Compton Observatory Science Report #151 Thursday, February 18, 1994 Chris Shrader, Compton Observatory Science Support Center Questions or comments can be sent to the Compton SSC. Phone: 301/286-8434 e-mail: NSI_DECnet: GROSSC::SHRADER Internet: shrader@grossc.gsfc.nasa.gov Spacecraft Status The CGRO spacecraft and instrument packages remain healthy. A study of instrument and heater reconfiguration which could be implemented in the event of battery failures was carried out spacecraft engineers. The results are very encouraging for an extended mission. For example, even if all 3 of the MPS-1 batteries were to fail, calculations show that heater reconfiguration can be implemented which will allow for a mission lifetime of greater than 10 years. Science Support Center News The BATSE Burst Spectroscopy Catalog (Schaefer et al 1994 ApJ Supp, in press) is now available in machine readable form from the GRO- SSC; via anonymous ftp under /batse/spec_cat or DECnet copy from grossc::anon_dir:[batse.spec_cat]. Note: these files are NOT in FITS format. The EGRET galactic-plane map is available via electronic distribution from GROSSC: anonymous ftp, /egret/skymap or DECnet: grossc::anon_dir:[egret.sky_map]. Thinking about a Cycle-4 CGRO proposal? We're here to help - feel free to call and bounce any ideas off us or ask questions. The GRO- SSC Instrument Specialists are EGRET: John Mattox (jrm@geminga.gsfc.nasa.gov), Greg Stacy (gstacy@comptel.unh.edu), Chris Starr (OSSE::STARR) and Mark Finger (ANIMAL::FINGER). For questions about accessing the archive the best sources are Tom McGlynn (GROSSC::MCGLYNN) or Jim Jordan (GROSSC::JMJ). Guest Investigator News The Be X-Ray transient A0535+26 has undergone a major outburst as documented by BATSE (see the BATSE report below). A Guest Investigation to observe this source as a target of opportunity with OSSE (PI Eckhard Kendziorra) was activated by the project scientist. A0535+26 is a member of the "Be" class of transient X- ray sources, which consist of a neutron star in an eccentric orbit about a early type emission-line star. Transient outbursts occur when anamolous wind activity coincides approximately with periastron passage. This effort is also involves collaboration with members of the OSSE Instrument Team. Instrument Reports OSSE OSSE operations are normal. Since the last biweekly report, OSSE has observed the following targets. The target-of-opportunity pointings at Cyg X-1 in a very low state and A0535+26 in outburst have been very fruitful. An OSSE IAU circular on Cyg X-1 is appended. Period Targets and Owners 318.1 Cyg X-1 (PI team), NGC 7123 (PI team), PSR 0540-69 (PI team), Sun 321.1 A0535+26 (GI: Kendziorra), NGC 253 (GI: Zych) 317 3C120 (GI: Petre), QSO 2251+158 (Key proj), CTA 102 (Key proj) Data from viewing periods 34, 35, and 36.0 were delivered to the Compton GRO Science Support Center Archive last week. Data from periods 38, 40, and 41 will be delivered today. The targets during these periods are listed below. An update of the standard OSSE data analysis software system, Igore v7.3, has been delivered to the Archive. Period Targets 34 Cas A, Geminga 35 ESO 141-55, MCG-5-23-16 36.0 GRO J0422+32, MCG-5-23-16 38 ESO 141-55, MCG-5-23-16 40 NGC 4388, M87, Galactic center region 41 MCG-6-30-15, PKS 0528+134, QSO 0834-201 The following report from IAU Circular 5930 summarizes the OSSE measurement of the unusually low flux and soft spectrum of Cyg X-1. CYGNUS X-1: B. F. Phlips, Universities Space Research Association and Naval Research Laboratory (NRL); M. D. Leising, Clemson University; K. McNaron-Brown, George Mason University; and J. E. Grove, W. N. Johnson, J. D. Kurfess, and M. S. Strickman, NRL, report for the Compton Observatory OSSE Team: "OSSE is observing Cygnus X-1 between Feb. 1 and 8, based on information from BATSE that the source is in a low state. We measured the integrated 45- to 140-keV flux to be 1.5 and 1.1 (+/- 0.1) x 10E-2 photon cmE-2 sE-1 for the first two days of the observation, respectively. This is one-fifth of the flux in the low, or gamma1, state observed by HEAO-3 (Ling et al. 1987, Ap.J. 321, L117). Optically-thin thermal bremsstrahlung models fit the spectra well and produced temperatures of 56 +/- 8 and 62 +/- 8 keV for Feb. 1 and 2, respectively. These temperatures are half of the values typically measured by OSSE for Cygnus X-1. We find no evidence for the enhanced 1-MeV emission associated with low states as reported by Ling et al. Observations at other wavelengths are encouraged." EGRET EGRET operations were normal during this biweekly period. The spark chamber gas which had been deteriorating at the anticipated rate was replaced with fresh gas. This replacement occurred at about the expected time; so the integral lifetime prediction remains the same. The instrument performance is now at the expected level. The percentage of possible data that was recovered by EGRET was about 80% during the last two weeks, compared to between 60% and 65% before the TDRS satellite was moved and the Australian ground station was operative. All phase 1 data have been delivered to the Science Support Center (SSC). Phase 3 preliminary analyses and delivery of preliminary results to the SSC are on schedule. Interaction with guest investigators, primarily in relation to the Phase 2 data, remains at a good level. COMPTEL The COMPTEL instrument is performing well and continues routine observations. The proceedings of last year's solar conference at Waterville Valley, New Hampshire on "High-Energy Solar Phenomena" (editors J. M. Ryan and W.T. Vestrand) have just been published by AIP; copies have been sent to all conference participants. Plans are already underway for a follow-up conference. There will be a general team meeting of the collaboration next week at the University of New Hampshire, to assess instrument operation and performance to date, the status of the routine processing of incoming flight data, and the progress of numerous scientific investigations. And, there is no truth to the rumor that informal medal tallies from the Lillehammer Winter Games are being kept by team members from the half-dozen (or so) countries represented within the COMPTEL collaboration. BATSE The following notice was included in IAU Circular No. 5931: A0535+26 M. H. Finger, Computer Sciences Corporation; R. B. Wilson, Marshall Space Flight Center, NASA; and K. S. Hagedon, Universities Space Research Association, report for the Compton Gamma Ray Observatory BATSE team: "Hard x-ray pulses in the energy range 20-120 keV are currently being detected from the transient x-ray pulsar A0535+26. The present outburst was first detected on Jan. 28. The pulsed flux remained only marginally detectable until Feb. 1. A steady increase in flux began on Feb. 3.0, when the pulsed flux doubled. The spectrum of the phase-averaged pulsed flux on Feb. 3 is adequately fitted by an optically-thin thermal bremsstrahlung model F(E) = (A/E) exp(-E/kT), with a temperature kT = 27.7 +/- 1.0 keV, and F(50 keV) = (5.1 +/- 0.1) x 10E-4 photon cmE-2 sE-1 keVE-1. The pulse shape is very broad, with a sharp dip occupying about 0.2 in phase. This is the largest of a series of four consecutive outbursts of A0535+26 that has been detected by BATSE. Outbursts in 1993 March and July were reported on IAUC 5388. A weak outburst occurred in 1993 Oct.-Dec. The following preliminary binary orbital parameters have been determined from a pulse timing analysis of the previous three outbursts: P = 110.3 +/- 0.3 days, T = 1993 Mar. 12.7 +/- 0.6 UT, a sin i = 267 +/- 13 light-s, e = 0.47 +/- 0.02, and argument of periastron = 130 +/- 5 deg. An intrinsic pulse period of 103.4216 +/- 0.0010 s was determined for Feb. 3 using these orbital parameters." The following notice was included in IAU Circular No. 5933: A0535+26 R. B. Wilson and B. A. Harmon, Marshall Space Flight Center, NASA; and M. H. Finger, Computer Sciences Corporation, report for the Compton Gamma Ray Observatory BATSE team: "The outburst reported on IAUC 5931 has continued and intensified through Feb. 8. The daily phase-averaged pulsed intensity in the band 20-40 keV, relative to the total Crab Nebula flux (pulsed + steady), was 0.2, 0.5, 0.7, 0.8, 0.9, and 1.1 on Feb. 3-8. The daily total intensity (also 20-40 keV), measured using Earth occultation, was 0.4, 0.9, 1.4, 1.9, 2.2, and 2.7 relative to the total Crab Nebula flux. All values stated above are accurate to +/- 5 percent. The source is detectable to at least 100 keV." The following was submitted to be included in an IAU Circular: A0535+26 QPO M. H. Finger, Computer Sciences Corporation, R. B. Wilson, and B. A. Harmon, NASA/Marshall Space Flight Center, report for the BATSE/Compton GRO Observatory team: "The hard x-ray flux currently being observed from the x-ray binary pulsar A 0535+26 (IAUC 5931,5933) has significant aperiodic variability in addition to coherent pulsations. Power spectra contain the pulsed component and an underlying continuum with a significant bump between approximately 20 and 80 mHz, in both the 20-50 and 50-100 keV energy bands. The continuum in the 20-50 keV power spectra on Feb 9 and Feb 12 have been fit with a sum of power law and gaussian components. On both days the power law component has an index of near 1.0, and a fractional RMS (10-500 mHz) near 20%. On Feb 9 the bump was centered at 50 +- 1 mHz, had a FWHM of 28 +- 3 mHz, and had a fractional RMS of 14.1% +- 0.5%. On Feb 12 the bump was centered at 65 +- 1 mHz, had a FWHM of 37 +- 3 mHz, and had a fractional RMS of 11.6% +- 0.4%. The 20-40 keV flux increased from 3.1 +- 0.2 Crab on Feb 9 to 5.6 +- 0.2 Crab on Feb 12. The shift in center frequency is therefore consistent with the luminosity dependence predicted by the beat frequency QPO model (Alpar, M.A. & Shaham, J., 1985, Nature 316,239). Both the total and pulsed flux have continued to increase. On Feb 14 the 20-40 keV total flux was 6.2 +/- 0.1 Crab, and the phase averaged pulsed flux was 1.6 +- 0.1 Crab (total)." Current BATSE A0535+26 observing plans include the collection of LAD high energy resolution (HER) data with 16.384 sec resolution, the onboard epoch folding of spectroscopy detector data, and the collection of LAD 31 ms resolution single sweep data using the pulsar hardware. In addition, the CONT data (16 channels, 2.048 s resolution) energy channel edges are being changed to increase resolution near 100 keV. We expect to operate in this mode for at least 7-10 days.