F. Daigne

DETECTION OF A THERMAL SPECTRAL COMPONENT IN THE PROMPT EMISSION OF GRB 100724B.

Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor find that the spectrum is dominated by the typical Band functional form, which is usually taken to represent a non-thermal emission component, but also includes a statistically highly significant thermal spectral contribution. The simultaneous observation of the thermal and non-thermal components allows us to confidently identify the two emission components. The fact that these seem to vary independently favors the idea that the thermal component is of photospheric origin while the dominant non-thermal emission occurs at larger radii. Our results imply either a very high efficiency for the non-thermal process or a very small size of the region at the base of the flow, both quite challenging for the standard fireball model. These problems are resolved if the jet is initially highly magnetized and has a substantial Poynting flux.

The expected thermal precursors of gamma‐ray bursts in the internal shock model

The prompt emission of gamma-ray bursts probably comes from a highly relativistic wind which converts part of its kinetic energy into radiation via the formation of shocks within the wind itself. Such ‘internal shocks’ can occur if the wind is generated with a highly non-uniform distribution of the Lorentz factor. We estimate the expected photospheric emission of such a relativistic wind when it becomes transparent. We compare this thermal emission (temporal profile + spectrum) with the non-thermal emission produced by the internal shocks. In most cases, we predict a rather bright thermal emission that should already have been detected. This favours acceleration mechanisms for the wind where the initial energy input is under magnetic rather than thermal form. Such scenarios can produce thermal X-ray precursors comparable to those observed by Ginga and WATCH/GRANAT.

The redshift distribution of Swift gamma-ray bursts: evidence for evolution

We predict the redshift distribution of long gamma-ray bursts (GRBs) with Monte Carlo simulations. Our improved analysis constrains free parameters with three kinds of observation: (i) the log N - log P diagram of Burst and Transient Source Experiment (BATSE) bursts; (ii) the peak energy distribution of bright BATSE bursts and (iii) the High Energy Transient Explorer (HETE2) fraction of X-ray rich GRBs and X-ray flashes. The statistical analysis of the Monte Carlo simulation results allows us to carefully study the impact of the uncertainties in the GRB intrinsic properties on the redshift distribution. The comparison with Swift data then leads to the following conclusions. The Amati relation should be intrinsic, if observationally confirmed by Swift. The progenitor and/or the GRB properties have to evolve to reproduce the high mean redshift of Swift bursts. Our results favour an evolution of the efficiency of GRB production by massive stars, that would be nearly six to seven times higher at z ∼ 7 than at z ∼ 2. We finally predict around 10 GRBs detected by Swift at redshift z > 6 for a 3-yr mission. These may be sufficient to open a new observational window over the high redshift Universe.

Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Context. Gamma-ray burst emission in the prompt phase is often interpreted as synchrotron radiation from high-energy electrons accelerated in internal shocks. Fast synchrotron cooling of a power-law distribution of electrons leads to the prediction that the slope below the spectral peak has a photon index α = −3/ 2( N(E) ∝ E α ). However, this differs significantly from the observed median value α ≈− 1. This discrepancy has been used to argue against this scenario. Aims. We quantify the influence of inverse Compton (IC) and adiabatic cooling on the low energy slope to understand whether these processes can reconcile the observed slopes with a synchrotron origin. Methods. We use a time-dependent code developed to calculate the GRB prompt emission within the internal shock model. The code follows both the shock dynamics and electron energy losses and can be used to generate lightcurves and spectra. We investigate the dependence of the low-energy slope on the parameters of the model and identify parameter regions that lead to values α> −3/2. Results. Values of α between −3/ 2a nd−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 (GeV) 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 ( 50%) when adiabatic cooling is comparable with radiative cooling (marginally fast cooling). This requires collisions at small radii and/or with low magnetic fields. Conclusions. Amending the standard fast cooling scenario to account for IC cooling naturally leads to values α up to −1. Marginally fast cooling may also account for values of α up to −2/3, although the conditions required are more difficult to reach. About 20% of GRBs show spectra with slopes α> −2/3. Other effects, not investigated here, such as a thermal component in the electron distribution or pair production by high-energy gamma-ray photons may further affect α. Still, the majority of observed GRB prompt spectra can be reconciled with a synchrotron origin, constraining the microphysics of mildly relativistic internal shocks.

Prompt high-energy emission from gamma-ray bursts in the internal shock model

Context. Gamma-ray bursts (GRB) are powerful, short duration events with a spectral luminosity peaking in the keV-MeV (BATSE) range. The prompt emission is thought to arise from electrons accelerated in internal shocks propagating within a highly relativistic outflow. Aims. The launch of Fermi offers the prospect of observations with unprecedented sensitivity in high-energy (HE, >100 MeV) gammarays. The aim is to explore the predictions for HE emission from internal shocks, taking into account both dynamical and radiative aspects, and to deduce how HE observations constrain the properties of the relativistic outflow. Methods. The prompt GRB emission is modelled by combining a time-dependent radiative code, solving for the electron and photon distributions, with a dynamical code giving the evolution of the physical conditions in the shocked regions of the outflow. Synthetic lightcurves and spectra are generated and compared to observations. Results. The HE emission deviates significantly from analytical estimates, which tend to overpredict the IC component, when the time dependence and full cross-sections are included. The exploration of the parameter space favors the case where the dominant process in the BATSE range is synchrotron emission. The HE component becomes stronger for weaker magnetic fields. The HE lightcurve can display a prolonged pulse duration due to IC emission, or even a delayed peak compared to the BATSE range. Alternatively, having dominant IC emission in the BATSE range requires most electrons to be accelerated into a steep power-law distribution and implies strong second order IC scattering. In this case, the BATSE and HE lightcurves are very similar. Conclusions. The combined dynamical and radiative approach allows a firm appraisal of GRB HE prompt emission. A diagnostic procedure is presented to identify from observations the dominant emission process and derive constrains on the bulk Lorentz factor, particle density and magnetic field of the outflow.

Can the early X-ray afterglow of gamma-ray bursts be explained by a contribution from the reverse shock

We propose to explain the recent observations of GRB early X-ray afterglows with SWIFT by the dissipation of energy in the reverse shock which crosses the ejecta as it is decelerated by the burst environment. We compute the evolution of the dissipated power and discuss the possibility that a fraction of it can be radiated in the X-ray range. We show that this reverse shock contribution behaves in a way very similar to the observed X-ray afterglows if the following two conditions are satisfied: (i) the Lorentz factor of the material which is ejected during the late stages of source activity decreases to small values < 10 and (ii) a large part of the shock dissipated energy is transferred to a small fraction (� ∼ 10 2 ) of the electron population. We also discuss how our results may help to solve some puzzling problems raised by multiwavelength early afterglow observations such as the presence of chromatic breaks.

A new X-ray flare from the galactic nucleus detected with the XMM-Newton photon imaging cameras

Sgr A*, the compact radio source believed to be the counterpart of the massive black hole at the Galactic nucleus, was observed to undergo rapid and intense flaring activity in X-rays with Chandra in 2000 October. We report here the detection with XMM-Newton European Photon Imaging Cameras of the early phase of a similar X-ray flare from this source, which occurred on 2001 September 4. The source 2-10 keV luminosity increased by a factor of ≈20 to reach a level of 4 × 1034 ergs s-1 in a time interval of about 900 s, just before the end of the observation. The data indicate that the source spectrum was hard during the flare. This XMM-Newton observation confirms the results obtained by Chandra and suggests that in Sgr A* rapid and intense X-ray flaring is not a rare event. This can constrain the emission mechanism models proposed for this source and also implies that the crucial multiwavelength observation programs planned to explore the behavior of the radio/submillimeter and hard X-ray/gamma-ray emissions during the X-ray flares have a good chance of success.

Variable Polarization Measured in the Prompt Emission of GRB 041219A Using IBIS on Board INTEGRAL

Polarization measurements provide direct insight into the nature of astrophysical processes. Unfortunately, only a few instruments are available for this kind of measurements at gamma-ray energies, and the sources need to be very bright. Gamma-Ray Bursts (GRBs) are ideal candidates due to their large flux over limited time intervals, maximizing the available signal-to-noise ratio. To date a few polarization measurements have been reported, claiming of a high degree of polarization in the prompt emission of GRBs but with low statistical evidence. We used the IBIS telescope on board the INTEGRAL satellite to measure the polarization of the prompt gamma-ray emission of the long and bright GRB 041219A in the 200-800 keV energy band. We find a variable degree of polarization ranging from less than 4% over the first peak to 43+/-25% for the whole second peak. Time resolved analysis of both peaks indicates a high degree of polarization, and the null average polarization in the first peak can be explained by the rapid variations observed in the polarization angle and degree. Our results are consistent with different models for the prompt emission of GRBs at these energies, but they favor synchrotron radiation from a relativistic outflow with a magnetic field which is coherent on an angular size comparable with the angular size of the emitting region (~1/Gamma) . Indeed this model has the best capabilities to maintain a high polarization level, and to produce the observed variability.

Time-resolved spectroscopy of the three brightest and hardest short gamma-ray bursts observed with the Fermi gamma-ray burst monitor

From 2008 July to 2009 October, the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope has detected 320 gamma-ray bursts (GRBs). About 20% of these events are classified as short based on their T90 duration below 2 s. We present here for the first time time-resolved spectroscopy at timescales as short as 2 ms for the three brightest short GRBs observed with GBM. The time-integrated spectra of the events deviate from the Band function, indicating the existence of an additional spectral component, which can be fit by a power law with index ∼-1.5. The time-integrated Epeak values exceed 2 MeV for two of the bursts and are well above the values observed in the brightest long GRBs. Their Epeak values and their low-energy power-law indices (a) confirm that short GRBs are harder than long ones. We find that short GRBs are very similar to long ones, but with light curves contracted in time and with harder spectra stretched toward higher energies. In our time-resolved spectroscopy analysis, we find that the Epeak values range from a few tens of keV up to more than 6MeV. In general, the hardness evolutions during the bursts follow their flux/intensity variations, similar to long bursts. However, we do not always see the Epeak leading the light-curve rises and confirm the zero/short average light-curve spectral lag below 1 MeV, already established for short GRBs. We also find that the time-resolved low-energy power-law indices of the Band function mostly violate the limits imposed by the synchrotron models for both slow and fast electron cooling and may require additional emission processes to explain the data. Finally, we interpreted these observations in the context of the current existing models and emission mechanisms for the prompt emission of GRBs.

Cosmic Star Formation, Reionization, and Constraints on Global Chemical Evolution

Motivated by the Wilkinson Microwave Anisotropy Probe (WMAP) results indicating an early epoch of reionization, we consider alternative cosmic star formation models that are capable of reionizing the early intergalactic medium. We develop models that include an early burst of massive stars (with several possible mass ranges) combined with standard star formation. We compute the stellar ionizing flux of photons, and we track the nucleosynthetic yields for several elements: D, 4He, C, N, O, Si, S, Fe, and Zn. We compute the subsequent chemical evolution as a function of redshift, both in the intergalactic medium and in the interstellar medium of forming galaxies, starting with the primordial objects that are responsible for the reionization. We apply constraints from the observed abundances in the Ly? forest and in damped Ly? clouds in conjunction with the ability of the models to produce the required degree of reionization. We also consider possible constraints associated with the observations of the two extremely metal-poor stars HE 0107-5240 and CS 22949-037. We confirm that an early top-heavy stellar component is required, since a standard star formation model is unable to reionize the early universe and reproduce the abundances of the very metal-poor halo stars. A bimodal (or top-heavy) initial mass function (IMF; 40-100 M?) is our preferred scenario, compared with the extreme mass range (100 M?) often assumed to be responsible for the early stages of reionization. A mode of even more extreme stellar masses in the range ?270 M? has also been considered. All massive stars in this mode collapse entirely into black holes, and as a consequence, chemical evolution and reionization are decorrelated. The ionizing flux from these very massive stars can easily reionize the universe at z ~ 17. However, the chemical evolution in this case is exactly the same as in the standard star formation model, and the observed high-redshift abundances are not reproduced. We show that the initial top-heavy mode, which originally was introduced to reionize the early universe, produces rapid initial metal pollution. The existence of old, C-rich halo stars with high [O/Fe] and [C/Fe] ratios is predicted as a consequence of these massive stars. The recently observed abundances in the oldest halo stars could trace this very specific stellar population. The extreme mass range is disfavored, and there is no evidence, nor any need, for a hypothesized primordial population of very massive stars in order to account for the chemical abundances of extremely metal-poor halo stars or of the intergalactic medium. The combined population of early-forming normal (0.1-100 M?) and massive (40-100 M?) stars can simultaneously explain the cosmic chemical evolution and the observations of extremely metal-poor halo stars and also account for early cosmological reionization.