Exploring the broadband spectral and timing characteristics of GRS 1915+105 with AstroSat and NICER observations

TL;DR

This study analyzes the spectral and timing properties of the black hole binary GRS 1915+105 using simultaneous AstroSat and NICER observations, revealing complex variability features and emphasizing the need for advanced models.

Contribution

It presents a combined spectral-timing analysis of GRS 1915+105 with simultaneous AstroSat and NICER data, proposing a model where coronal heating variations drive observed spectral-timing features.

Findings

Detection of a broad 2 Hz feature in power spectra.

Energy dependence of R.M.S. and time lag suggests complex coronal behavior.

Simultaneous observations are crucial for understanding black hole variability.

Abstract

In this study, we undertake a spectral-timing analysis of the black hole X-ray binary source GRS 1915+105 using simultaneous observations carried out by AstroSat (LAXPC and SXT) and NICER in 2017. The source showed two flux levels (high and low), whose energy spectra can be described by the thermal comptonization of disk photons. The spectral parameters obtained by the joint fitting of SXT/LAXPC and NICER/LAXPC were consistent. The power density spectra from LAXPC and NICER revealed a broad, prominent feature at approximately 2 Hz. The energy dependence of the fractional R.M.S. and time lag of this feature cannot be explained by only variations of coronal spectral parameters. Instead, a model where the coronal heating rate varies first and induces a change in the disk temperature and inner radius can explain the variation. Our results underline the importance of simultaneous observations by AstroSat and NICER and highlight the need for more sophisticated models to explain the spectral-temporal behavior of black hole systems.