Central Bureau for Astronomical Telegrams
IAUC 7427: XTE J1118+480
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Circular No. 7427
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1118+480
C. A. Haswell, Open University; D. Skillman, Laurel, MD; J.
Patterson, Columbia University; R. I. Hynes, Southampton University;
and W. Cui, Massachusetts Institute of Technology, on behalf of a
larger collaboration, report: "Hubble Space Telescope observed XTE
J1118+480 for a total integration time of 2.54 hr (range 115-170 nm)
on Apr. 8. The power density spectrum (PDS) is roughly flat below
about 0.02 Hz and breaks into a power law (roughly 1/f) above.
Such a PDS shape is characteristic of blackhole candidates or
'atoll-type' low-mass x-ray binaries in the low state. In addition,
the ultraviolet PDS reveals a broad quasiperiodic oscillation (QPO)
at 0.08 Hz with FWHM 0.02 Hz. Unfiltered CCD observations (0.68-m
f/2.9 telescope) during Apr. 30-May 3 with 0.32-Hz sampling show a
QPO at 0.101 Hz (FWHM 0.008 Hz). Both the low-state PDS shape and
the QPO are confirmed by preliminary results from RXTE, which
indicate that the source has remained in such a low state
throughout the outburst. Comparison of simultaneous HST near-
ultraviolet (160-235 nm) and RXTE lightcurves reveals a correlation
on timescales of seconds. A preliminary analysis indicates that
ultraviolet variability lags behind x-rays by 1-2 s. This is
likely due to light-echoes caused by reprocessing of the x-ray
flares within the binary system; the delay is consistent with
expected light-travel-time delays within a 4-hr x-ray binary (IAUC
7397). Further high-time-resolution observations at all
wavelengths are urged to investigate light-echoes further (see
http://phys-ftp.open.ac.uk/pub/1118/satobs)."
K. Yamaoka, Y. Ueda, and T. Dotani, Institute of Space and
Astronautical Science; and P. Durouchoux and J. Rodriguez, Service
d'Astrophysique, Saclay, report observations of XTE J1118+480 with
the ASCA satellite during from May 11.527-12.031 UT: "The x-ray
intensity showed rapid variability over an averaged flux level of 8
x 10**-10 erg cm**-2 s**-1 (2-10 keV). A QPO feature was found at
0.115 +/- 0.002 Hz with an amplitude of about 6 percent rms in the
averaged power spectrum; this QPO was confirmed by simultaneous
RXTE data, which covered two epochs during May 11.729-11.748 and
11.781-11.931. The QPO frequency has shifted from the value
reported before May 4 (0.085 +/- 0.002 Hz) by Revnivtsev et al.
(http://xxx.lanl.gov/abs/astro-ph/?0005212). The ASCA GIS spectrum
in the range 0.7-10 keV is about represented by a power law, over
which a slight soft excess is seen below 2 keV. Applying a disk
blackbody and power-law model yields an acceptable fit with a
temperature of the innermost disk of 0.2 +/- 0.1 keV, and a power-
law photon index of 1.76 +/- 0.02, with an upper limit to an
absorption column density of 10**21 cm**-2."
(C) Copyright 2000 CBAT
2000 May 20 (7427) Daniel W. E. Green
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