1,721,180 research outputs found
Gravitational lensing by the supermassive black hole in the center of M31
No full textWe examine the possibility of observing gravitational lensing in the weak deflection regime by the supermassive black hole in the center of the galaxy M31. This black hole is significantly more massive than the black hole in the center of our Galaxy, qualifying itself as a more effective lens. However, it is also more distant, and the candidate stellar sources appear consequently fainter. We separately consider as potential sources stars belonging to the bulge, to the disk, and to the triple nucleus formed by P1 + P2 and by the recently discovered inner cluster P3. We calculate the number of simultaneously lensed stars at a given time as a function of the threshold magnitude required for the secondary image. For observations in the K band we find 1.4 expected stars having secondary images brighter than K = 24 and 182 brighter than K = 30. For observations in the V band we expect 1.3 secondary images brighter than V = 27 and 271 brighter than V = 33. The bulge stars have the highest chance of being lensed by the supermassive black hole, whereas the disk and the composite nucleus stars contribute 10% each. The typical angular separation of the secondary images from the black hole range from 1 mas to 0.1 ''. For each population we also show the distribution of the lensed sources as a function of their distance and absolute magnitude, the expected angular positions and velocities of the generated secondary images, and the rate and the typical duration of the lensing events
Microlensing towards the LMC: self lensing for OGLE-II and OGLE-III
No full textWe present an analysis of the results of the OGLE-III microlensing campaign towards the Large Magellanic Cloud (LMC). We evaluate for all the possible lens populations along the line of sight the expected microlensing quantities, number of events and duration. In particular we consider lensing by massive compact halo objects (MACHOs) in the dark matter haloes of both the Milky Way (MW) and the LMC, and "self lensing" by stars in the LMC bar and disc, in the MW disc and in the stellar haloes of both the LMC and the MW. As a result we find that the self-lensing signal is able to explain the 2 OGLE-III microlensing candidates. In particular, we estimate the expected MW disc signal to be almost as large as that from LMC stars and able, by itself, to explain the observed rate. We evaluate a 95% CL upper limit for f, the halo mass fraction in form of MACHOs, in the range 10-20% for (0.01-0.5) Msun and f=24%$ for 1 solar mass MACHOs (and below 10% in this full range, and in particular below 5% for (0.01-0.1) Msun) for the Bright (All) samples of source stars. Furthermore, we find that these limits do not rise much even if we assume the observed events \emph{are} MACHOs. For the All sample we also evaluate a rather significant constraint on f for larger values of the MACHO mass, in particular about f=50% (95% CL) for 100 Msun, to date the stronger bound coming from microlensing analyses in this mass range. Finally, we discuss these results in the framework of the previous observational campaigns towards the LMC, that of the MACHO and the EROS collaborations, and we present a joint analysis of the OGLE-II and the OGLE-III campaigns
LMC self lensing for OGLE-II microlensing observations
No full textIn the framework of microlensing searches towards the Large Magellanic Cloud (LMC), we discuss the results presented by the Optical Gravitational Lensing Experiment (OGLE) collaboration for their OGLE-II campaign (Wyrzykowski et al). We evaluate the optical depth, the duration and the expected rate of events for the different possible lens populations: both luminous, dominated by the LMC self-lensing, and `dark', the would be compact halo objects (massive compact halo objects) belonging to either the Galactic or the LMC halo. The OGLE-II observational results, two microlensing candidate events located in the LMC bar region with duration of 24.2 and 57.2 days, compare well with the expected signal from the luminous lens populations: n<SUB>exp</SUB> = 1.5, with typical duration, for LMC self-lensing, of about 50 days. Because of the small statistics at disposal, however, the conclusions that can be drawn as for the halo mass fraction, f, in the form of compact halo objects are not too severe. By means of a likelihood analysis we find an upper limit for f, at 95 per cent confidence level, of about 15 per cent in the mass range (10<SUP>-2</SUP>-10<SUP>-1</SUP>)M<SUB>solar</SUB> and 26 per cent for 0.5M<SUB>solar</SUB>
The M31 pixel lensing plan campaign : macho lensing and self-lensing signals
Full text linkA.G. was supported by NSF grant AST-1103471. M.D. is thankful to Qatar National Research Fund (QNRF), member of Qatar Foundation, for support by grant NPRP 09-476-1-078. P.J. acknowledges support by the Swiss National Science Foundation.We present the final analysis of the observational campaign carried out by the PLAN (Pixel Lensing Andromeda) collaboration to detect a dark matter signal in form of MACHOs through the microlensing effect. The campaign consists of about 1 month/year observations carried out over 4 years (2007-2010) at the 1.5 m Cassini telescope in Loiano (Astronomical Observatory of BOLOGNA, OAB) plus 10 days of data taken in 2010 at the 2 m Himalayan Chandra Telescope monitoring the central part of M31 (two fields of about 13′ × 12.′6). We establish a fully automated pipeline for the search and the characterization of microlensing flux variations. As a result, we detect three microlensing candidates. We evaluate the expected signal through a full Monte Carlo simulation of the experiment completed by an analysis of the detection efficiency of our pipeline. We consider both "self lensing" and "MACHO lensing" lens populations, given by M31 stars and dark matter halo MACHOs, in M31 and the Milky Way, respectively. The total number of events is consistent with the expected self-lensing rate. Specifically, we evaluate an expected signal of about two self-lensing events. As for MACHO lensing, for full 0.5(10-2) M☉ MACHO halos, our prediction is for about four (seven) events. The comparatively small number of expected MACHO versus self-lensing events, together with the small number statistics at our disposal, do not enable us to put strong constraints on that population. Rather, the hypothesis, suggested by a previous analysis, on the MACHO nature of OAB-07-N2, one of the microlensing candidates, translates into a sizeable lower limit for the halo mass fraction in form of the would-be MACHO population, f, of about 15% for 0.5 M☉ MACHOs.Peer reviewe
Pixel lensing as a way to detect extrasolar planets in M31
No full textWe study the possibility to detect extrasolar planets in M31 through pixel-lensing observations. Using a Monte Carlo approach, we select the physical parameters of the binary lens system, a star hosting a planet, and we calculate the pixel-lensing light curve taking into account the finite source effects. Indeed, their inclusion is crucial since the sources in M31 microlensing events are mainly giant stars. Light curves with detectable planetary features are selected by looking for significant deviations from the corresponding Paczyński shapes. We find that the time-scale of planetary deviations in light curves increase (up to 3-4d) as the source size increases. This means that only few exposures per day, depending also on the required accuracy, may be sufficient to reveal in the light curve a planetary companion. Although the mean planet mass for the selected events is about , even small mass planets (MP < 20M⊕) can cause significant deviations, at least in the observations with large telescopes. However, even in the former case, the probability to find detectable planetary features in pixel-lensing light curves is at most a few per cent of the detectable events, and therefore many events have to be collected in order to detect an extrasolar planet in M31. Our analysis also supports the claim that the anomaly found in the candidate event PA-99-N2 towards M31 can be explained by a companion object orbiting the lens star
Endometriosi, immagine corporea e rischio di sviluppare disturbi dell’alimentazione: confronto tra un gruppo di donne con endometriosi e un gruppo di donne senza la patologia
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