Addendum to CGRO March Status report

[SHRADER@grossc.gsfc.nasa.gov]

Addendum to CGRO March Status report

OSSE Instrument Team report:

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 remains disabled because of the bursting pulsar, GRO J1744-28.

OSSE observations of this new source in December and January were particularly
fruitful.  In the following IAU circular, we reported the discovery that pulses
within bursts lag those at times away from bursts.  A paper summarizing
spectroscopy and timing of the bursts and the pulsar is in the final stages of
preparation. 

    GRO J1744-28		(IAUC 6321)
         G. V. Jung and B. F. Phlips, Universities Space Research
    Association; J. D. Kurfess, J. E. Grove, M. S. Strickman, W. N.
    Johnson, C. D. Dermer, and G. H. Share, Naval Research Laboratory
    (NRL); and S. J. Sturner, NRL and National Research Council, on
    behalf of the Gamma Ray Observatory OSSE team, report:  "OSSE has
    observed the 467-ms pulsed emission from the bursting x-ray pulsar
    GRO J1744-28 (IAUC 6285), in both the steady source and during
    bursts.  Using the pulsar ephemeris supplied by BATSE (M. Finger,
    private communication), we epoch-folded and co-added data from the
    brightest 2 s of each burst in a sample of 104 bursts observed by
    OSSE between Jan. 18 and 30.  We find that the pulsed emission
    during the bursts lags the non-bursting pulsed emission; the lag
    amounts to 0.155 +/- 0.007 of a pulsar period at the peak of the
    burst.  The magnitude of the phase lag follows the average
    intensity profile of the burst sample.  This result suggests that
    the burst-emitting region is physically offset from the region that
    produces the persistent emission, which could be caused by
    azimuthal asymmetries from accreting matter constrained to move
    along different field lines."

An important paper summarizing OSSE spectra of Cyg X-1 has been accepted for
publication in ApJ (Phlips et al.).   Although we have measured a new minimum
in the hard X-ray flux from the source, no evidence was found for either a
broad 1 MeV feature or a narrow 511 keV line previously reported in association
with a low flux state. Preprints are available through on the World Wide Web at
URL http://osse-www.nrl.navy.mil/osselib.htm. 

Data through viewing period 405 have been delivered to the Compton GRO
Science Support Center archive.