Ground-based optical CCD experiments for GRB and optical transient detection

We present details of one operational ground-based experiment for optical detection of GRBs and two which are under consideration/development. The wide-field CCD camera is already in manual burst alert operation with promising results. The Optical Transient Monitor is a CCD-based double monitor suitable for network use for reliable detection of short-lived phenomena in the sky. The system is well suited for correlated efforts with GRB projects. The third experiment is a robotic telescope with automatic response to GRB burst alert messages received via the Internet link. It is expected to get CCD frames with a FOV of 20 deg (needed for BACODINE triggers) of positions of newly detected GRBs within 1 minute.     

Search for a radio counterpart to GRB 940301 with westerbork

The COMPTEL instrument onboard theCompton Gamma-Ray Observatory imaged the bright gamma-ray burst GRB 940301 within 1.6 hours of the event, with a mean 1 error radius of 1.5°. The error region was subsequently refined by combining the COMPTEL location with the arc derived from differences in the event arrival time at the Ulysses and BATSE detectors. Westerbork observations of the COMPTEL error region began on March 4 1994 at 21 cm, however coverage of the refined position was not obtained until 32 days after the GRB occurrence, by which time the operating wavelength had changed to 92 cm. We have constrained the level of variability of sources within the triangulation arc-COMPTEL 2 error box region to be less than 40 mJy (5 upper limit) at 92 cm 41 days after the burst.     

BATSE/OSSE rapid burst response

The BATSE and OSSE instrument teams have modified flight software to promptly (within 2 min of trigger) slew the OSSE detectors to burst locations determined on-board by BATSE. This enables OSSE to make sensitive searches for prompt and delayed post-burst line and continuum emission above 50 keV. In the best cases our sensitivity will be more than an order of magnitude better than any other search in this energy range. We expect to slew to 1–2 bursts per month, based on the OSSE FOV and BATSE event rate. Detections or limits from continued operation of this system may provide significant constraints on burst models. As an example of the observations made using this system, we present preliminary limits for post-burst emission from GRB 950223 on several time scales.     

Radio counterpart searches of gamma ray bursts

Radio observations may be one of the most promising but least explored bands of the spectrum to search for the counterparts of gamma ray bursters. We describe several ongoing experiments with demonstrated high sensitivity to monitor gamma ray bursts for evidence of a flaring or fading counterpart in the days, weeks and months following the original event.     

ARGO-YBJ constraints on very high energy emission from GRBs

The ARGO-YBJ (Astrophysical Radiation Ground-based Observatory at YangBaJing) experiment is designed for very high energy γ-astronomy and cosmic ray researches. Due to the full coverage of a large area (5600 m²) with resistive plate chambers at a very high altitude (4300 m a.s.l.), the ARGO-YBJ detector is used to search for transient phenomena, such as Gamma-ray bursts (GRBs). Because the ARGO-YBJ detector has a large field of view (2 sr) and is operated with a high duty cycle (>90%), it is well suited for GRB surveying and can be operated in searches for high energy GRBs following alarms set by satellite-borne observations at lower energies. In this paper, the sensitivity of the ARGO-YBJ detector for GRB detection is estimated. Upper limits to fluence with 99% confidence level for 26 GRBs inside the field of view from June 2006 to January 2009 are set in the two energy ranges 10–100 GeV and 10 GeV–1 TeV.     

EGRET observations of > 30 MeV emission from the brightest bursts detected by BATSE

EGRET on the Compton Gamma Ray Observatory has detected 5 gammaray bursts above 30 MeV. The sub-MeV emission, as detected by BATSE, for these 5 bursts has the largest fluence,F, and peak intensity,I, of any of the BATSE detected bursts within the EGRET field of view. The BATSE data reported in the 2B catalog and the EGRET exposure,E, are combined to select additional bursts with largeF ×E andI ×E. The EGRET data for these bright BATSE bursts are searched for prompt, as well as delayed, emission above 30 MeV. The average properties of the >30 MeV emission are obtained by adding the EGRET data from the 5 bursts. On average the fluence is greater than 15% of the fluence detected by BATSE below an MeV, and the average spectrum is flatter than the spectrum from 1-30 MeV.     

Systematics in the interpretation of aggregated neutrino flux limits and flavor ratios from gamma-ray bursts

Gamma-ray burst analyses at neutrino telescopes are typically based on diffuse or stacked (i.e., aggregated) neutrino fluxes, because the number of events expected from a single burst is small. The interpretation of aggregated flux limits implies new systematics not present for a single burst, such as by the integration over parameter distributions (diffuse fluxes), or by the low statistics in small burst samples (stacked fluxes). We simulate parameter distributions with a Monte Carlo method computing the spectra burst by burst, as compared to a conventional Monte Carlo integration. With this approach, we can predict the behavior of the flux in the diffuse limit as well as in low statistics stacking samples, such as used in recent IceCube data analyses. We also include the flavor composition at the detector (ratio between muon tracks and cascades) into our considerations. We demonstrate that the spectral features, such as a characteristic multi-peak structure coming from photohadronic interactions, flavor mixing, and magnetic field effects, are typically present even in diffuse neutrino fluxes if only the redshift distribution of the sources is considered, with z ? 1 dominating the neutrino flux. On the other hand, we show that variations of the Lorentz boost can only be interpreted in a model-dependent way, and can be used as a model discriminator. For example, we illustrate that the observation of spectral features in aggregated fluxes will disfavor the commonly used assumption that bursts with small Lorentz factors dominate the neutrino flux, whereas it will be consistent with the hypothesis that the bursts have similar properties in the comoving frame.     

Gamma-ray bursts: post-burst evolution of fireballs

The post-burst evolution of fireballs that produce γ-ray bursts (GRBs) is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Owing to the adoption of a new relation between t R and γ, our results differ markedly from previous studies. Synchrotron radiation from the shocked interstellar medium is carefully calculated, using a conventional set of equations. The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory agreement with observations. We also find that the expansion will no longer be highly relativistic about 4 d after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later.     

Spectral characterisation of gamma-ray bursts with COMPTEL and BATSE

Although the earliest observed gamma-ray burst spectra were well described by thermal bremsstrahlung models, subsequent observations above 1 MeV showed the existence of high energy power law tails in 60% of events. In order to accurately characterise burst spectra, both the low energy turnover and the high energy tail must be accounted for. We have addressed this issue by jointly deconvolving spectra obtained by BATSE and COMPTEL onboard theCompton Observatory. We present preliminary results obtained by application of this method to the gamma-ray burst of February 17, 1994.     

Match probability statistics and gamma-ray burst recurrences in the BATSE catalog

We developmatch probability statistics to test the recurrences of gamma-ray bursts in the BATSE catalog 1B and 2B. We do not find a signal of repetitions at the match level of 10^–3.     

On detection of high-energy neutrinos from sources of gamma-ray bursts

We show that, contrary to a recent suggestion, fluxes of 30 GeV-1 TeV neutrinos that may accompany-ray bursts are at least a few orders of magnitude too weak to be detected by the current or planned neutrino detectors.     

Gamma-burst positions - are they reliable?

Ever since the discovery of the Cosmic Gamma-Ray Bursts we have searched for the quiescent counterparts of these mysterious flashes. Without definite counterparts our pictures of the sources must remain vague. The review discusses a number of techniques which have been applied to the gamma burst localization problem and their limitations. The most accurate technique to-date is the Interplanetary Network (IPN) timing technique. Considering the lack of success in the searches for counterparts in even the smallest gamma burst error boxes, it seems, that the next generation of satellite gamma burst detectors must be designed to allow immediate follow-up with sensitive telescopes. Concepts to achieve this goal are discussed.     

MARIA: A large area balloon instrument

MARIA (Maxi Array for Rapid Imaging of -ray burst Activity) is a large-area instrument that could be flown on a balloon in 1997–1999. It is expected to contribute significantly to the GRB log N - log S distribution and to provide rapid GRB localizations by means of a rapidly moving gamma-camera.     

Gamma-ray bursts: the time domain

Temporal aspects of the gamma-ray burst phenomenon are reviewed in a hierarchical schema. The macrocosm - burst profiles taken as a whole - is fairly well characterized. The bimodal duration distribution can be framed in terms of discretization of pulse structures. The average burst envelope is slightly asymmetric, an aspect possibly related to spectral softening. Burst durations are longer for dim BATSE bursts, an effect explainable by either cosmic time dilation or a luminosity function governed by special relativistic beaming, or a combination. GeV emission, persisting up to thousands of seconds after burst cessation at keV-MeV energies is one of the most challenging features of bursts. On the timescale of pulses structures (the mesocosm), some properties mirror the macrocosm: rise/decay asymmetry; wider pulses and longer intervals between pulses in dim bursts than in bright ones; and the tendency of pulses to soften with time. A central clue to the burst mechanism may be the organization in time and energy, manifest as pulses, for both long and short bursts. Burst profiles appear to be well represented by pulses, accounting for the vast majority of emission in the BATSE energy band. In the microcosm, existence of a higher frequency component - with properties possibly unlike those of pulses - has not been well addressed.     

Wide field X-ray monitor on board the High Energy Transient Experiment (HETE)

The High-Energy Transient Experiment (HETE) is designed for the multiwavelengths study of Gamma-Ray Bursts (GRBs) in UV, X-ray and gamma-ray range with three scientific instruments. The X-ray instrument, Wide-field X-ray Monitor (WXM), consists of four units of one-dimensional position sensitive gas proportional counters and two perpendicularly oriented one-dimensional coded apertures. The WXM has a wide FOV of 1.5 steradian together with the capability to locate GRBs with 10 arcmin accuracy, and covers photon energies of 2 to 25 keV with an energy resolution of typically 18 % at 6 keV, measuring wide band spectra together with the gamma-ray spectrometer (FREGATE). The coded X-ray image will be deconvolved on board and the GRB location will be provided to the UV camera within 1 sec . GRB locations will also be broadcast in real time to ground-based observers for follow-up observations.     

EDGE: Explorer of diffuse emission and gamma-ray burst explosions

How structures of various scales formed and evolved from the early Universe up to present time is a fundamental question of astrophysical cosmology. EDGE (Piro et al., 2007) will trace the cosmic history of the baryons from the early generations of massive stars by Gamma-Ray Burst (GRB) explosions, through the period of galaxy cluster formation, down to the very low redshift Universe, when between a third and one half of the baryons are expected to reside in cosmic filaments undergoing gravitational collapse by dark matter (the so-called warm hot intragalactic medium). In addition EDGE, with its unprecedented capabilities, will provide key results in many important fields. These scientific goals are feasible with a medium class mission using existing technology combined with innovative instrumental and observational capabilities by: (a) observing with fast reaction Gamma-Ray Bursts with a high spectral resolution. This enables the study of their star-forming and host galaxy environments and the use of GRBs as back lights of large scale cosmological structures; (b) observing and surveying extended sources (galaxy clusters, WHIM) with high sensitivity using two wide field of view X-ray telescopes (one with a high angular resolution and the other with a high spectral resolution). The mission concept includes four main instruments: a Wide-field Spectrometer (0.1–2.2 eV) with excellent energy resolution (3 eV at 0.6 keV), a Wide-Field Imager (0.3–6 keV) with high angular resolution (HPD = 15”) constant over the full 1.4 degree field of view, and a Wide Field Monitor (8–200 keV) with a FOV of ? of the sky, which will trigger the fast repointing to the GRB. Extension of its energy response up to 1 MeV will be achieved with a GRB detector with no imaging capability. This mission is proposed to ESA as part of the Cosmic Vision call. We will outline the science drivers and describe in more detail the payload of this mission.     

Hot core chemistry

The BATSE/COMPTEL/NMSU Rapid Response Network (RRN) was formed to provide rapid follow-up of-ray bursts imaged by the COMPTEL instrument on the Compton Gamma-Ray Observatory. The RRN consists of 22 professional observatories located around the world spanning both northern and southern hemispheres. Also included in the RRN is the Airforce GEODSS network. The goal of the RRN is to perform optical and radio observations of COMPTEL error boxes as soon after the burst as possible. We present results from the first two years of operation of this network.     

Cosmological models of gamma-ray bursts

We review models of cosmological gamma-ray bursts (GRBs). The statistical and -ray transparency issues are summarized. Neutron-star and black-hole merger scenarios are described and estimates of merger rates are summarized. We review the simple fireball models for GRBs and the recent work on non-simple fireballs. Alternative cosmological models, including models where GRBs are analogs of active galactic nuclei and where they are produced by high-field, short period pulsars, are also mentioned. The value of neutrino astronomy to solve the GRB puzzle is briefly reviewed.     

γ爆与不同富性的Abell团的交叉相关分析

我们分析了强γ爆和弱γ爆与不同富性的Ａｂｅｌｌ团之间的交叉相关特性。发现弱γ爆与所有富性的Ａｂｅｌｌ团都没有交叉相关，而强γ爆除了与Ｒ≥２的Ａｂｅｌｌ团没有交叉相关外，与Ｒ＝０，Ｒ＝１的Ａｂｅｌｌ团都有１σ以上的交叉相关。     

Internal shocks in a relativistic wind as a source for gamma-ray bursts?

We investigate the recent proposal made by Rees and Mészáros (1994) that GRBs result from internal shocks in the relativistic wind emerging from two coalesced neutron stars. Using a simple model of that wind, where a large number of layers with different Lorentz factors interact through a series of mildly relativistic shocks we compute the efficiency of the process and the time profile of the resulting burst. We show that a great variety of profiles can be obtained using plausible initial distributions of Lorentz factors in the wind. However, the rather low overall efficiency (< 10%) of the process and limits imposed on the burst duration can be used to put severe constraints on the nature of the energy source.