Origin Of Gamma-ray Bursts Research Articles

Gamma-Ray Bursts: Afterglows and Central Engines

Gamma-ray bursts (GRBs) are most intense transient gamma-ray events in the sky when they are on together with the strong evidences (i.e. the isotropic and inhomogeneous distribution of GRBs detected by BASTE) that they are located at cosmological distances, which make them the most energetic events ever known. For example, the observed radiation energies of some GRBs are equivalent to convert more than one solar mass energy into radiation completely. This is thousand times stronger than that of supernova explosion. Unconventional energy mechanisms and extremely high conversion efficiency for these mysterious events are required. The discovery of host galaxies and association with supernovae in the cosmological 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 central engines of GRBs. 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.

A Windy Gamma-Ray Burst

ASTROPHYSICS The origin of gamma-ray bursts (GRB)—energetic, short-duration flashes of bright gamma rays—is unknown. Recent observations of iron-intensified x-ray emissions in the afterglow of GRB991216 about one and one-half days after the main burst (see [Piro][1] et al. , Reports, 3 November 2000) have underscored the need for explanatory models. An existing two-stage model posits a supernova explosion followed several days later by the burst of gamma rays; this allows the stellar remnant to expand before the burst-induced blast wave interacts with the envelope and results in the iron-intensified emissions. Rees and Meszaros offer an alternative scenario, beginning with a stellar collapse of either a rapidly spinning neutron star (collapsar model) or the highly magnetized torus of a black hole (magnetar model) into the GRB progenitor. The burst would occur soon afterwards, producing the bright flash followed by the decay of the x-ray continuum in the afterglow from the GRB blast wave. The iron-intensified x-ray emissions would develop later, due to the interaction of the GRB blast wave with the magnetohydrodynamic wind emanating from the recently collapsed and possibly not quite stable neutron star or black hole. — LR Astrophys. J. 545 , L73 (2000). [1]: /lookup/doi/10.1126/science.290.5493.955

“Hot Topics” in astrophysics

Three current topics in astrophysics are described on the occasion of the joint meeting of the AAPT and the American Astronomical Society (Jan. 7-11, 2001) in San Diego, CA. They are the habitability of Mars (evidence for ancient and contemporary water and indications of current volcanism); black holes and their intimate relationship with galaxy bulges, including their involvement in the x-ray background; and the nature and origin of gamma-ray bursts.

Implications of recent observational discoveries for the nature and origin of gamma-ray bursts

The discoveries that GRBs have X-ray, optical and radio afterglows have connected the study of GRBs to the rest of astronomy, and revolutionized the field. In this review, I discuss the implications that the observation of these afterglows have for burst energies and luminosities, and for models of the bursts and their afterglows. I describe recent evidence linking the long, softer, smoother GRBs detected by BeppoSAX and core collapse supernovae. Finally, I summarize recent work showing that, if these GRBs are due to the collapse of massive stars, they may provide a powerful probe of the very high redshift universe.

Gamma-ray bursts and the history of star formation

Popular models for the origin of gamma-ray bursts (GRBs) include short-lived massive stars as the progenitors of the fireballs. Hence the redshift distribution of GRBs should track the cosmic star formation rate of massive stars accurately. A significant proportion of high-mass star formation activity appears to occur in regions that are obscured from view in the optical waveband by interstellar dust. The amount of dust-enshrouded star formation activity taking place has been estimated by observing the thermal radiation from the dust that has been heated by young stars in the far-infrared and submillimetre wavebands. Here we discuss an alternative probe — the redshift distribution of GRBs. GRBs are detectable at the highest redshifts, and because gamma-rays are not absorbed by dust, the redshift distribution of GRBs should therefore be unaffected by dust extinction. At present the redshifts of GRBs can only be determined from the associated optical transient emission; however, useful information about the prevalence of dust-obscured star formation can also be obtained from the ratio of GRBs with and without an associated optical transient. Eight GRBs currently have spectroscopic redshifts. Once about a hundred redshifts are known, the population of GRBs will provide an important test of different models of the star formation history of the Universe.

Current Topics in Gamma-Ray Astrophysics.

This paper reports on recent progress toward unraveling the origin of gamma-ray bursts. It is concluded that neutron-star binaries are one of the few remaining candidates. A model is proposed based upon general relativistic hydrodynamic studies which indicate a new physical process by which to power a gamma-ray burst. Relativistically driven compression, heating, and collapse of the individual neutron stars can occur many seconds before inspiral and merger. This compression may produce a neutrino burst of ∼1053 ergs lasting several seconds. The associated thermal neutrino emission produces an e+–e − pair plasma by annihilation. We show first results of a simulated burst which produces ∼1051 erg in γ rays of the correct spectral and temporal properties.

Ballerina – pirouettes in search of gamma bursts

The cosmological origin of gamma ray bursts has now been established with reasonable certainty. Many more bursts will need to be studied to establish the typical distance scale, and to map out the large diversity in properties which have been indicated by the first handful of events. We are proposing Ballerina, a small satellite to provide accurate positions and new data on the gamma-ray bursts. We anticipate a detection rate an order of magnitude larger than obtained from Beppo–SAX.

Highlights of the Rome Workshop on gamma-ray bursts in the afterglow Era

I review some of the highlights of the Rome Workshop on Gamma-Ray Bursts, and discuss some of the questions these results pose about the nature and origin of gamma-ray bursts.

An intrinsic anisotropy in the angular distribution of gamma-ray bursts

The anisotropy of the sky distribution of 2025 gamma-ray bursts (GRBs) collected in Current BATSE catalog is confirmed. It is shown that the quadrupole term being proportional to $\sim \sin 2b \sin l$ is non-zero with a probability 99.9%. The occurrence of this anisotropy term is then supported by the binomial test even with the probability 99.97%. It is also argued that this anisotropy cannot be caused exclusively by instrumental effects due to the non-uniform sky exposure of BATSE instrument; there should exist also some intrinsic anisotropy in the angular distribution of GRBs. Separating GRBs into short and long subclasses, it is shown that the 251 short ones are distributed anisotropically, but the 681 long ones seem to be distributed still isotropically. The 2-sample Kolmogorov-Smirnov test shows that they are distributed differently with a 98.7% probability. The character of anisotropy suggests that the cosmological origin of short GRBs further holds, and there is no evidence for their Galactical origin. The work in essence contains the key ideas and results of a recently published paper (\cite{balazs}), to which the new result following from the 2-sample Kolmogorov-Smirnov test is added, too.

Muon Detection of TeV Gamma Rays from Gamma‐Ray Bursts

Because of the limited size of the satellite-borne instruments, it has not been possible to observe the flux of gamma ray bursts (GRB) beyond GeV energy. We here show that it is possible to detect the GRB radiation of TeV energy and above, by detecting the muon secondaries produced when the gamma rays shower in the Earth's atmosphere. Observation is made possible by the recent commissioning of underground detectors (AMANDA, the Lake Baikal detector and MILAGRO) which combine a low muon threshold of a few hundred GeV or less, with a large effective area of 10^3 m^2 or more. Observations will not only provide new insights in the origin and characteristics of GRB, they also provide quantitative information on the diffuse infrared background.

A possible origin of gamma ray bursts and axionic boson stars

We indicate a possible mechanism of generating gamma ray bursts. They are generated by a collision between an axionic boson star and a neutron star. The axionic boson star can dissipates its whole energy 10 50 erg in the magnetized conducting medium of the neutron star. This dissipation arises only around envelope of the neutron star so that a fire ball with small baryon contamination is expected. Under plausible assumptions we have evaluated roughly a rate of the collision per year and per galaxy, which is consistent with observations. We also point out that cosmic rays with extremely high energy, 10 21 eV, can be produced in the similar collisions with the neutron stars with strong magnetic fields ∼10 14 G.

Search for low-energy neutrino radiation accompanying gamma-ray bursts on the Baksan subterranean scintillation telescope

An analysis aimed at finding possible neutrino radiation accompanying gamma-ray bursts in a 24-h period about them is performed on the basis of the data in the 4B BATSE Gamma-Ray Burst Catalog and data from the Baksan scintillation telescope according to a program for finding neutrinos from collapsing stars. Values significantly exceeding the background are not discovered. A lower bound for the distance to the source is established under the assumption that the anticipated radiation has characteristics similar to the characteristics of collapse neutrinos. It attests to the cosmological origin of gamma-ray bursts with a high degree of probability.

On the Association of Gamma-Ray Bursts with Supernovae

The recent discovery of a supernova (SN 1998bw) seemingly associated with GRB 980425 adds a new twist to the decades-old debate over the origin of gamma-ray bursts. To investigate the possibility that some (or all) bursts are associated with supernovae, we performed a systematic search for temporal/angular correlations using catalogs of BATSE and BATSE/Ulysses burst locations. We find no associations with any of the precise BATSE/Ulysses locations, which allows us to conclude that the fraction of high-fluence gamma-ray bursts associated with known supernovae is small (<0.2%). For the more numerous weaker bursts, the corresponding limiting fraction of 2.5% is far less constraining due to the imprecise locations of these events. This fraction (2.5%) of bursts corresponds to approximately 30% of the recent supernovae used as a comparison data set. Thus, although we find no significant evidence to support a burst/supernova association, the possibility cannot be excluded for weak bursts.

Can Fireball Models Explain Gamma-Ray Bursts?

The observed afterglows of gamma-ray bursts (GRBs), in particular the afterglow of GRB 970228 after 6 months, seem to rule out, as the origin of GRBs, relativistic fireballs driven by the mergers or accretion-induced collapse of compact stellar objects in galaxies. GRBs can be produced by superluminal jets from such events.

Gamma‐Ray Bursts from Evolved Galactic Nuclei

A new cosmological scenario for the origin of gamma-ray bursts (GRBs) is proposed. In our scenario, a highly evolved central core in the dense galactic nucleus is formed, containing a subsystem of compact stellar remnants (CSRs), such as neutron stars and black holes. Those subsystems result from the dynamical evolution of dense central stellar clusters in the galactic nuclei through merging of stars, thereby forming (as has been realized by many authors) the short-lived massive stars and then CSRs. We estimate the rate of random CSR collisions in the evolved galactic nuclei by taking into account, in a procedure similar to that of Quinlan & Shapiro, the dissipative encounters of CSRs, mainly due to radiative losses of gravitational waves, which result in the formation of intermediate short-lived binaries, with further coalescence of the companions to produce GRBs. We also consider how the possible presence of a central supermassive black hole, formed in a highly evolved galactic nucleus, influences the CSR binary formation. This scenario does not postulate ad hoc a required number of tight binary neutron stars in the galaxies. Instead, it gives, for the most realistic parameters of the evolved nuclei, the expected rate of GRBs consistent with the observed one, thereby explaining the GRB appearance as a natural part of the dynamical evolution of galactic nuclei. In addition, this scenario provides an opportunity for a cosmological GRB recurrence, previously considered to be a distinctive feature of GRBs of a local origin only. We also discuss some other observational tests of the proposed scenario.

Gamma-ray bursts from internal shocks in a relativistic wind: temporal and spectral properties

We construct models for gamma-ray bursts where the emission comes from internal shocks in a relativistic wind with a highly non uniform distribution of the Lorentz factor. We follow the evolution of the wind using a very simplified approach where a large number of layers interact by direct collisions but where all pressure waves have been suppressed. We suppose that the magnetic field and the electron Lorentz factor reach large equipartition values in the shocks. Synchrotron photons emitted by the relativistic electrons have a typical energy in the gamma-ray range in the observer frame. Synthetic bursts are constructed as the sum of the contributions from all the internal elementary shocks and their temporal and spectral properties are compared to the observations. We reproduce the diversity of burst profiles, the ``FRED'' shape of individual pulses and the short time scale variability. Synthetic bursts also satisfy the duration-hardness relation and individual pulses are found to be narrower at high energy, in agreement with the observations. These results suggest that internal shocks in a relativistic wind may indeed be at the origin of gamma-ray bursts. A potential problem is however the relatively low efficiency of the dissipation process. If the relativistic wind is powered by accretion from a disc to a stellar mass black hole it implies that a substantial fraction of the available energy is injected into the wind.

Spectroscopy of gamma-ray bursts: An overview

More than two decades of accumulated data on the spectra of gamma-ray bursts (GRB's) are now in hand. In general, burst spectra display a wide variety of both spectral and temporal behavior. In essentially all bursts where spectral measurements are possible, the spectrum evolves during the burst. This spectral complexity and variability has presented researchers with a challenging task. Early spectral measurements contained evidence for cyclotron-line-like absorption and emission features. These results were the main anchor for the galactic neutron star paradigm for the origin of gamma-ray bursts. Ginga provided striking confirmation with evidence in three bursts for cyclotron absorption with both first and second harmonics present in two of them. The absorption line energies were consistent with the teragauss magnetic fields expected for neutron stars. However, the BATSE experiment on CGRO has dealt a severe blow to this paradigm. BATSE has found no evidence for the expected galactic anisotropy and also no evidence for cyclotron lines. This paper will: 1) provide an historical overview of GRB spectroscopic measurements 2) a discussion of the BATSE/Ginga controversy and 3) a sampling of the most recent results and analyses of GRB spectral data.

The spectral features of gamma-ray bursts at lower and higher energy components: a possible way to distinguish between cosmological and Galactic origins of gamma-ray bursts

The spectral features of gamma-ray bursts at lower and higher energy components: a possible way to distinguish between cosmological and Galactic origins of gamma-ray bursts

The Collapse of Neutron Stars in High-Mass Binaries as the Energy Source for the Gamma-Ray Bursts

The energy source has remained to be the great mystery in understanding of the gamma-ray bursts (GRBs), if the events are placed at cosmological distances as indicated by a number of recent observations. The currently popular models include (1) the merger of two neutron stars or a neutron star and a black hole binary and (2) the hypernova scenario, i.e., the collapse of a massive member in a close binary. Since a neutron star will inevitably collapse into a black hole if its mass exceeds the limit Mmax ≈ 3 M☉, releasing a total binding gravitational energy of ~1054 ergs, we explore semiempirically the possibility of attributing the energy source of GRBs to the accretion-induced collapse of a neutron star (AICNS) in a massive X-ray binary system consisting of a neutron star and a type O/B companion. This happens because a significant mass flow of ~10 -->−3-10 -->−4 M☉ yr-1 may be transferred onto the neutron star through the Roche-lobe overflow and primarily during the spiral-in phase when it plunges into the envelope of the companion, which may eventually lead to the AICNS before the neutron star merges with the core of the companion. In this scenario, a dirty fireball with a moderate amount of beaming is naturally expected because of the nonuniformity of the stellar matter surrounding the explosion inside the companion, and a small fraction (~0.1%) of the energy is sufficient to create the observed GRBs. In addition, the bulk of the ejecting matter of the companion star with a relatively slow expansion rate may act as the afterglow. Assuming a nonevolutionary model for galaxies, we estimate that the birthrate of the AICNS events is about two per day within a volume to redshift z=1 for an Ω -->0=1 universe, consistent with the reported GRB rate. It appears that the AICNS scenario, as a result of stellar evolution, may provide a natural explanation for the origin of GRBs and therefore deserves to be further investigated in the theoretical study of GRBs.

Evidence for the Galactic Origin of Gamma-Ray Bursts

We propose a new criterion to divide the classical gamma-ray bursts into two classes on the basis of their distribution in the duration-hardness plane. We study the two proposed classes separately, dividing them into subsets of different hardness-ratio ranges. The intensity distribution (log N vs. log S) and the corresponding space distribution of the subsets provide us with a different scenario for each of the two classes. For the first time, in both cases, clear indications for a Galactic origin of gamma-ray bursts have been found. Two possibilities are favored: (1) two kinds of Galactic gamma-ray bursts coming from the disk and from the halo, respectively, and (2) two kinds of gamma-ray bursts deriving from two distinct source populations located at different positions in the Galactic disk.