Oleg V. Terekhov

The Ulysses Supplement to the Granat/WATCH Catalog of Cosmic Gamma-Ray Bursts

We present third Interplanetary Network (IPN) localization data for 56 gamma-ray bursts in the Granat/WATCH catalog that occurred between 1990 November and 1994 September. These localizations are obtained by triangulation using various combinations of spacecraft and instruments in the IPN, which consisted of Ulysses, BATSE, Pioneer Venus Orbiter, Mars Observer, WATCH, and PHEBUS. The intersections of the triangulation annuli with the WATCH error circles produce error boxes with areas as small as 16 arcmin2, reducing the sizes of the error circles by factors of up to 800.

Observation of quasi-periodic pulsations in the solar flare SF 900610

A quasi-periodic component was found at the maximum of the X-ray light curve for the June 10, 1990 solar flare detected by the Granat observatory. The pulsation period was 143.2±0.8 s. The intensity of the pulsing component is not constant; the maximum amplitude of the pulsations is ∼5% of the total flare intensity. An analysis of the data showed the characteristic size of the magnetic loop responsible for these pulsations to be ∼(1–3)×1010 cm.

The first hours of the optical afterglow from the cosmic gamma-ray burst 030329

We describe the first results of our observations of the exceptionally bright optical afterglow from the cosmic gamma-ray burst (GRB) of March 29, 2003 (030329), with the 1.5-m Russian-Turkish telescope (RTT150) installed at the TUBITAK National Observatory (Turkey) at Mount Bakyrlytepe. RTT150 was one of the first medium-class telescopes pointed at the afterglow. The observations began as early as about six hours after the GRB. During the first five hours of our observations, the BV RI flux fell off exactly as a power law with the same slope −1.19±0.01. Subsequently, in all of the BV RI bands, we observed the same increase in the power-law slope of the light curve to a value that was later recorded during the observations at observatories in the western hemisphere. The break in the power-law light curve occurs at t − t0 ≈ 0.57 days (13.5 h) and lasts for about 0.2 days. Apart from this smooth decrease in the flux, the afterglow exhibited no flux variability. The upper limits on the variability are 10–1% on time scales of 0.1–1000 s, respectively. The BV RI spectral flux distribution during the first night of our observations closely corresponds to a power-law spectrum with a spectral index α=0.66±0.01. The change in the power-law slope of the light curve at the end of our observations is probably attributable to the deceleration of the ultrarelativistic jet to a gamma factor when its structural features begin to show up in the light curve. The radio, optical, and X-ray broadband spectrum is consistent with the assumption about the synchrotron radiation of the ultrarelativistic jet. This unique object continues to be observed with RTT150.We present the first results of the observations of the extrem ely bright optical afterglow of gamma-ray burst (GRB) 030329 with the 1.5m Russian-Turkish telescope RTT150 (T ¨ UBITAK National Observatory, Bakyrlytepe, Turkey). RTT150 was one of the first 1.5m-class telescopes pointed to the aftergl ow. Observations were started approximately 6 hours after the burst. During the first 5 hours of our observations the afterg low faded exactly as a power law with index −1.19 ± 0.01 in each of the BVRI Bessel filters. After that, in all BVRI filters simultaneously we observe a steepening of the power law light curve. The power law decay index smoothly approaches the value ≈ −1.9, observed by other observatories later. This power law break occurs at t − t0 ≈ 0.57 days and lasts for ≈ ±0.1 days. We observe no variability above the gradual fading with the upper limits 10‐1% on time scales 0.1‐1000 s. Spectral flux distribution in four BVRI filters corr esponds to the power law spectrum with spectral index α = 0.66 ± 0.01. The change of the power law decay index in the end of our observations can be interpreted as a signature of coll imated ultrarelativistic jet. The afterglow flux distribut ion in radio, optical and x-rays is consistent with synchrotron sp ectrum. We continue our observations of this unique object with RTT150.

WATCH observations of gamma ray bursts during 1990–1992

The first WATCH/GRANAT Gamma‐Ray Burst Catalogue comprises 70 events which have been detected by WATCH during the period December 1989–September 1992. 32 GRBs could be localized within a 3σ error radii of 1°. We have found a weak (2.2σ) clustering of these 32 bursts towards the Galactic Center. However we conclude that there is no strong evidence of concentration of the bursts towards the Galactic Center or Plane. Around ∼10% of the 70 bursts showed x‐ray precursor or/and X‐ray tail. We discuss the possibility that two events, GRB 900126 and GRB 920311, would have been produced by the same source.

Measured radiation damage in charge coupled devices exposed to simulated deep orbit proton fluxes

Measurements were made to quantify the effects of radiation damage in large area, deep-depletion CCDs developed for the joint European X-ray telescope (JET-X). Two devices were tested — the first, a standard X-ray CCD and the second, otherwise identical, including supplementary buried channels to improve its radiation hardness. Both devices were exposed to the expected in-orbit proton fluxes at the proton irradiation facility (PIF) at the Paul Scherrer Institute. The experimental conditions included the predicted orbital energy spectrum, its evolution and the proposed JET-X proton shielding. Energy resolution and charge transfer efficiency were measured after exposure to incremental doses corresponding to 0.25, 0.5, 1, 2 and 5 years in orbit. The radiation-hard device performed substantially better than the conventional device, showing a 20% degradation in energy resolution after 5 years equivalent dose (corresponding to ∼ 109 protons cm−2), as compared to ∼ 60% for the conventional device. Finally, the devices were exposed to fluences appropriate to the August 1972 solar flare. These results have been used to develop a model to predict the fwhm energy resolution of radiation damaged CCD.

Radiation shielding study for the joint European X-ray Telescope focal plane Detectors

The requirements, philosophy and implementation of inorbit radiation shielding for the Charge Coupled Devices (CCDs) on-board the Joint European X-ray Telescope (JET-X) are described in detail. Relevant trade-offs between displacement damage, spectral degradation, instrument mass and mission lifetime are examined and a maximum permissible fluence at the CCDs derived. The calculations show that for the ambient JET-X radiation environment no benefit is obtained by increasing the shield thickness above 30 mm of aluminum due to the local production of cascade nucleons. However, a large flare of the August 1972 type will exceed the required maximum fluence by a factor of ∼ 2. In order to survive such a flare, a thicker shield is required. Because of mass constraints, JET-X will fly a composite shield composed of 20 mm of aluminum on the outside and 5 mm of tungsten on the inside. Such a shield is designed to ensure that the degradation in the CCD FWHM energy resolution is no more than 40% around the Fe line over the nominal two year mission lifetime (a factor of 2 x the intrinsic line broadening). The predicted degradation in energy resolution and the efficacy of the shield design has been recently verified by experiment (Owens et al., Nucl. Instr. and Meth., A361 (1995) 602).

Comparison of the properties of short and long gamma-ray bursts observed with Phebus

The observation of subclasses within the classical Gamma-Ray Burst population is the first clue that the bursts may be produced by a small number of distinct mechanisms, sources, or initial conditions. Events in one subclass are characterized by a short duration (less than 2 seconds) and, on average, harder spectra compared to longer bursts. Here, we present a statistical study of GRB properties as observed with the PHEBUS experiment such as intensity, duration, spectral hardness, and the shape of their time profiles. We show which burst characteristics are common to both the long and short populations and which are not. Finally, we compare our results with BATSE observations. Specifically, we discuss the possible causes for the striking difference observed in the hardness-duration diagrams for the two experiments.

Discovery of an Optical Afterglow from the Cosmic GRB 920925C

A systematic study of the archival images for the error boxes of cosmic gamma-ray bursts (GRBs) obtained at the Palomar (USA) and Siding Spring (Australia) Observatories during the DSS allsky survey has revealed an optical transient with a magnitude of 17.8 within the error circle of the bright event GRB 920925C on the plate taken 6 h after the burst. The position of the object falls within the IPN error box for the burst. Analysis of the event properties suggests that the detected transient is most likely the optical afterglow from GRB 920925C. This event occurred 4.5 yr before GRB 970228, which has been considered to be the first optically identified GRB up until now.

Localization of the solar flare SF900610 in X-rays with the WATCH instrument of the GRANAT observatory

During the solar flare of June 10, 1990, the WATCH instrument of the GRANAT space observatory obtained 110 localizations of the X-ray source in the X-ray range 8–20 keV. Its coordinates were measured with an accuracy of ∼2 arcmin at a 3σ confidence level. The coordinates of the X-ray source do not coincide with the coordinates of the Hα-line flare. The X-ray source moved over the solar disk during the flare. This probably implies that, as the X-ray emission was generated, different parts of one loop or a system of magnetic loops dominated at different flare times.

Observations of cosmic gamma-ray bursts with the main detector of the SIGMA telescope onboard the Granat Observatory

We present the observations of cosmic gamma-ray bursts (GRBs) with the main detector of the SIGMA telescope onboard the Granat Observatory from January 1990 through September 1994. The observations were carried out in the energy range 35–1300 keV. We detected 36 GRBs and 31 high-energy solar flares during this period. No GRB fell within the main field of view; they were all recorded by the “secondary optics” of the telescope. The SIGMA telescope recorded relatively bright bursts with peak fluxes of 10−6–10−4 erg s−1 cm−2 in the 100–500-keV energy band. Stable detector background allows the long-term variability of GRB sources on a time scale of ∼1000 s to be studied. The results of our search for early afterglows of GRBs are presented. The flux averaged over all bursts in the interval 100–800 s after the main event is 0.36±0.14 counts s−(35–300 keV), suggesting that there is soft gamma-ray emission on this time scale after a considerable number of GRBs.