Optical and near-infrared observations of the microquasar V4641 Sagittarii during the 1999 September outburst

We present photometric and spectroscopic optical and near-infrared (NIR) observations (Based on observations collected at the European Southern Observatory, Chile (ESO ID 63.H-0493 and 64.H-0382)) taken during the outburst of the microquasar V4641 Sgr = SAX J1819.3-2525 (in'tzand et al., 2000) in September 1999. We observed an increase in the J-Ks colour between 5 and 8 days after the outburst, which we interpret as likely evidence for the presence of dust around the source. We also observed an extraordinarily strong, broad and variable H_alpha line, with a velocity width of 4560 km/s suggesting the presence of a high-velocity outflow component. We constrain the distance of the system between 3 and 8 kpc, locating it further away than previously derived from radio observations (Hjellming et al., 2000), but consistent with Orosz et al. (2001). We then discuss the nature of this system, showing that the companion star is either a B3-A2 main sequence star, or a B3-A2 sub-giant crossing the Hertzsprung gap. The system is therefore an Intermediate or High Mass X-ray Binary System (IMXB or HMXB). The distance derived by these optical/NIR observations implies that the jets observed by Hjellming et al. (2000) would then exhibit apparent velocities of ~ 10 c. We finally discuss the possibility of an interaction between the jets and surroundings of the source, and also of this source being a ``microblazar''.Comment: 7 pages, 6 figures, accepted for publication in MNRA

Fitting the Gamma-Ray Spectrum from Dark Matter with DMFIT: GLAST and the Galactic Center Region

We study the potential of GLAST to unveil particle dark matter properties with gamma-ray observations of the Galactic center region. We present full GLAST simulations including all gamma-ray sources known to date in a region of 4 degrees around the Galactic center, in addition to the diffuse gamma-ray background and to the dark matter signal. We introduce DMFIT, a tool that allows one to fit gamma-ray emission from pair-annihilation of generic particle dark matter models and to extract information on the mass, normalization and annihilation branching ratios into Standard Model final states. We assess the impact and systematic effects of background modeling and theoretical priors on the reconstruction of dark matter particle properties. Our detailed simulations demonstrate that for some well motivated supersymmetric dark matter setups with one year of GLAST data it will be possible not only to significantly detect a dark matter signal over background, but also to estimate the dark matter mass and its dominant pair-annihilation mode.Comment: 37 pages, 16 figures, submitted to JCA