Compton Gamma Ray Observatory Status Report #198 Friday November 8, 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 Cycle 6 officially began on October 15, 1996. There were several minor changes to the Viewing Plan prior to the start of the cycle, and an additional one very recently - refer to the CGRO hompepage on the World Wide Web for the latest (http://cossc.gsfc.nasa.gov). Cycle-6 grant requests have now all been evaluated and are being processed at Goddard. In a special arrangement between NASA and NRAO, the Greenbank Interferometer (GBI) has been (or very nearly so) restored into operational condition. Both the CGRO and RXTE projects have contributed to support this effort and to operate the facility for at least one year. The understanding between NASA and NRAO is that the data obtained will be made available to CGRO and RXTE Guest Investigators, and that individual Guest investigators on either NASA facility will have input into how the GBI observing program is defined. A steering committee, consisting of CGRO and RXTE scientists, as well as several radio astronomers who are highly familiar with the GBI facility, will consider specific requests when constructing the GBI observing program. The limiting sensitivity of the GBI is approximately 10-20 mJy (2-8GHz), and the program is likely to consist of nominal-daily monitoring of 10-12 sources. Examples of CGRO targets include galactic binaries such as Cyg X-3, GRO J1655-40, GRS 1915+105 and LSI 61 303. Additionally, quasars such as PKS 1622-297 may be possible as well. If you are currently participating in a CGRO Cycle-6 investigation that could potentially benefit from this program, please send e-mail with a brief scientific justification to Chris Shrader (shrader@grossc.gsfc.nasa.gov). Please note: - the position of any proposed source (should be at the level of 1 arcsec). - the proposed duration of GBI monitoring (typically not to exceed 1 month or so). - any time constraint for GBI monitoring (requests for simultaneous CGRO/GBI observations should be made well in advance to incorporate the proposed radio source in the GBI schedule. * Note that all the GBI data will be made publicly available promptly after they are obtained. Instrument Team Reports 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. EGRET has now observed high-energy gamma rays from six pulsars for certain and a seventh with high probability. For three of them, there are energy spectra as a function of the phase in the pulse period which may be used to test details of the theoretical models that exist. In addition, there is, for all of the pulsars seen by EGRET, the information on the phase relative to the radio phase, the knowledge of the energy flux observed, and a general spectrum. These results, together with other information, allow the determination of the energy emitted as a function of age, the fraction of the total energy going into gamma radiation, and the spectral index of the gamma rays as a function of age. All of these characteristic may be compared against the theoretical predictions to choose between the models and make refinements to the theory. In order to conserve the remaining gas supply on EGRET for observations determined to be of high priority by the review committee, the high voltage for the spark chambers was turned off on October 29, 1996 and will remain off until December 10, 1996 when the 3C 279 observations begin. COMPTEL The COMPTEL instrument is performing well and continues routine observations. The collaboration looks forward to further successful operation during the upcoming CGRO Cycle 6. The proceedings of the 3rd Compton Symposium, held in Munich, are scheduled to appear imminently in an upcoming issue of the Astronomy and Astrophysics Supplement Series. A new paper on the "COMPTEL detection of low-energy gamma rays from the HVC complex M and A region?" by Blom et al. has been accepted for publication in Astronomy and Astrophysics. A further delivery of COMPTEL Cycle 4 data products to the COSSC public archive is currently in progress. This will include both low-level and high-level data products, including standard-processing skymaps, for viewing periods up to 420. Members of the COMPTEL group at UNH were pleased to recently co-host a symposium in honor of Bill Webber, in recognition of 40 years of research in cosmic-ray astrophysics; numerous luminaries were in attendance, at the peak of the fall foliage season in New England. OSSE OSSE operations are currently normal. The instrument is working as designed, with all subsystems in complete and full operation. The slewing response to BATSE burst triggers continues to be enabled. OSSE slewed in response to BATSE trigger #5623 (GRB961004) and viewed the position estimated on-board for three hours. Twenty-five of the 26 classical bursts in the last month had 16 ms shield data available; 13 of these produced significant responses in the shields. Only 5628 produced a significant central detector response (in MLL), with a possible precursor. Largest signals were seen from 5629, a long (30 s), chaotic event, and 5647, a short (~0.2 s) event with peak flux of ~600 cts/16 ms. The new bursts are available on the WWW at: http://www.astro.nwu.edu/astro/osse/bursts/newlist.html The ApJ has recently accepted the paper, "Spallation of Iron in Black Hole Accretion Flows" by J.G. Skibo. Papers recently submitted to ApJ include "OSSE Observations of the Soft Gamma Ray Continuum from Galactic Longitude l=95 deg" by Skibo et al. (ApJ Letters) and "Determining the Hubble Constant from Coordinated Gamma Ray and Radio Observations of MeV Blazars", by Skibo et al. A number of papers were also recently submitted to the proceedings of the 2nd Integral Workshop in St. Malo: "OSSE Observations of the Ultraluminous IR Galaxies Arp 220 and Mrk 273" by Dermer et al.; "Two Gamma-Ray Spectral Classes of Black Hole Transients" by Grove et al.; "The Source and Distribution of Galactic Positrons" by Purcell et al.; "Spallation of Iron in Seyfert AGN" by Skibo; "The Galactic Soft Gamma Ray Continuum" by Skibo; and "X-Rays and Gamma-Rays from Accretion Flows onto Black Holes in Seyferts and X-Ray Binaries" by Zdziarski et al. Recent observations are listed in the following table. View period Dates Target (owner) 531 3-15 Oct Carina region (D.Grabelsky) Arp 220 (C. Dermer) Mrk 841 (public) 601.1 15-29 Oct GRS 1915+105 (C. Robinson & D.Smith) NGC 4507 (G. Madejski) 520.9 29 Oct - 12 Nov PKS 0528+134 (W.N.Johnson) IC 4329A (J.E. Grove) Low-level OSSE data products through viewing period 425 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. BATSE The following items were reported in IAU circular 6501: GRO J1655-40 B. A. Harmon, C. R. Robinson, G. J. Fishman, S. N. Zhang, and W. S. Paciesas, report for the Compton Observatory BATSE Team: "Hard x-ray flux from the superluminal jet source GRO J1655-40 has been increasing since mid-October, and has approximately doubled in intensity since Oct. 26, with fluxes near 1.0 Crab on Oct. 30-31 (20-100 keV). This activity follows an extended, bright, and highly-variable outburst (IAUC 6436) between June and Sept. Spectra obtained between Oct. 26 and 30 are adequately fit with a power law between 20 and 200 keV, with photon index changing from -2.6 to -2.9 (+/- 0.1) as the source brightened." SGR 1806-20 C. Kouveliotou, Universities Space Research Association; G. J. Fishman, C. A. Meegan, Marshall Space Flight Center, NASA; J. van Paradijs, University of Amsterdam and University of Alabama in Huntsville (UAH); M. S. Briggs and G. Richardson, UAH; and K. Hurley, University of California, Berkeley, report: "The BATSE has triggered on renewed activity from the soft gamma-ray repeater SGR 1806-20 on Oct. 30.95220 and on Oct. 31.54094 and 31.97024 UT. The fluence and peak flux of the first event are 10E-7 erg cmE-2 and 10E-6 erg cmE-2 sE-1, respectively. The reactivation of the source has initiated several Target of Opportunity observations with other satellites and groundbased observatories; currently RXTE has detected several bursts from the source and is observing during Nov. 6. We have changed the BATSE trigger criteria to trigger on (weaker) bursts during the RXTE observations. We strongly encourage follow-up observations in other wavelengths." During the last month the following pulsed sources have been detected by the BATSE pulsed source monitor: Her X-1, Cen X-3, 4U 1626-67, OAO 1657-415, GX 1+4, Vela X-1, 4U 1145-619,and GX 301-2. On October 29, BATSE recorded the longest gamma-ray burst of the mission in its energy band. The event consists of several pulses over an interval of about23 minutes and generated two burst triggers. The different pulses are consistent with a common direction. Since November 5, the burst trigger has been using rates from channels 1+2 (20 - 100 keV), with thresholds of 5.5 sigma for 64ms and 256ms timescales and 7.0 sigma for the 1.024s timescale. As of November 7 BATSE has detected 1689 gamma-ray bursts out of a total of 5555 on-board triggers in 2025 days of operation. There have been 768 triggers due to solar flares, 15 due to SGR events, 63 due to terrestrial gamma-ray flashes, and 1477 due to the bursting pulsar GRO J1744-28.