1112.4480 (A. Melandri et al.)

The Dark Bursts population in a complete sample of bright Swift Long
Gamma-Ray Bursts    [PDF]

A. Melandri, B. Sbarufatti, P. D'Avanzo, R. Salvaterra, S. Campana, S. Covino, S. D. Vergani, L. Nava, G. Ghisellini, G. Ghirlanda, D. Fugazza, V. Mangano, M. Capalbi, G. Tagliaferri

We study the properties of the population of optically dark events present in
a carefully selected complete sample of bright Swift long gamma-ray bursts. The
high level of completeness in redshift of our sample (52 objects out of 58)
allow us to establish the existence of a genuine dark population and we are
able to estimate the maximum fraction of dark burst events (~30%) expected for
the whole class of long gamma-ray burst. The redshift distribution of this
population of dark bursts is similar to the one of the whole sample.
Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux)
of the sub-class of dark bursts is slightly higher than the average luminosity
of the non-dark events. At the same time the prompt properties do not differ
and the optical flux of dark events is at the lower tail of the optical flux
distribution, corrected for Galactic absorption. All these properties suggest
that dark bursts events generate in much denser environments with respect to
normal bright events. We can therefore exclude the high-z and the low-density
scenarios and conclude that the major cause of the origin of optically dark
events is the dust extinction.

View original: http://arxiv.org/abs/1112.4480