Simultaneous Observations of GRS 1758-258 in 1997 by VLA, IRAM,
SEST, RXTE and OSSE: Spectroscopy and Timing
Lin, D., Smith, I. A., Liang, E. P., Bridgman, T., Smith, D. M.,
Marti, J., Durouchoux, Ph., Mirabel, I. F., & Rodriguez, L. F.
We report the results of our multiwavelength observations of GRS 1758-258
made in 1997 August. The energy bands include radio, millimeter, X-ray, and
gamma-ray. 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 fit by the Sunyaev-Titarchuk
(ST) Compton scattering model with a plasma temperature of 45 keV and a
Thomson depth of 3.3. Taking relativistic effects into account, we get a
little higher plasma temperature (52 keV) by using the improved version of
the ST model (HT model) plus a soft blackbody component. The spectrum is
also fit by a power law with an exponential cutoff (PLE) plus a soft
blackbody component. The temperature of the soft components in both models
is about 1.2 keV, and the energy flux is less than 1.5% of the total X-ray
and gamma-ray flux. The deduced hydrogen column density is in the range of
(0.93 - 2.0) x 10^22 cm^-2. No significant iron lines are detected. The
radio emission has a flat energy spectrum. 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 timescales in X-rays
and gamma rays. The power density spectra are typical for the low state, but
we find the photon flux for the 5 - 10 keV band to be more variable
than that in the other two energy bands (2 - 5 keV and 10 - 40
keV). Harmonically spaced quasi-periodic oscillations are observed in the
power spectra. 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. We compare these results with
two variation models. Our millimeter observations did not reveal any
conclusive signatures of an interaction between the jet from GRS 1758-258
and the molecular cloud that lies in the direction of GRS 1758-258.
Status:
2000, ApJ, 532, 548.
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