高能暂现源 · 每日文献综述

We analysed Nuclear Spectroscopic Telescope Array data of GRS 1915 + 105 (2012─2024) to trace its outburst evolution and clues to the physical processes involved through different phases of intensity, especially those related to the radiation pressure instability. We examined its 1-s binned light curves and observation-long spectra to investigate the short- and long-term variability of GRS 1915 + 105, as an extension of the previous studies, which did not cover the fainter period that started in mid-2018. We assigned a new spectral group of variability, 'states group 2' (SG2), which consists of known classes, <inline-formula><tex-math>$\gamma$</tex-math></inline-formula> and <inline-formula><tex-math>$\omega$</tex-math></inline-formula>, variation of <inline-formula><tex-math>$\phi$</tex-math></inline-formula> (<inline-formula><tex-math>$\phi _1$</tex-math></inline-formula>), and an unclassified feature 'uc'. SG2 is evident when GRS 1915 + 105 entered its most luminous state, when its innermost part of the accretion disc transformed to a slim disc state. Other previously known peculiar classes, <inline-formula><tex-math>$\kappa$</tex-math></inline-formula> and <inline-formula><tex-math>$\rho$</tex-math></inline-formula>, can be considered as the transition class between standard and slim disc, which act as signatures of the starting phase of radiation pressure instability. Only the basic variability classes (<inline-formula><tex-math>$\chi$</tex-math></inline-formula> and <inline-formula><tex-math>$\phi _2$</tex-math></inline-formula>), which represent the standard disc, were commonly found in other black hole X-ray transients (e.g. GX 339─4, MAXI J1820 + 070, and 4U 1543─475). However, a class that represents the slim disc was found in 4U 1543─475, possibly due to both GRS 1915 + 105 and 4U 1543─475 having successfully reached a luminosity of <inline-formula><tex-math>$\ge 0.49\ L_{\rm Edd}$</tex-math></inline-formula>. Our analysis of the fainter period (mid-2018─2024) provides more evidence that GRS 1915 + 105 has not slipped into a quiescent phase.