Central Bureau for Astronomical Telegrams
IAUC 2833: CYGNUS X-1; 3U 0900-40; N Cyg 1975
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Circular No. 2833
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Cable Address: SATELLITES, NEWYORK
Western Union: RAPID SATELLITE CAMBMASS
CYGNUS X-1
F. Primini, Massachusetts Institute of Technology, reports
that the SAS-3 Group has observed a marked increase in the x-ray
flux from Cyg X-1. The intensity in the 1.3 to 5-keV band rose by
a factor of between three and four from Sept. 6 to 8. It is now
fluctuating by a factor of two on a timescale of hours and is at a
level comparable to the pre-transition maximum listed in the Uhuru
Catalogue. The 5 to 13-keV and 8 to 35-keV intensities have not
increased over their pre-outburst values. The event appears similar
to the x-ray and radio outburst in May (IAUC 2778, 2779).
3U 0900-40
S. Rappaport and J. McClintock, Massachusetts Institute of
Technology, report that the SAS-3 Group has measured the Doppler
shift in the 282s.9 x-ray pulse period of 3U 0900-40 = Vel X-1 (IAUC
2794). Observations were made between June 18.8 and 24.2 and
between July 19.3 and 25.3 UT. The Doppler shifts are in phase with
the 8.96-day orbital period of the compact object around the B0.5Ib
star HD 77581. The best-fit parameters (and 90-percent confidence
error limits) for the binary system are: P = 282s.8913 +/- 0s.0004
(heliocentric); a sin i = 109 +/- 4 light seconds; K_x = 268 +/- 12
km/s; f(M) = 17.3 +/- 2.0 M_sol; e = 0.15 +/- 0.05; w = 157o +/- 24o;
geometric center of eclipse = 1975 July 18.6 +/- 0.2 UT. Emersion from
eclipse was observed on July 19.3, but the preceding orbital
elements, combined with the eclipse duration (1.9 days) determined
from Uhuru data (Forman et al. 1973, Astrophys. J. 182, L103),
predict emersion at least 0.1 day later than this time. The
determination of the semimajor axis (hence the mass function) is fairly
insensitive to the choice of eccentricity. These measurements,
when coupled with the minimum likely orbital velocity (K_opt = 23
km/s) inferred for HD 77581 (Avni and Bahcall 1975, Astrophys. J.
Lett. in press), yield a lower limit for the mass of the x-ray star
of m_x >= 1.7 M_sol. The existence of a high 'Q' periodicity rules out
the possibility that Vel X-1 is a black hole, and the mass makes it
unlikely that it is a white dwarf.
NOVA CYGNI 1975
Corrigenda. IAUC 2826: line 21, for Deterwagne read De
Terwagne; line 44, for D. Tallant read D. Talent. IAUC 2828: line
6, for Boudewijn and Baud read B. Baud.
1975 September 9 (2833) Brian G. Marsden
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