AstroSat/LAXPC reveals the high energy variability of GRS 1915+105 in the chi class

Abstract

We present the first quick look analysis of data from nine {\it AstroSat}'s LAXPC observations of GRS 1915+105 during March 2016 when the source had the characteristics of being in Radio-quiet $\chi$ class. We find that a simple empirical model of a disk blackbody emission, with Comptonization and a broad Gaussian Iron line can fit the time averaged 3--80 keV spectrum with a systematic uncertainty of 1.5\% and a background flux uncertainty of 4\%. A simple deadtime-corrected Poisson noise level spectrum matches well with the observed high frequency power spectra till 50 kHz and as expected the data show no significant high frequency ($> 20$ Hz) features. Energy dependent power spectra reveal a strong low frequency (2 - 8 Hz) Quasi-periodic oscillation (LFQPO) and its harmonic along with broad band noise. The QPO frequency changes rapidly with flux (nearly 4 Hz in ~ 5 hours). With increasing QPO frequency, an excess noise component appears significantly in the high energy regime (> 8 keV). At the QPO frequencies, the time-lag as a function of energy has a non-monotonic behavior such that the lags decrease with energy till about 15 -20 keV and then increase for higher energies. These first look results benchmark the performance of LAXPC at high energies and confirms that its data can be used for more sophisticated analysis such as flux or frequency-resolved spectro-timing studies.