EXOSAT observations of the suspected flare star BD + 48°1958A

Abstract

We present the first X-ray observations of the BY Dra type of variable BD + 48 ° 1958A using the low-energy (LE) and the medium-energy (ME) detectors of Exosat observatory. The star is detected in both the LE and the ME detectors, and we find some evidence for a long-duration (< 2-hr) X-ray flare. We have carried out a combined spectral fit to the LE and ME data and derived an X-ray plasma temperature of 1.6 ± 0.4 keV during the quiescent state and a temperature of 5.0 ± 1.4 keV during the flare, using the plasma emission models given by Raymond and Smith. The X-ray luminosity in the 0.1-4.0 keV energy band is 2.7 × 10²⁹ ergs s^-1 during the quiescent state, and a nearly 20 % increase in the X-ray flux is detected during the flare. The X-ray luminosity and the temperature are typical of a dMe type flare star. We suggest that (a) BY Dra type of variability is conducive to flare activity and (b) the study of X-ray emission from BY Dra type of variables is an important tool for understanding the flare activity among them.