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

Abstract

Abstract To understand the nature of supercritical accretion, we systematically analyzed the RXTE/PCA data of GRS 1915$+$105 in its quasi-steady states, by choosing data with small variability during 1999–2000. We applied a multicolor disk plus a thermal Comptonization model, and took into consideration accurate interstellar absorption, a reflection component (with an iron-K emission line), and absorption features from the disk wind self-consistently. The total luminosity ranges from $\sim $0.2$L_{\rm E}$ to slightly above $L_{\rm E}$. There is a strong correlation between the inner disk temperature and the fraction of the disk component. Most of the Comptonization-dominated ($\gt$50% total flux) spectra show $T_{\rm in} \sim $ 1 keV with a high electron temperature of $\gt$10 keV, which may correspond to the very high state in canonical black hole X-ray binaries (BHBs). In contrast, the disk-dominated spectra have $T_{\rm in} \sim $ 2 keV with a low temperature ($<$10 keV) and optically thick Comptonization, and show two separate branches in the luminosity vs. innermost temperature ($L$–$T_{\rm in}$) diagram. The lower branch clearly follows the $L \propto T_{\rm in}^4$-track. Furthermore, by applying the extended disk blackbody (or $p$-free disk) model, we found 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 the source 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 the spectral hardening factor ($\sim $4) that would be required for a non-rotating black hole. The spectra in the upper branch are consistent with the picture of a truncated disk with low-temperature Comptonization. This state was uniquely observed from GRS 1915$+$105 among BHBs, which may be present at near-Eddington luminosity.