S. Calchi Novati

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 rand 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 rand ifilters. 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 rflux is more than five times greater

POINT-AGAPE Pixel Lensing Survey of M31 : Evidence for a MACHO contribution to Galactic Halos

The POINT-AGAPE collaboration is carrying out a search for gravitational microlensing toward M31 to reveal galactic dark matter in the form of MACHOs (Massive Astrophysical Compact Halo Objects) in the halos of the Milky Way and M31. A high-threshold analysis of 3 years of data yields 6 bright, short-duration microlensing events, which are confronted to a simulation of the observations and the analysis. The observed signal is much larger than expected from self lensing alone and we conclude, at the 95% confidence level, that at least 20% of the halo mass in the direction of M31 must be in the form of MACHOs if their average mass lies in the range 0.5-1 M⊙. This lower bound drops to 8% for MACHOs with masses � 0.01 M⊙. In addition, we discuss a likely binary microlensing candidate with caustic crossing. Its location, some 32 away from the centre of M31, supports our conclusion that we are detecting a MACHO signal in the direction of M31.

Classical novae from the POINT–AGAPE microlensing survey of M31 – I. The nova catalogue

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. Here we describe the automated detection and selection pipeline used to identify M31 classical novae (CNe) and we present the resulting catalogue of 20 CN candidates observed over three seasons. CNe are observed both in the bulge region as well as over a wide area of the M31 disc. Nine of the CNe are caught during the final rise phase and all are well sampled in at least two colours. The excellent light-curve coverage has allowed us to detect and classify CNe over a wide range Qf speed class, from very fast to very slow. Among the light curves is a moderately fast CN exhibiting entry into a deep transition minimum, followed by its final decline. We have also observed in detail a very slow CN which faded by only 0.01 mag d -1 over a 150-d period. We detect other interesting variable objects, including one of the longest period and most luminous Mira variables. The CN catalogue constitutes a uniquely well-sampled and objectively-selected data set with which to study the statistical properties of CNe in M31, such as the global nova rate, the reliability of novae as standard-candle distance indicators and the dependence of the nova population on stellar environment. The findings of this statistical study will be reported in a follow-up 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 sixty nights during the six months that M31 is visible. The two 33 ′ ×33 ′ fields of view straddle the central bulge, northwards and sou thwards. We have calculated the locations, periods and brightness of 35414 variable stars i n M31 as a by-product of the microlensing search. The variables are classified according t o their period and brightness. Rough correspondences with classical types of variable star (suc h as population I and II Cepheids, Miras and semi-regular long-period variables) are established. The spatial distribution of population I Cepheids is clearly associated with the spiral arm s, while the central concentration of the Miras and long-period variables varies noticeably, t he 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 fiel ds as measured against those in the northern or nearer fields. It was initially assumed that t he 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 distributi ons are asymmetric primarily because of the effects of differential extinction associated with the dust lanes. The siz e 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 completi on of the microlensing experiments towards M31 are discussed.

The Anomaly in the Candidate Microlensing Event PA-99-N2

The light curve of PA-99-N2, one of the recently announced microlensing candidates toward M31, shows small deviations from the standard Paczynski form. We explore a number of possible explanations, including correlations with the seeing, the parallax effect, and a binary lens. We find that the observations are consistent with an unresolved red giant branch or asymptotic giant branch star in M31 being microlensed by a binary lens. We find that the best-fit binary lens mass ratio is ~1.2 × 10-2, which is one of the most extreme values found for a binary lens so far. If both the source and lens lie in the M31 disk, then the standard M31 model predicts the probable mass range of the system to be 0.02-3.6 ☉ (95% confidence limit). In this scenario, the mass of the secondary component is therefore likely to be below the hydrogen-burning limit. On the other hand, if a compact halo object in M31 is lensing a disk or spheroid source, then the total lens mass is likely to lie between 0.09 and 32 ☉, which is consistent with the primary being a stellar remnant and the secondary being a low-mass star or brown dwarf. The optical depth (or, alternatively, the differential rate) along the line of sight toward the event indicates that a halo lens is more likely than a stellar lens, provided that dark compact objects comprise no less than 15% (or 5%) of halos.

LMC self-lensing from a new perspective

We present a new analysis of the location of the observed microlensing events in the direction of the Large Magellanic Cloud (LMC). This is carried out starting from a recently drawn coherent picture of the geometrical structure and dynamics of the LMC disk and by considering different configurations for the LMC bar. In this framework it clearly emerges that the spatial distribution of the events observed so far shows a near-far asymmetry. This turns out to be compatible with the optical depth calculated for the LMC halo objects. Our main conclusion, supported by a statistical analysis of the outcome of an evaluation of the microlensing rate, is that self-lensing cannot account for all the observed events. Finally we propose a general inequality to calculate quickly an upper limit to the optical depth along a line of view through the LMC center.

Microlensing towards M 31 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 M 31 halo using data collected during the 1998-1999 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 that is 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.

Monte Carlo analysis of MEGA microlensing events towards M 31

We perform an analytical study and a Monte Carlo (MC) analysis of the main features for microlensing events in pixel lensing observations towards M31. Our main aim is to investigate the lens nature and location of the 14 candidate events found by the MEGA collaboration. Assuming a reference model for the mass distribution in M31 and the standard model for our galaxy, we estimate the MACHO-to-self lensing probability and the event time duration towards M31. Reproducing the MEGA observing conditions, as a result we get the MC event number density distribution as a function of the event full-width half-maximum duration t1/2 and the magnitude at maximum Rmax. For a MACHO mass of 0.5 M⊙ we find typical values of t1/2 ≃ 20 day and Rmax ≃ 22, for both MACHO-lensing and self-lensing events occurring beyond about 10 arcminutes from the M31 center. A comparison of the observed features (t1/2 and Rmax) with our MC results shows that for a MACHO mass > 0.1 M⊙ the four innermost MEGA events are most likely self-lensing events, whereas the six outermost events must be genuine MACHO-lensing events.

Microlensing towards the Large Magellanic Cloud : a study of the LMC halo contribution

We carry out a new analysis of the sample of MACHO microlensing candidates towards the LMC. Our main purpose is to determine the lens population to which the events may belong. We give particular emphasis to the possibility of characterising the Milky Way dark matter halo population with respect to the LMC one. Indeed, we show that only a fraction of the events have characteristics that match those expected for lenses belonging to the MACHO population of the Galactic halo. This suggests that this component cannot explain all the candidates. Accordingly, we challenge the view that the dark matter halo fraction of both the Galaxy and the LMC halos are equal, and indeed we show that, for a MACHO mass in the range 0.1–0.3xa0

Microlensing towards the SMC: a new analysis of OGLE and EROS results

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