Simultaneous Observations of GRS 1758-258 in 1997 by VLA, RXTE and OSSE: Spectroscopy and Timing

D. Lin, I. A. Smith, E. P. Liang

T. Bridgman

D. M. Smith

J. Martí

I.F. Mirabel

L.F. Rodrí guez

Abstract

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 0.3490.003, 0.271 0.004, and 0.225 0.005 respectively. Two harmonic QPOs with central frequencies of 0.210.2 Hz and 0.42 0.4 Hz are identified in the averaged power spectrum. A pair of harmonic QPOs at frequencies of 1.410.04 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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