S. Brandt

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.

The INTEGRAL ? Burst Alert System

ESA’s INTEGRAL high-energy observatory is scheduled for launch in 2001. INTEGRAL carries two gamma-ray instruments, optimized respectively for spectroscopy and high-resolution imaging, complemented by an X-ray and an optical monitor. The high sensitivity of the INTEGRAL instruments will allow the detection and detailed studies of relatively faint gamma-ray bursts. The INTEGRAL Burst Alert System (IBAS) is implemented as a ground-based system, working on the near-real time telemetry stream. It is expected that the system will detect more than one GRB per month in the field of view of the main instruments. Positions with an accuracy of a few arc-minutes will be distributed to the community for follow-up observations within a few tens of seconds of the event. Furthermore, the system will upload commands to optimize the detection of an associated transient with the INTEGRAL optical monitor.

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.

Gamma ray bursts observed with WATCH‐EURECA

The WATCH wide field x‐ray monitor has the capability of independently locating bright Gamma Ray Bursts to 1° accuracy. We report the preliminary positions of 12 Gamma Ray Bursts observed with the WATCH monitor flown on the ES spacecraft EURECA during its 11 month mission. Also the recurrence of the Soft Gamma Repeater SGR 1900+14 in 1992 is verified.

Optical follow‐up of gamma‐ray bursts observed by WATCH

44 Gamma‐Ray Bursts have been localized by the WATCH experiments on GRANAT and EURECA. For some of them, Schmidt plates were taken within days after the burst. In other cases, time‐correlated plates were found in some of the main astronomical archives. No obvious optical counterpart has been found in any of the investigated plates.

JEM-X: the x-ray monitor on INTEGRAL

The INTEGRAL X-ray monitor, JEM-X, (together with the two gamma ray instruments, SPI and IBIS) provides simultaneous imaging with arcminute angular resolution in the 3-35 keV band. The good angular resolution and low energy response of JEM-X plays an important role in the detection and identification of gamma ray sources as well as in the analysis and scientific interpretation of the combined X-ray and gamma ray data. JEM-X is a coded aperture X-ray telescope consisting of two identical detectors. Each detector has a sensitive area of 500 cm2, and views the sky through its own coded aperture mask. The coded masks are located 3.4 m above the detector windows. The detector field of view is constrained by X-ray collimators (6.6° FOV, FWHM).

Gamma‐ray bursts observed by the watch experiment

After two years in orbit the WATCH instruments on the GRANAT space observatory have localized seven gamma burst sources with better than 1° accuracy. In several cases, follow‐up observations with Schmidt telescopes have been made within a few days. Some of the bursts have also been detected by the distant space probes PVO and ULYSSES and there are, therefore, good prospects for obtaining much improved positions using the burst arrival times. The existence of the almost concurrent Schmidt plates could then become particularly interesting.

The scientific role of JEM-X: The X-ray monitor on INTEGRAL

INTEGRAL, ESA’s next venture in gamma-ray astronomy is due for launch in 2001. In addition to the two large gamma-ray instruments INTEGRAL will carry an X-ray monitor, JEM-X, and an optical monitor, OMC. The primary role of JEM-X will be to provide data on the X-ray flux and its variations from the targets observed by the gamma-ray instruments. JEM-X will also provide accurate positions for transient X- and gamma-ray sources observed during the mission. Together with the optical monitor, JEM-X can carry out a special program on stellar activity, independent of the two high energy instruments on INTEGRAL.

Wide Field Monitoring of the X-Ray Sky Using Rotation Modulation Collimators

Wide field monitoring is of particular interest in X-ray astronomy due to the strong time-variability of most X-ray sources. Not only does the time-profiles of the persistent sources contain characteristic signatures of the underlying physical systems, but, additionally, some of the most intriguing sources have long periods of quiesense in which they are almost undetectable as X-ray sources, interspersed with relatively brief periods of intense outbursts, where we have unique opportunities of studying dynamical effects, in, for instance, the evolution of accretion discs. Another question for which wide field monitors may provide key information, is the origin and nature of the cosmic gamma ray bursts.Rotation Modulation Collimators (RMCs) were originally introduced in X-ray astronomy to provide accurate source localizations over extended fields. This role has since been taken over by the grazing incidence telescope systems. The potential of the RMCs as wide field monitors have recently been demonstrated by the WATCH instruments on GRANAT and EURECA. It now appears likely, that for use on large, 3-axis stabilized spacecraft, a pinhole camera system may provide better sensitivity than an RMC-system of corresponding physical dimensions. But due to its simplicity, low data rate, and ability to work on spin stabilized (micro)satellites, the RMC wide field monitor may still have a role to play in the X-ray astronomy of the future.

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.