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IAUC 4247: 4U 1820-30; Poss. CATACLYSMIC Var IN NGC 6637

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                                                  Circular No. 4247
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
TWX 710-320-6842 ASTROGRAM CAM    Telephone 617-495-7244/7440/7444

4U 1820-30
     W. Priedhorsky, Los Alamos National Laboratory; and L. Stella
and N. E. White, EXOSAT Observatory, telex: "EXOSAT observations
of the x-ray-burst and QPO source 4U 1820-30 in the globular
cluster NGC 6624 (IAUC 4117) have revealed that the x-ray flux is
modulated at a period of 685 s with a peak-to-peak amplitude of 3
percent.  This coherent sinusoidal modulation was independently
detected in three observations when the source was in a high,
nonbursting state (L = 6 x 10**37 erg/s).  The measured pulse periods
were 686.5 +/- 1.3 s (2 sigma) on 1984 Sept. 26, 683.8 +/- 1.2 s on 1985
Apr. 16 and 686.0 +/- 2.2 s on 1985 Sept. 23.  In a low-bursting-
state observation (with L a factor of three lower) made on 1985
Aug. 19 the modulation amplitude was a factor of two lower and the
periodicity was only marginally detected at 693.0 +/- 6.0 s.  This
periodicity is probably not that of the rotation of a neutron star
because accretion torques would cause the period to change by at
least 10 s/yr for the mass-transfer rate inferred from the observed
L.  The period stability (varying by < 3 s/yr) suggests that
it is an orbital period, the shortest of any known binary system.
The possible existence of such an ultra-short-orbital-period, low-
mass x-ray binary system has been discussed by Rappaport et al.
(1982, Ap.J. 254, 616) and by Savonije et al. (1986, A.Ap. 155,
51).  The inferred mass-accretion rate is consistent with the
companion being an He white dwarf with a mass of 0.07 MO."

     R. E. Williams, Cerro Tololo Interamerican Observatory,
reports: "J. Rose, University of North Carolina; H. Cohn, Indiana
University; and T. Armandroff, Yale University, have discovered a
possible cataclysmic variable in the globular cluster NGC 6637.
On Aug. 10.17 UT V was ~ 12-12.5, ~ 1 mag brighter than on a CCD
frame taken a month earlier at Mauna Kea.  A spectrogram obtained
on Aug. 10.12 with the CTIO 1-m telescope exhibits strong Balmer
emission lines.  The star is located in the cluster core, 29" east
and 39" south of star No. II-35 of Hartwick and Sandage (1986,
Ap.J. 153, 715); at present, it is brighter than any cluster member.
The possibility that the star may only be chance superposition
is indicated by Mv = -3 if it is a cluster member, which is
too bright for a dwarf nova."

1986 August 28                 (4247)              Brian G. Marsden

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