Optical search for gamma-ray bursts

The preliminary results from optical search for light pulses associated with gamma ray bursts by means of the Czechoslovak Fireball Network plate collection at the Ondřejov observatory are given. Optical monitoring represents more than 7700 hours, but no real optical counterpart was found. Problems associated with the optical search for gamma ray bursts are discussed.     

Thermonuclear flashes on magnetized neutron stars as a model for gamma-ray bursts

We propose, as a model for gamma ray bursts, that the energy released by a thermonuclear runaway at the surface of a slowly accreting, magnetized neutron star, is converted into gamma ray photons by small scale magnetic field reconnection. Most of the energy flux is transported up to the atmosphere by Alfvén waves, driven by oscillatory magnetoconvection. The electric field created by reconnection accelerates electrons to very high energies. The excitation of Landau levels by collisions is followed by gamma ray synchrotron emission. The time scales, energies, and luminosities involved agree with the observations.     

New results on the time histories of gamma-ray bursts

A study of the morphology of 14 short ( 1 s) gamma ray bursts observed by the Franco Soviet SIGNE detectors onboard the VENERA spacecraft between 1978 and 1982 is presented. We find two major groups of short bursts characterised not only by their different durations ( 1 s and 100 ms respectively), but also by different e-folding rise and decay times. A study of the time history of the impulsive portion of the 1979 March 5 event at 2 ms resolution shows evidence for a previously undiscovered 23ms quasi periodicity. These results are discussed in the context of neutron star models for gamma ray bursters.     

Medium energy gamma ray astronomy with transpacific balloon flights

Transpacific balloon flights with the University of California, Riverside (UCR) double scatter telescope are discussed. With flight durations from 5 days up to perhaps 15 days the long observation times necessary for medium energy (1–30 MeV) gamma ray astronomy can be obtained. These flights would be made under the auspices of the Joint U.S.-Japan Balloon Flight Program at NASA. We propose that flights can provide at least 30 hours of observation time per flight for many discrete source candidates and 120 hours for detecting low intensity cosmic gamma ray bursts.     

Nature of gamma-ray burst sources

Observational evidence suggests that gamma ray bursts have a local galactic origin involving neutron stars. In this light we make a critical review of physics of the thermonuclear runaway model placing emphasis on self-consistency. We further show that some of the proposed models can be observationally excluded in the light of existing data from the Einstein Observatory. The possibility of gamma bursts arising in low mass binaries is finally discussed in the light of evolutionary scenarios leading to low luminosity systems.     

Gamma-ray bursts — the current status

The field of gamma ray burst astronomy is reviewed with emphasis on the results obtained since 1978 by numerous spacecraft experiments. Burst energy spectra are now known to display complex and rapidly varying shapes; however, the detection of line emission poses both experimental and theoretical problems. The log N-log S curves, when properly corrected for instrumental effects, are substantially in agreement at high intensities, although the shape of the curves is inconsistent with the observed spatial distribution of the bursts. Precise localizations using the method of arrival time analysis between widely separated spacecraft have given small error boxes which have in many cases been searched down to magnitude 23.5 and beyond. The results of these searches, as well as those of archibal and real-time optical searches, are reviewed.     

Observations of cosmic gamma-ray bursts

The recent discovery of a late-1983 cluster of soft transients /1,2/ adds a new aspect to the study of gamma ray bursts. Its source is consistent with that of an isolated, > 4-year earlier event from the galactic bulge /3/. It is the third repeating series found to date with gamma-ray burst instrumentation; typical events of all three are fairly brief in duration and have energies below those typical of the harder, > 150-keV events and well above those of X-ray bursts. One may speculate that these soft repeaters form a separate population with sources in high-density (galactic or LMC) regions, given the disk and the N49 source directions for the three series.Gamma ray burst workshops and conferences of recent years are cited. Current viewpoints include a size spectrum based on peak intensity that can fit the −1.5-index power law /4/; this, consistent with the continuing isotropy of hard bursts /5/, implies the absence of any source region information. Observations of spectral evolution /6,7/ and of very high every components /8/, together with uncertainties as to the low-energy features, suggest that the understanding of hard burst spectra may be premature. Evidence for regular features in time histories has been inferred /9/; less convincing than in the soft prototype of 1979 March 5, the effects of quasi-periodic processes may be indicated instead.Since the hard, “classical” bursts and the soft repeaters may not necessarily have a great deal in common, and with no conclusive counterpart studies, the assumption that nearby neutron stars are the sources of the hard events remains plausible but unproved.     

Gamma rays from giant-molecular clouds

An examination of the evidence favouring the detection of gamma rays from the irradiation of giant molecular clouds by cosmic rays is given. Data relevant to specific nearby clouds are studied in some detail and the evidence favouring some of the so-called gamma ray sources being due to the irradiation of these and more distant clouds is critically assessed. The need for enhancement of cosmic ray intensity in some clouds is discussed. The relevance of this work to interpretation of current and planned X-ray measurements as well as future gamma-ray space missions is discussed.     

Real time localization of gamma ray bursts with INTEGRAL

The INTEGRAL satellite has been successfully launched in October 2002 and has recently started its operational phase. The INTEGRAL burst alert system (IBAS) will distribute in real time the coordinates of the gamma ray bursts (GRBs) detected with INTEGRAL. After a brief introduction on the INTEGRAL instruments, we describe the main IBAS characteristics and report on the initial results. During the initial performance and verification phase of the INTEGRAL mission, which lasted about two months, two GRBs have been localized with accuracy of 2–4 arcmin. These observations have allowed us to validate the IBAS software, which is now expected to provide quick (few seconds delay) and precise (few arcmin) localization for 10–15 GRBs per year.     

Soft gamma repeaters

The observational properties of the soft gamma repeaters are reviewed briefly, starting with the time histories and energy spectra of their bursts. The short bursts and giant flares are compared. Their quiescent emission is presented, and the context of the magnetar model is discussed.     

Measurement of gamma-ray line intensities from the earth's atmosphere

We report measurements of gamma ray lines from the Earth's atmosphere observed by the gamma ray spectrometer on the Solar Maximum Mission satellite. The data are summed over 3.5 years and have excellent statistical accuracy. 20 line features within the range 0.3 to 8 MeV have been observed. Line intensities are consistent with models attributing them to neutron absorption by nitrogen and oxygen. The relative intensity of scattered radiation from the 0.511 MeV line suggests production at an average atmospheric depth of 21 g/cm². These data allow a detailed comparison of gamma ray production models with experiment in a medium with known composition, geometry, and irradiation level and therefore serve as a good test subject for models of astrophysical gamma ray line sources.     

Background-sensitive event selection in the search for gamma-ray sources

Poor angular resolution of present day gamma ray telescopes (1° at 100 MEV) together with the complicated gamma ray sky structures at compareable angular extents and poor statistics available necessitate the selection of those gammas which will increase the observable signal from a suspected source. A method for such a selection using the pre-known time signature of the source as its signal, has been developed (Özel and Mayer-Haβelwander, 1983). In an iterative procedure based on the improvement of source signal, generalized zones around the source are defined, from which gamma rays should be accepted to construct the optimum light curve of the source. These zones depend on the energy of the probed gamma rays, on the spatial structure and intensity of the background around the source, on the (estimated and/or tentative) intetnsity of the source and finally on the observing instrument's angular and energy response functions. The method, when applied to simulated and observed COS-B data demonstrated its feasibility and merits over the previonsly used circular event-acceptance cones method which was the basis of searches for gamma ray emission from radio pulsars (see for example Kniffen et al., 1974; Ögelman, et al. 1976; Kanbach, et al., 1977; and references therein). Present method does not introduce “trials” which usually reduce the significance of the results obtained, fully accounts the available knowledge on the topography of gamma ray sky, makes use of all the information about the sky, source and the instrument and therefore is the proper way of handling gamma ray data in pulsar-type searches.Presently, method is applied in the analyses of COS-B data in the context of searches for gamma ray emitting X-ray and radio pulsars. Results will be published elsewhere.     

Relationships between neutron fluxes and rain flows

Registration of secondary cosmic ray neutrons is a convenient tool for investigation of primary cosmic ray variations and meteorological effects as well. At present a large network of neutron monitors exists, providing the studies of cosmic ray variations related to the interplanetary conditions and geomagnetic activity. At the same time cosmic ray variations may be caused by some atmospheric processes. In this connection, using the data from standard and lead-free neutron monitors, and gamma and muon detectors, we studied relations between rain flows and neutron, gamma and ionization component behavior. To explain observable results the calculations of neutron and gamma absorption and albedo neutron spectra have been performed on the basis of universal software package FLUKA-2006. In this study we used hourly data on the neutron flux, corrected for barometric pressure and data from local meteorological stations. It was shown that secondary neutron radiation, recorded by lead-free NM, and gamma radiation as well are strongly effected by meteorological factors. The neutron component behavior depends on the moisture content in the soil, and above its surface.     

The observation of the galactic anticenter region by the balloon borne gamma-ray telescope Natalya-I

This paper presents the results of measurements of gamma radiation with energies above 5 MeV, from the galactic anticenter region. The balloon-borne gamma ray telescope “Natalya-I”, was launched on 6 November, 1980 from the Tata Institute of Fundamental Research's Balloon Facility (Hyderabad, India) and reached ceiling altitude of 35 km. The results on the accelerator calibration of the telescope, using a “tagged” gamma ray beam are also presented.     

An imaging telescope for soft gamma-ray astronomy: The preliminary in-flight tests

A large area (6000 cm²) actively shielded low energy gamma-ray telescope is going to be built by an Anglo-Italian collaboration. The telescope, named ZEBRA, will be capable of producing images of the X and gamma ray sky in the energy range 0.015–20 MeV with an intrinsic angular resolution of a few tenths of a degree. A prototype detector has been built in order to experimentally study the main characteristics of the detection plane. The preliminary results obtained during a balloon flight from Trapani, Sicily in July 1981 are presented.     

SIGMA observations of two new hard X-ray and soft gamma-ray transients: GX 354-0 and Nova Persei

We report the first detection of two soft gamma ray transients GX 354-0 and Nova Persei (GRO J0422+32) by the coded-mask telescope SIGMA. Only preliminary results are presented with regard to Nova Persei while special emphasis is given to the data on GX 354-0 which has been monitored during the 1992 February–April survey of the Galactic Center. GX 354-0 underwent two flares of about two week duration and its average spectrum is well described by a thermal Bremsstrahlung model or a broken power law whereas the Nova Persei spectrum seems to be similar to the one observed for GRS 1124-684 in Musca.     

Magnetospheric structure and high-energy photon emission of radio pulsars

Some elementary aspects of particle acceleration and flow along the polar field lines of an isolated, rotating magnetized neutron star are described. Semi-quantitaive consequences of these ideas for the emission of high energy gamma rays and their relation to gamma ray observations of radio pulsars are outlined. A global model of the magnetosphere in which these polar flows can occur is described briefly.     

X射线天文卫星观测需求分析与控制总体设计

对X射线天文卫星观测需求进行了分析,提炼了观测任务对观测模式、源的高精度定位与对准、轨道、热控、测控数传等多项需求与约束;针对X射线观测的多需求、多约束难点,设计了集巡天观测、定点观测与小天区扫描观测于一体的观测模式,解决了一颗卫星同时实现全天扫描、银道面深度扫描、重要惯性区域扫描、重要及机遇目标深度观测以及伽马暴全天监测的多种观测需求的难题,该技术已在我国硬X射线调制望远镜卫星上得到应用.     

The Swift GRB MIDEX mission

Swift is a first-of-its-kind multiwavelength transient observatory for γ-ray burst astronomy. It has the optimum capabilities for the next breakthroughs in determining the origin of γ-ray bursts and their afterglows, as well as for using bursts to probe the early Universe. Swift will also monitor the soft gamma repeaters and perform the first sensitive hard X-ray survey of the sky. The mission is being developed by an international collaboration and consists of three instruments, the Burst Alert Telescope (BAT), the X-ray Telescope (XRT), and the Ultraviolet and Optical Telescope (UVOT). The BAT, a wide-field γ-ray detector, will detect >100 γ-ray bursts per year with a sensitivity 5× that of BATSE. The sensitive narrow-field XRT and UVOT will be autonomously slewed to the burst location within 20–70 s to determine 0.3–5.0″ positions and perform optical, UV, and X-ray spectrophotometry. Strong education/public outreach and follow-up programs will help to engage the public and the astronomical community. Swift launch is planned for late 2004.