323,530 research outputs found
The M31 pixel lensing plan campaign : macho lensing and self-lensing signals
No full textA.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
Le opere minori di Dante e i grandi maestri (Sabbadini e Novati)
No full textL'articolo analizza il contributo di Remigio Sabbadini e di Francesco Novati sul testo delle così dette opere minori di Dante (Egloghe ed Epistole). In una serie di lavori (e, per quanto riguarda Novati, di appunti inediti) Sabbadini e Novati hanno lasciato preziose considerazioni su queste opere di Dante, con una serie di proposte testuali ed esegetiche che ancora oggi hanno la loro validità. In particolare le carte inedite di Novati, che era stato incaricato di curare un'edizione critica delle lettere dell'Alighieri, offrono preziose indicazioni sulle possibili fonti adoperate da Dante
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
Some transpose-free CG-like solvers for nonsymmetric ill-posed problems
No full textThis paper introduces and analyzes an original class of Krylov subspace methods that provide an efficient alternative to many well-known conjugate-gradient-like (CG-like) Krylov solvers for square nonsymmetric linear systems arising from discretizations of inverse ill-posed problems. The main idea underlying the new methods is to consider some rank-deficient approximations of the transpose of the system matrix, obtained by running the (transpose-free) Arnoldi algorithm, and then apply some Krylov solvers to a formally right-preconditioned system of equations. Theoretical insight is given, and many numerical tests show that the new solvers outperform classical Arnoldi-based or CG-like methods in a variety of situations
- …
