arXiv subjects

M. Nardini

Publications and source records attributed to M. Nardini.

42 records · Page 3Linked to original sources

arXiv2009

The puzzling clustering and bimodality of long GRBs optical afterglow luminosities

The study of the rest frame properties of long Gamma-Ray bursts (GRBs) afterglows is a fundamental aspect for a better understanding of the nature of these powerful explosions. The launch of the Swift satellite (November 2004) marked a strong improvement of the observational capabilities of X-rays and optical afterglows. We studied the intrinsic optical afterglows of a sample of long GRBs finding an unexpected clustering and a hint of bimodality of the optical luminosity distribution (at 12h after the trigger). Through a Montecarlo simulation we proved that both the observed clustering and bimodality are not simply due to selection effects but should hide important information in the understanding of the nature of the afterglow emission. These findings can shed also light on the nature of the large fraction of optically dark GRBs.

astro-ph.HE↗

arXiv2008

A unifying view of Gamma Ray Burst Afterglows

We selected a sample of 33 Gamma Ray Bursts (GRBs) detected by Swift, with known redshift and optical extinction at the host frame. For these, we constructed the de-absorbed and K-corrected X-ray and optical rest frame light curves. These are modelled as the sum of two components: emission from the forward shock due to the interaction of a fireball with the circum-burst medium and an additional component, treated in a completely phenomenological way. The latter can be identified, among other possibilities, as "late prompt" emission produced by a long lived central engine with mechanisms similar to those responsible for the production of the "standard" early prompt radiation. Apart from flares or re-brightenings, that we do not model, we find a good agreement with the data, despite of their complexity and diversity. Although based in part on a phenomenological model with a relatively large number of free parameters, we believe that our findings are a first step towards the construction of a more physical scenario. Our approach allows us to interpret the behaviour of the optical and X-ray afterglows in a coherent way, by a relatively simple scenario. Within this context it is possible to explain why sometimes no jet break is observed; why, even if a jet break is observed, it is often chromatic; why the steepening after the jet break time is often shallower than predicted. Finally, the decay slope of the late prompt emission after the shallow phase is found to be remarkably similar to the time profile expected by the accretion rate of fall-back material (i.e. proportional to t^{-5/3}), suggesting that this can be the reason why the central engine can be active for a long time.

astro-ph↗

arXiv2008

Precursors in Swift Gamma Ray Bursts with redshift

We study a sample of Gamma-Ray Bursts detected by the Swift satellite with known redshift which show a precursor in the Swift-BAT light curve. We analyze the spectra of the precursors and compare them with the time integrated spectra of the prompt emission. We find neither a correlation between the two slopes nor a tendency for the precursors spectra to be systematically harder or softer than the prompt ones. The energetics of the precursors are large: on average, they are just a factor of a few less energetic (in the source rest frame energy range 15-150 keV) than the entire bursts. These properties do not depend upon the quiescent time between the end of the precursor and the start of the main event. These results suggest that what has been called a "precursor" is not a phenomenon distinct from the main event, but is tightly connected with it, even if, in some case, the quiescent time intervals can be longer than 100 seconds.

astro-ph↗

arXiv2008

Optical afterglow luminosities in the Swift epoch: confirming clustering and bimodality

We show that Gamma Ray Bursts (GRBs) of known redshift and rest frame optical extinction detected by the Swift satellite fully confirm earlier results concerning the distribution of the optical afterglow luminosity at 12 hours after trigger (rest frame time). This distribution is bimodal and relatively narrow, especially for the high luminosity branch. This is intriguing, given that Swift GRBs have, on average, a redshift larger than pre-Swift ones, and is unexpected in the common scenario explaining the GRB afterglow. We investigate if the observed distribution can be the result of selection effects affecting a unimodal parent luminosity distribution, and find that either the distribution is intrinsically bimodal, or most (60 per cent) of the bursts are absorbed by a substantial amount of grey dust. In both cases we suggest that most dark bursts should belong to the underluminous optical family.

astro-ph↗

arXiv2006

The unexpected clustering of the optical afterglow luminosities

We studied the behaviour of the optical afterglow lightcurves of a sample of 24 Gamma--Ray Bursts (GRBs) with known redshift and published estimates of the optical extinction in the source frame, detected before the SWIFT satellite launch. We found an unexpected clustering of the optical luminosities at 12 hours in the source frame. The distribution of the optical luminosities is narrower than the distribution of X-ray luminosities at the same time. Few (3) bursts stand apart from the main optical distribution, being fainter by a factor of about 15. We also analysed the optical luminosities of the SWIFT burst with known redshift finding that the luminosity distribution is similar to the pre SWIFT GRBs one, even if they have a different mean redshift. These results can suggest the existence of a family of intrinsically optically under--luminous dark GRBs.

astro-ph↗

arXiv2006

The clustering of the luminosities of optical afterglows of long Gamma Ray Bursts

We studied the optical afterglows of the 24 pre-Swift Gamma-Ray Bursts with known spectroscopic redshift and published estimates of the optical extinction in the source frame. We find an unexpected clustering of the optical afterglow luminosities measured 12 hours (source frame time) after the trigger. For 21 out of 24 bursts, the distribution of the optical luminosities is narrower than the distribution of the X-ray luminosities, and even narrower than the distribution of the ratio between the monochromatic optical luminosities and the total isotropic emitted prompt energy. Three bursts stand apart from the distribution of the other sources, being underluminous by a factor ~15. We compare this result with the somewhat analogous result concerning the luminosity of the X-ray afterglows studied by Gendre & Boer. For all our GRBs we construct the optical to X-ray spectral energy distribution. For all but a minority of them, the optical and the X-ray emissions are consistent with being produced by the same radiation process. We discuss our results in the framework of the "standard" external shock synchrotron model. Finally, we consider the behavior of the first GRBs of known redshifts detected by Swift. We find that these Swift GRBs entirely confirm our findings.

astro-ph↗