Circular No. 6544
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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GRB 970111
     A. Guarnieri, C. Bartolini, and A. Piccioni, University of
Bologna; N. Masetti, University of Padua; M. Teodorani,
Osservatorio Astronomico, Napoli; E. Costa, Istituto Astrofisica
Spaziale, Frascati; and D. Dal Fiume and F. Frontera, TESRE,
Bologna, communicate:  "A preliminary analysis of CCD observations
of the GRB 970111 field (IAUC 6533), obtained with the Bologna
University 1.5-m telescope in B and R bands (limiting magnitudes B
about 20, R about 21) on Jan. 14.19, 15.19, and 17.19 UT, shows the
presence of some objects that are probably variable; however, on
this timescale, no object reveals a decline > 0.3 mag.  A
comparison between these frames and the same field from the Digital
Sky Survey (DSS) shows that all the objects brighter than B about
19 and R about 20 in our frames are also present in the DSS field
and that their luminosities have not noticeably changed since then
(1950.294).  Many galaxies are present in the GRB 970111 field.  We
also observed the error-box fields of sources 'a' and 'b' (IAUC
6539) in the R band, and we detected five and eight objects,
respectively, brighter than R about 21.  None of these seems to be
variable in this time interval (or if compared with the DSS).  Our
group continues the optical observations of the field."


CYGNUS X-1
     R. P. Fender and C. Brocksopp, University of Sussex; and G. G.
Pooley, University of Cambridge, report:  "We have detected the
5.6-day orbital modulation of the blackhole candidate Cyg X-1 in
radio monitoring at 15 GHz with the Ryle Telescope (Cambridge),
undertaken since 1996 Oct., during which time the source has been
in a hard x-ray state.  This confirms the apparent orbital
modulation in previous Very Large Array data (Han and Hjellming,
Ap.J., submitted).  The modulation is approximately sinusoidal,
with a semiamplitude of about 3 mJy superimposed on a mean flux
density of about 14 mJy for the entire data set.  Orbital flux
minimum occurs at superior conjunction of the compact object, as
observed also in ASM/XTE x-ray data (IAUC 6510).  This implies that
the radio-emitting region is associated with the compact object,
and the orbital modulation may be due to varying line-of-sight
optical depth through the stellar wind of the OB-type companion
star.  The orbital modulation is superimposed upon other trends
that also appear to be correlated with the x-ray activity."

                      (C) Copyright 1997 CBAT                    
1997 January 24                (6544)            Daniel W. E. Green