[Paper Review] The 1-12 Hz QPO and dips in GRS 1915+105: tracers of Keplerian and viscous time scales?

S. Trudolyubov, E. Churazov+1 more|arXiv (Cornell University)|Oct 4, 1999

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper investigates the connection between 1–12 Hz quasi-periodic oscillations (QPOs) and spectral dips in the microquasar GRS 1915+105, proposing that QPO frequency variations trace the viscous time scale through changes in the inner disk boundary. The authors find a tight correlation between QPO frequency and hard spectral episode duration, consistent with viscous evolution timescales at radii where Keplerian frequency matches the QPO frequency, suggesting the QPO arises from viscously driven modulations of the disk-comptonization boundary.

ABSTRACT

We analysed 9 RXTE/PCA observations of GRS 1915+105 in the flaring state, when the hardness of the source spectrum was changing on the time scales from few seconds up to 1000 seconds. The quasi periodic oscillations (QPOs) with the frequency varying between ~ 1 and 12 Hz are associated with the episodes of harder source spectrum. In each observation we found tight correlation between the duration of the hard episode and the characteristic QPO frequency. For a half of the observations this correlation matches the relation between the viscous time scale and the Keplerian frequency, when both quantities are evaluated for various radii in the radiation pressure dominated accretion disk. Assuming that the QPO frequency is proportional to the Keplerian frequency at the boundary between an optically thick accretion disk and a hot comptonization region, the changes of the QPO frequency can then be understood as due to variations of this boundary position on the viscous time scales.

Motivation & Objective

• To understand the physical origin of 1–12 Hz quasi-periodic oscillations (QPOs) in the microquasar GRS 1915+105.
• To investigate the relationship between QPO frequency and the duration of hard spectral states observed in RXTE/PCA data.
• To test whether the observed QPO frequency evolution correlates with viscous timescales in a radiation-pressure-dominated accretion disk.
• To determine if the QPO frequency is linked to the Keplerian frequency at the boundary between the cold disk and hot Comptonizing region.
• To explore whether the QPO mechanism is driven by viscous evolution of the inner disk boundary.

Proposed method

• Analysis of 9 RXTE/PCA observations of GRS 1915+105 in the flaring state, focusing on spectral hardness and timing variability.
• Measurement of the duration of hard spectral episodes and correlation with the associated QPO frequency.
• Calculation of viscous timescale τ_vis = r² / (3ν) and Keplerian frequency f_K = (GM/r³)¹ᐟ² for various radii in a radiation-pressure-dominated accretion disk.
• Comparison of observed QPO frequency–duration correlation with theoretical relations between viscous time scale and Keplerian frequency.
• Assumption that QPO frequency is proportional to Keplerian frequency at the inner disk boundary, implying the QPO traces viscous evolution of the boundary position.
• Use of spectral and timing data to infer the location of the transition between the optically thick disk and the hot Comptonizing corona.

Experimental results

Research questions

• RQ1 Is there a correlation between the duration of hard spectral states and the frequency of 1–12 Hz QPOs in GRS 1915+105?
• RQ2 Can the observed QPO frequency evolution be explained by viscous time scales in the inner accretion disk?
• RQ3 Does the QPO frequency track the Keplerian frequency at the inner disk boundary, suggesting a physical link to disk dynamics?
• RQ4 Is the observed correlation between QPO frequency and hard state duration consistent with viscous evolution of the disk boundary?
• RQ5 What physical mechanism could produce the observed QPOs, and how does it relate to the accretion disk structure?

Key findings

• A tight correlation was found between the duration of hard spectral episodes and the QPO frequency in half of the analyzed RXTE observations.
• The observed correlation between QPO frequency and hard state duration matches the theoretical relation between viscous time scale and Keplerian frequency for radii in a radiation-pressure-dominated accretion disk.
• The QPO frequency is interpreted as proportional to the Keplerian frequency at the boundary between the cold disk and the hot Comptonizing region.
• Variations in QPO frequency are explained as due to viscous evolution of the inner disk boundary position over viscous timescales.
• The results support a model in which the QPO arises from modulations driven by viscous processes at the inner edge of the accretion disk.
• The study provides observational evidence linking QPOs to fundamental disk timescales, suggesting a physical connection between QPOs and disk accretion physics.