Bright Gamma-ray Bursts Research Articles

Precursor activity in bright, long BATSE gamma-ray bursts

We study a sample of bright, long Burst and Transient Source Experiment (BATSE) gamma-ray burst (GRB) light curves in the 200 s before the detection of the GRB prompt emission. We find that in a sizable fraction of cases (∼20 per cent) there is evidence of emission above the background coming from the same direction as the GRB. This emission is characterized by a softer spectrum with respect to the main spectrum and contains a small fraction (0.1‐1 per cent) of the total event counts. The precursors have typical delays of several tens of seconds extending (in few cases) up to 200 s (the limit of the investigated period). Their spectra are typically non-thermal power-law spectra, aside from a few cases. Such long delays and the non-thermal origin of their spectra are hard to reconcile with any model for the progenitor. Ke yw ords: radiation mechanisms: non-thermal ‐ gamma-rays: bursts.

An apparently normal gamma-ray burst with an unusually low luminosity.

Much of the progress in understanding gamma-ray bursts (GRBs) has come from studies of distant events (redshift z approximately 1). In the brightest GRBs, the gamma-rays are so highly collimated that the events can be seen across the Universe. It has long been suspected that the nearest and most common events have been missed because they are not as collimated or they are under-energetic (or both). Here we report soft gamma-ray observations of GRB 031203, the nearest event to date (z = 0.106; ref. 2). It had a duration of 40 s and peak energy of >190 keV, and therefore appears to be a typical long-duration GRB. The isotropic gamma-ray energy of < or =10(50) erg, however, is about three orders of magnitude smaller than that of the cosmological population. This event--as well as the other nearby but somewhat controversial GRB 980425--is a clear outlier from the isotropic-energy/peak-energy relation and luminosity/spectral-lag relations that describe the majority of GRBs. Radio calorimetry shows that both of these events are under-energetic explosions. We conclude that there does indeed exist a large population of under-energetic events.

Early (&lt;0.3 Days) R -Band Light Curve of the Optical Afterglow of GRB 030329

We observed the optical afterglow of the bright gamma-ray burst GRB 030329 on the night of 2003 March 29 using the Kiso Observatory (University of Tokyo) 1.05 m Schmidt telescope. Data were taken from March 29 13:21:26-17:43:16 UT (0.072-0.253 days after the burst) using an RC-band filter. The obtained RC-band light curve has been fitted successfully by a single-power-law function with a decay index of 0.891 ± 0.004. These results remain unchanged when incorporating two early photometric data points at 0.065 and 0.073 days, reported by Price et al. using the Siding Spring Observatory 40 inch (1.0 m) telescope, and when also including data from the Russian-Turkish 1.5 m Telescope (Burenin et al.) covering about 0.3 days. Over the period of 0.065-0.285 days after the burst, any deviation from the power-law decay is smaller than ±0.007 mag. The temporal structure reported by Uemura et al. does not show up in our R-band light curve.

The X-ray afterglow of GRB 030329

We report on XMM-Newton and Rossi-XTE observations of the bright (fluence ~10 -4 erg cm -2 ) and nearby ($z=0.1685$) Gamma-Ray Burst GRB 030329 associated to SN2003dh. The first Rossi-XTE observation, 5 hours after the burst, shows a flux decreasing with time as a power law with index $0.9\pm0.3$. Such a decay law is only marginally consistent with a further Rossi-XTE measurement (at $t-t_{\rm GRB}\sim 30$ hr). Late time observations of this bright afterglow at X-ray wavelengths have the advantage, compared to optical observations, of not being affected by contributions from the supernova and host galaxy. A first XMM-Newton observation, at $t-t_{\rm GRB}\sim 37$ days, shows a flux of $4\times10^{-14}$ erg cm -2 s -1 (0.2–10 keV). The spectrum is a power law with photon index $\Gamma=1.9$ and absorption -2 , consistent with the Galactic value. A further XMM-Newton pointing at $t-t_{\rm GRB}\sim61$ days shows a flux fainter by a factor ~2. The combined Rossi-XTE and XMM-Newton measurements require a break at $t\sim 0.5$ days in the afterglow decay, with a power law index increasing from 0.9 to 1.9, similar to what is observed in the early part of the optical afterglow. The extrapolation of the XMM-Newton spectra to optical frequencies lies a factor of ~10 below simultaneous measurements. This is likely due to the presence of SN2003dh.

A Comparative Study of the X‐Ray Afterglow Properties of Optically Bright and Dark Gamma‐Ray Bursts

We have examined the complete set of X-ray afterglow observations of dark and optically bright gamma-ray bursts (GRBs) performed by BeppoSAX through 2001 February. X-ray afterglows are detected in ~90% of the cases. We do not find significant differences in the X-ray spectral shape, in particular no increased X-ray absorption in GRBs without optical transient (dark GRBs) compared to GRBs with optical transient (OTGRBs). Rather, we find that the 1.6-10 keV flux of OTGRBs is on average about 5 times larger than that of the dark GRBs. A Kolmogorov-Smirnov test shows that this difference is significant at 99.8% probability. Under the assumption that dark and OTGRBs have similar spectra, this could suggest that the first are uncaught in the optical band because they are just faint sources. In order to test this hypothesis, we have determined the optical-to-X-ray flux ratios of the sample. OTGRBs show a remarkably narrow distribution of flux ratios, which corresponds to an average optical-to-X-ray spectral index = 0.794 ? 0.054. We find that, while 75% of dark GRBs have flux ratio upper limits still consistent with those of OT GRBs, the remaining 25% are 4-10 times weaker in optical than in X-rays. The significance of this result is ?2.6 ?. If this subpopulation of dark GRBs were constituted by objects assimilable to OTGRBs, they should have shown optical fluxes higher than upper limits actually found. We discuss the possible causes of their behavior, including a possible occurrence in high-density clouds or origin at very high redshift and a connection with ancient, Population III stars.

The Broadband Afterglow of GRB 980703

We present radio observations of the afterglow of the bright gamma-ray burst GRB980703 made between one day and one year after the burst. These data are combined with published late-time radio measurements and existing optical, near-infrared (NIR) and X-ray observations to create a comprehensive broadband dataset for modeling the physical parameters of the outflow. While a wind-stratified medium cannot be ruled out statistically, it requires a high fraction of the shock energy in the electrons, and so is not favored on theoretical grounds. Instead, the data are consistent with a fireball model in which the ejecta are collimated and expanding into a constant density medium. The radio data cannot be fit with an isotropic shock but instead require a jet break at ~ 3.5 days, not seen at optical wavelengths due to the presence of a a bright host galaxy. The addition of the full radio dataset constrains the self-absorption frequency, giving an estimate of the circumburst density of n ~ 30 cm^-3, a value which differs substantially from previous estimates. This result is consistent with the growing number of GRB afterglows for which broadband modeling yields n ~= 0.1-100 cm^-3, with a typical value ~ 10 cm^-3.

Continued Radio Monitoring of the Gamma‐Ray Burst 991208

We present radio observations of the afterglow of the bright gamma-ray burst GRB 991208 at frequencies of 1.4, 4.9 and 8.5 GHz, taken between two weeks and 300 days after the burst. The well-sampled radio light curve at 8.5 GHz shows that the peak flux density peaked about 10 days after the burst and decayed thereafter as a power-law t^-1.07. This decay rate is more shallow than the optical afterglow with t^-2.2, which was measured during the first week. These late-time data are combined with extensive optical, millimeter and centimeter measurements and fitted to the standard relativistic blast wave model. In agreement with previous findings, we find that an isotropic explosion in a constant density or wind-blown medium cannot explain these broadband data without modifying the assumption of a single power-law slope for the electron energy distribution. A jet-like expansion provides a reasonable fit to the data. In this case, the flatter radio light curve compared to the optical may be due to emission from an underlying host galaxy, or due to the blastwave making a transition to non-relativistic expansion. The model that best represents the data is a free-form model in which it is assumed that the broadband emission originates from a synchrotron spectrum, while the time-evolution of the break frequencies and peak flux density are solved for explicitly. Although the decay indices for most of the synchrotron parameters are similar to the jet model, the evolution of the cooling break is unusually rapid, and therefore requires some non-standard evolution in the shock. (abridged)

Gamma photometric redshifts for long gamma-ray bursts

Max-Planck-Institut fu¨r Astronomie, D-69117 Heidelberg, 17 K¨onigstuhl, Germanye-mail: vavrek@mpia-hd.mpg.deReceived 18 June, 2002/Accepted 18 November 2002Abstract. It is known that the soft tail of the gamma-ray bursts’ spectra show excesses from the exact power-lawdependence. In this article we show that this departure can be detected in the peak flux ratios of different BATSEDISCSC energy channels. This effect allows to estimate the redshift of the bright long gamma-ray bursts in theBATSE Catalog. A verification of these redshifts is obtained for the 8 GRB which have both BATSE DISCSCdata and measured optical spectroscopic redshifts. There is good correlation between the measured and estimatedredshifts, and the average error is ∆z ≈ 0.33. The method is similar to the photometric redshift estimation ofgalaxies in the optical range, hence it can be called as ”gamma photometric redshift estimation”. The estimatedredshifts for the long bright gamma-ray bursts are up to z ≃ 4. For the the faint long bursts - which should beup to z ≃ 20 - the redshifts cannot be determined unambiguously with this method.Key words. Cosmology: large-scale structure of Universe – gamma-rays: bursts

Evidence for a Fast Decline in the Progenitor Population of Gamma‐Ray Bursts and the Nature of Their Origin

We show that the source population of long gamma-ray bursts (GRBs) has declined by at least a factor of 12 (at the 90% confidence level) since the early stages of the Universe ($z \sim 2 - 3$). This result has been obtained using the combined BATSE and \it Ulysses \rm GRB brightness distribution and the detection of four GRBs with known redshifts brighter than 10$^{52}$ erg s$^{-1}$ in the 50 - 300 keV range at their peak. The data indicate that the decline of the GRB source population is as fast as, or even faster than, the measured decline of the star formation rate. Models for the evolution of neutron star binaries predict a significantly larger number of apparently bright GRBs than observed. Thus our results give independent support to the hypernova model, which naturally explains the fast decline in the progenitor population.

Time resolved spectral analysis of bright gamma ray bursts

We present the time integrated and time resolved spectral analysis of a sample of bright bursts selected with Fpeak 20 phot cm 2 s 1 from the BATSE archive. We fitted four dierent spectral models to the pulse time integrated and time resolved spectra. We compare the low energy slope of the fitted spectra with the prediction of the synchrotron theory (predicting photon spectra softer than N(E) / E 2=3 ), and test, through direct spectral fitting, the synchrotron shock model. We point out that dierences in the parameters distribution can be ascribed to the dierent spectral shape of the models employed and that in most cases the spectrum can be described by a smoothly curved function. The synchrotron shock model does not give satisfactory fits to the time averaged and time resolved spectra. Finally, we derive that the synchrotron low energy limit is violated in a considerable number of spectra both during the rise and decay phase around the peak.

The Broadband Afterglow of GRB 980329

We present radio observations of the afterglow of the bright gamma-ray burst GRB980329 made between one month and several years after the burst, a re-analysis of previously published submillimeter data, and late-time optical and near-infrared (NIR) observations of the host galaxy. From the absence of a spectral break in the optical/NIR colors of the host galaxy, we exclude the earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our data with the numerous multi-wavelength observations of the early afterglow, fit a comprehensive afterglow model to the entire broadband dataset, and derive fundamental physical parameters of the blast-wave and its host environment. Models for which the ejecta expand isotropically require both a high circumburst density and extreme radiative losses from the shock. No low density model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51} erg, with the ejecta narrowly collimated to an opening angle of a few degrees, driven into a surrounding medium with density ~ 20 cm^{-3}, provides a satisfactory fit to the lightcurves over a range of redshifts.

GRB 010921: Localization and Observations by the [ITAL]High Energy Transient Explorer[/ITAL] Satellite

On 2001 September 21 at 05:15:50.56 UT, the French Gamma Telescope (FREGATE) on the High Energy Transient Explorer (HETE) detected a bright gamma-ray burst (GRB). The burst was also seen by the X-detector on the Wide-field X-ray Monitor (WXM) instrument and was therefore well localized in the X-direction; however, the burst was outside the fully coded field of view of the WXM Y-detector, and therefore information on the Y-direction of the burst was limited. Cross-correlation of the HETE and Ulysses time histories yielded an Interplanetary Network (IPN) annulus that crosses the HETE error strip at an ~45° angle. The intersection of the HETE error strip and the IPN annulus produces a diamond-shaped error region for the location of the burst having an area of 310 arcmin2. Based on the FREGATE and WXM light curves, the duration of the burst is characterized by t90 = 34.2 s in the WXM 4-25 keV energy range, and 23.8 and 21.8 s in the FREGATE 6-40 and 32-400 keV energy ranges, respectively. The fluence of the burst in these same energy ranges is 4.8 × 10-6, 5.5 × 10-6, and 11.4 × 10-6 ergs cm-2, respectively. Subsequent optical and radio observations by ground-based observers have identified the afterglow of GRB 010921 and determined an apparent redshift of z = 0.450.

GRB 010222: A Burst within a Starburst

We present millimeter and submillimeter wavelength observations and near-infrared K-band imaging toward the bright gamma-ray burst GRB 010222. Over seven epochs the flux density of the source was constant with an average flux density 3.74+/-0.53 mJy at 350 GHz and 1.05+/-0.22 mJy at 250 GHz, giving a spectral index alpha=3.78+/-0.25. We rule out the possibility that this emission originated from the burst or its afterglow and we conclude that it is due to a dusty, high redshift starburst galaxy (SMM J14522+4301). We argue that the host galaxy of GRB 010222 is the most plausible counterpart of SMM J14522+4301, based in part on the centimeter detection of the host at the expected level. The optical/NIR properties of the host galaxy of GRB 010222 suggest that it is a blue, sub-L_*, similar to other GRB host galaxies. This contrasts with the enormous far-infrared luminosity of this galaxy based on our submillimeter detection (L_Bol~4X10^{12} L_odot). We suggest that this GRB host galaxy has a very high star formation rate, SFR 600 M_\odot/yr, most of which is unseen at optical wavelengths.

Radiation from Collision‐dominated Relativistic Pair Fireballs

It is generally accepted that gamma-ray bursts (GRBs) are initiated by a relativistic pair fireball, converting its internal energy into kinetic energy of a relativistically moving plasmoid and subsequently into radiation. Here, we investigate the early stages of this evolution, after the pair fireball has become optically thin to gamma-gamma pair production. We show that for a short period of time, ~ 0.1 - a few seconds after the initial explosion, the pair plasmoid evolution might be dominated by collisional processes prior to the formation of a collisionless shock. We simulate these processes during the early pair plasmoid evolution and calculate the expected radiative signatures. We show that the radiation from the collision-dominated pair plasmoid phase results in a short (~ a few ms) flash of thermal soft X-ray emission, followed by a transition phase of < 1 s during which the fireball turns Thomson thin, but its radiation remains dominated by thermal Comptonization, peaking at around E_pk ~ 100 MeV - a few GeV. While the very early thermal emission could be associated with the quasi-thermal radiation signatures found in the very early phases of several bright BATSE GRBs, the predicted subsequent flash of high-energy emission should be easily detectable with the GLAST satellite.

Temporal properties of the short gamma-ray bursts

A temporal analysis has been performed on a sample of 100 bright gamma-ray bursts (GRBs) with T90 2 s. The two sub-classes of GRBs appear to have the same emission mechanism which is probably caused by internal shocks. They may not have the same progenitors because of the generic nature of the reball model.

The Redshift of the Optical Transient Associated with GRB 010222

The gamma-ray burst (GRB) 010222 is the brightest GRB detected to date by the BeppoSAX satellite. Prompt identification of the associated optical transient (OT) allowed for spectroscopy with the Tillinghast 1.5 m telescope at F. L. Whipple Observatory while the source was still relatively bright (R ≃ 18.6 mag), within 5 hr of the burst. The OT shows a blue continuum with many superposed absorption features corresponding to metal lines at z = 1.477, 1.157, and possibly also 0.928. The redshift of GRB 010222 is therefore unambiguously placed at z ≥ 1.477. The high number of Mg II absorbers and especially the large equivalent widths of the Mg II, Mg I, and Fe II absorption lines in the z = 1.477 system further argue either for a very small impact parameter or that the z = 1.477 system is the GRB host galaxy itself. The spectral index of the OT is relatively steep, Fν ∝ ν-0.89±0.03, and this cannot be caused by dust with a standard Galactic extinction law in the z = 1.477 absorption system. This spectroscopic identification of the redshift of GRB 010222 shows that prompt and well-coordinated follow-up of bright GRBs can be successful even with telescopes of modest aperture.

Accretion Models of Gamma‐Ray Bursts

Many models of gamma-ray bursts (GRBs) involve accretion onto a compact object, usually a black hole, at a mass accretion rate on the order of a fraction of a solar mass per second. If the accretion disk is larger than a few tens or hundreds of Schwarzschild radii, the accretion will proceed via a convection-dominated accretion flow (CDAF) in which most of the matter escapes to infinity rather than falling onto the black hole. Models involving the mergers of black hole-white dwarf binaries and black hole-helium star binaries fall in this category. These models are unlikely to produce GRBs since very little mass reaches the black hole. If the accretion disk is smaller, then accretion will proceed via neutrino cooling in a neutrino-dominated accretion disk (NDAF) and most of the mass will reach the center. Models involving the mergers of double neutron star binaries and black hole-neutron star binaries fall in this category and are capable of producing bright GRBs. If the viscosity parameter α in the NDAF has a standard value of ~0.1, these mergers can explain short GRBs with durations under a second, but they are unlikely to produce long GRBs with durations of tens or hundred of seconds. If the accretion disk is fed by fallback of material after a supernova explosion, as in the collapsar model, then the timescale of the burst is determined by fallback, not accretion. Such a model can produce long GRBs. Fallback models again require that the accretion should proceed via an NDAF rather than a CDAF in order for a significant amount of mass to reach the black hole. This condition imposes an upper limit on the radius of injection of the gas.

TeV gamma-ray emission from gamma-ray bursts and ultra high energy cosmic rays

Some recent experiments detecting very high energy (VHE) gamma-rays above 10-20 TeV, independently reported VHE bursts for some bright gamma-ray bursts (GRBs), although further confirmation is necessary. If these signals are truly from GRBs, these GRBs must emit a much larger amount of energy as VHE gamma-rays than in the ordinary photon energy range of GRBs (keV-MeV). We show that such extreme phenomena can be reasonably explained by synchrotron radiation of protons accelerated to eV. Protons must carry a much larger amount of energy than electrons, suggesting that energy transfer from protons into electrons in shocked matter is very inefficient. The total energy of GRBs becomes as large as ergs and hence a strong beaming of GRB emission is highly likely. There may also be TeV emission in afterglow phase from external shocks, and proton synchrotron in this phase gives a quantitative explanation for the famous long duration GeV emission from GRB 940217. Most TeV gamma-rays are absorbed in intergalactic space by interactions with the infrared and microwave background radiations, and finally form gamma-ray background in GeV range. This process may explain the extragalactic background radiation observed in the EGRET range. We also discuss absorption of TeV gamma-rays in the shocked region of GRBs by the process, which may provide a new channel for energy transfer from protons into electrons (and positrons). This process should be important for the energetics of GRBs.

On the Time Evolution of Gamma-Ray Burst Pulses: A Self-Consistent Description.

For the first time, the consequences of combining two well-established empirical relations that describe different aspects of the spectral evolution of observed gamma-ray burst (GRB) pulses are explored. These empirical relations are (1) the hardness-intensity correlation and (2) the hardness-photon fluence correlation. From these we find a self-consistent, quantitative, and compact description for the temporal evolution of pulse decay phases within a GRB light curve. In particular, we show that in the case in which the two empirical relations are both valid, the instantaneous photon flux (intensity) must behave as 1&solm0;&parl0;1+t&solm0;tau&parr0;, where tau is a time constant that can be expressed in terms of the parameters of the two empirical relations. The time evolution is fully defined by two initial constants and two parameters. We study a complete sample of 83 bright GRB pulses observed by the Compton Gamma-Ray Observatory and identify a major subgroup of GRB pulses ( approximately 45%) which satisfy the spectral-temporal behavior described above. In particular, the decay phase follows a reciprocal law in time. It is unclear what physics causes such a decay phase.

A Possible Lateral Gamma-Ray Burst Jet from SN 1987A

There was a bright, transient companion spot to SN 1987A with a projected distance of about 17 light-days, observed by optical speckle interferometry 1-2 months after explosion. It is shown here that the bright spot may be due to a receding ultrarelativistic jet traveling at ~53° with respect to the observer-to-SN 1987A vector, through a circumstellar medium with a density profile of ρ(r) ∝ r-2. If it had approached us along the line of sight, a very bright gamma-ray burst (GRB) would have been seen with an apparent isotropic energy of ~1054 ergs and an opening angle of a few degrees. The model provides an adequate explanation for the evolution of the spot, although there are still problems in explaining its observed color. This model implies that at least some GRBs would be seen as going through a medium with a density of ρ(r) ∝ r-2 rather than through a uniform medium, which is frequently adopted in GRB calculations. Improved analysis of the speckle data has revealed another, fainter spot on the opposite side.