Compton Observatory Science Report #118 Thursday, May 6, 1993 Eric Chipman, Compton Observatory Science Support Center Questions or comments can be sent to the Compton SSC. Phone 301/286-7764, e-mail SPAN GROSSC::CHIPMAN, Internet chipman@grossc.gsfc.nasa.gov. The Compton spacecraft continues operations. This is a big week for the spacecraft, as the test and calibration burns of the propulsion system are taking place. There were three maneuvers this week. One to the engineering burn attitude on Tuesday, another back to a science attitude on Wednesday, and a third to the calibration burn attitude on Friday morning (May 7). The first maneuver was from 1345Z to 1416Z on May 4, covering an angle of 28.4 degrees, with a final boom angle wrt the equator of 28.4 degrees. This maneuver aligned the spacecraft z-axis with the planned thrust direction for the test firing of the propulsion system. There was a second maneuver on Wednesday, May 5 to the attitude of period 219.4, where the z-axis target is the Galactic Center and the x-axis target is MCG-2-58-22. The times of the maneuver were 1355Z to 1439Z, and the maneuver slew angle was 141.8 degrees. The HGA boom angle wrt the equator after the maneuver was 13.2 degrees. The third maneuver, on May 7, was from 1320Z to 1348Z, covering an angle of 81.8 degrees. The Boom angle from the equator is 32 degrees. The test firing on Tuesday was a success, in that the thrusters burned correctly for a short time. However, a miscompare of a gyro direction sensor beyond the planned tolerance caused the spacecraft to enter a safehold mode from which it was brought out a few hours later. Science was begun again as planned after the maneuver of May 5, for about 2 days up to the test of May 7. Today's test sequence went perfectly. The tolerance on the gyro compare was increased somewhat, and no miscompares were found during today's 16-minute sequence of firings. The orbital altitude was raised by about 5 km, as planned. BATSE ----- During viewing period 219 BATSE folded-on-board pulsar data will be collected for the Crab pulsar, and 31 ms resolution single sweep data will be collected for pulsar searches. The single sweep data will be collected from LAD's 0 and 6 separately. A shipment of IBDB's for 69 gamma-ray bursts and 119 solar flares was sent to the Science Support Center for archiving last week. (This is expected to be available in the archive by Friday or Monday). As of May 5, BATSE has detected 637 cosmic gamma-ray bursts out of a total of 2223 on-board triggers in 743 days of operation. There have been 644 triggers due to solar flares with emission above 60 keV. COMPTEL ------- The COMPTEL experiment was placed in a "safe" mode, with all primary detector high voltages turned off, prior to the test burns of the spacecraft propulsion system on Tuesday, 4 May. The COMPTEL operations group reports that the instrument is operating normally after switch-on of the detector modules, and continues observations of scientific targets. The telescope will be placed in a similar safe, low-voltage configuration during future planned maneuvers involving the spacecraft thrusters. EGRET ----- EGRET operations were normal this week except for adjustments for the reboost test. As noted last week, we are working hard to finish the work on Phase 1 and expect a series of papers to be submitted over the next few months. Meanwhile, four more preprints are being mailed this week. Work with guest investigators for Phase 2 is increasing with data being ready for them or work starting with them as they chose between three and four months after the observation as promised in spite of the major computer problem in March. OSSE ---- OSSE is once again in Science Mode today (6 May) and operating normally. The OSSE team would like to thank the GRO Flight Operations Team for their quick action in response to the spacecraft safehold on Tuesday. OSSE had been placed in its Safehold Mode on Monday evening in preparation for the thruster test, and was not affected by the spacecraft contingency. OSSE was commanded to return to Science Mode on Wednesday, about 5 hours after the maneuver to the target attitude for viewing period 219.4. OSSE will once again be placed in Safehold Mode tonight in preparation for Friday's calibration burn. For viewing period 219.4, the Z-axis target is 1E1740.7-2942 (GI: N. Gehrels) and the X-axis target is the Sy galaxy MCG-2-58-22 (GI: J. Schachter). The Sun is not available on OSSE's scan plane, so the slewing response to BATSE solar flare triggers is disabled. Phase 2 Guest Investigator R. Nemiroff and collaborators visited NRL last week to analyze their data from the unidentified COS-B source 2CG010-31. Data from viewing period 7.5 will be delivered to the GRO Science Support Center Archive this week. The targets for this pointing were the galactic plane at 25 degrees longitude and the active galaxy NGC 5548. OSSE team members have completed the technical reviews of Phase 3 proposals, and the reviews have been delivered to the Science Support Center.