Compton Observatory Science Report #125 Thursday, June 24, 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. Science operations were begun on June 19. The planned timeline was picked up starting with period 227, but starting that observation on June 19 rather than June 29 as previously scheduled. The current plan is to continue the planned timeline, but shifted forward in time by 10 days. Analyses of the reboost situation continue. A decision on a choice of procedure will take several days at least. Other Items: ------------ Reminder on Call for Papers for the Second Compton Symposium All those planning to attend the Second Compton Symposium, to be held September 20-22 at the University of Maryland, are reminded that abstracts are due by July 1, 1993. If you need the Abstract Form or the Information Request Form, contact the Science Support Center by phone or e-mail (Ms. Sandra Barnes, 301-286-7780, barnes@grossc.gsfc.nasa.gov). The regular registration fee is $160. A special registration fee of $60 will be available for students, and registrations after September 1 will be $200. On July 1, the conference managers Jorge Scientific Corp. will be mailing out a logistics package containing registration form, hotel listings, shuttle service information, etc. to all those who have requested more information. If you wish to receive this logistics package you should contact the Science Support Center. BATSE ----- Last week data (IBDB's) for 95 additional solar flares and 65 additional gamma-ray bursts were delivered to the science support center. BATSE coinvestigators from USC and Goddard are visiting at Marshall this week for a BATSE team meeting on burst spectroscopy. As of June 22th, BATSE has detected 687 cosmic gamma-ray bursts out of a total of 2303 on-board triggers in 791 days of operation. There have been 653 triggers due to solar flares with emission above 60 keV. COMPTEL ------- Following the decision to delay orbital reboost operations for GRO, the COMPTEL instrument was reactivated without incident on Saturday, 19 June, to resume scientific observations with the pointing to begin observing period 227. Telescope operations are normal and routine observations are underway. The delivery of flight data tapes to MPE for production data processing was also scheduled to resume last Saturday, on TJD 9157. EGRET ----- EGRET operations were normal this week after the instrument was turned on again on June 19, 1993. It is planned that the instrument will remain on and collect data until the Compton Observatory reboost is resumed. OSSE ---- For viewing period 226, the Z-axis target is the galactic plane near 355 deg longitude (PI team), and the X-axis target is the Coma cluster (GI: Y. Rephaeli). The viewing strategy near the galactic center employs large source and background offsets from the galactic plane to measure the extent of the bulge in the diffuse continuum and annihilation radiation. On day 93/169 (last Friday UT morning), the elevation drive for detector 3 went off-line after a positioning error. This followed a similar error for detector 4 the day before. Both drives are recalibrated and now operating properly. In both instances, the main positioning readout potentiometer became noisy near the end of travel in the negative scan direction, while the redundant pot continued to operate properly. The viewing strategy for those two days was unusual in that it required detectors 3 and 4 to slew through a large angle (38 degrees) to a point near the end of travel every two minutes. Because in all cases the drives actually reached the proper positions, the problem appears to be confined to the readout pots, and it should not affect normal operations, which do not have such large slews to that point and have not shown any readout noise. The OSSE team submitted the following IAU Circular this week. CYGNUS X-1 M.D. Leising, Clemson University, M.S. Strickman, W.N. Johnson, J.D. Kurfess, and J.E. Grove, Naval Research Laboratory report for the Compton Observatory/OSSE Team: OSSE observed Cyg X-1 for two intervals, 13--14 May 1993 and 31 May--3 June, during the period when it was reported to be in a low hard x-ray state (IAU Circ. 5813). We confirm that the 45- to 140-keV flux was low during these periods, 0.08 and 0.06 photons cmE-2 sE-1, respectively. We find no evidence for MeV emission in either observation, with 99% confidence upper limits on the integral flux from 0.6- to 1.5-MeV of 2.8E-3 and 1.9E-3 photons cmE-2 sE-1. These limits are factors of 4 and 6, respectively, below the flux observed by Ling et al. (Ap. J. 321, L117, 1987) when the hard x-ray flux was similarly low.