Can all breaks in gamma-ray burst afterglows be explained by jet effects?

Whether gamma-ray bursts are highly beamed or not is a very important question, as it has been pointed out that the beaming will lead to a sharp break in the afterglow light curves during the ultrarelativistic phase, with the breaking point determined by  Γ∼1/ θ ₀  , where Γ is the bulk Lorentz factor and θ ₀ is the initial half opening angle of the ejecta, and such a break is claimed to be present in the light curves of some GRBs. In this paper we will examine whether all the observed breaks in GRB afterglow light curves can be explained by jet effects. Here we present a detailed calculation of the jet evolution and emission, and have obtained a simple formula of bulk Lorentz factor evolution. We show that the light curves are very smoothly steepened by jet effect, and the shape of the light curve is determined by only one parameter –     , where E and n are the fireball energy and surrounding medium density, respectively. We find that for GRB 990123 and GRB 991216, the jet model can approximately fit their light curves, and the values of     are about 0.17 and 0.22, respectively. On the other hand, the light curves of GRB 990510, GRB 000301c, GRB 000926 and GRB 010222 cannot be fitted by the jet model, which suggests that the breaks may be caused by some other reasons, and the jet effect should be not the unique reason.     

The Synchrotron-self-Compton Radiation Accompanying Shallow Decaying X-Ray Afterglow： the Case of GRB 940217

High energy emission (> tens MeV) of Gamma-Ray Bursts (GRBs) provides an important clue on the physical processes occurring in GRBs that may be correlated with the GRB early afterglow. A shallow decline phase has been well identified in about half of Swift Gamma-ray Burst X-ray afterglows. The widely considered interpretation involves a significant energy injection and possibly time-evolving shock parameter(s). We calculate the synchrotron-self-Compton (SSC) radiation of such an external forward shock and show that it could explain the well-known long term high energy (i.e., tens MeV to GeV) afterglow of GRB 940217. We propose that cooperation of Swift and GLAST will help to reveal the nature of GRBs.     

The visibility of gamma-ray burst afterglows in dusty star-forming regions

Recent observations of the environments of gamma-ray bursts (GRBs) favour massive stars as their progenitors, which are likely to be surrounded by gas and dust. The visibility of the optical and UV emission of a GRB is expected to depend on the characteristics of both the dust and the GRB emission itself. A reasonable distribution of surrounding dust is capable of absorbing all the optical and UV emission of the optical flash and afterglow of a GRB, unless the optical flash has a peak isotropic luminosity L _peak≳10⁴⁹ erg s⁻¹ . This means that dark bursts should exist and these bursts will have to be studied at infrared rather than optical wavelengths. In this paper details will be given about the infrared GRB dust emission. The reprocessed dust emission peaks at a rest-frame wavelength of about 8 μm. Forthcoming space telescopes, in particular the IRAC camera on board the Space Infrared Telescope Facility , could detect this emission out to a redshift of about two. However, an accurate position of the GRB afterglow must be provided for this emission to be identified, because the light curve of the reprocessed dust emission does not vary on time-scales less than several years.     

Metallicities of high redshift GRB hosts: The case of GRB 100219A

GRB 100219A at z = 4.667 has been the highest redshift gamma‐ray burst observed with the X‐shooter spectrograph up to now. The spectrum covering the range from 5000 to 24000 Å and a large number of absorption lines allows to make a detailed study of the interstellar medium in a high redshift galaxy. The ISM in the low ionisation state and the kinematics of the absorption line components reveal a complex velocity field. The metallicity measured from different absorption lines is around 0.1 solar. Other GRB hosts at redshift beyond ∼3 have similar metallicities albeit with a large scatter in the metallicity distribution. X‐shooter will allow us to determine metallicities of a larger number of GRB hosts beyond redshift 5, to probe the early chemical enrichment of the Universe and to study its evolution from redshift 2 to beyond 10 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ring-Shaped Jets in Gamma-Ray Bursts

When the axis of a gamma-ray burst (GRB) does not coincide with the spin axis of its source, there may result a ring-shaped jet. Using some refined jet dynamics, we calculate multi-wavelength afterglow light curves for such ring-shaped jets. In the R-band we find an obvious break in the afterglow light curve due to the beaming effect and the break is affected by many parameters, such as the electron energy fraction ξe, the magnetic energy fraction ξ2B, the width of ring A0 and the medium number density n. The overall light curve can be divided into three power-law stages, I.e., an ultra-relativistic stage, an after-break stage and a deep Newtonian stage. For each stage the power-law index is larger in the ring-shaped jet than in the corresponding conical jet.     

AMRVAC and relativistic hydrodynamic simulations for gamma-ray burst afterglow phases

We apply a novel adaptive mesh refinement (AMR) code, AMRVAC (Adaptive Mesh Refinement version of the Versatile Advection Code), to numerically investigate the various evolutionary phases in the interaction of a relativistic shell with its surrounding cold interstellar medium (ISM). We do this for both 1D isotropic and full 2D jet-like fireball models. This is relevant for gamma-ray bursts (GRBs), and we demonstrate that, thanks to the AMR strategy, we resolve the internal structure of the shocked shell–ISM matter, which will leave its imprint on the GRB afterglow. We determine the deceleration from an initial Lorentz factor  γ= 100  up to the almost Newtonian     phase of the flow. We present axisymmetric 2D shell evolutions, with the 2D extent characterized by their initial opening angle. In such jet-like GRB models, we discuss the differences with the 1D isotropic GRB equivalents. These are mainly due to thermally induced sideways expansions of both the shocked shell and shocked ISM regions. We found that the propagating 2D ultrarelativistic shell does not accrete all the surrounding medium located within its initial opening angle. Part of this ISM matter gets pushed away laterally and forms a wide bow-shock configuration with swirling flow patterns trailing the thin shell. The resulting shell deceleration is quite different from that found in isotropic GRB models. As long as the lateral shell expansion is merely due to ballistic spreading of the shell, isotropic and 2D models agree perfectly. As thermally induced expansions eventually lead to significantly higher lateral speeds, the 2D shell interacts with comparably more ISM matter and decelerates earlier than its isotropic counterpart.     

High-energy afterglow emission from gamma-ray bursts

We calculate the very high-energy (sub-GeV to TeV) inverse Compton emission of GRB afterglows. We argue that this emission provides a powerful test of the currently accepted afterglow model. We focus on two processes: synchrotron self-Compton emission within the afterglow blast wave, and external inverse Compton emission which occurs when flare photons (produced by an internal process) pass through the blast wave. We show that if our current interpretations of the Swift X-ray telescope (XRT) data are correct, there should be a canonical high-energy afterglow emission light curve. Our predictions can be tested with high-energy observatories such as GLAST , Whipple, HESS and MAGIC. Under favourable conditions we expect afterglow detections in all these detectors.     

Gamma-ray burst efficiency and possible physical processes shaping the early afterglow

The discovery by Swift that a good fraction of gamma-ray bursts (GRBs) have a slowly decaying X-ray afterglow phase led to the suggestion that energy injection into the blast wave takes place several hundred seconds after the burst. This implies that right after the burst the kinetic energy of the blast wave was very low and in turn the efficiency of production of γ-rays during the burst was extremely high, rendering the internal shocks model unlikely. We re-examine the estimates of kinetic energy in GRB afterglows and show that the efficiency of converting the kinetic energy into γ-rays is moderate and does not challenge the standard internal shock model. We also examine several models, including in particular energy injection, suggested to interpret this slow decay phase. We show that with proper parameters, all these models give rise to a slow decline lasting several hours. However, even those models that fit all X-ray observations, and in particular the energy injection model, cannot account self-consistently for both the X-ray and the optical afterglows of well-monitored GRBs such as GRB 050319 and GRB 050401. We speculate about a possible alternative resolution of this puzzle.     

Time resolved spectroscopy of GRB 100418A and its host galaxy with X‐shooter

GRB 100418A was an intermediate duration GRB detected by Swift. It showed an initially dim optical afterglow that had a late increase in brightness, reaching its maximum several hours after the burst onset, unlike typical afterglows that peak tens of seconds after. It also displayed a bright X‐ray and radio counterpart. In this paper we present the observations of the afterglow obtained with X‐shooter. Three epochs were obtained, 0.4, 1.4, and 2.4 days after the burst. In these spectra, each covering the range from 3000 to 24800 Å, we detect abundant absorption features with 4 velocity components, and emission lines from the host galaxy with 2 additional velocity components. In one single velocity component, we detect a Fe II* 2396 Å fine structure feature which disappears from the first to the second epoch indicating that it is due to the effect of the GRB radiation on its environment. We consider it to be the closest absorption component to the GRB itself, for which we determine a redshift of z = 0.6239 ± 0.0002. From the Hα to [N II] ratio we determine a host galaxy metallicity of 0.5 solar (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Probing the neutral fraction of the IGM with GRBs during the epoch of reionization

We show that near-infrared observations of the red side of the Lyα line from a single gamma-ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM),     , at the GRB's redshift to better than     . Some GRB sightlines will encounter more neutral hydrogen than others at fixed     owing to the patchiness of reionization. GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra. We discuss the constraints on     from the   z = 6.3  burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows. Assuming an observation with signal-to-noise ratio similar to the Subaru FOCAS spectrum of GRB050904 and that the column density distribution of damped Lyα absorbers is the same as measured at lower redshifts, a GRB from an epoch when     can be used to detect a partly neutral IGM at 97 per cent confidence level ≈10 per cent of the time (and, for an observation with three times the sensitivity, ≈30 per cent of the time).     

On the probability of microlensing in gamma-ray burst afterglows

The declining light curve of the optical afterglow of gamma-ray burst (GRB) GRB000301C showed rapid variability with one particularly bright feature at about t − t ₀=3.8 d. This event was interpreted as gravitational microlensing by Garnavich, Loeb & Stanek and subsequently used to derive constraints on the structure of the GRB optical afterglow. In this paper, we use these structural parameters to calculate the probability of such a microlensing event in a realistic scenario, where all compact objects in the universe are associated with observable galaxies. For GRB000301C at a redshift of z =2.04, the a posteriori probability for a microlensing event with an amplitude of Δ m 0.95 mag (as observed) is 0.7 per cent (2.7 per cent) for the most plausible scenario of a flat Λ-dominated Friedmann–Robertson–Walker (FRW) universe with Ω_m=0.3 and a fraction f ∗=0.2 (1.0) of dark matter in the form of compact objects. If we lower the magnification threshold to Δ m 0.10 mag, the probabilities for microlensing events of GRB afterglows increase to 17 per cent (57 per cent). We emphasize that this low probability for a microlensing signature of almost 1 mag does not exclude that the observed event in the afterglow light curve of GRB000301C was caused by microlensing, especially in light of the fact that a galaxy was found within 2 arcsec from the GRB. In that case, however, a more robust upper limit on the a posteriori probability of ≈5 per cent is found. It does show, however, that it will not be easy to create a large sample of strong GRB afterglow microlensing events for statistical studies of their physical conditions on microarcsec scales.     

Analytical Afterglow Light Curves of Gamma-ray Bursts: the Case of a Flat Electron Spectrum

In the standard afterglow model, the swept electrons have a single power-law energy distribution dn/dγe ∝ γ−p e (p ∼ 2.3), owing to the first order Fermi acceleration process. However, in some events people find a lot of evidence for a flat electron spectrum (i.e., p < 2). In this work, the analytical afterglow light curves in the case of a flat electron energy distribution are presented respectively for a single power-law spectrum and a broken power-law spectrum, then the results are applied to the specific burst GRB 060908. Besides, we have also speculated a possible solution of the so-called low energy spectrum crisis of Gamma-ray Bursts     

具平缓电子能谱的伽玛射线暴的解析光变曲线

在标准的伽玛暴余辉模型中,电子通过费米一级加速后形成单幂律能谱分布dn/dγe∝γe-p(p≈2.3),但在某些伽玛暴事件中观测到了平缓的电子能谱分布(即p＜2).在单幂律谱和分段幂律谱两种情况下,分别给出了具有平缓电子能谱的伽玛暴余辉的解析光变曲线,并以GRB 060908为例进行了讨论.同时提出了伽玛暴低能谱危机的...     

Optical observations of GRB 050401 afterglow : a case for double-jet model

The afterglow of GRB 050401 presents several novel and interesting features. (i) An initially faster decay in optical band than in X-rays. (ii) A break in the X-ray light curve after ∼0.06 d with an unusual slope after the break. (iii)The X-ray afterglow does not show any spectral evolution across the break while the R -band light curve does not show any break. We have modelled the observed multiband evolution of the afterglow of GRB 050401 as originating in a two-component jet, and interpreting the break in X-ray light curve as due to lateral expansion of a narrow collimated outflow which dominates the X-ray emission. The optical emission is attributed to a wider jet component. Our model reproduces all the observed features of multiband afterglow of GRB 050401. We present optical observations of GRB 050401 using the 104-cm Sampurnanand Telescope at the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. Results of the analysis of multiband data are presented and compared with GRB 030329, the first reported case of double jet.     

The circumstellar environment of Wolf–Rayet stars and gamma-ray burst afterglows

We study the evolution of the circumstellar medium of massive stars. We pay particular attention to Wolf-Rayet stars that are thought to be the progenitors of some long gamma-ray bursts (GRBs). We detail the mass-loss rates we use in our stellar evolution models and how we estimate the stellar wind speeds during different phases. With these details we simulate the interactions between the wind and the interstellar medium to predict the circumstellar environment around the stars at the time of core-collapse. We then investigate how the structure of the environment might affect the GRB afterglow. We find that when the afterglow jet encounters the free-wind/stalled-wind interface, rebrightening occurs and a bump is seen in the afterglow light curve. However, our predicted positions of this interface are too distant from the site of the GRB to reach while the afterglow remains observable. The values of the final wind density,   A _*  , from our stellar models are of the same order (≲1) as some of the values inferred from observed afterglow light curves. We do not reproduce the lowest   A _*  values below 0.5 inferred from afterglow observations. For these cases, we suggest that the progenitors could have been a WO-type Wolf–Rayet (WR) star or a very low-metallicity star. Finally, we turn our attention to the matter of stellar wind material producing absorption lines in the afterglow spectra. We discuss the observational signatures of two WR stellar types, WC and WO, in the afterglow light curve and spectra. We also indicate how it may be possible to constrain the initial mass and metallicity of a GRB progenitor by using the inferred wind density and wind velocity.     

Gamma-ray burst afterglows: effects of radiative corrections and non-uniformity of the surrounding medium

The afterglow of a gamma-ray burst (GRB) is commonly thought to be the result of continuous deceleration of a relativistically expanding fireball in the surrounding medium. Assuming that the expansion of the fireball is adiabatic and that the density of the medium is a power-law function of shock radius, i.e. n _ext ∝  R ^{− k} , we study the effects of the first-order radiative correction and the non-uniformity of the medium on a GRB afterglow analytically. We first derive a new relation among the observed time, the shock radius and the Lorentz factor of the fireball: t _⊕ =  R /4(4− k ) γ²c, and also derive a new relation among the comoving time, the shock radius and the Lorentz factor of the fireball: t _co = 2 R /(5− k ) γc. We next study the evolution of the fireball by using the analytic solution of Blandford &38; McKee. The radiation losses may not significantly influence this evolution. We further derive new scaling laws both between the X-ray flux and observed time and between the optical flux and observed time. We use these scaling laws to discuss the afterglows of GRB 970228 and GRB 970616, and find that if the spectral index of the electron distribution is p  = 2.5, implied from the spectra of GRBs, the X-ray afterglow of GRB 970616 is well fitted by assuming k  = 2.     

Gamma—Ray Bursts：Afterglows and Central Engines

Gamma-ray bursts (GRBs) are the most intense transient gamma-ray events in the sky; this, together with the strong evidence (the isotropic and inhomogeneous distribution of GRBs detected by BASTE) that they are located at cosmological distances, makes them the most energetic events ever known. For example, the observed radiation energies of some GRBs are equivalent to the total convertion into radiation of the mass energy of more than one solar mass. This is thousand times stronger than the energy of a supernova explosion. Some unconventional energy mechanism and extremely high conversion efficiency for these mysterious events are required. The discovery of host galaxies and association with supernovae at cosmoligical distances by the recently launched satellite of BeppoSAX and ground based radio and optical telescopes in GRB afterglow provides further support to the cosmological origin of GRBs and put strong constraints on their central engine. It is the aim of this article to review the possible central engines, energy mechanisms, dynamical and spectral evolution of GRBs, especially focusing on the afterglows in multi-wavebands.     

The extreme, red afterglow of GRB 060923A: distance or dust?

Gamma-ray bursts (GRBs) are powerful probes of the early Universe, but locating and identifying very distant GRBs remain challenging. We report here the discovery of the K -band afterglow of Swift GRB 060923A, imaged within the first hour post-burst, and the faintest so far found. It was not detected in any bluer bands to deep limits, making it a candidate very high- z burst  ( z ≳ 11)  . However, our later-time optical imaging and spectroscopy reveal a faint galaxy coincident with the GRB position which, if it is the host, implies a more moderate redshift (most likely   z ≲ 2.8  ) and therefore that dust is the likely cause of the very red-afterglow colour. This being the case, it is one of the few instances so far found of a GRB afterglow with high-dust extinction.     

The possible high-energy emission from GRB 080319B and origins of the GeV emission of GRBs 080514B, 080916C and 081024B

We calculate the high-energy (sub-GeV to TeV) prompt and afterglow emission of GRB 080319B that was distinguished by a naked-eye optical flash and by an unusual strong early X-ray afterglow. There are three possible sources for high-energy emission: the prompt optical and γ-ray photons IC scattered by the accelerated electrons, the prompt photons IC scattered by the early external reverse-forward shock electrons, and the higher band of the synchrotron and the synchrotron self-Compton emission of the external shock. There should have been in total hundreds of high-energy photons detectable for the Large Area Telescope onboard the Fermi satellite, and tens of photons of those with energy >10 GeV. The >10 GeV emission had a duration about twice that of the soft γ-rays. Astro-rivelatore Gamma a Immagini Leggero (AGILE) could have observed these energetic signals if it was not occulted by the Earth at that moment. The physical origins of the high-energy emission detected in GRB 080514B, GRB 080916C and GRB 081024B are also discussed. These observations seem to be consistent with the current high-energy emission models.     

GeV emission from gamma-ray burst afterglows

We calculate the GeV afterglow emission expected from a few mechanisms related to gamma-ray bursts (GRBs) and their afterglows. Given the brightness of the early X-ray afterglow emission measured by Swift /X-Ray Telescope, Gamma-ray Large Area Space Telescope (GLAST)/Large Area Telescope (LAT) should detect the self-Compton emission from the forward shock driven by the GRB ejecta into the circumburst medium. Novel features discovered by Swift in X-ray afterglows (plateaus and chromatic light-curve breaks) indicate the existence of a pair-enriched, relativistic outflow located behind the forward shock. Bulk and inverse-Compton upscattering of the prompt GRB emission by such outflows provide another source of GeV afterglow emission detectable by LAT. The large-angle burst emission and synchrotron forward-shock emission are, most likely, too dim at high photon energy to be observed by LAT. The spectral slope of the high-energy afterglow emission and its decay rate (if it can be measured) allow the identification of the mechanism producing the GeV transient emission following GRBs.