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Henze M.,Max Planck Institute for Extraterrestrial Physics | Pietsch W.,Max Planck Institute for Extraterrestrial Physics | Haberl F.,Max Planck Institute for Extraterrestrial Physics | Della Valle M.,National institute for astrophysics | And 21 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Classical novae (CNe) represent the main class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. Only three confirmed novae and three SSSs have been discovered in globular clusters (GCs) of any galaxy so far, of which one nova and two SSSs (including the nova) were found in M 31 GCs. Aims. To study the SSS state of CNe we carried out a high-cadence X-ray monitoring of the M 31 central area with XMM-Newton and Chandra. This project is supplemented by regular optical monitoring programmes at various observatories. Methods. We analysed X-ray and optical monitoring data of a new transient X-ray source in the M 31 GC Bol 126, discovered serendipitously in Swift observations. Our optical data set was based on regular M 31 monitoring programmes from five different small telescopes and was reduced using a homogeneous method. Additionally, we made use of Pan-STARRS 1 data obtained during the PAndromeda survey. We extracted light curves of the source in the optical and X-rays, as well as X-ray spectra. Results. Our observations reveal that the X-ray source in Bol 126 is the third SSS in an M 31 GC and can be confirmed as the second CN in the M 31 GC system. This nova is named M31N 2010-10f. Its properties in the X-ray (high black-body temperature, short SSS phase) and optical (relatively high maximum magnitude, fast decline) regimes agree with a massive white dwarf (MWD ≳ 1.3 M⊙) in the binary system. Incorporating the data on previously found (suspected) novae in M 31 GCs we used our high-cadence X-ray monitoring observations to estimate a tentative nova rate in the M 31 GC system of 0.05 yr-1 GC-1. An optical estimate, based on the recent 10.5-year WeCAPP survey, gives a lower nova rate, which is compatible with the X-ray rate on the 95% confidence level. Conclusions. Although still based on small-number statistics, there is growing evidence that the nova rate in GCs is higher than expected from primordial binary formation and under conditions as in the field. Dynamical binary formation and/or additional accretion from the intracluster medium are possible scenarios for an increased nova rate, but observational confirmation for this enhancement has been absent, so far. Regular X-ray monitoring observations of M 31 provide a promising strategy to find these novae. © 2013 ESO. All Rights Reserved.

Henze M.,Institute Of Ciencies Of Lespai Csic Ieec | Darnley M.J.,Liverpool John Moores University | Kabashima F.,Miyaki Argenteus Observatory | Nishiyama K.,Miyaki Argenteus Observatory | And 2 more authors.
Astronomy and Astrophysics | Year: 2015

The Andromeda Galaxy recurrent nova M31N 2008-12a has been caught in eruption nine times. Six observed eruptions in the seven years from 2008 to 2014 suggested a duty cycle of ~1 yr, which makes this the most rapidly recurring system known and the leading single-degenerate Type Ia Supernova progenitor candidate; but no 2010 eruption has been found so far. Here we present evidence supporting the recovery of the 2010 eruption, based on archival images taken at and around the time of eruption. We detect the 2010 eruption in a pair of images at 2010 Nov. 20.52 UT, with a magnitude of mR = 17.84 ± 0.19. The sequence of seven eruptions shows significant indications of a duty cycle slightly shorter than one year, which makes successive eruptions occur progressively earlier in the year. We compared three archival X-ray detections with the well-observed multi-wavelength light curve of the 2014 eruption to accurately constrain the time of their optical peaks. The results imply that M31N 2008-12a might actually have a recurrence period of ~6 months (175 ± 11 days), making it even more exceptional. If this is the case, then we predict that two eruptions per year will be observable soon. Furthermore, we predict that the next eruption will occur in late Sep. 2015. We encourage additional observations. © ESO, 2015.

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