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IAUC 7121: N SMC; XTE J1550-564

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                                                  Circular No. 7121
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7244/7440/7444 (for emergency use only)

     D. L. Welch, McMaster University, and the MACHO collaboration
(cf. IAUC 6312) report their discovery of an apparent nova at R.A.
= 1h02m21s.8, Decl. = -73o18'27" (equinox 2000.0), which peaked in
brightness sometime between consecutive observations on 1998 Dec.
25.513 (V = 19.0) and 1999 Jan. 3.519 UT (V < 13.5; image
saturated).  Prior to eruption, a stellar object at the nova's
position had a mean magnitude of V = 19.0, with peak-to-peak
brightness changes of 0.5 mag on a timescale of hundreds of days.
On 1999 Mar. 3.463, it was at V about 16, and it continues to fade.

XTE J1550-564
     J. Homan, R. Wijnands, and M. van der Klis, Astronomical
Institute 'Anton Pannekoek', University of Amsterdam, report:  "The
blackhole candidate x-ray transient source XTE J1550-564 (IAUC
7008) seems to have undergone a sudden major state transition.  The
source was observed with the RXTE PCA instrument on Mar. 4.80 UT.
The source flux (2-20.0 keV) was 6.0 x 10E-8 erg sE-1 cmE-2.  A
quasiperiodic oscillation (QPO) was detected at 284.2 +/- 1.3 Hz,
with a width (FWHM) of 29 +/- 4 Hz and an rms amplitude of 3.9 +/-
0.2 percent (6.4-60 keV; 11.5 sigma). The QPO amplitudes in the
energy ranges 2-60, 2-6.4, 6.4-13, and 13-60 keV, are 1.3 +/- 0.2,
< 0.8, 3.2 +/- 0.2, and 8.4 +/- 0.9 percent, respectively.  This
high-frequency QPO is most likely related to the 184-Hz QPO
previously observed in XTE J1550-564 (IAUC 7025; Remillard et al.
1999, Ap.J. Let., submitted).  Simultaneously with the high-
frequency QPO, QPOs at 5.92 +/- 0.02 Hz [FWHM 2.02 +/- 0.05 Hz; rms
amplitude 2.98 +/- 0.03 percent (2-60 keV; 43.6 sigma)] and at 10.4
+/- 0.1 Hz [FWHM 5.8 +/- 0.3 Hz; rms amplitude 2.55 +/- 0.06
percent (2-60 keV; 23.7 sigma)] are observed.  An RXTE observation
taken the day before (on Mar. 3.06), shows a 2-20.0-keV x-ray flux
about 20 percent lower than that of Mar. 4.80 (4.9 x 10E-8 erg sE-1
cmE-2), a softer x-ray spectrum, and a power spectrum with no QPO,
dominated by a weak (2.7 +/- 0.2 percent; 2-60 keV) power-law
component with index -1.23 +/- 0.04.  These properties are typical
for those of XTE J1550-564 during the last few months.  This
pronounced change in x-ray properties, which took place within 1.8
days, shows that XTE J1550-564 has undergone a major source state
transition.  Observations at all wavelengths are encouraged.  More
x-ray observations with the RXTE/PCA instrument are required to
study the high-frequency QPO in more detail."

                      (C) Copyright 1999 CBAT
1999 March 5                   (7121)            Daniel W. E. Green

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