Short-timescale X-ray variability of the blazar 1ES 1959+650 observed with AstroSat/SXT

Abstract

We analyse the AstroSat Soft X-ray Telescope (SXT) observation of the high-synchrotron-peaked blazar 1ES 1959+650 taken on 2023 September 2, which covers 362 ks with about 95 ks of good exposure. We build 0.6-7 keV light curves in six sub-bands, binned at 5830 s (the orbital period) and 1000 s. All sub-bands show excess variance, with fractional variability of 3.7 to 6.5 per cent that increases with energy ($\propto E^{0.32\pm0.07}$). The power spectrum of the total band, fitted with a model that includes the window function of the orbital gaps, is a red-noise power law with slope $-1.80$ (95 per cent range $-2.8$ to $-1.4$). A Lomb-Scargle search finds no significant periodicity (global $p>0.57$) and would have detected, with 90 per cent probability, a sinusoid with an rms amplitude above 1.9 to 3.7 per cent at periods of 3 to 50 ks. The soft (0.6-1.4 keV) and hard (1.9-7 keV) light curves are correlated near zero lag, but only marginally (global $p=0.04$ and 0.07 for power-spectrum slopes of $-1.7$ and $-2.0$), and the lag is $-1^{+6}_{-10}$ ks, consistent with zero. No pair of sub-bands that includes 0.6-0.8 keV shows a significant correlation or lag, so an energy-dependent lag of a few ks can be neither confirmed nor excluded. The 0.6-7 keV light curve contains one hump with rise and decay times of about 0.4 to 0.6 d, which is only marginally distinguishable from red noise ($p\approx0.05$ to $0.07$). The count rate distribution is consistent with a lognormal distribution, and the hardness ratio does not depend significantly on flux.