Class Of Gamma-ray Bursts Research Articles

Swift observations of gamma-ray bursts

Since its launch on 20 November 2004, the Swift mission has been detecting approximately 100 gamma-ray bursts (GRBs) each year, and immediately (within approx. 90s) starting simultaneous X-ray and UV/optical observations of the afterglow. It has already collected an impressive database, including prompt emission to higher sensitivities than BATSE, uniform monitoring of afterglows and a rapid follow-up by other observatories notified through the GCN. Advances in our understanding of short GRBs have been spectacular. The detection of X-ray afterglows has led to accurate localizations and the conclusion that short GRBs can occur in non-star-forming galaxies or regions, whereas long GRBs are strongly concentrated within the star-forming regions. This is consistent with the NS merger model. Swift has greatly increased the redshift range of GRB detection. The highest redshift GRBs, at z approximately 5-6, are approaching the era of reionization. Ground-based deep optical spectroscopy of high redshift bursts is giving metallicity measurements and other information on the source environment to a much greater distance than other techniques. The localization of GRB 060218 to a nearby galaxy, and the association with SN 2006aj, added a valuable member to the class of GRBs with detected supernova.

Swift-BAT results on the prompt emission of short bursts

This is a brief review of short hard bursts (SHBs) from previous missions and from Swift-BAT; in particular, a review of the developing class of gamma-ray bursts which are similar to SHBs in that they have the short hard initial spike (0.1 to a few seconds), but that they also have a long extended phase of soft emission (50-200s). Further, we suggest that a class of events discovered by Horvath in the T90 versus hardness ratio plane is this SHB with extended emission.

Gamma-ray bursts in the Swift era

Gamma-ray burst (GRB) research has undergone a revolution in the last two years. The launch of Swift, with its rapid slewing capability, has greatly increased the number and quality of GRB localizations and x-ray and optical afterglow lightcurves. Over 160 GRBs have been detected, and nearly all have been followed up with the on-board narrow field telescopes. Advances in our understanding of short GRBs have been spectacular. The detection of x-ray afterglows has led to accurate localizations from ground based observatories, which have given host identifications and redshifts. Theoretical models for short GRB progenitors have, for the first time, been placed on a sound foundation. The hosts for the short GRBs differ in a fundamental way from the long GRB hosts: short GRBs tend to occur in non-star forming galaxies or regions, whereas long GRBs are strongly concentrated within star forming regions. Observations are consistent with a binary neutron star merger model, but other models involving old stellar populations are also viable. Swift has greatly increased the redshift range of GRB detection. The highest redshift GRBs, at z∼5–6, are approaching the era of reionization. Ground-based deep optical spectroscopy of high redshift bursts is giving metallicity measurements and other information on the source environment to much greater distance than other techniques. The localization of GRB 060218 to a nearby galaxy, and association with SN 2006aj, added a valuable member to the class of GRBs with a detected supernova. The prospects for future progress are excellent given the >10 year orbital lifetime of the Swift satellite.

X-ray flashes or soft gamma-ray bursts?

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.

Properties of X-ray rich gamma ray bursts and X-ray flashes detected with BeppoSAX and Hete-2

We study the spectrum of the prompt emission and the X-ray and optical afterglow fluxes of 54 X-Ray Rich Gamma Ray Burst (XRRs) and X-Ray Flashes (XRFs), observed by BeppoSax and HETE-2. A comparison is then performed with classical Gamma Ray Bursts (GRBs). The goal of this paper is to investigate the nature of XRRs/XRFs, as high redshift GRBs or off-axis GRBs, analyzing both their prompt and afterglow properties. We find that the XRR/XRF spectral indexes of the Band function are similar to those of classical GRBs, whereas the peak energy is lower by a factor of 4. We study the optical and X-Ray afterglow properties of the XRRs/XRFs; in particular we analyze the XRR 011030 afterglow. We find that the X-ray and optical flux distributions and the lightcurves of the XRRs/XRFs sample are consistent with those of classical GRBs; in particular, they show evidence of a break and no rising temporal slope. We compare these results with the afterglow predictions of the high redshift scenario, where XRFs are GRBs at higher redshift and of the off-axis scenario, where the observed differences are due to viewing angle effects. In this last framework, we consider jets with a homogeneous, a -2 power-law shaped and a Gaussian luminosity angular distribution. We find that the high redshift scenario can explain some events but not the total sample of XRRs/XRFs. The off-axis model may be consistent with our findings when a homogeneous jet is considered. However, given the uncertainties on the selection effects in our sample, a Gaussian jet viewed at small angles from the Gaussian core and a power-law shaped cannot be ruled out.

An optical supernova associated with the X-ray flash XRF 060218

Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae that are more luminous than average and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB-supernovae were thought to be rare events. Whether X-ray flashes--analogues of GRBs, but with lower luminosities and fewer gamma-rays--can also be associated with supernovae, and whether they are intrinsically 'weak' events or typical GRBs viewed off the axis of the burst, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB-supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output and the supernova radio flux. Our data, combined with radio and X-ray observations, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB-supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB-supernovae.

The Ep,i-Eiso correlation in gamma-ray bursts: updated observational status, re-analysis and main implications

The correlation between the cosmological rest-frame vF v spectrum peak energy, E p,i , and the isotropic-equivalent radiated energy, E iso , discovered by Amati et al. in 2002 and confirmed/extended by subsequent osbervations, is one of the most intriguing and debated observational evidences in gamma-ray burst (GRB) astrophysics. In this paper, I provide an update and a re-analysis of the E p,i -E iso correlation basing on an updated sample consisting of 41 long GRBs/X-ray flashes (XRFs) with firm estimates of z and observed peak energy, E p,obs , 12 GRB s with uncertain values of z and/or E p,obs , two short GRBs with firm estimates of z and E p,obs and the peculiar subenergetic events GRB 980425/SN1998bw and GRB 031203/SN20031w. In addition to standard correlation analysis and power-law fitting, the data analysis here reported includes modelling that accounts for sample variance. All 53 classical long GRBs and XRFs, including 11 Swift events with published spectral parameters and fluences, have E p,i and E iso values, or upper/lower limits, consistent with the correlation, which shows a chance probability as low as ∼7 x 10 -15 , a slope of ∼0.57 (∼0.5 when fitting by accounting for sample variance) and an extra-Poissonian logarithmic dispersion of ∼0.15, it extends over ∼5 orders of magnitude in E iso and ∼3 orders of magnitude in E p,i and holds from the closer to the higher z GRBs. Subenergetic GRBs (980425 and possibly 031203) and short GRBs are found to be inconsistent with the E p,i -E iso correlation, showing that it can be a powerful tool for discriminating different classes of GRBs and understanding their nature and differences. I also discuss the main implications of the updated E p,i -E iso correlation for the models of the physics and geometry of GRB emission, its use as a pseudo-redshift estimator and the tests of possible selection effects with GRBs of unknown redshift.

The Importance of Off‐Jet Relativistic Kinematics in Gamma‐Ray Burst Jet Models

Gamma-Ray Bursts (GRBs) are widely thought to originate from collimated jets of material moving at relativistic velocities. Emission from such a jet should be visible even when viewed from outside the angle of collimation. I summarize recent work on the special relativistic transformation of the burst quantities Eiso and Epeak as a function of viewing angle, where Eiso is the isotropic-equivalent energy of the burst and Epeak is the peak of the burst spectrum in the power nu F_nu. The formulae resulting from this work serve as input for a Monte Carlo population synthesis method, with which I investigate the importance of off-jet relativistic kinematics as an explanation for a class of GRBs termed X-ray Flashes (XRFs). I do this in the context of several top-hat shaped variable opening-angle jet models. I find that such models predict a large population of off-jet bursts that are observable and that lie away from the Epeak ~ Eiso^(1/2) relation. The predicted burst populations are not seen in current datasets. I investigate the effect of the bulk gamma value upon the properties of this population of off-jet bursts, as well as the effect of including an Omega0-Egamma correlation to jointly fit the Epeak ~ Eiso^(1/2) and Epeak ~ Egamma^(beta) relations, where Omega0 is the opening solid angle of the GRB jet. I find that the XRFs seen by HETE-2 and BeppoSAX cannot be easily explained as classical GRBs viewed off-jet. I also find that an inverse correlation between gamma and Omega0 has the effect of greatly reducing the visibility of off-jet events. Therefore, unless gamma > 300 for all bursts or unless there is a strong inverse correlation between gamma and Omega0, top-hat variable opening-angle jet models produce a significant population of bursts away from the Epeak ~ Eiso^(1/2) and Epeak ~ Egamma^(beta) relations.

Producing Ultrastrong Magnetic Fields in Neutron Star Mergers

We report an extremely rapid mechanism for magnetic field amplification during the merger of a binary neutron star system. This has implications for the production of the short class of gamma-ray bursts, which recent observations suggest may originate in such mergers. In detailed magnetohydrodynamic simulations of the merger process, the fields are amplified by Kelvin-Helmholtz instabilities beyond magnetar field strength and may therefore represent the strongest magnetic fields in the universe. The amplification occurs in the shear layer that forms between the neutron stars and on a time scale of only 1 millisecond, that is, long before the remnant can collapse into a black hole.

Study of Very Short Gamma-Ray Bursts: New Results from BATSE and Konus

Our recent studies of very short gamma-ray bursts (GRBs) of T90 ≤ 100 ms duration have indicated a significant angular asymmetry and a uniform V/Vmax distribution from the BATSE (Burst and Transient Source Experiment on the Compton Gamma Ray Observatory [CGRO]) data. Here we update these studies, and we extend our research to events observed with Konus, a GRB experiment from Russia on the International Solar Terrestrial Physics/Wind satellite, that gives us new insight into the spectra that was not possible with the BATSE data. Konus has observed 18 events with T90 ≤ 100 ms duration. These events display a considerable number of photons with energies above 1 MeV and have photons above 5 MeV in all cases. These appear to be some of the most energetic photons observed from any class of GRB to date.

Gamma-ray bursts from quark stars

Abstract Long gamma-ray bursts (GRBs) are believed to be related to the explosion of Type Ic supernovae, which have been stripped of their hydrogen and helium envelopes. There appear to be two types of these explosions: those that are approximately spherical (GRB 980425/SN 1998bw), and which are associated with weak bursts; and the classical GRBs which generate ultrarelativistic jets (GRB 030329/SN 2003dh). If this bimodality is real, Swift will provide clear evidence for it. We propose that classical powerful GRBs, which generate ultrarelativistic outflows, are a result of the formation of quark stars. Quark stars may provide additional energy for the explosion of Type Ic supernovae, but far more important is the creation of a surface which acts as a membrane that cannot be penetrated by baryons. A surface of a quark star allows only ultrarelativistic matter to escape: photons, neutrinos, electron–positron pairs and magnetic fields. The formation of a quark star follows several minutes after the initial core collapse. Possible evidence for this time delay is provided by BATSE precursors to GRBs, as analysed by Lazzati.

On the nature of X-ray flashes

We discuss the origin of X-Ray Flashes (XRFs), a recently discovered class of Gamma-Ray Bursts (GRBs). Using a simplified model for internal shocks we check if XRFs can be intrinsically soft due to some specific values of the parameters describing the relativistic outflow emerging from the central engine. We generate a large number of synthetic events and find that XRFs are obtained when the contrast $\Gamma_{\rm max}/\Gamma_{\rm min}$ of the Lorentz factor distribution is small while the average Lorentz factor $\bar \Gamma$ is large. A few XRFs may be GRBs at large redshifts but we exclude this possibility for the bulk of the population. If outflows with a small contrast are commonly produced, even a large population of XRFs could be explained. If conversely the Lorentz factor distribution within the wind is broad, one should then rely on extrinsic causes, such as viewing angle effects or high redshift.

X-ray Lines in Gamma-ray Bursts and Cerenkov Line Mechanism

X-ray emission and absorption features are of great importance in our understanding the nature and environment of gamma-ray bursts (GRBs). So far, iron emission lines have been detected in at least four GRB afterglows. In this paper, the observational properties and physical constraints on materials surrounding GRB sources are reviewed, and several classes of theoretical models are also discussed. We will specially concentrate on the Cerenkov line mechanism, in which the broad iron lines are expected, and a small mass of Fe is required to produce the large line luminosity. In addition, our interpretation can favor the recent jet unified model for different classes of gamma-ray bursts with a standard energy reservoir.

Simulating a faint gamma-ray burst population

Abstract There have now been three supernova-associated gamma-ray bursts (GRBs) at redshift z < 0.17, namely 980425, 030329 and 031203, but the nearby and under-luminous GRBs 980425 and 031203 are distinctly different from the ‘classical’ or standard GRBs. It has been suggested that they could be classical GRBs observed away from their jet axes, or they might belong to a population of under-energetic GRBs. Recent radio observations of the afterglow of GRB 980425 suggest that different engines may be responsible for the observed diversity of cosmic explosions. Given this assumption, a crude constraint on a luminosity function for faint GRBs with a mean luminosity similar to that of GRB 980425 and an upper limit on the rate density of 980425-type events, we simulate the redshift distribution of under-luminous GRBs assuming BATSE and Swift sensitivities. A local rate density of about 0.6 per cent of the local supernova Type Ib/c rate yields simulated probabilities for under-luminous events to occur at rates comparable to the BATSE GRB low-redshift distribution. In this scenario the probability of BATSE/HETE detecting at least one GRB at z < 0.05 is 0.78 over 4.5 years, a result that is comparable with observation. Swift has the potential to detect 1–5 under-luminous GRBs during one year of observation.

Distributions of the hardness ratio of short and long gamma-ray bursts in different time intervals within the first two seconds

In the present paper, we investigate the distribution of the hardness ratio (HR) for short and long gamma-ray bursts (GRBs) in different time-scales for the first two seconds. After including and subtracting the background count, we performed a Kolmogorov-Smirnov (K-S) test on the HR distributions of the two classes of GRBs in each time interval. Our analysis shows that the probabilities of the K-S test to the distributions are very small, suggesting that the two classes of bursts are unlikely to arise from the same HR distributions. The result indicates that the two kinds of bursts probably originate from different mechanisms or have different central engines. In addition, we found that the HR of short bursts within the time interval 0-0.96 s changes from hard to soft; the HR of long bursts does not. The two kinds of bursts have different characteristics in the first two seconds, which might be associated with different physical mechanisms.

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.

Gamma-ray Bursts

SummarySince their discovery in 1967 Gamma-ray bursts (GRBs) have been puzzling to astrophysicists. With the advent of a new generation of X–ray satellites in the late 90’s, it was possible to carry out deep multi-wavelength observations of the counterparts associated with the long duration GRBs class just within a few hours of occurrence, thanks to the observation of the fading X-ray emission that follows the more energetic gamma-ray photons once the GRB event has ended. The fact that this emission (the afterglow) extends at longer wavelengths, led to the discovery of optical/IR/radio counterparts in 1997-2003, greatly improving our understanding of these sources. The classical, long duration GRBs, have been observed to originate at cosmological distances in a range of redshifts with 0.1685 ≤ z ≤ 4.50 implying energy releases of ~ 1051 ergs. The recent results on GRB 021004 and GRB 030329 confirm that the central engines that power these extraordinary events are due to be collapse of massive stars rather than the merging of compact objects as previously also suggested. Short GRBs still remain a mystery as no counterparts have been detected so far.

The Role ofT50in the Classification of Gamma-ray Bursts

The role of T-50 in classifying gamma-ray bursts (GRBs) is investigated. We take T-50 = 0.7 s as the line of division and find that some bursts belonging to the class of long bursts defined by T-90 > 2 s now become short bursts (sample 1) while some belonging to the class of short bursts defined by T-90 < 2 s now become long bursts (sample 2). We study how these sources are affected by the two methods of classification and find the change of classes of sample 1 is due to some peculiar properties of the light curves. Based on their characters, most of the bursts of sample 1 should be taken as short bursts.

On the Afterglow of the X‐Ray Flash of 2003 July 23: Photometric Evidence for an Off‐Axis Gamma‐Ray Burst with an Associated Supernova?

We present optical and near-infrared follow-up observations of the X-ray flash (XRF) of 2003 July 23. Our observations in the R band cover the temporal range from 4.2 hr to 64 days after the high-energy event. We also present the results of multicolor imaging extending to the K band on three epochs. The light curve of the R-band afterglow the first week after the burst is similar to the light curve for long-duration gamma-ray bursts (GRBs), i.e., a broken power law with a late time slope of � � 2: 0( F� / t � � ). Furthermore, the spectral energy distribution (SED) has a power-law (F� / � � � ) shape with slope � � 1:0. However, the decay slope at t < 1d ay is shallow, consistent with zero. This is in qualitative agreement with the prediction that XRFs are off-axis classical GRBs. After the first week there is a strong bump in the light curve, which peaks at around 16 days. The SED after the peak becomes significantly redder. We discuss the possible interpretations of this bump and conclude that an underlying supernova is the most likely explanation since no other model appears consistent with the evolution of the SED. Finally, we present deep spectroscopy of the burst both in the afterglow and in the bump

Analysis Methods and Results of a Search for Weak Gamma‐Ray Bursts in the BATSE Data

We report initial results on the statistical properties of the dimmest gamma-ray bursts (GRBs) observed with the Burst and Transient Source Experiment (BATSE), using new ground-based methods to obtain a sample of GRBs from 502 days of BATSE data. Using the most sensitive ground-based detection of GRBs, the sample extends to GRBs much fainter than those detected by the onboard trigger, but because of the temporal resolution of the data, the sample is limited to GRBs of duration of at least 2 s. For each detected event, Bayesian probabilities are calculated for the event to belong to each of seven classes of differing physical origins. The sample of GRB candidates is defined by the requirement that the Bayesian probability for belonging to the GRB class is higher than 0.5. The intensity distribution of the GRB sample is corrected using a Monte Carlo simulation of the postflight detection efficiency. The sky distribution of the dimmest BATSE bursts is consistent with isotropy. The dimmest bursts are softer than the brightest triggered bursts.