Compton Observatory Science Report #140 Thursday, October 21,1993 Chris Shrader, Compton Observatory Science Support Center Questions or comments can be sent to the Compton SSC. Phone: 301/286-8434 e-mail: NSI_DECnet: GROSSC::SHRADER Internet: shrader@grossc.gsfc.nasa.gov Spacecraft Status: ------------------ The first phase of the CGRO orbit reboost operation was successfully completed last Sunday, 10/17/93! Thanks to all involved for the great work. A full slate of science operations have subsequently resumed. The apogee height was increased 346 to 450 km by using 10 burns of approximately 1-minute duration each over a 2 week period, 10/4-10/17. The second phase of the CGRO orbit reboost procedure will circularize the orbit by increasing the perigee height from its present 345 km to 450 km. This activity is expected to commence about 11/22/93. The success of the first phase, and the experience gained in carrying it out make the spacecraft engineers highly optimistic of a successful second phase. Scientific Highlights: ---------------------- A milestone discovery in gamma-ray burst research was made during the past several weeks - the probable identification of a soft gamma repeater (SGR) with counterpart sources of X-ray and radio emission. The discovery of renewed activity in the source SGR 1806-20 during late September is described in the BATSE Instrument Team report below. Follow up observations at X-ray and radio wavelengths led to the probable identification of the SGR with a "Plerion", which is a pulsar powered super-nova remnant, associated with SNR G10.0-0.3. For details refer to Kulkarni, S. 1993 (IAU circ. 5879) and Tanaka, Y. 1993 (IAU circ. 5880). Soft gamma repeaters are widely believed to comprise a distinct subclass of cosmic gamma-ray bursts, so the more general gamma- ray burst mystery is by no means solved. Instrument Team Reports: ----------------------- OSSE ---- OSSE operations are normal. The noise in the main positioning readout potentiometer on elevation drive 4 has disappeared. To provide additional redundancy in measuring the elevation angle of detector 4 should the noise return, we calibrated the stepper motor by counting steps to two additional fiducial points, one near each of the Z and X limits. In viewing period 304, the Z-axis target is the Virgo sky survey region (Key project) centered at (l,b) = (278,+67), and the X-axis target is the radio pulsar PSR B1800-21 (GI: J. Cordes). In addition, the X-axis field contains the soft gamma-ray repeater SGR 1806-20 (PI/GI mix), which has recently returned to its bursting state. Fortunately, the instrument configuration that optimizes the probability of detecting the pulsar (i.e., 8-ms rate samples in a variety of energies with all 4 detectors staring at the pulsar) also provides very high sensitivity for detecting bursts from the soft repeater. We have loaded this configuration and have already begun searches for weak bursts. Data from viewing period 17 will be delivered to the Compton GRO Science Support Center Archive today. The targets during vp 17 were the Galactic Center region and SN 1987A. Data from viewing period 16 are being screened. EGRET ----- EGRET was in a low power mode for the first half of the last week for the end of the reboost; currently the deep exposure has just begun. Next to the all-sky survey, the deep study is probably the most important step for high energy gamma ray astronomy. It should lead to information on the distribution in intensity of AGN's and time variations and assist in determining whether the gamma ray intensity increases are very large relative to some low state or fluctuations about an average level. I am happy to take this opportunity to congratulate personally the entire group that was involved in the first phase of the Compton Observatory reboost operation that was just completed so successfully. It was careful, professional work from the planning stages through the final execution. -- Carl Fichtel COMPTEL ------- During the period of the orbital reboost of the GRO spacecraft the COMPTEL instrument was placed in low-power mode, and remained "off" for the two-week duration of the reboost maneuver. The instrument was reactivated on Sunday, 17 October, and is now conducting normal observations. There was a general team meeting of the COMPTEL collaboration last week at ESA/ESTEC, in Noordwijk, The Netherlands. Among the decisions taken was to proceed with the release to the GRO public archive of COMPTEL low- and first high-level data products for the balance of Phase 1 (i.e., Viewing Periods 14 through 44). Since this represents an extremely large collection of data, a staged release is anticipated over the coming months (in batches of four to eight viewing periods), to allow for smooth export from the team, and import at the SSC. Finally, we note with regret that, in a strong reaffirmation of Murphy's Law, BATSE reported that an intense gamma-ray burst occurred within the field of view of COMPTEL last week. Unfortunately, since the instrument had been placed in a low-power mode for the reboost maneuver, this burst was not detected by COMPTEL. BATSE ----- A counterpart has been identified for the soft gamma-ray repeater SGR 1806-20. On September 29th SGR 1806-20 triggered the BATSE detectors three times in a period of 14 hour (IAUC 5875). Two more bursts were detected on October 5th and 9th. After the first bursts the BATSE team quickly alerted other observers and further observations of the source were obtained by radio observatories (VLA, Westerbock) and X-ray observatories (ASCA, ROSAT). The Japanese satellite ASCA detected a new steady source and a burst from the SGR region. Both emissions coincided in position with the supernova remnant previously discovered (Kulkarni et al.) within the SGR error box. The Huntsville Gamma Ray Burst Workshop is currently being held. More then 170 Astrophysicists are attending, and over 180 papers are being presented. The discussions of the evidence for cyclotron lines, the galactic or extra-galactic origin of bursts, and the evidence for gamma-ray burst repeaters, have been very lively. During viewing period 306 BATSE folded-on-board pulsar data is being collected for the Crab Pulsar, PSR 0525-21, PSR 1259-63, Cen X-3, and Her X-1.