227 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
Novae In External Galaxies From The POINT-AGAPE Survey And The Liverpool Telescope
We have recently begun a search for Classical Novae in M31 using three years
of multicolour data taken by the POINT-AGAPE microlensing collaboration with
the 2.5m Isaac Newton Telescope (INT) on La Palma. This is a pilot program
leading to the use of the Liverpool Telescope (LT) to systematically search for
and follow novae of all speed classes in external galaxies to distances up to
around 5Mpc.Comment: 5 pages, 4 figures, To appear in the Proceedings of the Conference
``Classical Nova Explosions'', M. Hernanz and J. Jose eds., American Inst. of
Physics, 200
A remarkable recurrent nova in M 31: The predicted 2014 outburst in X-rays with Swift
The M 31 nova M31N 2008-12a was recently found to be a recurrent nova (RN)
with a recurrence time of about 1 year. This is by far the fastest recurrence
time scale of any known RNe. Our optical monitoring programme detected the
predicted 2014 outburst of M31N 2008-12a in early October. We immediately
initiated an X-ray/UV monitoring campaign with Swift to study the
multiwavelength evolution of the outburst. We monitored M31N 2008-12a with
daily Swift observations for 20 days after discovery, covering the entire
supersoft X-ray source (SSS) phase. We detected SSS emission around day six
after outburst. The SSS state lasted for approximately two weeks until about
day 19. M31N 2008-12a was a bright X-ray source with a high blackbody
temperature. The X-ray properties of this outburst were very similar to the
2013 eruption. Combined X-ray spectra show a fast rise and decline of the
effective blackbody temperature. The short-term X-ray light curve showed
strong, aperiodic variability which decreased significantly after about day 14.
Overall, the X-ray properties of M31N 2008-12a are consistent with the average
population properties of M 31 novae. The optical and X-ray light curves can be
scaled uniformly to show similar time scales as those of the Galactic RNe U Sco
or RS Oph. The SSS evolution time scales and effective temperatures are
consistent with a high-mass WD. We predict the next outburst of M31N 2008-12a
to occur in autumn 2015.Comment: 13 pages, 7 figures, 3 tables; accepted for publication in A&
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