Phase Lag Variability Associated with the 0.5–10 Hz Quasi‐Periodic Oscillations in GRS 1915+105

Abstract

We have used Rossi X-ray Timing Explorer data to measure the lags between soft (2-5 keV) and hard (5-13 keV) photons and to study the aperiodic variability of the superluminal black hole candidate GRS 1915+105 during low-flux states. The power density spectra exhibit quasi-periodic oscillations (QPO) whose frequency increases with increasing count rate and varies in the frequency range 0.6-8 Hz. A correlation between the QPO frequency and the phase lag spectra is reported for the first time. This correlation is found for both the phase lag continuum and the phase lag at the QPO frequency. We find that as the QPO frequency moves to higher values the phase lags reverse sign from positive to negative. The absolute value of the lag always increases with photon energy. The negative (soft) lags are associated with a softer energy spectrum, whereas the positive (hard) lags are seen when the source is harder. We describe a possible scenario that may account for the change in the sign of the lags.

Wide-band X-ray variability of GRS 1915+105 observed with BeppoSAX

“…This effect suggest that there is a delay like the emission would be due to different components. Such behavior is reminiscent of that observed in the so called plateau intervals, during which a QPO frequency lower than 2 Hz is observed (Reig et al 2000), and interpreted by Nobili et al (2000) in terms of a comptonization model. Another possibility is that the non-regular mode is associated with a greater oscillation amplitude of the innermost region of the accretion disk where higher temperatures can be reached.…”

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“…The second and most obvious di †erence between the timing properties of these three observations is in the Fourier phase lags. In region 2 (between the low-frequency break and the QPO), the phase lags are negative when the radio emission is faint (left and right panels) and positive during radio plateau emission (center panels ; compare Reig et al 2000). This behavior is in sharp contrast to sources such as GX 339[4 and Cyg X-1, for which the phase lags are always positive.…”

“…The thermalized photons are then Compton downscattered in the cool corona, which produces soft lags. This model does not attempt to explain the phase lags as a function of Fourier frequency, but it does predict that if the QPO frequency tracks the radius of the inner disk, negative phase lags are expected when the QPO frequency is high, exactly as Reig et al (2000) observed in GRS 1915]105. However, a corona with an optical depth of q [ 100 would produce a thermal spectrum, while the energy spectrum of GRS 1915]105 between 25 and 200 keV cannot be described by an extremely optically thick spectrum (e.g., a spherical Compton cloud with optical depth q [ 5 ; Sunyaev & Titarchuk 1980) when the phase lags are negative.…”

“…Additional timing characteristics, such as the Fourier phase lags and the coherence function, may further constrain the relationship between hard and soft X-ray components. Reig et al (2000) have found that the signs of the phase lags change from positive to negative as the frequency of the 0.5È10 Hz QPO increases, which has provided impetus for others (e.g., Nobili et al 2000) to develop models describing the structure of the Comptonizing corona and how it is related to the optically thick accretion disk.…”

On the origin of the low-frequency QPO in GRS 1915+105 ρ state

“…The LFQPO amplitude is inversely correlated with the source flux or LFQPO frequency (e.g., Muno et al 1999;Reig et al 2000). As the LFQPO frequency increases, the temperature of the inner accretion disk increases and the radius of the inner accretion disk decreases (e.g., Rodriguez et al 2002a).…”