Compton Observatory Science Report #168, Tuesday October 18, 1994 Chris Shrader, Compton Observatory Science Support Center Questions or comments can be sent to the CGRO SSC. Phone: 301/286-8434 e-mail: NSI_DECnet: GROSSC::SHRADER Internet: shrader@grossc.gsfc.nasa.gov Spacecraft Status The SAA prediction tables uplinked to the spacecraft on Saturday morning, October 8 had errors that resulted in several SAA exit commands being executed prematurely. The instruments are designed to take full SAA passes with their high voltages turned on, so this was not a serious problem. However, there were false BATSE gamma-ray burst triggers due to SAA particles. Unfortunately, these went out on the BACODINE network and may have resulted in astronomers attempting observations of non-existent bursts, a situation for which the project my apologizes. The source of the problem was traced to changes to the shape of the SAA exclusion zone in response to false BATSE triggers uplinked several weeks ago. The BATSE burst trigger criteria had been modified to allow improved studies of atmospheric gamma-ray flashes and by the gradual expansion of the SAA region as solar minimum approaches. We now have a stable situation with an acceptable SAA box and accurate predictions and software fixes are underway. Instrument Reports EGRET EGRET operations were normal this biweekly period except for the matter discussed in the next paragraph. Delivery of data to the CGRO- SSC remains on schedule. Interaction with guest investigators remains at a good level. Over the last several months, EGRET has been receiving about 82% of the possible data that it could have received if there had been no tape recorder failure, indicating that the Compton Observatory is continuing to receive good coverage. A change was made in the SAA boundary two weeks ago to alleviate false triggers that BATSE was experiencing, ad to reduce the regions of high background for OSSE and COMPTEL. The new definition was found to be incompatible with the existing software in Flight Dynamics. A second minor adjustment in the region definition was made beginning last Saturday to correct the problem until the software could be modified. A new error then occurred. The processing system was confused when the spacecraft crossed the prime meridian. Consequently, SAA passages were terminated prematurely at 0-degrees longitude. All of the instruments, including EGRET, were turned on at that location resulting in high count rates in EGRET and EGRET triggers. Fortunately, the intense region of the SAA was not penetrated. The problem is now believed to be corrected. OSSE OSSE operations have returned to normal. Because an error in the GRO-wide definition of the boundary of the SAA activated OSSE during periods of high particle flux, OSSE automatically entered a safe-hold mode for one orbit on each of five nights. We commanded the instrument back into science mode each time without incident. In addition, we deactivated the on-board slewing to bursts to prevent accidental triggering. The error in the SAA definition has been fixed, and we expect to reactive the slewing shortly. In viewing period 401 (4-18 Oct), the Z-axis target is PSR 2334+61 (Guest Investigator S. Sturner), and the X-axis target is M51 (Guest Investigator D. Bhattacharya). Two interesting OSSE results have recently been accepted for publication and will be distributed as preprints shortly. The average OSSE and Ginga Seyfert spectrum (Zdziarski et al.) and the detection of a (presumably cyclotron) absorption line at 110 keV from A0535+26 (Grove et al.) will appear in ApJL. Data from viewing period 227 were delivered to the Compton GRO Science Support Center archive this week. The targets were SN1993J and PSR 0740-28. COMPTEL The COMPTEL instrument is performing well and continues routine observations. The collaboration toasted the successful completion of CGRO Phase 3 observations, and looks forward to continued productive operation during the Cycle 4 observing cycle. A paper discussing a possible repeating source of gamma-ray bursts at MeV energies has recently been accepted for publication in Astronomy and Astrophysics (Letters). In "COMPTEL detection of two spatially-coincident gamma-ray bursts," Kippen et al. present evidence, based on COMPTEL and IPN localizations, and supported by Monte Carlo simulations, that GRB 930704 and GRB 940301 may have arisen from a single source. The random probability of the occurrence of two such bursts from unrelated sources is estimated to be on the order of ~1.5 %. A second announcement has recently been released regarding the upcoming 29th ESLAB Symposium "Towards the Source of Gamma-Ray Bursts" to be held on 25-27 April 1995 at the European Space and Technology Centre ESTEC in Noordwijk, The Netherlands. In this second circular, the organizers at the Space Science Department of ESA issue a call for papers and provide information concerning the submission of abstracts and the preparation of contributions for publication in the proceedings. The deadline for the receipt of abstracts is 13 January 1995. Further information can be found at the following URL on the WWW, "http://astro.estec.esa.nl/", or via anonymous ftp to estsa2.estec.esa.nl within the directory /pub/ESLAB29. BATSE The x-ray transient GRO J1719-24 = GRS 1716-249 which was first seen in November 1993 appears to again be in outburst, with low flux levels being detected at least since early September. Its intensity has gradually increased to about 150 mCrab in the 20-100 keV band. The following sources were detected by the BATSE pulsed source monitor in the past two weeks: Her X-1, Cen X-3, 4U 1626-67, 2S 1417-624, OAO 1657-415, EXO 2030+375, GX 1+4, Vela X-1, and GX 301-2. The outburst of 2S 1417-624 which began 50 days ago is still continuing. The pulse frequency increased steadily until September 30, and then fell for five days, probably indicating periastron passage, and has since continued to increase. The current e-folding time is 25 years! Since the BATSE burst trigger criteria was change to use channels 3 and 4 rather then 2 and 3, there has been two triggers due to atmospheric gamma-ray flashes. As of October 13th, BATSE has detected 1134 cosmic gamma-ray bursts out of a total of 3135 on-board triggers in 1269 days of operation. There have been 740 triggers due to solar flares with emission above 60 keV.