LGRBs are associated with massive stars and are therefore linked to star formation. The conditions necessary to produce LGRBs can affect the relation between the LGRB rate and star formation. By using the power of a complete LGRB sample, our aim is t
o understand whether such a bias exists and, if it does, what is its origin. In this first paper, we build the SED of the z<1 host galaxies of the BAT6 LGRB sample, and determine their stellar masses from SED fitting. We compare the resulting stellar mass distribution (i) with star-forming galaxies observed in deep surveys (UltraVISTA); (ii) with semi-analitical models of the z<1 star forming galaxy population and (iii) with numerical simulations of LGRB hosts having different metallicity thresholds for the progenitor star environment. We find that at z<1 LGRBs tend to avoid massive galaxies and are powerful in selecting faint low-mass star-forming galaxies. The stellar mass distribution of the hosts is not consistent with that of the UltraVISTA star-forming galaxies weighted for their SFR. This implies that, at least at z<1, LGRBs are not unbiased tracers of star formation. To make the two distributions consistent, a much steeper faint-end of the mass function would be required, or a very shallow SFR-Mass relation for the low mass galaxy population. GRB host galaxy simulations indicates that, to reproduce the stellar mass distribution, a metallicity threshold of the order of Z_th=0.3-0.5Z_sun is necessary. Models without a metallicity threshold or with an extreme threshold of Z_th = 0.1Z_sun are excluded at z<1. The use of the BAT6 complete sample makes this result not affected by possible biases which could have influenced past results based on incomplete samples. The preference for low metallicities (Z<~0.5Z_sun) can be a consequence of the particular conditions needed for the progenitor star to produce a GRB. (Abridged)
Gamma-ray burst are thought to be produced by highly relativistic outflows. Although upper and lower limits for the outflow initial Lorentz factor $Gamma_0$ are available, observational efforts to derive a direct determination of $Gamma_0$ have so fa
r failed or provided ambiguous results. As a matter of fact, the shape of the early-time afterglow light curve is strongly sensitive on $Gamma_0$ which determines the time of the afterglow peak, i.e. when the outflow and the shocked circumburst material share a comparable amount of energy. We now comment early-time observations of the near-infrared afterglows of GRB 060418 and GRB 060607A performed by the REM robotic telescope. For both events, the afterglow peak was singled out and allowed us to determine the initial fireball Lorentz, $Gamma_0sim 400$.
We present prompt gamma-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt gamma-ray pulse as well as for the early and la
te multi-band afterglow of GRB 070311. We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays. The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the gamma-ray burst and possibly accompanied by a faint gamma-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3x10^4 and 2x10^5 s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt gamma-ray pulse and an underlying power law. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock (abridged).
