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IAUC 7191: XTE J1550-564; 1999cl

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IAUC number

                                                  Circular No. 7191
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1550-564
     W. Cui and L. Wen, Massachusetts Institute of Technology; S.
N. Zhang and X.-B. Wu, University of Alabama, Huntsville, report:
"We have detected a periodic signal in the x-ray intensity of
blackhole candidate XTE J1550-564, using public data from the
All-Sky Monitor (ASM) aboard RXTE.  A Lomb-Scargle periodogram is
constructed from the ASM light curve of this transient source for
the period when it was in a recent x-ray outburst (1998 Sept. 4-
1999 May 22).  A peak is apparent at 1.398 +/- 0.003 days in the
periodogram, with a power of about 40 (the Poisson noise power is
normalized to 1 in the same units).  Taking into account the
intrinsic red noise of the source, we estimate that the detection
bears a false-alarm probability of about 10E-4.  We have also tried
to break up the outburst light curve into segments and have
performed the same analysis on each segment.  We have found that
the signal is always present at the same position in the
periodogram (within measurement uncertainties).  Similar results
are obtained from Fourier analyses of the light curve.  The epoch-
folded light curve is roughly of sinusoidal form, with a mean peak-
to-peak amplitude about 60 percent in each of the three ASM bands
(1.3-3, 3-5, and 5-12 keV).  The detected period is remarkably
close to what was suggested for the binary orbital period of XTE
J1550-564 (Soria and Wu, IAUC 7184), based on a theoretical
relationship between the orbital period and the mean density of the
secondary star, which is assumed to have filled its Roche lobe.
Considering that few blackhole candidates have been observed to
show any x-ray modulations at the binary orbital period, the
detection of such modulation in XTE J1550-564 perhaps implies a
special environment or geometry of the binary system, like in the
well-known case of Cyg X-1 (Wen et al. 1999, Ap.J., in press).
Future optical observations of the source in x-ray quiescence can
hopefully shed more light on the issue."

SUPERNOVA 1999cl in NGC 4501
     G. Masi, Ceccano, Italy, reports the following precise
position for SN 1999cl:  R.A. = 12h31m56s.01, Decl. = +14 25'35".3
(equinox 2000.0); on June 5.858 UT, he obtained an unfiltered CCD
magnitude of 14.1.  D. Hanzl, Brno, Czech Republic, reports the
following CCD magnitude:  June 2.914, R = 14.7.  Visual magnitude
estimates:  June 3.881, 14.6 (K. Hornoch, Lelekovice, Czech
Republic); 4.96, 14.2 (J. Carvajal, Madrid, Spain); 5.883, 14.4

                      (C) Copyright 1999 CBAT
1999 June 7                    (7191)            Daniel W. E. Green

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