The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr À1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z > 10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a newgeneration wide-field gamma-ray (15-150 keV ) detector that will detect bursts, calculate 1 0 -4 0 positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5 00 positions and perform spectroscopy in the 0.2-10 keV band; and a narrow-field UV/optical telescope that will operate in the 170-600 nm band and provide 0B3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of $1 mcrab ($2 ; 10 À11 ergs cm À2 s À1 in the 15-150 keV band ), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow-up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of $8 yr.
UHECRs are roughly isotropic and attain very large energies, E ∼ > 3 × 10 20 eV . Conventional models fail to explain both facts. I show here that acceleration of UHECRs in GRBs satisfies both observational constraints. Using Mészáros and Rees' (1994) model of GRBs as due to hyperrelativistic shocks, I show that the highest energies that can be attained thusly are E ≃ 10 20 θ −5/3 n −5/6 1 eV , explaining the energy of the Bird et al. (1995) event even without beaming. The traditional photopion catastrophe affecting UHECR acceleration in AGNs is circumvented. An order of magnitude estimate shows that the total energy flux of UHECRs at the Earth is also correctly reproduced. A test of the model based upon the UHECRs' distribution upon the plane of the sky is briefly discussed. Subject headings: acceleration of particles -gamma-rays: burstsWe should thus be scouting around for a class of objects that is roughly isotropically distributed, and where sufficient amounts of concentrated energy are available to accelerate UHECRs. One such category, so far unexplored, is gamma-ray bursts (GRBs, see Paczyński, 1993 for a review). I shall consider only cosmological models of GRBs, with a total energy release of E GRB ≃ 10 51 erg. I shall need in the following discussion no detailed property of the mechanism proposed to explain the energy injection mechanism, but I shall need the details of the hydrodymanical expansion of the fireball leading to the GRB. In particular, the really attractive feature that I shall try to exploit in the following is the suggestion by Mészáros and Rees (1994, MR from now on) that hyperrelativistic shocks (whether due to the impact of different parts of the same flow, endowed with different Lorenz bulk factors, or to the impact of the flow on the surrounding interstellar medium) are responsible for GRBs.In Section 2, I shall give two generic arguments (i.e., independent of the actual acceleration mechanism) in favor of GRBs as sites for the acceleration of UHECRs. Those features of the hydrodynamics of GRBs which are relevant to the problem at hand, are briefly reviewed in Section 3, which is entirely based upon the results of Mészáros and Rees (1994, and references therein). In section 4, I describe the acceleration process in this model for the evolution of the fireball. In particular, in Section 4.1 I discuss qualitatively two acceleration mechanisms, and I compute in subsections 4.2 and 4.3 the highest energies that UHECRs can attain. A mixed bag of limitations and caveats are discussed in Sections 5. I discuss the results in Section 6, and summarize them in Section 7. Gamma-ray bursts as accelerators of UHECRsThere are two arguments that make GRBs appealing, the first one being a numerological coincidence. The total energy of UHECRs striking the Earth can be estimated as 5 × 10 −13 erg s −1 cm −2 sr −1 . This
A remarkable correlation between the centroid frequencies of quasi periodic oscillations, QPOs, (or peaked noise components) from low mass X-ray binaries, has been recently discovered by Psaltis, Belloni & van der Klis (1999). This correlation extends over nearly 3 decades in frequency and encompasses both neutron star and black hole candidate systems. We discuss this result in the light of the relativistic precession model, which has been proposed to interpret the kHz QPOs as well as some of the lower frequency QPOs of neutron star low mass X-ray binaries of the Atoll and Z classes. Unlike other models the relativistic precession model does not require the compact object to be a neutron star and can be applied to black hole candidates as well. We show that the predictions of the relativistic precession model match both the value and dependence of the correlation to a very good accuracy and without resorting to additional assumptions.
We report on the discovery of two emission features observed in the x-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 December 1999 by the Chandra X-ray Observatory. These features are identified with the Ly α line and the narrow recombination continuum by hydrogenic ions of iron at a redshift z = 1.00 ± 0.02, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, a quantity of iron ∼0.01 of the mass of the sun at a velocity ∼0.1 of the speed of light, probably by a supernova explosion.
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