Simultaneous Observations of GRS 1758-258 in 1997
by VLA, RXTE and OSSE: Spectroscopy and Timing
D. Lin, I. A. Smith, E. P. Liang
D. M. Smith
L.F. Rodrí guez
We report the results of our multi-wavelength observations of GRS 1758-258 made
in August 1997. The observations enable us to obtain a complete spectrum of the
source over an energy range of 2 - 500 keV. The spectrum shows that GRS 1758-258
was in its hard state. It is well fitted by the Sunyaev-Titarchuk (ST) Compton
scattering model with a plasma temperature of 45 keV and a Thomson depth of 3.3.
The spectrum is also fit by a power law with an exponential cutoff (PLE) with a
photon index of 1.40 and a cutoff energy of 207 keV plus a soft
black-body component with a temperature of 1.27 keV. No significant iron lines are
detected. The daily radio, X-ray and gamma-ray light curves show that
GRS 1758-258 was stable during the observation period, but was highly variable
on smaller time scales in X- and gamma-rays. The power density spectra are
typical for the low-state, but
we find the photon flux for the 5 to 10 keV band to be more variable
than that in the other two energy bands (2 - 5 keV and 10 - 40 keV). The
integrated fractional rms amplitudes (IFRA) in the three energy bands are
0.349±0.003, 0.271 ±0.004, and 0.225 ±0.005 respectively.
Two harmonic QPOs with central frequencies of 0.21±0.2 Hz and
0.42 ±0.4 Hz are identified in the averaged power spectrum. A pair of
harmonic QPOs at frequencies of 1.41±0.04 Hz and 2.82 ±0.08 Hz
are found in one observation segment. The phase lags between
the hard photons and the soft photons have a flat distribution over
a wide range of frequencies. A high coherence of about 1.0 (0.01 - 1 Hz)
between the hard photons and the soft photons
is also obtained in our observations.
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On 16 Jul 1999, 09:19.