Central Bureau for Astronomical Telegrams
IAUC 6277: HD 49798 AND 2E 0050.1-7247
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Circular No. 6277
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
Phone 617-495-7244/7440/7444 (for emergency use only)
HD 49798 AND 2E 0050.1-7247
G. L. Israel, International School for Advanced Studies,
Trieste; L. Stella, Astronomical Observatory of Rome; L. Angelini
and N. E. White, Goddard Space Flight Center, NASA; and P. Giommi,
Satellite for Astronomy in X-rays, Scientific Data Center,
communicate: "As part of a systematic search for periodicities in
the ROSAT PSPC light curves (energy range 0.1-2 keV) of sources
listed in the WGACAT (IAUC 6100), we discovered two new x-ray
pulsators. A periodic signal at about 13 s (significance 15-sigma)
was detected in the x-ray flux emitted by HD 49798, a hydrogen-
deficient, sdO6, single-component spectroscopic binary with an
orbital period of 1.54 day. The observation was carried out on
1992 Nov. 12 with a total exposure of 5450 s. The modulation was
nearly sinusoidal and energy independent, with a pulsed fraction
(i.e., semi-amplitude of modulation divided by the mean source
count rates) of about 60 percent. The barycentric period was
determined to be 13.1787 +/- 0.0004 s. The approximately 900
photons detected from the source show a very soft spectrum with a
high energy excess. A blackbody-plus-power-law model provides a
good fit; for column density between NH = 0.4 and 1 x 10E21 cmE-2,
the blackbody temperature ranges between 32 and 12 eV. For the
estimated distance of 650 pc, a very wide of range of unabsorbed
0.1- to 2-keV luminosity (roughly between 10E33 and 10E37 erg/s)
is allowed. These results reveal the compact nature of the HD
49798 companion. It is unclear if this is a white dwarf or a
neutron star, but a simple wind accretion model would favor the
latter. Perhaps HD 49798 is the result of post high-mass x-ray
binary common envelope evolution.
We detected also 8.9 s pulsations from 2E 0050.1-7247, an
Einstein x-ray source in the Small Magellanic Cloud (significance
of 6-sigma) identified with the variable B1 star AV 111 (Azzopardi
and Vigneau 1982, A.Ap. Suppl. 50, 291). The source was observed
at a level of about 0.13 count/s (0.1-2 keV) during a 17600-s ROSAT
PSPC observation during 1993 May 9-12. The signal had a
barycentric period of 8.88163 +/- 0.00001 s and had a nearly-
sinusoidal shape with a 25-percent pulsed fraction. The source was
hard (power law with photon index of 1.1 and NH = 8 x 10E20 cmE-2)
with a luminosity (0.1-2 keV) of about 10E36 erg/s for a 60-kpc
distance. During other ROSAT pointed observations in 1991-1992,
the source was substantially fainter (factor of 20 or more) and
softer. These results reveal that 2E 0050.1-7247 contains an
accreting magnetic neutron star, likely in a transient Be high-mass
x-ray binary. X-ray, ultraviolet, and optical monitoring of these
sources is necessary."
1995 December 21 (6277) Daniel W. E. Green
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