Central Bureau for Astronomical Telegrams
IAUC 2778: X-RAY SOURCES
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Circular No. 2778
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
X-RAY SOURCES
H. Gursky, J. Grindlay, H. Schnopper and E. Schreier, Center
for Astrophysics; D. Parsignault, American Science and Engineering;
and A. Brinkman, J. Heise, J. Schrijver, R. Mewe, E. Gronenschild
and A. denBoggende, Laboratory for Space Research, Utrecht, using
the soft and hard x-ray detectors aboard ANS, report the observation
of an upward transition in the intensity of Cyg X-1 that appears
to be the inverse of the downward transition seen by Uhuru in
Apr. 1971. On 1975 May 1, when observations began, the intensity
between 1.5 and 7 keV was 59 cts/s (equivalent to 575 cts/s Uhuru,
2-6 keV). By May 3 it had increased to 83 cts/s. Subsequently,
the source remained at about this intensity, with excursions as
high as 130 cts/s. During Nov. 1974, ANS saw Cyg X-1 at an average
intensity of 20 cts/s (1.5-7 keV). The bulk of the increase occurs
at low energy; between 1 and 2 keV, the intensity increased by a
factor of 10 over the Nov. 1974 intensity, while above 8 keV there
is no significant change. Since Cyg X-1 has remained high for > 5
days after the transition with enhanced soft x-ray emission, we believe
that it has reverted to its high state. Additional observations
in radio, optical and x-rays are urgently needed, since there
is little information about the duration of the high state. The
radio emission may be expected to disappear, since it was only
first seen following the Apr. 1971 downward transition.
C. Chevalier, Observatoire de Meudon, and S. A. Ilovaisky,
Centre d'Etudes Nucleaires de Saclay, report that observations
carried out in April at the European Southern Observatory have yielded
a possible optical counterpart for the highly variable x-ray source
A1118-61 (Eyles et al. and Ives et al. 1975, Nature 254, 577 and
578). UBV photometry of about 20 stars in or near the error box
has revealed a heavily reddened, variable, ultraviolet star lying
about 4' southwest of the cited position (R.A. = 11h18m59s, Decl. =
-61o35'.3, equinox 1950.0). Observations on two nights give: V =
12.12, B-V = +0.94, U-B = -0.43 to -0.28. Monitoring in the B
filter on one night yielded an upper limit of 0.008 magnitude for
any 6.755-min periodicity present in the signal. Image-tube spectra
at 100 A/mm show H-beta and H-gamma in emission, a strong, diffuse,
interstellar band at 4430 A and an intense ultraviolet continuum.
Many other weak lines are present, including H, He I, N II, O II
and Si IV in absorption, the 4501 and 4882 A interstellar bands and
He I 5015 A and Fe II in emission. The evidence suggests that this
object is an early-type Be star resembling X Per in several aspects.
1975 May 9 (2778) Brian G. Marsden
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