2. The observations
A summary of the observations analysed in this paper is given in Table 1. The observations were obtained with different instruments onboard the X-ray satellites ROSAT , Ginga , and ASCA . For information about the ROSAT instruments, the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI), we refer to Tr"umper (1982). The Ginga Large Area Counter (LAC) has been described by Turner et al. (1989), and a description of ASCA and its instrumentation can be found in Tanaka et al. (1994).
Table 1. Flare observations
The classical TTS (CTTS) SR 13 was detected in X-rays by Montmerle et al. (1983). It is located in the cloud at position , . On a speckle imaging survey SR 13 was discovered to be a binary system (Ghez et al. 1993) with separation, which remains unresolved in the PSPC observation of 1991 September 07/08 we present here. The period of SR 13 is unknown. However, as shown below, a period of days is consistent with the rotating X-ray flare model.
P1724 is a weak line TTS (WTTS) located 15 arc min north of the Trapezium cluster in Orion (, ). Neuh"auser et al. (1998) confirm the rotational period of 5.7 d first discovered by Cutispoto et al. (1996) applying two independent numerical period search methods on the V-band lightcurve. In addition, they report systematic variations of the X-ray count rate of P1724 during an observation with the ROSAT HRI in October 1991. However, they cannot find any rotational modulation in the X-ray data. Neuh"auser et al. (1998) also find no indications for a circumstellar disk nor a close binary companion.
The WTTS V773 Tau is a double-lined spectroscopic binary (Welty 1995) which is located in the Barnard 209 dark cloud at optical position (, ). Upper limits for the rotation period derived by Welty (1995) are and for the K2 and K5 components respectively. These estimates are somewhat lower than the values previously reported by Rydgren & Vrba (1983). In the ASCA observation obtained on 1995 September 16/17 the V773 Tau binary system is not resolved from the classical TTS (CTTS) FM Tau, which lies at an offset of . For a more detailed discussion of this ASCA observation we refer to Skinner et al. (1997).
Algol is a triple system, with an inner close binary of period 2.87 d comprising a B8 V primary (Algol A) and an evolved K2 IV secondary (Algol B). In January 1989 Ginga observed a large flare from the Algol system (presumably Algol B; Stern et al. 1992). The shape of the Algol lightcurve resembles that of the TTS flares discussed before. Therefore, we decided to include this observation in our sample, although the Algol system represents a different class of flare stars.
We use the Extended Scientific Software Analysis System (EXSAS, Zimmermann et al. 1995) to analyse the two ROSAT observations, which were obtained from the ROSAT Public Data Archive. To take account of possible time variations in the background, the background count rate was computed for each satellite orbit and subtracted accordingly from the measured lightcurve. Ginga data were kindly supplied to us in computer readable format by Bob Stern, while Steve Skinner provided us the ASCA lightcurve of V773 Tau.
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999