Dimming GRS 1915+105 observed with NICER and Insight–HXMT

Abstract

The black hole X-ray binary GRS 1915+105 was bright for 26 years since its discovery and is well known for its disk instabilities, quasi-periodic oscillations, and disk wind signatures. We report a long-term spectral-timing tracing of this source from mid-2017 until the onset of the so-called obscured state based on the complete data from the Neutron Star Interior Composition Explorer (NICER) and the Insight–Hard X-ray Modulation Telescope (HXMT), whose hard coverage decisively informs the modeling at lower energies. In the soft state predating 2018, we observed highly ionized winds. However, in the hard state shortly before transitioning into the obscured state on May 14, 2019 (MJD 58617), the winds exhibited a discernible reduction in ionization degree (log ξ), which decreased from above 4 to approximately 3. Our analysis involves the measurement of the frequencies of the quasi-periodic oscillations and the estimation of the properties of the ionized winds and the intensities of different spectral components through spectroscopy during the decay phase. We studied the origin of these infrequently observed warm outflows in the hard state. The launching radius of the winds in the hard decay phase is similar to that in the soft state, which indicates that the launching mechanism of these winds likely is the same in both states. The presence of the ionized winds is preferentially dependent on the periphery of the accretion disk, but it is not directly related to the corona activities in the center of the binary system.