Cosmological Gamma-ray Bursts Research Articles

Gamma-Ray Bursts from Stellar Collisions

We propose that the cosmological gamma-ray bursts arise from the collapse of neutron stars to black holes, which is triggered by collisions or mergers with less evolved stars. This scenario represents a cosmological history qualitatively different from most previous theories because it contains a significant contribution from an old stellar population, namely the globular clusters. Furthermore, the gas-poor central regions of globular clusters provide an ideal environment for the generation of the recently confirmed afterglows via the fireball scenario. Collisions in close binaries resulting from neutron star birth kicks may also contribute to the overall rate and should lead to associations between some gamma-ray bursts and supernovae of Type Ib/c.

Gravitational Lensing Limits on the Average Redshift of Gamma‐Ray Bursts

The lack of bright host galaxies in several recently examined gamma-ray burst (GRB) error boxes suggests that the redshifts of cosmological GRBs may be significantly higher than previously believed. On the other hand, the non-detection of multiple images in the BATSE 4B catalog implies an upper limit on the average redshift of GRBs. Here, we calculate an upper limit to , independent of the physical model for GRBs, using a new statistical lensing method that removes distance ambiguities, and thus permits accurate computation of the lensing rate at high z. The upper limit on depends directly on the cosmological parameters Omega and Lambda. If there are no multiple images among the brightest 80% of the first 1802 bursts in the BATSE 4B catalog, then, at the 95% confidence level, is less than 2.2, 2.8, 4.3, or 5.3 for (Omega,Lambda) values of (0.3, 0.7), (0.5, 0.5), (0.5, 0.0), or (1.0, 0.0), respectively. The 68% upper limit to the average redshift is comparable to or less than the median redshift of GRBs in scenarios in which the GRB rate is proportional to the rate of star formation, for any cosmology. The uncertainty in the lensing rate --- arising from uncertainties in the cosmological parameters and in the number density and average velocity dispersion of galaxies --- will be reduced significantly in the next few years by a new generation of experiments and surveys. Moreover, the continued increase in the number of GRBs observed by BATSE will greatly constrain their redshift distribution.

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.

An Analysis of the Gamma‐Ray Burst No‐Host Problem

The lack of galaxies inside gamma-ray burst (GRB) error boxes has often been used to argue against the cosmological interpretation of this phenomenon. The fundamental problem with host galaxy tests is that our understanding of GRBs is too limited for constructing a sensitive measure of the presence of suitable hosts. In this paper the origin of the no-host problem is explained, and the tests used to make these inferences are evaluated. It is shown that these analyses are too dependent upon factors unrelated to bursts or to their host galaxies and that normal galaxies have not been eliminated as possible hosts. New data from a near-infrared sample of third Interplanetary Network error boxes, which reached levels deep enough to detect galaxies at every position, show the brightest galaxies to be at the distances (z ~ 0.2) and luminosities (L ~ 0.6L*) that are predicted by the simplest assumptions about cosmological GRBs.

Saturated Compton Cooling Model of Cosmological Gamma-Ray Bursts

We revisit the saturated Compton cooling model of gamma-ray burst pulse decay in the context of cosmological scenarios. We find that for pulse duration of ~1 s, this model is consistent with Gpc distance if the Compton cooling plasma is a relativistically expanding thin shell of bulk Lorentz factor ≥102, radius ~1015 cm, thickness ≤1011-1012 cm, comoving lepton density ≥1013 cm-3 and is dominated by a thermal population of keV comoving temperature, with less than a few percent nonthermal population.

An Explanation for the Bimodal Duration Distribution of Gamma-Ray Bursts: Millisecond Pulsars from Accretion-induced Collapse

Cosmological gamma-ray bursts (GRBs) could be driven by dissipation of pure electromagnetic energy (Poynting flux) extracted from rapidly rotating compact objects with strong magnetic fields. One such possibility is a young millisecond pulsar (MSP) formed from the accretion-induced collapse (AIC) of a white dwarf. The combination of an efficient magnetic dynamo, likely operating during the first seconds of the initially hot and turbulent MSP interior, and the subsequent modest beaming of gamma ray-emitting outflows, would easily account for energy constraints. But the remarkable feature of such models is that they may naturally explain the puzzling bimodal distribution in GRB time durations. The two burst classes could correspond to MSPs that form spinning above and below, respectively, a gravitationally unstable limit. In the former case, the spin-down timescale is caused by gravitational radiation emission (<1 s), while the spin-down timescale of the latter is caused by electromagnetic dipole emission (1 s). These two timescales account for the short and long GRB durations, i.e., the observed bimodal GRB duration distribution. A natural prediction is that the short-duration GRBs would be accompanied by strong gravitational radiation emission, which is absent from the longer class. Both would show millisecond variabilities.

Cosmological Gamma-Ray Bursts and Evolution of Galaxies

Evolution of the rate density of cosmological gamma-ray bursts (GRBs) is calculated and compared to the BATSE brightness distribution in the context of binary neutron-star mergers as the source of GRBs, taking account of the realistic star formation history in the universe and evolution of compact binary systems. We tried two models of the evolution of cosmic star formation rate (SFR): one is based on recent observations of SFRs at high redshifts, while the other is based on a galaxy evolution model of stellar population synthesis that reproduces the present-day colors of galaxies. It is shown that the binary merger scenario of GRBs naturally results in the comoving rate-density evolution of \propto (1+z)^{2-2.5} up to z ~ 1, that has been suggested independently from the compatibility between the number-brightness distribution and duration-brightness correlation. If the cosmic SFR has its peak at z ~ 1--2 as suggested by recent observations, the effective power-index of GRB photon spectrum, \alpha >~ 1.5$ is favored, that is softer than the recent observational determination of \alpha = 1.1 \pm 0.3. However, high redshift starbursts (z >~ 5) in elliptical galaxies, that have not yet been detected, can alleviate this discrepancy. The redshift of GRB970508 is likely about 2, just below the upper limit that is recently determined, and the absorption system at z = 0.835 seems not to be the site of the GRB.

Microlensing of Gamma‐Ray Burst Afterglows

The afterglow of a cosmological gamma-ray burst (GRB) should appear on the sky as a narrow emission ring of radius ~3 × 1016 cm (t/day)5/8 that expands faster than light. After a day, the ring radius is comparable to the Einstein radius of a solar mass lens at a cosmological distance. Thus, microlensing by an intervening star can significantly modify the light curve and polarization signal from a GRB afterglow. We show that the achromatic amplification signal of the afterglow flux can be used to determine the impact parameter and expansion rate of the source in units of the Einstein radius of the lens, and we probe the superluminal nature of the expansion. If the synchrotron emission from the afterglow photosphere originates from a set of coherent magnetic field patches, microlensing would induce polarization variability as a result of the transient magnification of the patches behind the lens. The microlensing interpretation of the flux and polarization data can be confirmed by a parallax experiment that would probe the amplification peak at different times. The fraction of microlensed afterglows can be used to calibrate the density parameter of stellar-mass objects in the universe.

Poynting Jets from Black Holes and Cosmological Gamma-Ray Bursts

We discuss the properties of magnetically dominated jet-like outflows from stellar mass black holes surrounded by debris tori resulting from neutron star disruption. These jets may have narrow cores (along the rotation axis) which are almost free of baryons and attain very high bulk Lorentz factors $\simg 10^6$. The jets give rise to a characteristic MeV to TeV emission as well as to relativistic shocks producing the usual MeV bursts. Because the outflow is highly directional the properties of the observed gamma-rays would depend on the viewing angle relative to the rotation axis. Even for the most intense bursts, which under the assumption of isotropic emission and substantial redshifts would be inferred to emit $10^{52}-10^{53}$ erg, the efficiencies required are only $10^{-2}-10^{-4}$.

Formation of Millisecond Pulsars from Accretion‐induced Collapse and Constraints on Pulsar Gamma‐Ray Burst Models

We study accretion-induced collapse of magnetized white dwarfs as an origin of millisecond pulsars. We apply magnetized accretion disk models to the precollapse accreting magnetic white dwarfs and calculate the white dwarf spin evolution. If the pulsar magnetic field results solely from the flux-frozen fossil white dwarf field, a typical millisecond pulsar is born with a field strength ~1011-1012 G. The uncertainty in the field strength is mainly due to the uncertain physical parameters of the magnetized accretion disk models. A simple correlation between the pulsar spin Ω* and the magnetic field B*, (Ω*/104 s-1) ~ (B*/1011 G)-4/5, is derived for a typical accretion rate ~5 × 10-8 M☉ yr-1. This correlation remains valid for a wide precollapse physical conditions unless the white dwarf spin and the binary orbit are synchronized prior to accretion induced collapse. We critically examine the possibility of spin-orbit synchronization in close binary systems. Using idealized homogeneous ellipsoid models, we compute the electromagnetic and gravitational wave emission from the millisecond pulsars and find that electromagnetic dipole emission remains nearly constant while millisecond pulsars may spin up rather than spin down as a result of gravitational wave emission. We also derive the physical conditions under which electromagnetic emission from millisecond pulsars formed by accretion-induced collapse can be a source of cosmological gamma-ray bursts. We find that relativistic beaming of gamma-ray emission and precession of gamma-ray-emitting jets are required unless the dipole magnetic field strengths are greater than 1015 G; such strong dipole fields are in excess of those allowed from the accretion-induced collapse formation process except in spin-orbit synchronization. Strong dipole fields greater than 1015 G could in principle be produced in situ. If the millisecond pulsars spin up while emitting gravitational waves, the required values for the shortest bursts are greater than 1016 G, which pushes the limits of suggested neutron star dynamos.

Optical and Long‐Wavelength Afterglow from Gamma‐Ray Bursts

We discuss the evolution of cosmological gamma-ray burst remnants, consisting of the cooling and expanding fireball ejecta together with any swept-up external matter, after the gamma-ray event. We show that significant optical emission is predicted, which should be measurable for timescales of hours after the event, and in some cases radio emission may be expected days to weeks after the event. The flux at optical, X-ray, and other long wavelengths decays as a power of time, and the initial value of the flux or magnitude, as well as the value of the time-decay exponent, should help to distinguish between possible types of dissipative fireball models.

Cooling Timescales and Temporal Structure of Gamma‐Ray Bursts

A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultrarelativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in two shocks, a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is then radiated via synchrotron emission and Comptonization. We estimate the synchrotron cooling timescale of the shocked material in the forward and reverse shocks for electrons of various Lorentz factors, focusing in particular on those electrons whose radiation falls within the energy detection range of the BATSE detectors. We find that in order to produce the rapid variability observed in most bursts, the energy density of the magnetic field in the shocked material must be greater than about 1% of the thermal energy density. In addition, the electrons must be nearly in equipartition with the protons, since otherwise we do not have reasonable radiative efficiencies of GRBs. Inverse Compton scattering can increase the cooling rate of the relevant electrons, but the Comptonized emission itself is never within the BATSE range. These arguments allow us to pinpoint the conditions within the radiating regions in GRBs and to determine the important radiation processes. In addition, they provide a plausible explanation for several observations. The model predicts that the duty cycle of intensity variations in GRB light curves should be nearly independent of burst duration and should scale inversely as the square root of the observed photon energy. Both correlations are in agreement with observations. The model also provides a plausible explanation for the bimodal distribution of burst durations. There is no explanation, however, for the presence of a characteristic break energy in GRB spectra.

The Compatibility of Friedmann Cosmological Models with Observed Properties of Gamma‐Ray Bursts and a Large Hubble Constant

The distance scale to cosmic gamma-ray bursts (GRBs) is still uncertain by many orders of magnitude; however, one viable scenario places GRBs at cosmological distances, thereby permitting them to be used as tracers of the cosmological expansion over a significant range of redshifts z. Also, several recent measurements of the Hubble constant H0 appearing in the referred literature report values of 70H − 80 km s−1 Mpc−1. Although there is significant debate regarding these measurements, we proceed here under the assumption that they are evidence of a large value for H0. This is done in order to investigate the additional constraints on cosmological models that can be obtained under this hypothesis when combined with the age of the universe and the brightness distribution of cosmological gamma-ray bursts. We show that the range of cosmological models that can be consistent with the GRB brightness distribution, a Hubble constant of 70 − 80 km s−1 >Mpc−1, and a minimum age of the universe of 13–15 Gyr is constrained significantly, largely independent of a wide range of assumptions regarding the evolutionary nature of the burst population. Low-density, Λ q0 < 0 and density parameter σ0 in the range ≈ 0.10 − 0.25 (Ω0 ≈ 0.2 − 0.5) are strongly favored.

Cosmological Evolution and Luminosity Function Effects on Number Counts, Redshift, and Time Dilation of Bursting Sources

We present analytic formulae for the integral number count distribution of cosmological bursting or steady sources valid over the entire range of fluxes, including density evolution and either standard candle or a power law luminosity function. These are used to derive analytic formulae for the mean redshift, the time dilations and the dispersion of these quantities for sources within a given flux range for Friedmann models with $\Omega=1,~ \Lambda=0$ without K-corrections, and we discuss the extension to cases with $\Omega <1$ and inclusion of K-corrections. Applications to the spatial distribution of cosmological gamma ray burst sources are discussed, both with and without an intrinsic energy stretching of the burst time profiles, and the implied ranges of redshift $z$ are considered for a specific time dilation signal value. The simultaneous consideration of time dilation information and of fits of the number distribution versus peak flux breaks the degeneracy inherent in the latter alone, allowing a unique determination of the density evolution index and the characteristic luminosity of the sources. For a reported time dilation signal of 2.25 and neglecting [including] energy stretching we find that the proper density should evolve more steeply with redshift than comoving constant, and the redshifts of the dimmest sources with stretching would be very large. However, the expected statistical dispersion in the redshifts is large, especially for power law luminosity functions, and remains compatible with that of distant quasars. For smaller time dilation values of 1.75 and 1.35 the redshifts are more compatible with conventional ideas about galaxy formation, and the evolution is closer to a comoving constant or a slower evolution. More generally, we have considered a wide range of possible measured time dilation

Delayed GeV–TeV Photons from Gamma-Ray Bursts Producing High-Energy Cosmic Rays

A scenario in which cosmic rays (CRs) above 1020 eV are produced by cosmological gamma-ray bursts (GRBs) is consistent with observations, provided that deflections by the intergalactic magnetic field (IGMF) delay and spread the arrival time of the CRs over ≥50 yr. The energy lost by the CRs as they propagate and interact with the microwave background is transformed by cascading into secondary GeV-TeV photons. We show that a significant fraction of these photons can arrive with delays much smaller than the CR delay if much of intergalactic space is occupied by large-scale magnetic "voids," regions of a size ≳5 Mpc and a field weaker than 10-15 G. Such voids might be expected, for example, in models where a weak primordial field is amplified in shocked, turbulent regions of the intergalactic medium during the formation of large-scale structure. For a field strength ~4 × 10-11 G in the high-field regions, the value required to account for observed galactic fields if the IGMF were frozen in the protogalactic plasma, the delay of CRs produced by a burst at a distance of 100 Mpc is ~100 yr, and the fluence of secondary photons above 10 GeV on hour-day timescales is I(>E) ~ 10-6 E cm-2. This fluence is close to the detection threshold of current high-energy gamma-ray experiments. Detection of the delayed flux would support the GRB-CR association and would also provide information on the IGMF structure.

Luminosity Distributions of Cosmological Gamma-Ray Bursts

view Abstract Citations (24) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Luminosity Distributions of Cosmological Gamma-Ray Bursts Hakkila, Jon ; Meegan, Charles A. ; Horack, John M. ; Pendleton, Geoffrey N. ; Briggs, Michael S. ; Mallozzi, Robert S. ; Koshut, Thomas M. ; Preece, Robert D. ; Paciesas, William S. Abstract The intrinsic luminosities of gamma-ray bursts are found to be constrained by the BATSE/PVO combined intensity distribution, assuming that (1) bursts originate in {LAMBDA} = 0, {OMEGA} = 1 Friedmann cosmology with a nonevolving density distribution, (2) the nonevolving intrinsic luminosity function can be modeled as a truncated power law, and (3) burst spectra are modeled as power laws with identical spectral indices. These simplifying assumptions allow constraints to be placed on luminosity functions in cosmological gamma-ray burst scenarios and indicate that standard-candle sources are not favored. In general, either the minimum burst luminosity L_min_ or the maximum burst luminosity L_max_ are known, with the opposite end of the luminosity function unconstrained. Both L_max_ and L_min_ must be specified for luminosity power-law indices near 2. When these results are combined with other studies measuring other cosmological burst signatures, it is possible that the intrinsic luminosity function contains more low-luminosity bursts than high-luminosity ones. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177134 Bibcode: 1996ApJ...462..125H Keywords: GAMMA RAYS: BURSTS full text sources ADS |

A Constraint on the Distance Scale to Cosmological Gamma-Ray Bursts

If γ-ray bursts are cosmological in origin, the sources are expected to trace the large-scale structure of luminous matter in the universe. I use a new likelihood method that compares the counts-in-cells distribution of γ-ray bursts in the BATSE 3B catalog with that expected from the known large-scale structure of the universe, in order to place a constraint on the distance scale to cosmological bursts. I find, at the 95% confidence level, that the comoving distance to the edge of the burst distribution is greater than 630 h-1 Mpc (z > 0.25), and that the nearest burst is farther than 40 h-1 Mpc. The median distance to the nearest burst is 170 h-1 Mpc, implying that the total energy released in γ-rays during a burst event is of order 3 × 1051 h-2 ergs. None of the bursts that have been observed by BATSE are in nearby galaxies, nor is a signature from the Coma Cluster or the Great Wall likely to be seen in the data at present.

Signatures of the Origin of High-Energy Cosmic Rays in Cosmological Gamma-Ray Bursts

We derive observational consequences for the hypothesis that cosmic rays (CRs) of energy greater than 1019 eV originate in the same cosmological objects producing gamma-ray bursts (GRBs). Intergalactic magnetic fields 10-12 G are required in this model to allow CRs to be observed continuously in time by producing energy-dependent delays in the CR arrival times. This results in individual CR sources having very narrow observed spectra, since at any given time only those CRs having a fixed time delay are observed. Thus, the brightest CR sources should be different at different energies. The average number of sources contributing to the total CR flux decreases with energy much more rapidly than in a model of steady CR sources, dropping to one at Ecrit 2 × 1020 eV with very weak sensitivity to the intergalactic magnetic field strength. Below Ecrit, a very large number of sources is expected, consistent with observations. Above Ecrit, a source may be observed with a flux considerably higher than the time-averaged CR flux from all sources, if a nearby GRB occurred recently. If such a source is present, its narrow spectrum may produce a gap in the total spectrum. These signatures should be detectable by the planned Auger CR experiment.

An Estimate of the Motion-induced Dipole Moment in the Spatial Distribution of Cosmological Gamma-Ray Bursts

view Abstract Citations (3) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Estimate of the Motion-induced Dipole Moment in the Spatial Distribution of Cosmological Gamma-Ray Bursts Brainerd, J. J. Abstract If gamma-ray bursts have a cosmological redshift of order unity, then their distribution on the sky has a motion-induced dipole moment of 0.0019. One must have a sample on the order of 106 gamma-ray bursts to make this dipole moment larger than a 3 σ statistical fluctuation. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176670 Bibcode: 1996ApJ...456..449B Keywords: GAMMA RAYS: BURSTS; METHODS: STATISTICAL full text sources ADS |

Determination of Distance from Time Dilation of Cosmological Gamma-Ray Bursts

view Abstract Citations (91) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Determination of Distance from Time Dilation of Cosmological Gamma-Ray Bursts Fenimore, E. E. ; Bloom, J. S. Abstract The isotropic distribution of gamma-ray bursts (GRBs) as observed with the Burst and Transient Source Experiment (BATSE) strongly suggests that the bursts are at cosmological distances. At such distances, the expansion of the universe should redshift the spectra and stretch the temporal structure. Indeed, such time dilation has been observed through a variety of analyses and over all observed timescales in gamma-ray bursts. We relate the observed peak intensities, spectral shapes, and time dilation to absolute distance. We include the uncertainties in our knowledge of the intrinsic spectrum and correct for the coupling between the spectral shape and the temporal structure. Assuming a q0 = ½ cosmology, the reported time dilation between the dimmest BATSE bursts and the bright BATSE bursts (a factor of ∼2) requires a standard candle luminosity of ∼1052 ergs-1, which translates into a redshift of >6 for the dimmest BATSE bursts rather than a redshift of 1 or 2, as previously reported. An alternative method to determine the distance to cosmological GRBs is to use the log N-log P distribution. The large luminosity and distance determined from the time dilation is inconsistent with the observed log N-log P distribution (which requires a luminosity of 4.6 × 1050 ergs s-1 and z ∼0.8) unless there is very strong evolution. Cosmologies with q0 ≠ ½ give similar results. The implied distance for the dimmest bursts is beyond where galaxies are thought to form. If true, the gamma-ray bursts would be orphans: no known objects would have the same distance scale. We conclude that either a large fraction (65%) of the observed time dilation between the BATSE bright and dimmest bursts is intrinsic to the bursts or that there are strong evolutionary effects in the log N-log P distribution and that it is only a coincidence that log N-log P shows a -3/2 power law at high intensities. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176366 arXiv: arXiv:astro-ph/9504063 Bibcode: 1995ApJ...453...25F Keywords: COSMOLOGY: THEORY; GAMMA RAYS: BURSTS; Astrophysics E-Print: 41 pages including figures, uuencoded compressed Postscript. Accepted for publication in the Astrophysical Journal. The preprint can also be found in Postscript form at http://sstcx1.lanl.gov/~jsbloom/ or at ftp://sstcx1.lanl.gov/pub/grb/time_dilation.ps full text sources arXiv | ADS |