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IAUC 7139: MXB 1659-29; Her X-1; 1999aa

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IAUC number

                                                  Circular No. 7139
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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MXB 1659-29
     C. B. Markwardt, National Research Council and Goddard Space
Flight Center (GSFC); E. Smith and J. Swank, GSFC; and J. in 't
Zand, Space Research Organization of the Netherlands, Utrecht,
write:  "Observations of the transient source MXB 1659-29 (IAUC
7138) were performed by RXTE on Apr. 5.83-6.05 UT.  The source was
detected at a level of about 38 mCrab (2-10 keV), but was
punctuated by a series of x-ray dips that lasted 5-350 s.  A single
x-ray eclipse was observed between Apr. 5.97206 and 5.98257
(barycenter-corrected).  Cominsky and Wood (1989, Ap.J. 337, 485)
have published an orbital period of 7.1161138(17) hr, based on
previous activity in 1976-1978, and we conclude that the observed
eclipse is number 27708 according to their numbering convention.
The observed center of the eclipse arrived 7.0 +/- 2.6 min before
the predicted time, based on a constant period, implying an orbital
dP/dt = (-4.8 +/- 1.8) x 10E-11.  No x-ray bursts were seen, and no
pulsations or QPOs were detected.  Follow-up observations are being
performed by RXTE."

     A. M. Levine, Center for Space Research, Massachusetts
Institute of Technology (MIT); and R. Corbet, Goddard Space Flight
Center (GSFC), NASA, and Universities Space Research Association,
report on behalf of the RXTE/ASM team at MIT and NASA/GSFC:  "The
35-day longterm cycle has been obvious in the RXTE All-Sky Monitor
1.5-12-keV lightcurve for Hercules X-1, with the 'main high state'
(see, e.g., Scott and Leahy 1999, Ap.J. 510, 974) being clearly
detected every 35 days for > 3 yr.  The first exception to this
regular appearance of the main high states has recently occurred.
No main high state appeared as expected around Mar. 23, with an
upper limit of about 15 mCrab.  Earlier main high states typically
reached peak intensities of 40-100 mCrab.  This may represent an
expansion of the accretion disk, such that the neutron star remains
occulted through the entire 35-day cycle.  Optical observations to
determine whether the companion, HZ Her, is still being heated by
x-rays are encouraged."

SUPERNOVA 1999aa IN NGC 2595
     CCD magnitude estimates, unfiltered unless otherwise noted:
Mar. 13.666 UT, 16.5 (S. Yoshida and K. Kadota, Ageo City, Japan);
16.534, 16.2 (Yoshida); 18.826, V = 15.6 +/- 0.2 (L. Kiss and B.
Csak, Piszkesteto, Hungary); 20.92, 16.0 (S. Moretti and S.
Tomaselli, Forli, Italy).

                      (C) Copyright 1999 CBAT
1999 April 7                   (7139)            Daniel W. E. Green

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