Review of GRANAT observations of gamma-ray bursts

The GRANAT observatory was launched into a high apogee orbit on 1 December, 1989. Three instruments onboard GRANAT - PHEBUS, WATCH and SIGMA are able to detect gamma-ray bursts in a very broad energy range from 6 keV up to 100 MeV. Over 250 gamma-ray bursts were detected. We discuss the results of the observations of the time histories and spectral evolution of the detected events provided by the different instruments in different energy ranges. Short Gamma-Ray Bursts (< 2 s) have 10 ms structure in their time histories. They have harder energy spectra than the long (> 2 s) events. Evidence of the existence of four differently behaving componenents in gamma-ray burst spectra is discussed. Statistical properties of the gamma-ray burst sources based on the 5 years of observations with ( 10^–6 erg/cm²) sensitivity as well as the results of high sensitivity ( 10^–8 erg/cm²) search for Gamma-Ray Bursts within the SIGMA telescope field of view are reviewed.     

Gamma-ray bursts observed from the Hinotori satellite

During February, 1981 and June, 1982 the gamma-ray and the hard X-ray spectrometers on the Hinotori satellite observed four gamma-ray bursts on 28 February, 21 July, 1981, 26 February and 13 March, 1982. These gamma-ray bursts were simultaneously observed by other satellites. The time histories and energy spectra are shown for these gamma-ray bursts, and the burst sizes (erg cm^–2) are estimated. Two possible source locations for the burst of 21 July, 1981 are roughly determined from arrival time delays between two pairs of satellites, PVO-Hinotori and ISEE-3-Hinotori. The weak gamma-ray line peak structure around 1.8 MeV was observed for the burst of 13 March, 1982. The line could be interpreted in terms of gravitationally redshifted neutron capture line at 2.22 MeV.     

Recent results from the gamma-ray burst studies in the KONUS experiment

Observations of 85 gamma bursts by the KONUS instruments on the Venera 11 and Venera 12 spacecraft in the period September 1978 to May 1979 inclusive have provided proof of a galactic localization of the gamma-burst sources based on an analysis of the logN-logS plot and the revealed anisotropy in the angular distribution of sources over the celestial sphere. Evaluation of the energy released in the sources yields 10⁴⁰–10⁴¹ erg. There apparently exist several types of gamma bursts differing in time profile, duration and shape of their energy spectrum. In some cases, extensive evolution of the energy spectrum is observed during a burst. The discovery of a flaring X-ray pulsar in Dorado has provided the first observational evidence for a connection of gamma bursts with neutron stars. Repeated short bursts from this source have revealed for the first time the recurrent features of this phenomenon. Repeated bursts have been detected from one more source in the short burst class. The data obtained thus far impose a number of restrictions on the applicability of many theoretical suggestions concerning the nature of the gamma bursts. The most plausible model for the gamma-burst source appears to be a binary with a neutron star with strongly non-stationary accretion involving, possibly, non-stationary thermonuclear fusion of matter falling onto the surface of a degenerate star.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts, held at Toulouse, France, 26–29 November, 1979.     

Advances in gamma-ray burst astronomy

Work at Goddard is preséntly being carried out in three major areas of gamma-ray burst research: (1) A pair of simultaneously operating 0.8-m² burst detectors were successfully balloon-borne at locations 800 miles apart on 9 May, 1975, each to atmospheric depths of 3 to 4 g cm^–2, for a 20-h period of coincident data coverage. This experiment investigates the size spectrum of bursts in the 10^–7 to 10^–6 erg cm^–2 size region where dozens of events per day are expected on a –1.5 index integral power-law extrapolation. Considerable separation in latitude was used to avoid possible atmospheric and auroral secondary effects. Its results are not yet available. This experiment is the sequel to a single balloon flight in May 1974, in which candidate events were found to fit the –1.5 spectral extrapolation, indicating the need for positive event identification. (2) A deep-space burst detector, the first spacecraft instrument built specifically for gamma-ray burst studies, was recently successfully integrated into the Helios-B space probe. Its use at distances of up to 2 AU will make possible the first high-resolution directional study of gamma-ray burst source locations. Similar modifications to several other space vehicles are also being prepared. (3) Our gamma-ray instrument on the IMP-7 satellite is presently the most sensitive burst detector still operating in orbit. Its results have shown that all measured event-average energy spectra are consistent with being alike. Using this characteristic spectrum to select IMP-7 candidate events of smaller size than those detected using other spacecraft in coincidence, a size spectrum is constructed which fits the –1.5 index power law down to 2.5×10^–5 erg cm^–2 per event, at an occurrence rate of about once per month.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Analysis of COMPTEL gamma-ray burst locations and spectra

In its first three years of operation, the COMPTEL instrument on theCompton Gamma-Ray Observatory has measured the locations (mean accuracy 1°) and spectra (0.75-30 MeV) of 18 gamma-ray bursts and continues to observe new events at a rate of 1/month. With good angular resolution and sensitivity at MeV energies, the growing COMPTEL burst catalog is an important new piece of evidence in the on-going GRB mystery. The COMPTEL burst locations are consistent with an isotropic distribution of sources, yet the spatial coincidence of two of the bursts indicates the possibility of repetition. The COMPTEL burst spectra are in most cases consistent with a single power law model with spectral index in the range 2–3. However, two bursts show evidence of a spectral break in the MeV range. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed. We present an overview of analysis results obtained from the COMPTEL burst catalog concentrating on the search for burst repetition and the implications of highly variable MeV emission.     

Cosmic gamma-ray burst spectroscopy

A review is given of the gamma-ray burst energy spectrum measurements on Venera 11 and Venera 12 space probes. The gamma burst continuum approximates in shape thermal brems-strahlung emission of a hot plasma. The radiation temperature varies over a broad range, 50–1000 keV, for different events. Spectra of many bursts contain cyclotron absorption and/or redshifted annihilation lines. Strong variability is typically observed in both continuum and line spectra. These spectral data provide convincing evidence for the gamma-ray bursts being generated by neutron stars with superstrong magnetic fields 10¹²–10¹³ G.     

SOLAR ENERGETIC ION AND ELECTRON LIMITS FROM HIREGS OBSERVATIONS

The HIgh-REsolution Gamma-ray and hard X-ray Spectrometer (HIREGS) consists of an actively shielded array of twelve liquid-nitrogen-cooled germanium detectors designed to provide unprecedented spectral resolution and narrow-line sensitivity for solar gamma-ray line observations. Two long-duration, circumpolar balloon flights of HIREGS in Antarctica (10–24 January, 1992 and 31 December, 1992–10 January, 1993) provided 90.9 and 20.4 hours of solar observations, respectively. During the observations, eleven soft X-ray bursts at C levels and above (largest M1.7) occurred, and three small solar hard X-ray bursts were detected by the Compton Gamma-Ray Observatory. HIREGS detected a significant increase above 30 keV in one. No solar gamma-ray line emission was detected. Limits on the 2.223-MeV line and the hard X-ray emission are used to estimate the relative contribution of protons and electrons to the energy in flares, and to coronal heating. For the 2.223-MeV line, the upper limit fluence is 0.8 ph cm^-2 in the flares, and the upper limit flux is 1.8 × 10^-4 ph s^-1 cm^-2 in the absence of flares. These limits imply that 6 × 10³⁰ (2) protons above 30 MeV were accelerated in the flares, assuming standard photospheric abundances and a thick target model. The total energy contained in the accelerated protons >30 MeV is 4 × 10²⁶ ergs, but this limit can be more than 10³⁰ ergs if the spectrum extends down to 1 MeV. The upper limit on the total energy in accelerated electrons during the observed flares can also exceed 10³⁰ ergs if the spectrum goes down to 7 keV. Quiet-Sun observations indicate that 10²⁶erg s^-1 are deposited by energetic protons >1 MeV, well below the10²⁷ –10²⁸ erg s^-1 required for coronal heating, while <3 × 10²⁷ erg s^-1 are deposited by energetic electrons, which does not exclude the possibility of coronal heating by quiet-time accelerated electrons. The quiet-Sun observations also suggest that if protons stored in the corona are to supply the energy for flares, as suggested by Elliot (1964), the proton spectrum must extend down to at least 2 MeV. However, collisional losses at typical coronal-loop densities prevent those low-energy protons from being stored for 10⁴ s. It therefore seems unlikely that the energy for flares could come from energetic protons stored over long periods.     

Comments on the gamma-ray burst logN>S) — LogS point of Beurleet al.

The isotropic cumulative burst rate of 7030 _–6000 ⁺¹⁰⁰⁰⁰ yr^–1 at a fluence ofS=8.47×10^–9 erg^–1 cm^–2 determined by Beurleet al. from their observation of two gamma-ray bursts is shown to be statistically improbable. The difficulty arises from their assumption that the power law cumulative distribution function index equals one. Their observations are rediscussed and an upper limit ofN(>8.47×10^–9 erg cm^–2)<5400 yr^–1 is proposed.     

X-ray observations of gamma-ray burst sources

We report on preliminary results of EXOSAT observations of three gamma-ray burst error boxes. No source was detected down to a limit of 10^–10 erg cm^–2s^–1, assuming a black-body spectrum for the burst counterpart. Results are interpreted in the framework of current theoretical models.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Gamma-ray burst measurements with A 6.3 m2 array

Two flights from Alice Springs, Australia, were achieved in November 1977 and November 1978 with a plastic scintillator -burst detector, effective area 6.3 m², thickness 5 cm, energy response in the range 50 keV to 2 MeV. In 33 hr of good, high altitude data, two bursts were detected, yielding a rate corrected to an isotropic flux of at a size of 8.5×10^–9 erg cm^–2. One event, seen at 22.14 on 15 Nov 1978, was confirmed by spacecraft measurements. The second, too small to be detected by spacecraft, arrived from 0 hr RA, –13.2° Decl. ±12° and possibly comes from a confirmed -burst source location. A galactic origin with a source distribution originating from a relatively thick disk, is favoured by these results.     

A search for cosmic gamma-ray bursts with a balloon-borne NaI(T1) scintillation crystal

A search has been made for gamma-ray bursts in 15 hours of data obtained from a balloonborne gamma-ray detector on 10 October and 21 October, 1970. The event rate for photon energy losses in the 0.1–0.4 MeV range from the 13-in. diameter by 6-in. thick NaI(T1) scintillation crystal was examined for statistically significant fluctuations as an indication of gamma-ray bursts. Searches of the data were made with time resolutions varying from 2 ms to 64 s. Four statistically significant bursts were detected and are considered as possible cosmic gamma-ray burst events. The characteristic duration of all four of the observed events is 100 ms. Similar events can be generated in the laboratory following an extremely large (10³ GeV) thirty ns X-ray energy deposition in the NaI(T1) crystal. The implications of these short duration, low intensity events, if valid gamma-ray bursts, are discussed.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

The LASL gamma-ray burst astronomy program

Gamma-ray burst observations performed by LASL began with the identification and initial report of the phenomenon from data acquired by the Vela satellites. The Vela instruments have recorded responses to 73 gamma-ray bursts over a ten-year interval, and are continuing to contribute toward these observations. Similar instrumentation was included aboard the NRL SOLRAD 11 spacecraft. These performed well but suffered an early demise. Recently, the LASL gamma-ray burst astronomy program has been enhanced through the implementation of experiments aboard the Pioneer Venus Orbiter and ISEE-C spacecraft. Both of these experiments are continuing to contribute data vital to trigonometric directional analyses.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts, held at Toulouse, France, 26–29 November, 1979.     

Localization of gamma-ray bursts with wide field multiple pinhole camera system in near earth orbit

A multiple pinhole camera system has been designed and proposed for a small satellite of the SAS type for the detection and localization of gamma-ray bursts. The instrument consists of a three unit array of detectors each of which includes a semi-cylindrical collimator surrounding a twodimensional position-sensitive detector. The collimator contains slits of 1 mm width that are cut parallel to the axis of the cylinder. The slits are randomly arranged in azimuth around the cylinder. X-rays may enter the counter through several surfaces. The point at which photoelectric interaction takes place is determined in two dimensions in a plane perpendicular to the cylinder axis. Each unit of the system determines the position of a burst to a great circle. An intersection of two (or three) great circles provides the precise positions.The field of view of the instrument is 2.7 ster, essentially the entire region of sky not occulted by the Earth. It is designed to operate in the energy range 20–100 keV. An instrument sized to fit a SAS spacecraft has a sensitivity of better than 10^–6 erg cm^–2 for bursts whose intense phases occur in less than a total of three seconds. For stronger bursts (>10^–5 erg cm^–2) the location precision is better than a minute of arc.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

A review of recent results from the Franco-Soviet SIGNE gamma-burst experiments

The three-satellite SIGNE network has observed 26 confirmed gamma-ray bursts in the period September 1978–June 1979. Burst time histories may be used to define several classes of events, with a strong resemblance between events in a given class, but different arrival directions. The spectra of two gamma-ray bursts have been found to evolve from shapes which turn over at low energy during the start of the event, to near power law spectra at the end of the event.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts held at Toulouse, France, 26–29 November, 1979.     

Bremsstrahlung gamma radiation and interstellar electron spectrum in the local interstellar medium

We present a detailed study of the bremsstrahlung gamma-ray emissivity of the galactic disk. We show that there are large uncertainties in the production spectrum of photons in the medium energy range (10–100 MeV) due to our lack of knowledge of the interstellar electron spectrum below a few hundred MeV. In fact, gamma-ray observations can be of great help in determining this spectrum. At present, the spectral shape of the local gamma-ray emissivity above 30 MeV is available, thanks to the SAS-II and the COS-B satellites. Comparing it to our calculations, we determine the local interstellar electron flux in the 50–500 MeV range; the corresponding integrated gamma-ray emissivity above 100 MeV is equal to 2.4×10^–25 photons s^–1 (H-atom)^–1, 60% higher than previously accepted values.     

Search for ultrahigh-energy cosmic gamma-ray bursts on the Baksan underground scintillation telescope

Based on data from the Baksan underground scintillation telescope (BUST) for the period 2001–2004, we searched for cosmic gamma-ray bursts (GRBs) at primary photon energies of 0.5 TeV or higher. We obtained constraints on the rate of bursts with durations of 1–10 s for fluences within the range 4.6 × 10⁻³-1.8 × 10⁻² erg cm⁻² in the declination band 30° ≤ δ ≤ 80°. We searched for ultrahigh-energy gamma rays from GRBs detected on spacecraft during and within ±2 h of the burst. No statistically significant excesses above the background of random coincidences were found. The derived constraints on the ultrahigh-energy gamma-ray fluence during GRBs lie within the range 4.6 × 10⁻³-3.7 × 10⁻² erg cm⁻².     

Gamma-burst observations from the Pioneer Venus Orbiter

The LASL Pioneer Venus Orbiter Gamma Burst Detector (OGBD) is a vital element in the long base-line array of similar instruments intended to precisely locate sources of gamma-ray bursts. Results of early observations are described. The source of the gamma-ray burst of 5 March, 1979 (the first to be located with precision) has been determined to be consistent with the direction of supernova remnant N49 within the LMC. Approximate locations defined for a small number of events suggest no departure from an isotropic distribution.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts held at Toulouse, France, 26–29 November 1979.     

Spectral characteristics of solar S bursts

Observations of the solar radio spectrum have been made with high time and frequency resolution. Spectra were recorded over six 3-MHz bands between 30 and 82 MHz. The receivers used were capable of time and frequency resolutions of 1 ms and 2 kHz, respectively. A large number of radio bursts exhibiting a variety of find spectral structure were recorded.The bursts, referred to here as S bursts, were observed throughout the 30–82 MHz frequency range but were most numerous in the 33–44 MHz band and were very rare at 80 MHz. On a dynamic spectrum the bursts appeared as narrow sloping lines with the centre frequency of each burst decreasing with time. The rate of frequency drift was about 1/3 that of type III bursts. Most bursts were observed over only a limited frequency range (< 5 MHz) but some drifted for more than 10 MHz. The durations measured at a single frequency and the instantaneous bandwidths of S bursts were small; t = 49 ± 34 ms and f = 123 ± 56 kHz for bursts observed near 40 MHz. A significant number had t 20 ms. Flux densities of S burst sources were estimated to fall in the range 10²³-5 × 10²¹ Wm^–1 Hz^–1.A small proportion (1–2%) of bursts showed a fine structure in which the burst source apparently only emitted at discrete, regularly spaced frequencies causing the spectrogram to exhibit a series of bands or fringes. The fringe spacing increased with wave frequency and was f - 90 kHz for fringes near 40 MHz. The bandwidths of fringes was narrow, often less than 30 kHz and in some cases down to 10–15 kHz.New address: Astronomy Program, University of Maryland, College Park, MD, U.S.A.     

GRB 070912—A gamma-ray burst recorded from the direction to the galactic center

The detection of GRB 070912 recorded in the field of view of the SPI, IBIS/ISGRI, and JEMX telescope on September 12, 2007, at 07^h32^m19^s (UT) when analyzing the INTEGRAL archival data is reported. The burst is one of the well-localized events closest to the direction toward the Galactic center (less than from the source Sgr A*) over the entire history of burst observations. Since it was not promptly revealed by the INTEGRAL Burst Alert System (IBAS), no information about its coordinates was disseminated and no search for optical and soft X-ray afterglows was conducted. The 3–200 keV fluence was 2.8 × 10^?6 erg cm^?2 and the peak flux was 1.8 × 10^?7 erg cm^?2 s^?1 (1.9 ph cm^?2 s^?1). The burst was also observed in the KONUS/WIND experiment in the background mode, although it was not included in the list of recorded bursts. GRB 070912 is among a limited number of events for which a broadband (3 keV-2 MeV) spectrum of X-ray and gamma-ray emission has been obtained and their evolution from the first instants to complete decay has been traced. It shows how the fast evolution of its spectrum gives rise to absorption features at energies of ～100 keV.Within the first seconds after the onset of the burst, its spectrum was a power law with a photon index of ～0.8, but it exhibited a noticeable deficit of photons at energies below 20 keV. Such an initial deficit (a delay in appearance) of X-ray photons can be explained by their “high-latitude” origin relative to the line of sight. The spectrum rapidly softened and at the decay phase was well described by a blackbody (or Wien) law. This allows the distance (redshift) to the burst source to be estimated.     

Cosmic GRB 060428C detected in the field of view of the IBIS and SPI telescopes onboard the INTEGRAL observatory and its early afterglow

While analyzing the archival data of the INTEGRAL observatory, we detected and localized a cosmic gamma-ray burst recorded on April 28, 2006, by the IBIS/ISGRI and SPI telescopes in their fields of view. Since the burst was not revealed by the INTEGRAL burst alert system (IBAS), information about its coordinates was not distributed in time and no search for its afterglow was conducted. The burst was recorded by the KONUS/WIND and RHES SI satellites. Its 20–200-keV fluence was 2.3 × 10^?6 erg cm^?2, the peak flux was 3.6 × 10^?7 erg cm^?2 s^?1 (3.9 phot. cm^?2 s^?1). The burst had a complex multipeaked profile and stood out among typical bursts by an increase in its hardness with time. At the flux peak, the spectrum was characterized by a photon index α ? ?1.5 and a peak energy E _p ? 95 keV. The burst lasted for ～12 s, after which its afterglow decaying as a power law with an index γ ～ ?4.5 was observed at energies 15–45 keV. The spectral hardness decreased noticeably during the afterglow.