CGRO Status Report for July 1996

[SHRADER@grossc.gsfc.nasa.gov]

   Compton Gamma Ray Observatory Status Report #194
                 Monday, July 10 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:

The CGRO Cycle-6 proposal review was held late last
month. The broad consensus is that there is still an
abundance of top quality science forthcoming! No final
results will be announced until after the timeline
committee meeting which occurs later this month. Shortly
thereafter, letters of acceptance/rejection and, where
appropriate, budget solicitations will be sent via e-mail.

A proposal, prepared by the CGRO Users Committee, was
submitted to the NASA HQ MO&DA Senior Review for
evaluation later this month. The outcome of this review
will determine the level of support for the project
during FY 98 and beyond.  

High-level data products (BATSE, OSSE and COMPTEL)from
the recent Cyg X-1 ToO will be available imminently on
the CGRO SSC WWW site (http://cossc.gsfc.nasa.gov). These
include PostScript plots, ascii and FITS formatted data
files and "PHA" spectral-data files for use with the
HEASARC "XSPEC" software.


Instrument Team Reports:


EGRET

The high voltage for the track imaging system of EGRET
has been off since June 14, 1996 to conserve the
remaining gas, since the astrophysical objects in the
field of view were judged by the proposal review
committee to be of lesser priority.  It will be activated
on either July 23 or July 30, 1996 depending on whether
or not a target of opportunity is declared. 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 papers that have appeared in the Astrophysical
Journal recently include: "EGRET Observations of the
North Galactic Pole Region", by Sreekumar et al. (June
20), "Possible EGRET Gamma-Ray Burst Detection
Independent of BATSE Triggering", by Jones et al. (June
1), and "EGRET Observations of High- Energy Gamma
Radiation from PSR B1706-44", by Thompson et al. (July
1). At the June American Astronomical Society Meeting,
Carl Fichtel et al. showed that, within the limits of the
statistics available, the distribution in z of the high
energy gamma-ray emitting flat-spectrum radio-loud
blazars was the same as all flat-spectrum radio-loud
blazars, and a similar statement could be made for the
gamma-ray emitting BL Lac objects and all radio emitting
BL Lacs. Hence, the gamma-ray emitting BL Lacs are closer
and less luminous on the average.  No statistically
significant difference was found in the energy spectral
slopes of the high-energy gamma-ray emission from these
two types of blazars.


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 day 96/135. Since this time there have
been no burst triggers sent from BATSE that have met the
OSSE slewing criteria.

The OSSE Orion complex paper has been accepted for
publication in ApJ (Murphy et al.). Preprints are
available on the OSSE preprint WWW page at URL
http://osse-www.nrl.navy.mil/osselib.htm. Flux levels
reported by COMPTEL of gamma-ray line emission in the 3-7
MeV range suggest that if the origin of the emission were
a point source at the center of the OSSE field of view
then OSSE observations of the region would result in
significant detections of line emission near 4.4 and 6.1
MeV. The lack of compelling evidence for an OSSE
detection in this region requires that any source of this
emission must be more extended than the distribution of
the intense CO emission localized around Orion A and
Orion B. 

OSSE strongly detected Cyg X-1 in its gamma-ray low state
in the recent target of opportunity observation (VP
522.5). The Rossi XTE observed Cyg X-1 during the same
interval, providing an unprecedented opportunity to
obtain a broadband spectrum in this state. While
coordinated spectral analysis has not yet been performed
on these data sets, we expect to establish that the
gamma-ray low state, with its moderately steep power law
spectrum, is associated with the X-ray high (soft) state.
In VP 522.5, OSSE detects emission up to about 800 keV.
The spectrum is consistent with a single power law with
photon index = -2.5, with some evidence that the spectrum
is beginning to turn over with an exponential folding
energy of >600 keV. 

Recent observations are listed in the following table.
View period      Dates        Target (owner)

    522        11-14 June     Cen X-3      (T. Vestrand)
                              Orion GMC    (M. Harris)

    522.5      14-25 June     Cyg X-1      (public)
                              V 0332+53    (public)
                              Crab Pulsar  (public)

    523    25 June - 9 July   Orion GMC    (M. Harris)
                              Mrk 421      (public)
                              NGC 4151     (A. Zdziarski)

    524        9-23 July      GX 339-4     (E. Liang)
                              Vela Pulsar  (A. Harding)


Low-level OSSE data products through viewing period 419.1
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. All products for the VP 522.5
Cyg TOO will be delivered within the week. 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. Due to a small processing problem
discovered in the VP 522.5 Cygnus X-1 data set, it is
being reprocessed and is expected to be re-delivered to
the SSC by 7/17/96.


COMPTEL 

The COMPTEL instrument is performing well and continues
routine operations.

The collaboration has carried out an accelerated
processing of the data associated with the recent
target-of-opportunity observation of Cyg X-1 (VP 522.5).
Preliminary analysis indicates elevated flux levels from
this source, when compared to earlier epochs. Further
details on these first results, along with related data
products, will be made publicly available shortly via the
COMPTEL pages on the WWW.

The drought of non-detections of gamma-ray bursts
occurring within the field of view of COMPTEL continues:
neither GRB 960705 nor GRB 960702 was detected at MeV
energies by the instrument.

The organizers of the upcoming INTEGRAL workshop in St.
Malo, France announce that the scientific programme,
including abstracts and schedule information, is now
available on the WWW via the INTEGRAL home page at:
http://astro.estec.esa.nl/SA-general/Projects/Integral/integral.html 
The third and final circular for the INTEGRAL
workshop, with information on travel, list of
participants, paper-preparation kit, etc, is available on
these WWW pages, and will also be mailed to all
registered participants in early July.

Lastly, the 83rd Tour de France is well underway (in
snow!). Will Indurain become the first to achieve an
unprecedented sixth consecutive victory?!

BATSE

The following report was submitted to the IAU Circulars:

     2S 1417-624                                          
           
     M. H. Finger and M. Scott, Universities Space
     Research Association, T. A. Prince and B. Vaughan,
     Caltech, and R. B. Wilson, NASA/Marshall Space
     Flight Center, report for the BATSE/Compton
     Observatory team: "Renewed activity from the X-ray
     transient pulsar 2S1417-624 is being observed. Hard
     X-ray pulses were first detected on June 30, with
     the pulsed flux rising to 17 +- 3 mCrab (20-50 keV)
     by July 8. A barycentric frequency of 0.05694816(25)
     Hz on July 4.5 and barycentric frequency rate of
     3.35(14)x10E-11 Hz/s were obtained from a fit of
     frequency measurements from July 1-8. The source had
     been undetected by BATSE since a series of six
     outbursts were observed, beginning on August 26 1994
     (IAUC 6075) and ending July 8 1995."   
              
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, 2S1417-624, EXO 2030+375, OAO
1657-415, Vela X-1, and GX 301-2. The current outburst of
EXO 2030+375 is the third in a consecutive series of
outbursts occurring near periastron passage.

Earth occultation monitoring over the past month has
shown considerable activity from transient sources. GRO
J1655-40 has been bright (~0.6-0.8 Crab 20-100 keV), in
recent days; also GRS 1915+105, GX 339-4 and 4U 1630-47
have been detected. Cyg X-1 remains in a very low hard
X-ray/gamma ray state, dropping to 100-150 mCrab in the
20-100 keV band.

Another 150 days of Earth occultation data (TJD
9200-9350, Aug 1-Dec 29, 1993) for 25 hard X-ray sources
(list available at the COSSC) have been delivered to the
public database.

The burst trigger is currently using count rates from
above 100 keV. As of July 9 BATSE has detected 1593
gamma-ray bursts out of a total of 5429 on-board triggers
in 1904 days of operation. There have been 767 triggers
due to solar flares, 10 due to SGR events, 55 due to
terrestrial gamma-ray flashes, and 1477 due to the
bursting pulsar GRO J1744-28.