Central Bureau for Astronomical Telegrams

Central Bureau for Astronomical Telegrams -- Image credits

IAUC 6277: HD 49798 AND 2E 0050.1-7247

The following International Astronomical Union Circular may be linked-to from your own Web pages, but must not otherwise be redistributed (see these notes on the conditions under which circulars are made available on our WWW site).

Read IAUC 6276  SEARCH Read IAUC 6278

View IAUC 6277 in .dvi or .ps format.
IAUC number

                                                  Circular No. 6277
Central Bureau for Astronomical Telegrams
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)
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

Read IAUC 6276  SEARCH Read IAUC 6278

View IAUC 6277 in .dvi or .ps format.

Our Web policy. Index to the CBAT/MPC/ICQ pages.

Valid HTML 4.01!