Dust Echos from Gamma Ray Bursts

The deviation from the power-law decline of the optical flux observed in GRB 970228 and GRB 980326 has been used recently to argue in favor of the connection between GRBs and supernovae. We consider an alternative explanation for this phenomenon, based on the scattering of a prompt optical burst by 0.1 solar masses of dust located beyond its sublimation radius 0.1-1 pc from the burst. In both cases, the optical energy observed at the time of the first detection of the afterglow suffices to produce an echo after 20-30 days, as observed. Prompt optical monitoring of future bursts and multiband photometry of the afterglows will enable quantitative tests of simple models of dust reprocessing and a prediction of the source redshift.Comment: 4 pages including 3 postscript figures, LaTeX (emulateapj.sty; newapa.sty for bibliography definitions); accepted for publication in ApJ

Polarization in the prompt emission of gamma-ray bursts and their afterglows

Synchrotron is considered the dominant emission mechanism in the production of gamma-ray burst photons in the prompt as well as in the afterglow phase. Polarization is a characteristic feature of synchrotron and its study can reveal a wealth of information on the properties of the magnetic field and of the energy distribution in gamma-ray burst jets. In this paper I will review the theory and observations of gamma-ray bursts polarization. While the theory is well established, observations have prove difficult to perform, due to the weakness of the signal. The discriminating power of polarization observations, however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of Physics focus issue on Gamma Ray Burst

X-Shooter spectroscopy of FU Tau A

We have analyzed a broad-band optical and near-infrared spectrum of FU Tau A, a presumed young brown dwarf in the Taurus star forming region that has intrigued both theorists and observers by its over-luminosity in the HR diagram with respect to standard pre-main sequence evolutionary models. The new data, obtained with the X-Shooter spectrograph at the Very Large Telescope, include an unprecedented wealth of information on stellar parameters and simultaneously observed accretion and outflow indicators for FU Tau A. We present the first measurements of gravity (log g = 3.5 +- 0.5), radial velocity (RV = 22.5 +- 2.9 km/s), rotational velocity (v sin(i) = 20 +- 5 km/s) and lithium equivalent width (W_Li = 430 +- 20 mAA) for FUTau A. From the rotational velocity and the published period we infer a disk inclination of i ~ 50^deg. The lithium content is much lower than theoretically expected for such a young very low mass object, adding another puzzling feature to this object's properties. We determine the mass accretion rate of FU Tau A from comparison of the luminosities of 24 emission lines to empirical calibrations from the literature and find a mean of log (dM/dt)_acc [M_sun/yr] = -9.9 +- 0.2. The accretion rate determined independently from modeling of the excess emission in the Balmer and Paschen continua is consistent with this value. The corresponding accretion luminosity is too small to make a significant contribution to the bolometric luminosity. The existence of an outflow in FU Tau A is demonstrated through the first detection of forbidden emission lines from which we obtain an estimate for the mass loss rate, log (dM/dt)_out [M_sun/yr] < -10.4. The mass outflow and inflow rates can be combined to yield (dM/dt)_out / (dM/dt)_acc ~ 0.3, a value that is in agreement with jet launching models.Comment: 12 pages, accepted for publication in A&

Estimates of Active Region Area Coverage through Simultaneous Measurements of He I λλ\lambda\lambda 5876 and 10830 Lines

Simultaneous, high-quality measurements of the neutral helium triplet features at 5876~\AA\ and 10830~\AA, respectively, in a sample of solar-type stars are presented. The observations were made with ESO telescopes at the La Silla Paranal Observatory under program ID 088.D-0028(A) and MPG Utility Run for FEROS 088.A-9029(A). The equivalent widths of these features combined with chromospheric models are utilized to infer the fractional area coverage, or filling factor, of magnetic regions outside of spots. We find that the majority of the sample is characterized by filling factors less than unity. However, discrepancies occur among the coolest K-type and warmest and most rapidly rotating F-type dwarf stars. We discuss these apparently anomalous results and find that in the case of K-type stars they are an artifact of the application of chromospheric models best suited to the Sun than to stars with significantly lower $T_\mathrm{eff}$. The case of the F-type rapid rotators can be explained with the measurement uncertainties of the equivalent widths, but they may also be due to a non-magnetic heating component in their atmospheres. With the exceptions noted above, preliminary results suggest that the average heating rates in the active regions are the same from one star to the other, differing in the spatially integrated, observed level of activity due to the area coverage. Hence, differences in activity in this sample are mainly due to the filling factor of active regions.Comment: Accepted for publication in The Astrophysical Journa

Optical and near-infrared photometric monitoring of the transient X-ray binary A0538-66 with REM

The transient Be/X-ray binary A0538-66 shows peculiar X-ray and optical variability. Despite numerous studies, the intrinsic properties underlying its anomalous behaviour remain poorly understood. Since 2014 September we are conducting the first quasi-simultaneous optical and near-infrared photometric monitoring of A0538-66 in seven filters with the Rapid Eye Mount (REM) telescope, aiming to understand the properties of this binary system. We found that the REM lightcurves show fast flares lasting one or two days that repeat almost regularly every ~16.6 days, the orbital period of the neutron star. If the optical flares are powered by X-ray outbursts through photon reprocessing, the REM lightcurves indicate that A0538-66 is still active in X-rays: bright X-ray flares (L_x > 1E37 erg/s) could be observable during the periastron passages. The REM lightcurves show a long-term variability that is especially pronounced in the g band and decreases with increasing wavelength, until it no longer appears in the near-infrared lightcurves. In addition, A0538-66 is fainter with respect to previous optical observations most likely due to the higher absorption of the stellar radiation of a denser circumstellar disc. On the basis of the current models, we interpret these observational results with a circumstellar disc around the Be star observed nearly edge-on during a partial depletion phase. The REM lightcurves also show short-term variability on timescales of ~1 day possibly indicative of perturbations in the density distribution of the circumstellar disc caused by the tidal interaction with the neutron star.Comment: Accepted for publication in Astronomy & Astrophysic

The Chamaeleon II low-mass star-forming region: radial velocities, elemental abundances, and accretion properties

Radial velocities, elemental abundances, and accretion properties of members of star-forming regions (SFRs) are important for understanding star and planet formation. While infrared observations reveal the evolutionary status of the disk, optical spectroscopy is fundamental to acquire information on the properties of the central star and on the accretion characteristics. 2MASS archive data and the Spitzer c2d survey of the Chamaeleon II dark cloud have provided disk properties of a large number of young stars. We complement these data with spectroscopy with the aim of providing physical stellar parameters and accretion properties. We use FLAMES/UVES+GIRAFFE observations of 40 members of Cha II to measure radial velocities through cross-correlation technique, Li abundances by means of curves of growth, and for a suitable star elemental abundances of Fe, Al, Si, Ca, Ti, and Ni using the code MOOG. From the equivalent widths of the Halpha, Hbeta, and the HeI-5876, 6678, 7065 Angstrom emission lines, we estimate the mass accretion rates, dMacc/dt, for all the objects. We derive a radial velocity distribution for the Cha II stars (=11.4+-2.0 km/s). We find dMacc/dt prop. to Mstar^1.3 and to Age^(-0.82) in the 0.1-1.0 Msun mass regime, and a mean dMacc/dt for Cha II of ~7*10^(-10) Msun/yr. We also establish a relationship between the HeI-7065 Angstrom line emission and the accretion luminosity. The radial velocity distributions of stars and gas in Cha II are consistent. The spread in dMacc/dt at a given stellar mass is about one order of magnitude and can not be ascribed entirely to short timescale variability. Analyzing the relation between dMacc/dt and the colors in Spitzer and 2MASS bands, we find indications that the inner disk changes from optically thick to optically thin at dMacc/dt~10^(-10) Msun/yr. Finally, the disk fraction is consistent with the age of Cha II.Comment: 21 Pages, 15 Figures, 7 Tables. Accepted for publication in Astronomy and Astrophysics. Abstract shortene