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IAUC 4235: 1986i; GX 17+2; VY Aqr

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                                                  Circular No. 4235
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

     J. B. Tatum, University of Victoria, provides the following
precise positions.  The July 20 position is of a smudge on a plate
severely blackened by moonlight.

         1986 UT              R.A.   (1950.0)    Decl.

         July 19.42153      21 44 07.07      -15 11 23.7
              19.44861      21 44 03.88      -15 12 27.2
              20.45035      21 41 56.70      -15 49 47.9

     Total visual magnitude estimate by C. S. Morris, Whitaker
Peak, CA (0.26-m reflector): July 19.47 UT, 12.8; coma diameter
1.3, some central condensation, tail 1' long in p.a. ~ 280).

GX 17+2
     P. Hertz and K. S. Wood, Naval Research Laboratory, write:
"During continuing analysis of data obtained with the A-1 experiment
on HEAO 1 we discovered an apparent 19.5 +/- 0.2 hr x-ray periodicity
in the bright galactic bulge source GX 17+2 = 4U 1813-14 =
Ser X-2.  The data consist of x-ray flux determinations spaced
unevenly in time: 56 were obtained during 1977 Sept. 23-30 and 79
during 1978 Mar. 21-28.  During the 1978 observations GX 17+2 had a
steady flux of ~ 230 ufu, with several flares to ~ 365 ufu.  The
nonflaring data show an approximately sinusoidal variation with a
peak-to-peak flux range of ~ 50 ufu.  The probability that the
variation is spurious is < 0.01 percent, and evidence for several
subharmonics is seen in the power spectrum.  The flares appear to
be periodic with the 19.5-hr period, and they occur preferentially
near the peak of the sinusoidal variation.  In 1977 the source flux
decreased from ~ 315 to ~ 235 ufu over the 7-day period, and a
flare to ~ 420 ufu was observed at the end of the period.  No periodic
variations were observed in the 1977 data.  The 19.5-hr period
is consistent with that of a close binary if the companion star is
slightly evolved, as in the case of V818 Sco = Sco X-1.  Observers
with data covering these timescales are encouraged to search for
this period, particularly if their observations are near 1978 Mar."

     Magnitude estimates by R. H. McNaught, Coonabarabran, N.S.W.:
May 24.80 UT, 14.5; 25.64, 14.6:; 26.83, [14.8; 30.80, [15.6.

1986 July 22                   (4235)              Brian G. Marsden

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