Central Bureau for Astronomical Telegrams
IAUC 6876: SAX J1808.4-3658 = XTE J1808-369; 1998aq
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Circular No. 6876
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
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SAX J1808.4-3658 = XTE J1808-369
F. E. Marshall, Goddard Space Flight Center, on behalf of the
RXTE Science Operations Center, reports the detection of a
transient x-ray source with the Proportional Counter Array on the
Rosssi X-ray Timing Explorer at R.A. = 18h08m34s +/- 4s (90-percent
confidence interval), Decl. = -36o59'.4 +/- 3' (equinox 2000.0):
"The 2-10-keV flux was 8 x 10E-10 erg cmE-2 on Apr. 9.6 UT and
increased to 1.5 x 10E-9 erg cmE-2 on Apr. 11.8. The position is
consistent with the x-ray transient SAX J1808.4-3658 (in 't Zand et
al. 1998, A.Ap. 331, L25), for which type-I x-ray bursts were seen.
A continuing x-ray lightcurve from the RXTE All Sky Monitor is
available at http://heasarc.gsfc.nasa.gov/xte_weather.
Observations at other wavelengths are encouraged."
R. Wijnands and M. van der Klis, Astronomical Institute 'Anton
Pannekoek', University of Amsterdam, report the discovery of near-
coherent pulsations between 400.98 and 401.02 Hz (2.49 ms) in the
persistent emission of the x-ray transient XTE J1808-369, in RXTE
PCA observations taken between Apr. 11.32 and 11.37 UT: "An
approximately sinusoidal variation is observed in the frequency of
these oscillations, part of which is due to the Doppler shifts
caused by the satellite motion. The strength of the oscillations
was 4.2 percent rms in the photon energy range 2-60 keV, with
little dependence on photon energy. The 2-60-keV flux was about 60
mCrab. The presence of the pulsations was confirmed in data taken
on Apr. 13 using a different RXTE data mode. This is the first
x-ray source showing nearly coherent millisecond oscillations in
its persistent emission. The most likely interpretation is the
presence of an accretion-powered millisecond x-ray pulsar in the
system. XTE J1808-369 has a position that is consistent with that
of the known x-ray transient and x-ray burster SAX J1808.4-3658.
The best BeppoSAX coordinates are R.A. = 18h08m29s, Decl. =
-36o58'.6 (equinox 2000.0; error radius 2'; in 't Zand et al.,
op.cit.). More x-ray observations are needed in order to study
this unique system in more detail. Observations at other
wavelengths are encouraged in order to determine the exact nature
of this system."
SUPERNOVA 1998aq IN NGC 3982
Magnitude obtained by D. Hanzl, Brno, Czech Republic (0.4-m
reflector + CCD + V filter; GSC comparison stars): Apr. 14.834 UT,
14.3.
(C) Copyright 1998 CBAT
1998 April 15 (6876) Daniel W. E. Green
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