On the Two Types of Steady Hard X‐Ray States of GRS 1915+105

Abstract

Using data from Ðve years of Rossi X-Ray T iming Explorer observations, we investigate the X-ray spectral and timing properties of GRS 1915]105 during the hard steady states. The broadband energy spectrum of the source during these periods is dominated by an extended hard component with a characteristic cuto † or break at D10È120 keV. The power density spectrum of the sourceÏs rapid aperiodic variability shows a dominant band-limited white-noise component breaking at a few hertz, accompanied by a group of strong quasi-periodic oscillation peaks and in some cases an additional high-frequency noise component with a characteristic cuto † at D60È80 Hz. According to the results of our simultaneous X-ray spectral and timing analysis, the behavior of the source during the hard steady states can be reduced to two major, distinct types. (1) Type I states : The dominant hard component of the energy spectrum has characteristic quasi-exponential cuto † at 60È120 keV. The broadband power density spectrum of the source shows a signiÐcant high-frequency noise component with a cuto † at D60È80 Hz. (2) Type II states : The hard spectral component has a break in its slope at D12È20 keV. The high-frequency part of the power density spectrum fades quickly, lacking signiÐcant variability at frequencies higher than D30 Hz. These two types of X-ray hard states are also clearly distinguished by their properties in the radio band : while during the type I observations the source tends to be "" radio quiet,ÏÏ the type II observations are characterized by a high level of radio Ñux ("" plateau ÏÏ radio states). In this work we demonstrate the aforementioned di †erences using data from 12 representative hard steady state observations. We conclude that the di †erence between these two types can probably be explained in terms of a di †erence in accretion Ñow structure in the immediate vicinity of the compact object due to the presence of relativistic outÑow of matter.