270 research outputs found
A remarkable recurrent nova in M 31: The 2010 eruption recovered and evidence of a six-month period
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 year, 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. We detect the 2010 eruption in a pair of images at 2010
Nov 20.52 UT, with a magnitude of m_R = 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 have in
fact a recurrence period of ~6 months (175 +/- 11 days), making it even more
exceptional. If this is the case, then we predict that soon two eruptions per
year will be observable. Furthermore, we predict the next eruption will occur
around late Sep 2015. We encourage additional observations.Comment: 4 pages, 3 figures, 2 tables; submitted to A&A Letter
Difference image photometry with bright variable backgrounds
Over the last two decades the Andromeda Galaxy (M31) has been something of a
test-bed for methods aimed at obtaining accurate time-domain relative
photometry within highly crowded fields. Difference imaging methods, originally
pioneered towards M31, have evolved into sophisticated methods, such as the
Optimal Image Subtraction (OIS) method of Alard & Lupton (1998), that today are
most widely used to survey variable stars, transients and microlensing events
in our own Galaxy. We show that modern difference image (DIA) algorithms such
as OIS, whilst spectacularly successful towards the Milky Way bulge, may
perform badly towards high surface brightness targets such as the M31 bulge.
Poor results can occur in the presence of common systematics which add spurious
flux contributions to images, such as internal reflections, scattered light or
fringing. Using data from the Angstrom Project microlensing survey of the M31
bulge, we show that very good results are usually obtainable by first
performing careful photometric alignment prior to using OIS to perform
point-spread function (PSF) matching. This separation of background matching
and PSF matching, a common feature of earlier M31 photometry techniques, allows
us to take full advantage of the powerful PSF matching flexibility offered by
OIS towards high surface brightness targets. We find that difference images
produced this way have noise distributions close to Gaussian, showing
significant improvement upon results achieved using OIS alone. We show that
with this correction light-curves of variable stars and transients can be
recovered to within ~10 arcseconds of the M31 nucleus. Our method is simple to
implement and is quick enough to be incorporated within real-time DIA
pipelines. (Abridged)Comment: 12 pages. Accepted for publication in MNRAS. Includes an expanded
discussion of DIA testing and results, including additional lightcurve
example
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