Compton Gamma Ray Observatory Status Report #196 Friday September 6, 1996 Questions or comments can be sent to Chris Shrader at the CGRO-SSC. Phone: 301/286-8434 e-mail: shrader@grossc.gsfc.nasa.gov Guest Investigator News All notification letters for Cycle-6 proposals were mailed out last month. The second-phase (budget) review will be completed this month after all of the budget requests are received. Cycle-6 observations will begin on October 29, 1996. The cycle-6 timeline and information such as off-axis target summaries and program titles and abstracts are available on the CGRO SSC WWW site: http://cossc.gsfc.nasa.gov. EGRET EGRET operations were normal this monthly period. Delivery of the final phase 4 data to the GRO SSC is on schedule, and delivery of the phase 5 preliminary data to the GRO SSC is also on schedule. Interaction with guest investigators continues at a good level. As most of you are aware, the study of the extragalactic diffuse radiation is a very difficult problem requiring careful analysis to separate the Galactic component, and is best performed after most of the data are available. Several preliminary reports have been given, but a final paper is now in preparation and should be submitted shortly. We can say in advance that the analysis confirms earlier work of a power law spectrum in energy, and it extends this spectrum to 50 GeV. Currently the EGRET telescope is pointed at GRO J0004+73 and will be until October 3, 1996. OSSE OSSE operations are normal. The instrument is working as designed, with all subsystems in complete and full operation. The slewing response to BATSE burst triggers was enabled on 96/233. We have recently added new functionality to the OSSE flight software to collect "burst-regulated spectrum multiscale" data: in response to a BATSE burst trigger, high time-resolution spectra are accumulated from a single OSSE shield segment. In the standard configuration, 64-channel spectra covering 200 keV - 8 MeV are accumulated every 128 ms for a total of 8.192 sec. The energy range and sampling rate are user-definable parameters. In response to the recent increase in the rate of terrestrial gamma-ray flashes, we are currently collecting 64-channel spectra over 200 keV - 2 MeV every 16 ms for 1.024 sec. OSSE requested and was granted a target-of-opportunity observation of GRO J1655-40 following an extended outburst monitored by BATSE. Preliminary analysis of six days of OSSE observations near the peak of the recent flare indicate that the source is similar in intensity to previous outbursts. The spectrum is well described by a power law (photon index -2.73 to -3.00) from 50 to 600 keV. There is no evidence of line emission. Flaring with time scales on the order of 4-6 days have been characteristic of this extended outburst. By the completion of this viewing period in a few days, OSSE is likely to have data from one complete flare cycle. Recent observations are listed in the following table: View period Dates Target (owner) 527 13-20 Aug Crab Pulsar (PI team) IC 4329A (PI team) NGC 2992 (public) 528 20-27 Aug IC 443 (GI:C.Dermer) IC 4329A (PI team) 529.5 27 Aug - 6 Sep GRO J1655-40 (PI team) NGC 2992 (public) Low-level OSSE data products through viewing period 420 and high-level data products through viewing period 220 have been delivered to the Compton GRO Science Support Center archive. In addition, by special request all subsequent public Cyg X-1 data sets, both low and high level, have been delivered. Refer to the CGRO-SSC page on the WWW (http://cossc.gsfc.nasa.gov), or contact Tom Bridgman (bridgman@grossc.gsfc.nasa.gov) for more information. COMPTEL The COMPTEL instrument is performing well and continues routine operations. COMPTEL data associated with the current target-of-opportunity observations of the transient black-hole candidate GRO J1655-40 are presently undergoing accelerated processing. A major delivery of COMPTEL Cycle 4 data products is anticipated later this month to the CGRO public archive at the COSSC. A number of papers by COMPTEL team members and guest investigators will be presented later this month at the 2nd INTEGRAL Workshop in St. Malo, France. Among the topics to be covered will be gamma-ray line spectroscopy, all-sky mapping at MeV energies, and the latest results on emission from the Orion region, pulsars, and other sources. BATSE Follows is the BATSE contribution to the September monthly report: The following was reported in IAU circular 6447: CYGNUS X-1 S. N. Zhang, B. A. Harmon, and W. S. Paciesas report for the Compton Gamma Ray Observatory BATSE team: "The blackhole candidate Cyg X-1 is experiencing a transition from a soft state back to its normal hard state. It went into the soft state in mid-May, following a sudden x-ray flux increase in the band 1.3-12 keV (IAUC 6404) and a rapid hard-x-ray-flux decrease in the band 20-200 keV (IAUC 6405). Since July 26, its 20- to 200-keV flux increased from about 200 to about 600 mCrab (as of Aug. 7). Its energy spectrum also evolved from a steep power law (photon index about -2.5), without obvious break up to about 200 keV, to a harder power law (index about -2.0) with a break at about 100 keV, quite similar to its hard-state spectrum (typically with index -1.8 and with a break at 100-150 keV). If its current trend of hard-x-ray-flux increase continues, Cyg X-1 will be back to its normal hard state in about a week." The following was reported in IAU circular 6450: 4U 0115+634 M. Scott and M. H. Finger, Universities Space Research Association; R. B. Wilson, Marshall Space Flight Center, NASA; and T. A. Prince and B. Vaughan, California Institute of Technology, report for the Compton GRO Observatory BATSE pulsar team: "Pulsed hard x-ray flux is currently being detected from the transient Be/x-ray pulsar 4U 0115+634. The source was detected on Aug. 10, when the rms pulsed flux (20-50 keV) was approximately 25 mCrab. This is the first detection by BATSE of this source since a pair of outbursts that occurred during the interval 1995 Nov. 18-1996 Jan. 9 (IAUC 6266). On 1996 Aug. 12, the rms pulsed flux (20-50 keV) was approximately 30 mCrab. The pulse frequencies observed from Aug. 10 to 12 are consistent with a neutron-star spin rate of 0.27666712 +/- 0.00000022 Hz and no detectable spin-up, using the binary orbital ephemeris of Rappaport et al. (1978, Ap.J. 211, L1) with the epoch of periastron passage updated to 1996 Aug. 5.5 UT." This was corrected in IAUC 6451 as follows: 4U 0115+634 Corrigendum.On IAUC 6450, line 13, for 1996 Aug. 5.5 UT." read 1996 Aug. 5.583 TDB." The following was reported in IAU circular 6462: CYGNUS X-1 S. N. Zhang, B. A. Harmon, W. S. Paciesas, C. R. Robinson, D. M. Scott, and W. Yu, on behalf of the BATSE/CGRO team; and R. Remillard and W. Cui, on behalf of the ASM/RXTE team, report: "The blackhole candidate Cyg X-1 is returning to the hard state, following the recent hard x-ray flux increase (IAUC 6447). Hard x- ray (20-200 keV) and soft x-ray (1.3-12 keV) fluxes are very close to the values before the hard-to-soft-state transition in mid-May 1995 (IAUC 6405). Spectral hardening has also been observed in both bands. The ASM (1.3-12 keV) and BATSE (20-200 keV) long-term lightcurves are anticorrelated, with the total 1.3- to 200-keV luminosity remaining approximately unchanged within 15 percent during the entire state transition from mid-May to late August 1996. The BATSE and ASM soft-state power-density spectra (PDS) of Cyg X-1 (roughly 10E-7 to 10E-4 Hz), which can be approximately represented by a power law with an index of approximately -1.0, are apparently a low-frequency extension of the same soft-state PDS detected by PCA/RXTE above 10E-3 Hz (Cui et al., submitted to Ap.J. Lett.). Flaring episodes separated by approximately 45-50 days and of duration 5-30 days were observed superimposed on the spectral transition in both ASM and BATSE data. (The mid-1991 to early-1996 BATSE lightcurve for Cyg X-1 does not show evidence of any significant periodicity near 50 days.) A possible 15-day delay of the flares in the ASM band with respect to the flares in the BATSE band was also detected from the cross correlation between ASM and BATSE lightcurves." The following was reported in IAU circular 6468: SMC X-1 S. N. Zhang, C. R. Robinson, R. B. Wilson, B. A. Harmon, and D. M. Scott, on behalf of the BATSE/CGRO team; and R. Remillard, on behalf of the ASM/RXTE team, report: "Quasiperiodic flux variations have been observed from the 0.7-s pulsar SMC X-1 with a period around 60 days, in the energy bands 1.3-12 keV (ASM/RXTE) and 20-100 keV (BATSE/CGRO), confirming the HEAO-1 results in 1977-1978 (Gruber and Rothschild 1984, Ap.J. 283, 546). A sequence of five outbursts between early Oct. 1995 and mid-Aug. 1996 is apparent in the BATSE/CGRO lightcurve, of which the last three coincide with those observed from ASM/RXTE (the first two of the three ASM outbursts have been reported by Levine et al. 1996, Ap.J. 469, L33). The peak flux is about 30 mCrab in both bands. No significant outbursts were observed in the BATSE data between Apr. 1991 and Oct. 1995, though a weak level of emission at about 10-20 mCrab was sometimes detected. Eclipses with a duration of about 20 percent of the orbital period of 3.9 days are observed in both data sets. The current outburst was detected about Aug. 27." During the last month the following pulsed sources have been detected by the BATSE pulsed source monitor: Her X-1, 4U 0115+634,Cen X-3, 4U 1626-67, 2S1417-624, OAO 1657-415, GRO J2058+42, GX 1+4, Vela X-1, and GX 301-2. Since August 29, the burst trigger has been using rates from channels 2+3 (50 - 300 keV), with thresholds of 5.5 sigma for all three timescales. As of September 1 BATSE has detected 1638 gamma-ray bursts out of a total of 5486 on-board triggers in 1958 days of operation. There have been 768 triggers due to solar flares, 10 due to SGR events, 62 due to terrestrial gamma-ray flashes, and 1477 due to the bursting pulsar GRO J1744-28.