a very fast recurrent nova in M31 or a foreground CV?

ATel #2286; M. Henze, W. Pietsch, P. Podigachoski, V. Burwitz, F. Haberl (Max-Planck-Institut fuer extraterrestrische Physik, MPE), A. Updike, D. Hartmann (Clemson University), P. Milne, G. Williams (University of Arizona), G. Papamastorakis, P. Reig (FORTH & Dept. of Physics, University of Crete), A. Strigachev (Inst. of Astronomy, Bulgarian Academy of Sciences)
on 9 Nov 2009; 19:42 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: Optical, Ultra-Violet, Cataclysmic Variable, Nova

Referred to by ATel #: 2290, 9280

Detections of a new nova candidate in M 31 (which possibly is coincident with the nova candidate M31N 1997-11k in the MPE M 31 nova catalog at http://www.mpe.mpg.de/~m31novae/opt/m31/index.php ) were reported to us by K. Nishiyama and F. Kabashima (Miyaki-Argenteus observatory, Japan). We report multi-wavelength observations of the candidate.

The object is visible in several 12*60sec stacked R filter CCD images obtained with the robotic 60cm telescope with an E2V CCD (2kx2k) of the Livermore Optical Transient Imaging System (Super-LOTIS, located at Steward Observatory, Kitt Peak, Arizona, USA) at position RA = 00h42m39.61s, Dec = 41d09'03.2" (J2000, accuracy of 0.2") on 2009 October 29.18 UT: < 19.0mag; November 01.18: 18.8; 02.18: 19.0; 03.33: 18.4; 06.25: 18.6; 08.25: 18.6; 09.25: 18.6. All magnitudes are obtained from a photometric solution using R magnitudes of the Local Group Galaxy Survey M 31 catalogue (LGGS, Massey et al. 2006, AJ, 131, 2478).

The object position coincides with M31N 1997-11k (distance 0.8", error 0.7") and M31N 2001-12b (0.3", 0.1") (see MPE M 31 nova catalog). These two objects were already classified as recurrent nova (rec. time 4.1 yr). We suggest that we detected another outburst of the same system with a time lag of 12.0 yr and 7.9 yr, respectively.

However, there is evidence for a non-nova origin of the outbursts of this system:

The current outburst is already detected one month earlier in stacked H-alpha images obtained with the 1.3m Ritchey Chretien f/7.5 telescope at Skinakas Observatory, Crete, Greece, using an Andor DZ436-BV CCD Camera (with a Marconi 2k x 2k chip with 13.5 microns sq. pixels) and a broad (75\AA) H-alpha filter. Skinakas magnitudes were determined from a photometric solution using R magnitudes of the LGGS catalog. The nova candidate is first detected on 2009 October 9.10 UT with Rmag 18.7, but is already faintly visible on 2009 October 2.10. No object can be seen in images on 2009 September 27.10 (>19.5mag).

The object is also detected in UV with the Swift Ultraviolet/Optical Telescope (UVOT) UVW2 (112-264nm) filter in several observations: 2009 June 07.95 UT: 21.2+-0.3; October 22.61: 19.0+-0.1; 28.29: 18.9+-0.1; 31.24: 18.7+-0.1; November 03.58: 18.4+-0.1. The UV outburst of the object is clearly connected to the optical outburst but already detected ~10 d earlier.

At a distance of 1.0" there is an object with a V magnitude of 22.17 in the LGGS catalog. The color terms of this object (LGSS J004239.57+410902.3) are U-B = -0.525, B-V = 0.396, V-R = 0.374, and R-I = 0.873. A close examination of the stacked images provided by the LGGS group shows, that the object is brightest in fields 5 and 6 in the U band (obtained on 2001 September 22 and 2002 September 11, respectively), fainter in B and not visible at all in V, R, and I. However, due to the positional separation is not clear if the LGGS object is connected to the outbursts.

The earlier start of the outburst in UV and H-alpha compared to the R band does not fit to an optical nova. Additionally, the short recurrence time (4.1 yr) would be unprecedented and would require a very massive white dwarf in the system, which should produce a much faster outburst. Instead, we suggest that the nova-like outburst could point towards a cataclysmic variable in the Milky Way with a dwarf nova outburst (U Gem or SU UMa system). Events like this have been observed in the direction of M 31 in the past and have initially been mistaken for novae (see e.g. Sharov & Alksnis, SvAL, 15, 382).

An optical spectrum of the object could unveil its mystery (see e.g. the case of M31N 2008-11b in ATel #1867). We therefore strongly encourage follow-up spectroscopy on the supposed nova M31N 1997-11k. A finding chart is available at http://www.mpe.mpg.de/~m31novae/opt/m31/data/finding_charts/nova_R_09_11_07.jpg