Novae in M31

Novae are phenomena that appear to the observer as a new star in the sky, which gives their name. They arise in close binary systems consisting of cool star of late spectral class and hot white dwarf. Outburst is due to ignition of nuclear reactions at the base of the accretion disk, formed by the accreted gas from the companion star. The brightness of the system increase to one million times and fell to its previous state for months. The study of the change in magnitude over time (light curve) of the novae is important both for understanding the physical conditions and processes leading to such explosions and for calibration of these objects as indicators of distances in the Universe.

The nearby galaxy M31 gives an excellent opportunity for novae surveys. More than 800 novae (Pietsch et al. 2007) are known up to now as a considerable number of them are discovered in the last few years due to the growing number of searching teams.

In 2004, under the leadership of Assoc. Prof. Petko Nedialkov from the Department of "Astronomy" of the Faculty of Physics at University of Sofia "St. Kliment Ohridski", a group for novae searching in M31 galaxy was formed. The team involves professors, PhD and undergraduate students from the Department of Astronomy as well as scientists from the Institute of Astronomy at Bulgarian Academy of Sciences. Observations of M31 galaxy are carried out with the 2m RCC and 50/70cm Schmidt telescope at the National Astronomical Observatory Rozhen and 60cm Cassegrain telescope at the Astronomical Observatory Belogradchik.

By the end of 2009, the group discovered 14 novae, which represents ~ 15% of all discovered novae during these years. One of them, M31N2004-09b, is the closest to the center of M31 nova ever discovered. The large field of view (72x72 arcmin²) of 50/70cm Schmidt telescope, equipped with the CCD camera SBIG STL-11000M, enable simultaneous detection of two novae in the field of M31 in a single image - M31N2007-11f and M31N2007-11g. After spectroscopic observations, M31N2007-11g was rejected as a nova and was classified as a long-period variable of Mira.