452 research outputs found
AGAPE, an experiment to detect MACHO's in the direction of the Andromeda galaxy
The status of the Agape experiment to detect Machos in the direction of the
andromeda galaxy is presented.Comment: 4 pages, 1 figure in a separate compressed, tarred, uuencoded uufile.
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Microlensing towards M31 with MDM data
We report the final analysis of a search for microlensing events in the
direction of the Andromeda galaxy, which aimed to probe the MACHO composition
of the M31 halo using data collected during the 1998-99 observational campaign
at the MDM observatory. In a previous paper, we discussed the results from a
first set of observations. Here, we deal with the complete data set, and we
take advantage of some INT observations in the 1999-2000 seasons. This merging
of data sets taken by different instruments turns out to be very useful, the
study of the longer baseline available allowing us to test the uniqueness
characteristic of microlensing events. As a result, all the candidate
microlensing events previously reported turn out to be variable stars. We
further discuss a selection based on different criteria, aimed at the detection
of short--duration events. We find three candidates whose positions are
consistent with self--lensing events, although the available data do not allow
us to conclude unambiguously that they are due to microlensing.Comment: Accepted for publication in Astronomy and Astrophysic
Identification of 13 Cepheids and 333 Other Variables in M31
We present Cousins and band photometry of variable stars in a
region in the disk of M31 galaxy, obtained during 141
nights. Of the 26 Cepheid variables present in the region, two are newly
discovered, 11 are classified as Cepheids for the first time and 13 are
confirmed as Cepheids. The extensive photometry of these Cepheids enabled us to
determine precise phase and amplitude of pulsation which ranges from 0.11 to
0.48 mag in band. The period of variability ranges from 7.5 to 56
days. The period-luminosity diagram is used to derive a distance modulus of
24.490.11 mag for M31 galaxy. We also report variability in 333 other
stars, of them, 115 stars appear to be long period variables, 2 suspected
eclipsing binaries and remaining 216 are irregular variables.Comment: Accepted for publication in A&A, 9 figures, the paper and figures
with better resolution can be downloaded from
http://cdfinfo.in2p3.fr/Downloads/cosmobs/yogesh/paper
The POINT-AGAPE Survey - I. The variable stars in M31
For the purposes of identifying microlensing events, the POINT-AGAPE collaboration has been monitoring the Andromeda galaxy (M31) for three seasons (1999-2001) with the Wide Field Camera on the Isaac Newton Telescope. In each season, data are taken for one hour per night for roughly 60 nights during the six months that M31 is visible. The two 33 × 33 arcmin2 fields of view straddle the central bulge, northwards and southwards. We have calculated the locations, periods and brightness of 35 414 variable stars in M31 as a by-product of the microlensing search. The variables are classified according to their period and brightness. Rough correspondences with classical types of variable star (such as Population I and II Cepheids, Miras and semiregular long-period variables) are established. The spatial distribution of Population I Cepheids is clearly associated with the spiral arms, while the central concentration of the Miras and long-period variables varies noticeably, the brighter and the shorter period Miras being much more centrally concentrated. A crucial role in the microlensing experiment is played by the asymmetry signal - the excess of events expected in the southern or more distant fields as measured against those in the northern or nearer fields. It was initially assumed that the variable star populations in M31 would be symmetric with respect to the major axis, and thus variable stars would not be a serious contaminant for measuring the microlensing asymmetry signal. We demonstrate that this assumption is not correct. All the variable star distributions are asymmetric primarily because of the effects of differential extinction associated with the dust lanes. The size and direction of the asymmetry of the variable stars is measured as a function of period and brightness. The implications of this discovery for the successful completion of the microlensing experiments towards M31 are discusse
Photometric study of two novae in M31
We report light curves of 2 novae in the M31 galaxy which were
detected in the four year Nainital Microlensing Survey. One of these novae has
been tracked from the initial increase in flux while other has been observed
during its descending phase of brightness. The photometry of the first nova
during the outburst phase suggests its peak magnitude to be about 17.2
mag with a flux decline rate of 0.11 mag day which indicates that it was
a fast nova. A month after its outburst, it shows reddening followed by a
plateau in flux. The second nova exhibits a bump in and ,
possibly about three weeks after the outburst.Comment: 6 pages, 7 figs, Accepted to A&A, replaced with revised version Fri,
16 Oct 200
The POINT-AGAPE survey — II. An unrestricted search for microlensing events towards M31
An automated search is carried out for microlensing events using a catalogue of 44 554 variable superpixel light curves derived from our 3-yr monitoring programme of M31. Each step of our candidate selection is objective and reproducible by a computer. Our search is unrestricted, in the sense that it has no explicit time-scale cut. So, it must overcome the awkward problem of distinguishing long time-scale microlensing events from long-period stellar variables. The basis of the selection algorithm is the fitting of the superpixel light curves to two different theoretical models, using variable star and blended microlensing templates. Only if microlensing is preferred is an event retained as a possible candidate. Further cuts are made with regard to: (i) sampling, (ii) goodness of fit of the peak to a Paczyński curve, (iii) consistency of the microlensing hypothesis with the absence of a resolved source, (iv) achromaticity, (v) position in the colour-magnitude diagram and (vi) signal-to-noise ratio. Our results are reported in terms of first-level candidates, which are the most trustworthy, and second-level candidates, which are possible microlensing events but have a lower signal-to-noise ratio and are more questionable. The pipeline leaves just three first-level candidates, all of which have very short full-width at half-maximum time-scales (t1/2 < 5 d) and three second-level candidates, which have time-scales of t1/2 = 31, 36 and 51 d. We also show 16 third-level light curves, as an illustration of the events that just fail the threshold for designation as microlensing candidates. They are almost certainly mainly variable stars. Two of the three first-level candidates correspond to known events (PA 00-S3 and 00-S4) already reported by the POINT-AGAPE project. The remaining first-level candidate is new. This algorithm does not find short time-scale events that are contaminated with flux from nearby variable stars (such as PA 99-N1
The POINT-AGAPE survey II: An Unrestricted Search for Microlensing Events towards M31
An automated search is carried out for microlensing events using a catalogue
of 44554 variable superpixel lightcurves derived from our three-year monitoring
program of M31. Each step of our candidate selection is objective and
reproducible by a computer. Our search is unrestricted, in the sense that it
has no explicit timescale cut. So, it must overcome the awkward problem of
distinguishing long-timescale microlensing events from long-period stellar
variables. The basis of the selection algorithm is the fitting of the
superpixel lightcurves to two different theoretical models, using variable star
and blended microlensing templates. Only if microlensing is preferred is an
event retained as a possible candidate. Further cuts are made with regard to
(i) sampling, (ii) goodness of fit of the peak to a Paczynski curve, (iii)
consistency of the microlensing hypothesis with the absence of a resolved
source, (iv) achromaticity, (v) position in the colour-magnitude diagram and
(vi) signal-to-noise ratio. Our results are reported in terms of first-level
candidates, which are the most trustworthy, and second-level candidates, which
are possible microlensing but have lower signal-to-noise and are more
questionable. The pipeline leaves just 3 first-level candidates, all of which
have very short full-width half-maximum timescale (<5 days) and 3 second-level
candidates, which have timescales of 31, 36 and 51 days respectively. We also
show 16 third-level lightcurves, as an illustration of the events that just
fail the threshold for designation as microlensing candidates. They are almost
certainly mainly variable stars. Two of the 3 first-level candidates correspond
to known events (PA 00-S3 and PA 00-S4) already reported by the POINT-AGAPE
project. The remaining first-level candidate is new.Comment: 22 pages, 18 figures, MNRAS, to appea
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Impact of particles on the Planck HFI detectors: Ground-based measurements and physical interpretation
The Planck High Frequency Instrument (HFI) surveyed the sky continuously from
August 2009 to January 2012. Its noise and sensitivity performance were
excellent, but the rate of cosmic ray impacts on the HFI detectors was
unexpectedly high. Furthermore, collisions of cosmic rays with the focal plane
produced transient signals in the data (glitches) with a wide range of
characteristics. A study of cosmic ray impacts on the HFI detector modules has
been undertaken to categorize and characterize the glitches, to correct the HFI
time-ordered data, and understand the residual effects on Planck maps and data
products. This paper presents an evaluation of the physical origins of glitches
observed by the HFI detectors. In order to better understand the glitches
observed by HFI in flight, several ground-based experiments were conducted with
flight-spare HFI bolometer modules. The experiments were conducted between 2010
and 2013 with HFI test bolometers in different configurations using varying
particles and impact energies. The bolometer modules were exposed to 23 MeV
protons from the Orsay IPN TANDEM accelerator, and to Am and Cm
-particle and Fe radioactive X-ray sources. The calibration data
from the HFI ground-based preflight tests were used to further characterize the
glitches and compare glitch rates with statistical expectations under
laboratory conditions. Test results provide strong evidence that the dominant
family of glitches observed in flight are due to cosmic ray absorption by the
silicon die substrate on which the HFI detectors reside. Glitch energy is
propagated to the thermistor by ballistic phonons, while there is also a
thermal diffusion contribution. The implications of these results for future
satellite missions, especially those in the far-infrared to sub-millimetre and
millimetre regions of the electromagnetic spectrum, are discussed.Comment: 11 pages, 13 figure
Classical novae from the POINT-AGAPE microlensing survey of M31 - II. Rate and statistical characteristics of the nova population
The POINT-AGAPE (Pixel-lensing Observations with the Isaac Newton Telescope-Andromeda Galaxy Amplified Pixels Experiment) survey is an optical search for gravitational microlensing events towards the Andromeda galaxy (M31). As well as microlensing, the survey is sensitive to many different classes of variable stars and transients. In our first paper of this series, we reported the detection of 20 classical novae (CNe) observed in Sloan r′ and i′ passbands. An analysis of the maximum magnitude versus rate of decline (MMRD) relationship in M31 is performed using the resulting POINT-AGAPE CN catalogue. Within the limits of the uncertainties of extinction internal to M31, good fits are produced to the MMRD in two filters. The MMRD calibration is the first to be performed for Sloan r′ and i′ filters. However, we are unable to verify that novae have the same absolute magnitude 15 d after peak (the t15 relationship), nor any similar relationship for either Sloan filter. The subsequent analysis of the automated pipeline has provided us with the most thorough knowledge of the completeness of a CN survey to date. In addition, the large field of view of the survey has permitted us to probe the outburst rate well into the galactic disc, unlike previous CCD imaging surveys. Using this analysis, we are able to probe the CN distribution of M31 and evaluate the global nova rate. Using models of the galactic surface brightness of M31, we show that the observed CN distribution consists of a separate bulge and disc population. We also show that the M31 bulge CN eruption rate per unit r′ flux is more than five times greater than that of the disc. Through a combination of the completeness, M31 surface brightness model and our M31 CN eruption model, we deduce a global M31 CN rate of 65+16−15 yr−1, a value much higher than found by previous surveys. Using the global rate, we derive a M31 bulge rate of 38+15−12 yr−1 and a disc rate of 27+19−15 yr−1. Given our understanding of the completeness and an analysis of other sources of error, we conclude that the true global nova rate of M31 is at least 50 per cent higher than was previously thought and this has consequent implications for the presumed CN rate in the Milky Way. We deduce a Galactic bulge rate of 14+6−5 yr−1, a disc rate of 20+14−11 yr−1 and a global Galactic rate of 34+15−12 yr−1, consistent with the Galactic global rate derived elsewhere by independent method
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