Probing the Peculiar Behavior of GRS 1915+105 at Near-Eddington Luminosity

TL;DR

This study analyzes RXTE data of GRS 1915+105 during near-Eddington luminosity, revealing unique accretion states, disk structures, and spectral behaviors that differ from typical black hole binaries, highlighting the peculiar nature of this source.

Contribution

It provides a detailed spectral analysis of GRS 1915+105 at high luminosity, identifying two distinct accretion states and suggesting the presence of a slim disk and a rapidly spinning black hole.

Findings

Identification of two spectral branches with different disk properties.

Evidence for a slim disk state at high luminosity.

Spectral hardening factor around 4 indicating high spin.

Abstract

To understand the nature of supercritical accretion, we systematically analyze the {\it RXTE}/PCA data of GRS 1915+105 in its quasi-steady states, by choosing data with small variability during 1999 -- 2000. We apply a multicolor disk plus a thermal Comptonization model and take into consideration accurate interstellar absorption, a reflection component, and absorption features from the disk wind self-consistently. There is a strong correlation between the inner disk temperature and the fraction of the disk component. Most of the Comptonization-dominated spectra show $T_{\rm in} \sim 1$ keV with a high electron temperature of $>10$ keV, which may correspond to the very high state in canonical black hole X-ray binaries (BHBs). By contrast, the disk-dominated spectra have $T_{\rm in} \sim 2$ keV with a low temperature ($<10$ keV) and optically thick Comptonization, and show two separate branches in the ($L$ -- $T_{\rm in}$) diagram. The lower branch clearly follows the $L \propto T_{\rm in}^4$-track. Furthermore, applying the extended disk blackbody model, we find that 9 out of 12 datasets with disk luminosity above $0.3 L_{\rm E}$ prefer a flatter temperature gradient than that in the standard disk ($p < 0.7$). We interpret that, in the lower branch, the disk extends down to the innermost stable circular orbit, and is most probably in the slim disk state. A rapidly spinning black hole can explain both the lack of the $L \propto T_{\rm in}^2$-track and a high value of spectral hardening factor ($\sim 4$) that would be required for a non-rotating black hole. The spectra in the upper branch are consistent with the picture of a truncated disk with low temperature Comptonization. This state is uniquely observed from GRS 1915+105 among BHBs, which may be present at near-Eddington luminosity.