Binary phase correlated X-ray intensity variations and flaring activity in the RS CVn binary HR 1099

Abstract

A study of X-ray variability of the RS CVn binary HR 1099 has been carried out using observations made with the monitor proportional counter (MPC) and the imaging proportional counter (IPC) on board the Einstein Observatory. This star, with a total exposure of 8×104s, was observed once simultaneously with the MPC and the IPC covering all binary phases and with the MPC at three other times at different binary phases. Evidence is found for the variation of the X-ray intensity with the 2.837 day period of the binary. The extent of the correlated variations is ∼30 per cent in the IPC and ∼100 per cent in the MPC. The phase of the X-ray maximum coincides with that of the minimum of the optical wave and the X-ray minimum occurs close to the optical wave maximum reported by Bartolini et al. at the epoch 1979.11, which is close to the epoch of the X-ray observations. An intense X-ray flare, having rise time of |$\geqslant 16$| min, decay time of 76min, and total 2−10 keV X-ray energy release of |$\approx5\times10^{34}$| erg, was detected on 1980 February 17. The X-ray spectrum near the flare peak was hard and is best described by a thermal spectrum with |$kT=11.3^{+2.6}_{-1.8}\enspace\text {keV}$| compared to a quiescent state value of 2.0±0.4 keV. A second but less intense flare having decay time of > 33 min was detected on 1979 January 31. The binary phase correlated variations are explained in terms of the star spot model. From the measured values of the X-ray flare parameters and using the coronal loop model, it is concluded that the flare heated plasma cools either mainly by radiation or equally by conduction and radiation.