Early Danish Grb Experiments – and Some for the Future?

Gamma-ray Bursts: 15 Years of GRB Afterglows A.J. Castro-Tirado, J. Gorosabel and I.H. Park (eds) EAS Publications Series, 61 (2013) 15–25 EARLY DANISH GRB EXPERIMENTS – AND SOME FOR THE FUTURE? N. Lund1 Abstract. By 1975 the hunt for GRB counterparts had been on for almost ten years without success. Gamma burst instruments of that day provided little or no directional data in themselves. Positions could be extracted only using the time delay technique – potentially accurate but very slow. Triggered by a japanese report of a balloon instrument for GRB studies based on a Rotation Modulation Collimator we at the Danish Space Research Institute started the development of an RMC detector for GRBs, the WATCH wide field monitor. Four WATCH units were flown on the Soviet Granat satellites, and one on ESA’s EURECA satellite. The design and results will be summarized. Now, 35 years later, recent detector developments may allow the construction of WATCH-type instruments able to fit weight, power and data-wise into 1 kg cubesats. This could provide the basis for a true all-sky monitor with 100 percent duty cycle for rare, bright events. 1 Introduction The Danish Space Research Institute (DSRI) was set up in 1968 to provide a national focal point for a national participation in the European Space Research Organisation, ESRO. Prior to the formation of DSRI the national efforts in space had been directed towards the ionosphere because of the importance of this at- mospheric region for radio communications with Greenland. The first director of DSRI was Bernard Peters, a well known figure in the post war cosmic ray research. Not surprisingly, the first astrophysics project was a cosmic ray isotope experiment, constructed in collaboration with Centre d’Etudes Nucleaire, Saclay in France. This experiment was launched in 1979 on the NASA satellite HEAO-3. With a weight of 350 kg the HEAO-3 experiment was very large for its time, and even before the launch it was clear that DSRI had to find a new future research 1 DTU Space, Elektrovej Building 327, 2800 Lyngby, Denmark c EAS, EDP Sciences 2013 DOI: 10.1051/eas/1361002 16 Gamma-ray Bursts: 15 Years of GRB Afterglows theme for its astrophysics group, we could not dream of building anything larger or more complex to improve on the HEAO-3 results. At that time the Cosmic Gamma-Ray Bursts were the big mystery which in- trigued astrophysi-cists all over the world. Why had nobody succeeded in finding any counterparts? How could these events be so luminous in X- and gamma-rays, yet so completely absent at other wave-lengths? The distribution across the sky was peculiar to say the least – no hint of an origin within our Galaxy, yet it seemed obvious that such extraordinary flashes had to have a relatively local origin? A few good positions had been obtained from the early InterPlanetary Network (IPN), but nothing conspicuous was visible within the small IPN error boxes. Figure 1. In the early days it took quite a while to finalize the IPN positions – clock syn- chronization is not trivial! Therefore it was obvious that what was needed was an instrument which by itself could determine the burst position in near real time so ground based follow-up could start within hours or days rather than months. For the follow-up we planned to use classical astronomical search procedures – blink- ing Schmidt-plates, so we expected that a position accuracy better than 1 degree would be adequate – at least it would be far better than anything done in near real time on GRB’s up to that point. Fig. 1. Error box for GRB 790406 derived by the Interplanetary Network (Evans et al. 1980). 2 Instrument design For our instrument design we were inspired by a Japanese balloon experiment employing Rotation Modulation Collimators (RMC’s), Nishimura et al. (1976). RMC’s were originally developed by Mertz (1968), actually to be used for electronic read-out of optical Schmidt telescopes. However, it was in X-ray astronomy that the RMC technique really made its mark, with experiments on the British ARIEL V and the US SAS-3 satellites. N. Lund: Early Danish GRB Experiments 17 Nishimuras balloon experiment, specifically designed for GRB studies, em- ployed 3 detectors, two RMC detectors with orthogonal grid orientations and one “monitor counter” to provide an unmodulated time history of the burst. Figure 2. In the RMC technique the source positions are derived by an analysis of the “modulation pattern” arising through the rotation of the double grid structure. Figure 3. Fig. 2. Detector configuration of Fig. 3. Modulation patterns cor- Nishimuras balloon payload. Two or- responding to different off-axis and thogonal RMC detectors and one mon- phase angles. Off-axis: 10◦,20◦,30◦, itor counter. 40◦ and 50◦. The japanese design with the monitor counter took into account that GRB’s were known to have unpredictable time structures, and the derivation of the instrument modulation would be uncertain without an independent measurement of the true light curve. Two orthogonal RMC-units were used because the balloon payload rotated relatively slowly (two revolutions per minute) and many GRB’s would only last for a small fraction of a revolution. We realized that by suitable modifications of the RMC detector we could dis- pense with the monitor counter and achieve with one detector what was done with three in the balloon. The design of our WATCH (Wide Angle Telescope for Cosmic Hard X-rays) detector is shown together with the classical RMC in Figure 4. We eliminated the lower shadow grid and replaced it by two interleaved grid detectors with the same pitch as the top shadow grid. Now we can derive the un-modulated time history of a burst by adding the signals from the two detectors, and we can derive the instrument modulation of the signal independent of the signal amplitude from the ratio of the time history from one of the detectors to the un-modulated time history. 18 Gamma-ray Bursts: 15 Years of GRB Afterglows Fig. 4. Comparison of classical RMC (left) and WATCH design (right). The classical RMC uses two 50% open grids rotating synchronously and a single, large area detector which observes the time pattern of light and shadow as the grids rotate. WATCH uses only one shadow grid, but the co-rotating detector is now more complex with two interleaved grid-detectors. We also opted for a high spin rate of our detector: 60 revolutions per minute. More details on the design of the original WATCH instruments can be found in (Lund 1981 & Brandt et al. 1990). It should be noted that unlike most other GRB-instruments WATCH did not rely on the “burst”-nature of the GRBs to observe and localize them. WATCH performs equally well on persistent X-ray sources, it is a true wide field monitor. We build prototypes of the instrument and flew them in balloons from Spitzbergen in 1979 and 1980. Figure 5. No gamma bursts were observed during these flights but the design was proven and on this basis we got the instrument accepted for flight on ESA’s EURECA (EUropean REtrievable CArrier), a micro- gravity satellite with a planned launch in 1988. The main characteristics of the original WATCH instrument (Fig. 7) and the expected characteristics of a modern version of the instrument can be found in Table 1 of Section 5. 3 The challenger disaster and a new opportunity The construction of the WATCH flight model was well underway when in January 1986 the Challenger accident put a halt to the US shuttle program. For more than a year it was undecided whether EURECA would ever fly – in the aftermath of the disaster NASA had decided to transfer all future satellite launches back to expendable launchers, the Shuttle would only be used for manned flights – with a few exceptions. N. Lund: Early Danish GRB Experiments 19 In the summer of 1986, in the middle of this limbo I was fortunate to meet Rashid Sunyaev from the Moscow at a COSPAR meeting in Toulouse. We both made a presentation in a session devoted to future X-ray satellite missions. Rashid presented “Granat”, a Russian-French mission with a large French gamma-ray instrument, Sigma, and a cluster of Russian X-ray telescopes, ART-P and ART-S. In addition Granat carried two gamma-burst instruments, the French Phebus and the Russian Konus with an associated rapid moving platform, “Tournesol” with X-ray and optical cameras. (Unfortunately the downlink data connection to the entire Russian GRB instrument complex was lost soon after launch – otherwise the GRB afterglows may have been discovered with Granat in 1990 rather than with SAX in 1997.) In his presentation Rashid expressed regret that Granat did not carry an all-sky monitor. Such an instrument had been fore- seen, there was room for it as well as excess payload mass, but the instrument development had been delayed. Immediately after this I presented WATCH – an all-sky monitor which now appeared to be homeless! This was too much of a coincidence to be neglected. After the session Rashid and I met and after a good bottle of French red wine I could go back to Copenhagen with an offer from Rashid to fly four WATCH units on Granat – delivery of the flight units to be executed within a year! Of course we did not succeed to build four flight units adapted for Granat within a year, but the delivery of the flight units began in 1988.

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