CGRO biWeekly Status Report

[SHRADER@GROSSC.GSFC.NASA.GOV]

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.