An improved redshift indicator for Gamma-Ray Bursts, based on the prompt emission

We propose an improved version of the redshift indicator developed by Atteia (2003), which gets rid of the dependence on the burst duration and provides better estimates for high-redshift GRBs. We present the derivation and the definition of this redshift indicator, then its calibration with 17 GRBs with known redshifts detected by HETE-2 and 2 more detected by Konus-Wind. We also provide an estimation of the redshift for 59 bursts, and we finally discuss the redshift distribution of HETE-bursts and the possible other applications of this redshift indicator.Comment: To appear in the proceedings of the 16th Annual October Astrophysics Conference in Maryland, "Gamma Ray Bursts in the Swift Era", Washington DC., November 29-December 2, 2005, 4 pages, 3 figure

Inverse Compton cooling in Klein-Nishina regime and GRB prompt spectrum

Synchrotron radiation mechanism, when electrons are accelerated in a relativistic shock, is known to have serious problems to explain the observed gamma-ray spectrum below the peak for most Gamma-Ray Bursts (GRBs); the synchrotron spectrum below the peak is much softer than observed spectra. Recently, the possibility that electrons responsible for the radiation cool via Inverse Compton, but in the Klein-Nishina regime, has been proposed as a solution to this problem. We provide an analytical study of this effect and show that it leads to a hardening of the low energy spectrum but not by enough to make it consistent with the observed spectra for most GRBs (this is assuming that electrons are injected continuously over a time scale comparable to the dynamical time scale, as is expected for internal shocks of GRBs). In particular, we find that it is not possible to obtain a spectrum with \alpha>-0.1 (f_{\nu} \propto \nu^{\alpha}) whereas the typical observed value is \alpha\sim0. Moreover, extreme values for a number of parameters are required in order that \alpha\sim-0.1: the energy fraction in magnetic field needs to be less than about 10^{-4}, the thermal Lorentz factor of electrons should be larger than 10^6, and the radius where gamma-rays are produced should be not too far away from the deceleration radius. These difficulties suggest that the synchrotron radiation mechanism in internal shocks does not provide a self-consistent solution when \alpha>-0.2.Comment: 10 pages (single column), 2 figures, MNRAS in pres

Host galaxies of long gamma-ray bursts in the Millennium Simulation

(abridged) In this work, we investigate the nature of the host galaxies of long Gamma-Ray bursts (LGRBs) using a galaxy catalogue constructed from the Millennium Simulation. We developed an LGRB synthetic model based on the hypothesis that these events originate at the end of the life of massive stars following the collapsar model, with the possibility of including a constraint on the metallicity of the progenitor star. A complete observability pipeline was designed to calculate a probability estimation for a galaxy to be observationally identified as a host for LGRBs detected by present observational facilities. This new tool allows us to build an observable host galaxy catalogue which is required to reproduce the current stellar mass distribution of observed hosts. This observability pipeline predicts that the minimum mass for the progenitor stars should be ~75 solar masses in order to be able to reproduce BATSE observations. Systems in our observable catalogue are able to reproduce the observed properties of host galaxies, namely stellar masses, colours, luminosity, star formation activity and metallicities as a function of redshift. At z>2, our model predicts that the observable host galaxies would be very similar to the global galaxy population. We found that ~88 per cent of the observable host galaxies with mean gas metallicity lower than 0.6 solar have stellar masses in the range 10^8.5-10^10.3 solar masses in excellent agreement with observations. Interestingly, in our model observable host galaxies remain mainly within this mass range regardless of redshift, since lower stellar mass systems would have a low probability of being observed while more massive ones would be too metal-rich. Observable host galaxies are predicted to preferentially inhabit dark matter haloes in the range 10^11-10^11.5 solar masses, with a weak dependence on redshift.Comment: 11 pages, 10 figures, accepted for publication in MNRA

Reconciling observed GRB prompt spectra with synchrotron radiation ?

(abridged)Prompt GRB emission is often interpreted as synchrotron radiation from high-energy electrons accelerated in internal shocks. Fast synchrotron cooling predicts that the photon index below the spectral peak is alpha=-3/2. This differs significantly from the observed median value alpha \approx -1. We quantify the influence of inverse Compton and adiabatic cooling on alpha to understand whether these processes can reconcile the observations with a synchrotron origin. We use a time-dependent code that follows both the shock dynamics and electron energy losses. We investigate the dependence of alpha on the parameters of the model. Slopes between -3/2 and -1 are reached when electrons suffer IC losses in the Klein-Nishina regime. This does not necessarily imply a strong IC component in the Fermi/LAT range because scatterings are only moderately efficient. Steep slopes require that a large fraction (10-30%) of the dissipated energy is given to a small fraction (<~1%) of the electrons and that the magnetic energy density fraction remains low (<~ 0.1%). Values of alpha up to -2/3 can be obtained with relatively high radiative efficiencies (>50%) when adiabatic cooling is comparable with radiative cooling (marginally fast cooling). This requires collisions at small radii and/or with low magnetic fields. Amending the standard fast cooling scenario to account for IC cooling naturally leads to alpha up to -1. Marginally fast cooling may also account for alpha up to -2/3, although the conditions required are more difficult to reach. About 20% of GRBs show spectra with slopes alpha>-2/3. Other effects, not investigated here, such as a thermal component in the electron distribution or pair production by HE photons may further affect alpha. Still, the majority of observed GRB prompt spectra can be reconciled with a synchrotron origin, constraining the microphysics of mildly relativistic internal shocks.Comment: 14 pages, 10 figures, accepted for publication in A&A (10/10/2010

The luminosity function and the rate of Swift's Gamma Ray Bursts

We invert directly the redshift - luminosity distribution of observed long Swift GRBs to obtain their rate and luminosity function. Our best fit rate is described by a broken power law that rises like (1+z)^2.1{+0.5-0.6} for 0<z<3 and decrease like (1+z)^-1.4{+2.4-1.0} for z>3. The local rate is 1.3^{+0.6-0.7} [Gpc^-3 yr^-1]. The luminosity function is well described by a broken power law with a break at L* = 10^52.5{+-0.2}[erg/sec] and with indices alpha = 0.2^{+0.2-0.1} and beta = 1.4^{+0.3-0.6}. The recently detected GRB 090423, with redshift ~8, fits nicely into the model's prediction, verifying that we are allowed to extend our results to high redshifts. While there is a possible agreement with the star formation rate (SFR) for z<3, the high redshift slope is shallower than the steep decline in the SFR for 4<z. However we cannot rule out a GRB rate that follows one of the recent SFR models.Comment: Significantly revised version, including a comparison of the GRB rate to new results on the SFR, revisions in response to the referee comments and comparison with other works on the GRB rate. 28 pages, 14 figures, 5 tables. MNRAS

The Interplanetary Network Supplement to the BeppoSAX Gamma-Ray Burst Catalogs

Between 1996 July and 2002 April, one or more spacecraft of the interplanetary network detected 787 cosmic gamma-ray bursts that were also detected by the Gamma-Ray Burst Monitor and/or Wide-Field X-Ray Camera experiments aboard the BeppoSAX spacecraft. During this period, the network consisted of up to six spacecraft, and using triangulation, the localizations of 475 bursts were obtained. We present the localization data for these events.Comment: 89 pages, 3 figures. Submitted to the Astrophysical Journal Supplement Serie

A detailed spectral study of GRB 041219A and its host galaxy

GRB 041219A is one of the longest and brightest gamma-ray bursts (GRBs) ever observed. It was discovered by the INTEGRAL satellite, and thanks to a precursor happening about 300 s before the bulk of the burst, ground based telescopes were able to catch the rarely-observed prompt emission in the optical and in the near infrared bands. Here we present the detailed analysis of its prompt gamma-ray emission, as observed with IBIS on board INTEGRAL, and of the available X-ray afterglow data collected by XRT on board Swift. We then present the late-time multi-band near infrared imaging data, collected at the TNG, and the CFHT, that allowed us to identify the host galaxy of the GRB as an under-luminous, irregular galaxy of about 5x10^9 M_Sun at best fit redshift of z=0.31 -0.26 +0.54. We model the broad-band prompt optical to gamma-ray emission of GRB 041219A within the internal shock model. We were able to reproduce the spectra and light curve invoking the synchrotron emission of relativistic electrons accelerated by a series of propagating shock waves inside a relativistic outflow. On the other hand, it is less easy to simultaneously reproduce the temporal and spectral properties of the infrared data.Comment: 12 pages, 9 figures, accepted for publication in MNRAS, Figure 5 in reduced qualit

X-ray Flashes or soft Gamma-ray Bursts? The case of the likely distant XRF 040912

In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties. We performed a detailed spectral and temporal analysis of both the prompt and the afterglow emission and we estimated the distance scale of the likely host galaxy. We then used the currently available sample of XRFs with known distance to discuss the connection between XRFs and classical Gamma-ray Bursts (GRBs). We found that the prompt emission properties unambiguously identify this burst as an XRF, with an observed peak energy of E_p=17+/-13 keV and a burst fluence ratio S(2-30keV)/S(30-400keV)>1. A non-fading optical source with R~24 mag and with an apparently extended morphology is spatially consistent with the X-ray afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no afterglow optical counterpart is detected down to R>25 mag (3 sigma limiting magnitude) at 13.6 hours after the burst. The host galaxy spectrum detected from 3800A to 10000A, shows a single emission line at 9552A. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Ly alpha cut-off down to 3800A are consistent with the hypothesis of the [OII] line at redshift z=1.563+/-0.001. The intrinsic spectral properties rank this XRF among the soft GRBs in the E_peak-E_iso diagram. Similar results were obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903 represent a good example of instrinsic XRF(i-XRF) and are possibly associated with a different progenitor population. This scenario may calls for a new definition of XRFs.Comment: 10 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

Early emission of rising optical afterglows: The case of GRB 060904B and GRB 070420

We present the time-resolved optical emission of gamma-ray bursts GRB 060904B and GRB 070420 during their prompt and early afterglow phases. We used time resolved photometry from optical data taken by the TAROT telescope and time resolved spectroscopy at high energies from the Swift spacecraft instrument. The optical emissions of both GRBs are found to increase from the end of the prompt phase, passing to a maximum of brightness at t_{peak}=9.2 min and 3.3 min for GRB 060904B and GRB 070420 respectively and then decrease. GRB 060904B presents a large optical plateau and a very large X-ray flare. We argue that the very large X-flare occurring near t_{peak} is produced by an extended internal engine activity and is only a coincidence with the optical emission. GRB 070420 observations would support this idea because there was no X-flare during the optical peak. The nature of the optical plateau of GRB 060904B is less clear and might be related to the late energy injection.Comment: 11 pages, 5 color figues, 2 b&w figures, accepted for publication by Astronomy and Astrophysic