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Three M31 Microlensing/Nova Candidates

ATEL # 1845; M. J. Darnley (Liverpool John Moores University, UK), E. Kerins (University of Manchester, UK) A. M. Newsam (Liverpool John Moores University, UK), J. P. Duke (Liverpool John Moores University, UK), A. Gould (Ohio State University, USA), C. Han (Chungbuk National University, Korea), B.-G. Park (Korea Astronomy and Space Science Institute, Korea) and R. A. Street (Las Cumbres Observatory, USA)
on 14 Nov 2008; 16:39 UT
Password Certification: Matt Darnley (

Subjects: Optical, Microlensing Events, Nova, Transients

We report the discovery of three optical transients in M31 by the Angstrom M31 bulge microlensing survey using the Liverpool Telescope (La Palma, Steele et al. 2004). These transients were discovered using difference imaging techniques by the Angstrom Project Alert System (APAS, Darnley et al. 2007) in a series of Sloan i'-band images of the bulge of M31.

ANG-08B-M31-01: The first transient is located at RA=00:42:48.5 Dec=+41:13:49.8 (J2000), 137.12" South and 52.12" East of the core of M31. No significant variation has been observed at this position through the past four M31 observing seasons. This event was observed at a peak i'-band magnitude of 18.990.06 on October 20.02 2008 and was resolvable in a single 200s background subtracted image. Subsequent behaviour of the light-curve suggests this object may be either a nova or possibly a binary microlensing event.

ANG-08B-M31-02: A second transient is located at RA=00:42:56.4 Dec=+41:15:46.7 (J2000), 138.12" East and 18.21" South of the core of M31. Periodic lightcurve behaviour is seen throughout the previous four seasons, consistent with a long-period variable at or close to the position of this event. However a stronger and sharper peak in the i' band was observed to reach a maximum of mi=20.30.2 on November 07.81 2008 and was just resolvable in a stack of seven 200s images following background subtraction. The light-curve of this object suggests that it is likely to be due to microlensing with peak magnification occurring at MJD=54774.10.6 days and with a full width at half maximum duration of 73 days.

ANG-08B-M31-03: The third transient is located at RA=00:42:56.4 Dec=+41:16:18.1 (J2000), 137.15" East and 13.27" North of the core of M31. Periodic variation at or close to the position of this event has been observed throughout the previous four observing seasons. A stronger and sharper outburst was observed to peak at mi=18.740.05 in the i' band on November 07.81 2008 and was also just resolvable following background subtraction. This event shows a gradual rise consistent with microlensing rather than a nova but subsequent behaviour appears inconsistent with microlensing by a single lens.

Further details of these events, including lightcurves and discovery images can be found at:

Angstrom Project Web Site

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