Evidences for two Gamma-Ray Burst afterglow emission regimes

We applied cosmological and absorption corrections to the X-ray and optical afterglow fluxes of a sample of Gamma-Ray Burst sources of known distance. We find a good correlation in X-rays and that the GRBs in our sample form two well defined classes. We tentatively interpret them as radiative and adiabatic afterglow behaviours in the framework of the fireball model for GRBs. We do not observe this correlation at optical wavelengths. This discrepancy with the model may be due to the absorption in the source vicinity.Comment: 4 pages, 2 figures, letter to be published in Astronomy and Astrophysic

Simultaneous event detection rates by electromagnetic and gravitational wave detectors in the Advanced Era of LIGO and Virgo

We present several estimates of the rate of simultaneous detection of the merging of a binary system of neutron stars in the electromagnetic and the gravitational wave domains, assuming that they produce short GRBs. We have based our estimations on a carefully selected sample of short gamma-ray bursts, corrected from redshift effects. The results presented in this paper are based on actual observation only. In the electromagnetic spectrum, we considered observations by current (Swift and Fermi}) and future (LOFT and SVOM) missions. In the gravitational wave domain, we consider detections by the Advanced Virgo instrument alone and the network of both Advanced LIGO and Advanced Virgo. We discuss on the possible biases present in our sample, and how to fix them. For present missions, assuming a detection in the following years, we find that we should observe simultaneously between 0.11 and 4.2 gravitational wave events per year with Swift} and Fermi} respectively. For future projects (LOFT and SVOM) we can expect less than one common detection per year. We check the consistency of our results with several previously published rate of detection of gravitational waves.Comment: 7 pages, accepted for publication in MNRAS, with note added in proof correcting the rates for Fermi/SVOM experiment. Added tables 5 and 6 that are corrected and replace tables 2 and

Are Ultra-long Gamma-Ray Bursts different?

The discovery of a number of gamma-ray bursts with duration exceeding 1,000 seconds, in particular the exceptional case of GRB 111209A with a duration of about 25,000 seconds, has opened the question on whether these bursts form a new class of sources, the so called {\em ultra-long} GRBs, or if they are rather the tail of the distribution of the standard long GRB duration. In this Letter, using the long GRB sample detected by {\em Swift}, we investigate on the statistical properties of ultra-long GRBs and compare them with the overall long burst population. We discuss also on the differences observed in their spectral properties. We find that ultra-long GRBs are statistically different from the standard long GRBs with typical burst duration less than 100-500 seconds, for which a Wolf Rayet star progenitor is usually invoked. We interpret this result as an indication that an alternative scenario has to be found in order to explain the ultra-long GRB extreme energetics, as well as the mass reservoir and its size that can feed the central engine for such a long time.Comment: 6 pages, submitted to ApJ, minor typo

Simultaneous detection rates of binary neutron star systems in advanced Virgo/LIGO and GRB detectors

The coalescence of two compact objects is a key target for the new gravitational wave observatories such as Advanced-Virgo (AdV), Advanced-LIGO (aLIGO) and KAGRA. This phenomenon can lead to the simultaneous detection of electromagnetic waves in the form of short GRBs (sGRBs) and gravitational wave transients. This will potentially allow for the first time access to the fireball and the central engine properties. We present an estimation of the detection rate of such events, seen both by a Swift-like satellite and AdV/ALIGO. This rate is derived only from the observations of sGRBs. We show that this rate, if not very high, predicts a few triggers during the whole life time of Advanced LIGO-Virgo. We discuss how to increase it using some dedicated observational strategies. We apply our results to other missions such as the SVOM French-Chinese satellite project or LOFT.Comment: 7 pages, 1 figure, 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 18 in eConf Proceedings C130414

The Swift Gamma-Ray Burst redshift distribution: selection biases and optical brightness evolution at high-z?

We employ realistic constraints on astrophysical and instrumental selection effects to model the Gamma-Ray Burst (GRB) redshift distribution using {\it Swift} triggered redshift samples acquired from optical afterglows (OA) and the TOUGH survey. Models for the Malmquist bias, redshift desert, and the fraction of afterglows missing because of host galaxy dust extinction, are used to show how the "true" GRB redshift distribution is distorted to its presently observed biased distribution. We also investigate another selection effect arising from a correlation between $E_{{\rm iso}}$ and $L_{{\rm opt}}$. The analysis, which accounts for the missing fraction of redshifts in the two data subsets, shows that a combination of selection effects (both instrumental and astrophysical) can describe the observed GRB redshift distribution. Furthermore, the observed distribution is compatible with a GRB rate evolution that tracks the global SFR, although the rate at high-$z$ cannot be constrained with confidence. Taking selection effects into account, it is not necessary to invoke high-energy GRB luminosity evolution with redshift to explain the observed GRB rate at high-$z$.Comment: Version 2. Includes new data, figures and refined analysi

Discovery of a quiescent neutron star binary in the globular cluster M13

We have discovered with XMM-Newton an X-ray source in the core of the globular cluster M13, whose X-ray spectral properties suggest that it is a quiescent neutron star X-ray binary. The spectrum can be well fitted with a pure hydrogen atmosphere model, with T=76 +/- 3 eV, R=12.8 +/- 0.4 km and an X-ray luminosity of 7.3 +/- 0.6 x 10^{32} erg/s. In the light of this result, we have discovered a strong correlation between the stellar encounter rate and the number of quiescent neutron stars found in the ten globular clusters observed so far by either XMM-Newton or Chandra. This result lends strong support to the idea that these systems are primarily produced by stellar encounters in the core of globular clusters.Comment: 4 pages, 2 B&W figures and 1 color figure. Accepted for publication in A&A Letters. Revised reference list and minor correction

Constraints on the baryonic load of gamma-ray bursts using ultra-high energy cosmic rays

Ultra-high energy cosmic rays are the most extreme energetic particles detected on Earth, however, their acceleration sites are still mysterious. We explore the contribution of low-luminosity gamma-ray bursts to the ultra-high energy cosmic ray flux, since they form the bulk of the nearby population. We analyse a representative sample of these bursts detected by BeppoSAX, INTEGRAL and Swift between 1998-2016, and find they can produce a theoretical cosmic ray flux on Earth of at least $R_\text{UHECR} = 1.2 \times 10^{15}$ particles km$^{-2}$ century$^{-1}$ mol$^{-1}$. No suppression mechanisms can reconcile this value with the flux observed on Earth. Instead, we propose that the jet of low-luminosity gamma-ray bursts propels only the circumburst medium - which is accelerated to relativistic speeds - not the stellar matter. This has implications for the baryonic load of the jet: it should be negligible compared to the leptonic content.Comment: 5 pages, submitted to MNRA