摘要:

We examine galactic neutron‐star models for gamma‐ray bursts in light of the recent BATSE results. A population of neutron stars with birth velocities ≳900 km s−1could account for the statistics, but special bursting properties are required. The statistics are more simply explained by bursters at cosmological distances.

ISSN:0094-243X    

DOI:10.1063/1.42829

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The BATSE detectors aboard Compton Observatory record about one cosmic &ggr;‐ray burst (GRB) per day.1Preliminary data analysis shows a highly isotropic sky map and a non‐uniform brightness distribution.1–3Anisotropies expected from a Galactic neutron star population, the most frequently considered source model, did not emerge from the data. Taken at face value, the data seem to suggest a heliocentric solution of the GRB puzzle.4The observed isotropy can be achieved if sources are either very near or extra‐galactic, reminiscent of the great debate about the nature of galaxies. Pop I neutron stars in the disk do not simultaneously fit sky‐ and brightness distributions. A possibility are sources in an extended Galactic halo with scale length large enough to avoid strong anisotropies due to the solar offset from the galactic center. If GRBs are located in an extended halo we ask whether the neutron star paradigm can survive? We show that the recently discovered5high velocity radio pulsars may provide a natural source population for GRBs. If these pulsars formed in the halo, as suggested by the radio data, the possibility arises that GRBs and high velocity pulsars are two related phenomena that provide observational evidence of the dark Galactic corona. We also discuss cosmological redshift constraints that follow from the observed brightness distribution.

ISSN:0094-243X    

DOI:10.1063/1.42830

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

More than twenty years after their discovery,1the nature of gamma‐ray burst sources (GRBs) remains mysterious. The remarkable results from BATSE experiment aboard the Compton Observatory show however that most of the sources of gamma‐ray bursts cannot be distributed in the galactic disc. The possibility that a small fraction of sites of gamma‐ray bursts is of galactic disc origin cannot however be excluded. We point out in this paper that large numbers of neutron–star binaries with orbital periodsP∼10 hr and M dwarf companions of mass ∼0.2–0.3 M⊙are a natural result of the evolution of low‐mass x‐ray binaries (LMXBs). The numbers and physical properties of these systems suggest that some gamma‐ray burst sources may be identified with this endpoint of LMXB evolution. We suggest an observational test of this hypothesis.

ISSN:0094-243X    

DOI:10.1063/1.42831

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

BATSE measurements suggest1that the distribution of gamma‐ray burst sources is spatially nonuniform, but as yet shows no discernible anisotropy. This is not consistent with the distribution expected solely from a local galactic population of neutron star sources. We suggest, however, that the inconsistency can be reconciled if there are at least two distinct classes of bursts, both of which originate on galactic neutron stars, but which differ by a factor of 104or more, in mean luminosity. We show, as an example, that the observedV/Vmaxdistributions and anisotropy constraints of gamma‐ray bursts are consistent with those expected from a combination of two such classes, resulting from different burst mechanisms on a single galactic, high‐velocity, neutron star population.

ISSN:0094-243X    

DOI:10.1063/1.42832

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Recent observations of gamma‐ray bursts (e.g., x‐ray precursor, x‐ray tail, and cyclotron lines observed by Ginga) strongly suggest that the gamma‐ray burst source is associated with a magnetized neutron star. As an energy source model the shell flash is one of the most studied and its detonative wave has been suggested as an energy conversion mechanism from thermal energy to gamma‐ray photons. Here we present results of numerical simulations of shell flashes of a mass‐accreting neutron star. Models with mass accretion ratedM/dt≳10−13M⊙/yr do not trigger a detonative wave. A model withdM/dt≳10−15M⊙/yr is marginal and its recurrence time is ∼106yr, so that the detonative wave model is statistically inconsistent. An alternative mechanism of energy conversion would be an Alfve´n wave generated in a convective region of a deflagration wave. In this case, the Alfve´n wave is generated by a vibration of the magnetic field in the convective region where the thermal pressure is larger than the magnetic pressure. The amplitude of the Alfve´n wave is determined by the convective heat transport efficiency and its strength is weaker but its duration is longer than the case of a detonation shock wave.

ISSN:0094-243X    

DOI:10.1063/1.42812

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

If gamma‐ray bursts (GRBs), or a subset thereof, are caused by thermonuclear flashes on the surface of a strongly magnetized neutron star, accretion during quiescent intervals between the bursts will give rise to a diffuse background emission from a possibly very large number of sources, each of which is too faint to be detected. A comparison of the expected intensity in this background with observed diffuse background signals in the appropriate energy band may in principle provide a test of the thermonuclear‐flash model, which is independent of (i) the distance scale to GRB sources, (ii) the possible anisotropy of the emission in both the GRB and the accretion energy, (iii) the accretion rates of individual sources and (coupled to this) (iv) the recurrence time of GRBs.

ISSN:0094-243X    

DOI:10.1063/1.42814

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The cosmological distance scale to gamma‐ray bursts is indicated by the observed distribution of the bursts in angle and intensity. The redshifts of the weakest and the strongest bursts detected by the BATSE experiment are estimated to bez≊1.5 andz≊0.2, respectively, while the strongest bursts detect by the PVO in its 11 years of operation were atz≊0.05. The bursts’ rate is ∼10−6per year in a galaxy like ours, and a typical energy release if ∼1051ergs, assuming isotropic emission. The rate of the neutron star and black hole collisions in close binaries is somewhat higher, and those collision release 1053–1054ergs in a burst like form, making them the primary candidates for the observed gamma‐ray events. Contrary to the common belief no observations available to date are in conflict with the cosmological distance to the bursters. A detection of gravitational lensing of some bursts, the redshift of the weakest events, and the absorption of soft x‐rays by the interstellar matter in the plane of our galaxy, are the specific predictions of the cosmological scenario.

ISSN:0094-243X    

DOI:10.1063/1.42815

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We discuss the hypothesis that &ggr;‐ray bursts originate in mergers of double neutron star binaries and black hole‐neutrons star binaries at cosmological distances. Such events are estimated to occur in the Universe (out toz∼1) at a rate comparable to the GRO burst detection rate. The total energy released in a merger is ∼5×1053ergs, of which only a fraction ∼10−3has to be emitted in &ggr;‐rays to be detectable at cosmological distances.

ISSN:0094-243X    

DOI:10.1063/1.42816

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We find that an isolated canonical neutron star (1012Gdipole, 10 s rotational period) can accrete at most 106g/s from the interstellar medium regardless of its ambient density. Solitary neutron stars are therefore so fuel starved (even if placed in dense molecular clouds) that the thermonuclear model cannot account for the majority of gamma‐ray bursts. The derived upper limit on accretion rate is a consequence of the propeller mechanism and of the ionizing power of accretion‐caused radiation. Implications of this upper limit for the cyclotron‐line interpretation of GB 870303 are discussed. We also show that gamma‐ray bursts as a class cannot be powered by large crustal superfluid glitches of neutron stars produced at current birthrates. If gamma‐ray bursts are powered by internal sources of energy of neutron stars, such as rotational or nuclear free energy, typical rotation periods cannot fall in the range 10 s to 500 s.

ISSN:0094-243X    

DOI:10.1063/1.42817

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The photon‐photon optical depth for gamma‐ray bursts at cosmological distances is potentially so large that accurate calculations are needed to evaluate the conditions for which high energy photons can escape. one must explicitly included the rest frame emission pattern and determine the photon field at each point along the path of a high‐energy ‘‘test’’ photon to the observer. Relativistic expanding shell models have a key advantage over static relativistic wind models. For an expanding shell there is a sudden decrease of the optical depth with increasing Lorentz factor when the surface that contributes photons to the path of the test photon no longer subtends more than a critical angle regardless of how close the test photon is to the shell. As a result, expanding shell models require smaller relativistic flow rates than stationary surfaces. The minimum Lorentz factor that models must provide is roughly 100 for a 100 MeV photon to escape. We have made two crucial assumptions which require further study. The spectrum has been assumed to be a power law and a two component power law or a power law with a high‐energy cut‐off would decrease the required &ggr;. The expanding shell model uses a infinitly thin emitting surface and one with a finite thickness would increase the required &ggr;.

ISSN:0094-243X    

DOI:10.1063/1.42818

出版商:AIP    

年代:1991

数据来源: AIP