The radio monitoring of the microquasar GRS 1915+105 with FAST

Abstract

We report radio observations of the Galactic microquasar GRS 1915+105 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the band of 1.0−1.5 GHz. The observations were performed in 2020 with a total duration of ∼ 3.5 hours. In this work, we developed the pipeline to perform the calibration of the radio flux density and polarization for the persistent emitting radio source GRS 1915+105. The flux densities of the source during each observation are relatively stable, with the maximum fluctuation in flux density less than 40 mJy in one observation. And the linear polarization is between 20-30%, the polarization position angle is around 89.5∘, and the circular polarization is small and likely to be slightly left polarized. With the linear fitting of the polarization angles, we get a flat rotation measure of ∼ -0.5 rad⋅m−2. Meanwhile, the derived spectral index is near a constant for each observation of α∼−(0.4−0.3), indicating an optically thick emission source. The small variations of flux densities and flat rotation measure imply that GRS 1915+105 is likely in a plateau state during the FAST observations.