Analysis of Temporal Features of Gamma Ray Bursts in the Internal Shock Model

In a recent paper we have calculated the power density spectrum of Gamma-Ray Bursts arising from multiple shocks in a relativistic wind. The wind optical thickness is one of the factors to which the power spectrum is most sensitive, therefore we have further developed our model by taking into account the photon down-scattering on the cold electrons in the wind. For an almost optically thick wind we identify a combination of ejection features and wind parameters that yield bursts with an average power spectrum in agreement with the observations, and with an efficiency of converting the wind kinetic energy in 50-300 keV emission of order 1%. For the same set of model features the interval time between peaks and pulse fluences have distributions consistent with the log-normal distribution observed in real bursts.Comment: ApJ in press, 2000; with slight revisions; 12 pag, 6 fi

Observational Prospects for Afterglows of Short Duration Gamma-ray Bursts

If the efficiency for producing $\gamma$-rays is the same in short duration (\siml 2 s) Gamma-Ray Bursts (GRBs) as in long duration GRBs, then the average kinetic energy of short GRBs must be $\sim 20$ times less than that of long GRBs. Assuming further that the relativistic shocks in short and long duration GRBs have similar parameters, we show that the afterglows of short GRBs will be on average 10--40 times dimmer than those of long GRBs. We find that the afterglow of a typical short GRB will be below the detection limit (\siml 10 \microJy) of searches at radio frequencies. The afterglow would be difficult to observe also in the optical, where we predict R \simg 23 a few hours after the burst. The radio and optical afterglow would be even fainter if short GRBs occur in a low-density medium, as expected in NS-NS and NS-BH merger models. The best prospects for detecting short-GRB afterglows are with early (\siml 1 day) observations in X-rays.Comment: 5 pages, 2 figures, submitted to ApJ lette

GRB980923. A burst with a short duration high energy component

The prompt emission of Gamma Ray Bursts (GRBs) is usually well described by the Band function: two power-laws joined smoothly at a given break energy. In addition to the Band component, a few bursts (GRB941017, GRB090510, GRB090902B and GRB090926A) show clear evidence for a distinct high-energy spectral component, which in some cases evolves independently from the prompt keV component and is well described by a power-law (PL), sometimes with a cut-off energy; this component is found to have long duration, even longer than the burst itself for all the four bursts. Here we report the observation of an anomalous short duration high energy component in GRB980923. GRB980923 is one of the brightest Gamma-Ray Bursts (GRBs) observed by BATSE. Its light curve is characterized by a rapid variability phase lasting ~ 40 s, followed by a smooth emission tail lasting ~ 400 s. A detailed joint analysis of BATSE (LAD and SD) and EGRET TASC data of GRB980923 reveles the presence of an anomalous keV to MeV component in the spectrum that evolves independently from the prompt keV one. This component is well described by a PL with a spectral index of -1.44 and lasts only ~ 2 s; it represents one of the three clearly separated spectral components identified in GRB980923, the other two being the keV prompt emission, well described by the Band function and the tail, well fit by a Smoothly Broken Power Law (SBPL).Comment: 28 pages, 7 figures, ApJ, accepte

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.Comment: 5 pages 3 figures, minor revision, added reference, accepted for publication in MNRAS Letter

The puzzling temporally variable optical and X-ray afterglow of GRB 101024A

Aim: To present the optical observations of the afterglow of GRB 101024A and to try to reconcile these observations with the X-ray afterglow data of GRB 101024A using current afterglow models Method: We employ early optical observations using the Zadko Telescope combined with X-ray data and compare with the reverse shock/forward shock model. Results: The early optical light curve reveals a very unusual steep decay index of alpha~5. This is followed by a flattening and possibly a plateau phase coincident with a similar feature in the X-ray. We discuss these observations in the framework of the standard reverse shock/forward shock model and energy injection.We note that the plateau phase might also be the signature of the formation of a new magnetar.Comment: 5 pages, 2 figures. Accepted for publication in Astronomy and Astrophysic

Fundamental Physical Parameters of Collimated Gamma-Ray Burst Afterglows

We determine the basic physical characteristics of eight Gamma-Ray Bursts -- 980519, 990123, 990510, 991028, 991216, 000301c, 000926 and 010222 -- by modelling the broadband emission of their afterglows. We find that the burst kinetic energies after the GRB phase are well clustered around a mean value of 3E50 ergs. In contrast, the energy release in gamma-rays, after correcting for collimated explosion, varies among bursts by more than an order of magnitude. The jet initial apertures are the 2-14 degree range, mildly correlated with the energy, half of the jets being narrower than ~3 deg. This implies that, within 100 Mpc, there are about 10 GRB remnants (expanding at ~0.1c) which can be resolved with VLBA. For all eight afterglows the total energy in the shock-accelerated electrons is close to equipartition with protons. However the slope of the power-law electron distribution is not universal, varying between 1.4 and 2.8. In at least half of the cases, the density structure of the medium is inconsistent with an r^{-2} profile. A homogeneous medium with density in the 0.1-50 per cc range can accommodate the broadband emission of all afterglows, with the exception of 990123, for which we find the density to be less than 0.01 per cc. If GRBs arise from the core collapse of massive stars, then such low densities indicate the existence of superbubbles created by the supernovae and winds within a cluster of massive stars.Comment: 4 pages, color figures. Submitted to ApJ

Orphan Afterglows of Collimated Gamma-Ray Bursts: Rate Predictions and Prospects for Detection

We make a quantitative prediction for the detection rate of orphan GRB afterglows as a function of flux sensitivity in X-ray, optical, and radio wavebands, based on a recent model of collimated GRB afterglows. We find that the orphan afterglow rate strongly depends on the opening angle of the jet (roughly \propto \theta_jet^{-2}), as expected from simple geometrical consideration, if the total jet energy is kept constant as suggested by recent studies. The relative beaming factor b_rel, i.e., the ratio of all afterglow rate including orphans to those associated with observable prompt GRBs, could be as high as b_rel >~ 100 for searches deeper than R ~ 24, depending on afterglow parameters. To make the most plausible predictions, we average the model emission for ten sets of afterglow parameters obtained through fits to ten well-observed, collimated GRB jets, weighted by the sky coverage of each jet. Our model expectations are consistent with the results (or constraints) obtained by all past searches. We estimate the number of orphan afterglows in the first 1500deg^2 field of the SDSS to be about 0.2. The relative beaming factor b_rel is rapidly increasing with the search sensitivity: b_rel ~ 3 for the SDSS sensitivity to transient objects in the northern sky (R ~ 19), ~14 for the past high-z supernova searches (R ~ 23), and ~50 for the sensitivity of the Subaru Suprime-Cam (R ~ 26). Predictions are made for the current facilities and future projects in X-ray, optical, and radio bands. Among them, the southern-sky observation of the SDSS (sensitive to transients down to R ~ 23) could detect ~40 orphan afterglows during the five-year operation. Allen Telescope Array would find about 200 afterglows in a radio band at ~0.1-1mJy with b_rel ~ 15.Comment: Accepted to ApJ after minor changes. The afterglow sample is extended, and the predicted numbers are changed but only slightly. Received Apr 5, Accepted May

Decay properties of the X-ray afterglows of Gamma-ray bursts

We present a set of seventeen Gamma-Ray Bursts (GRBs) with known redshifts and X-ray afterglow emission. We apply cosmological corrections in order to compare their fluxes normalized at a redshift of 1. Two classes of GRB can be defined using their X-ray afterglow light curves. We show that the brightest afterglows seem to decay faster than the dimer ones. We also point out evidences for a possible flux limit of the X-ray afterglow depending on the time elapsed since the burst. We try to interpret these observations in the framework of the canonical fireball model of GRB afterglow emission.Comment: 7 pages, 3 color figures and 1 B&W figure. Accepted for publication in Astronomy and Astrophysic

Jet Breaks in Short Gamma-Ray Bursts. I: The Uncollimated Afterglow of GRB 050724

We report the results of the \chandra observations of the \swift-discovered short Gamma-Ray Burst GRB 050724. \chandra observed this burst twice, about two days after the burst and a second time three weeks later. The first \chandra pointing occurred at the end of a strong late-time flare. About 150 photons were detected during this 49.3 ks observation in the 0.4-10.0 keV range. The spectral fit is in good agreement with spectral analysis of earlier \swift XRT data. In the second \chandra pointing the afterglow was clearly detected with 8 background-subtracted photons in 44.6 ks. From the combined \swift XRT and \chandra-ACIS-S light curve we find significant flaring superposed on an underlying power-law decay slope of $\alpha$=0.98$^{+0.11}_{-0.09}$. There is no evidence for a break between about 1 ks after the burst and the last \chandra pointing about three weeks after the burst. The non-detection of a jet break places a lower limit of 25$^{\circ}$ on the jet opening angle, indicating that the outflow is less strongly collimated than most previously-reported long GRBs. This implies that the beaming corrected energy of GRB 050724 is at least $4\times 10^{49}$ ergs.Comment: 7 pages, ApJ acceped, scheduled for December 20, 2006, ApJ, 65

Challenging GRB models through the broadband dataset of GRB060908

Context: Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to disentangle the various possible emission processes and scenarios proposed to interpret the complex gamma-ray burst phenomenology. Aims: We collected a large dataset on GRB060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow. Methods: Data from Swift-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/NIR and millimeter telescopes allowed us to follow the afterglow evolution from about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions. Results: The prompt emission of GRB060908 was characterized by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterized by a rather flat optical/NIR spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution. Conclusions: GRB060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios showing that a satisfactorily modeling of this event is challenging. In the future, similar efforts will enable us to obtain optical/NIR coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research.Comment: A&A, in press. 11 pages, 5 figure