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Astron. Astrophys. 344, 154-162 (1999)

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1. Introduction

After the first stellar X-ray flares were discovered less than 25 years ago on dMe stars (Heise et al. 1975) it took almost another decade until the Einstein observatory (EO ) detected similar events on young T Tauri Stars (TTS) (Montmerle et al. 1983). Nowadays, X-ray flares are known to be entertained on stars all over the H-R diagram (see Pettersen 1989 for a review). The timescales and energetics involved in flare events on different types of stars vary strongly consistent with the observation that the level of X-ray activity decays with age.

TTS are late-type pre-main sequence stars with typical age of [FORMULA] yrs and rank among the most active young stars: energy outputs of up to [FORMULA] times the maximum X-ray emission observed from solar flares have been reported from TTS outbursts. Some of the largest X-ray flares ever observed were discovered by ROSAT on the TTS LH[FORMULA] 92 and P1724 (Preibisch et al. 1993, Preibisch et al. 1995). The energy released in these events ([FORMULA]) exceeds that of typical TTS flares by two orders or magnitude. Before the detection of these giant events, the record of X-ray luminosity was held for more than 10 years by the TTS ROX-20, where [FORMULA] were measured during an EO observation in February 1981 (Montmerle et al. 1983). A superflare from the optically invisible infrared Class I protostar YLW 15 in [FORMULA] Oph was presented by Grosso et al. (1997), with the intrinsic X-ray luminosity over the whole energy range being [FORMULA] to [FORMULA] erg/s, depending on the foreground absorption which is known to lie somewhere between 20 and 40 mag.

Although no model has been found yet that explains all aspects of flaring activity, the basic picture of all flare scenarios is - in analogy to the sun - that of dynamo driven magnetic field loops that confine a hot, optically thin, X-ray emitting plasma (see Haisch et al. 1991 for a summary of flare phenomena). Quasi-static cooling of such coronal loops has been described by van den Oord & Mewe (1989). An analysis of single flare events is of interest to determine physical parameters of the flaring region such as time scales, energies, temperature and plasma density, and ultimately decide whether coronal X-ray emission of TTS is scaled-up solar activity, or whether interaction between the star and either a circumstellar disk or a close binary companion are partly responsible for the X-ray emission.

In this paper we select a sample of four X-ray observations (three of TTS and one of Algol) which are in conflict with the standard modeling of the lightcurve as either a flare characterised by a quick rise and subsequent exponential decay or as simple sine-like rotational modulation of the quiescent emission. In the latter case X-ray emission would be larger when the more X-ray luminous area is on the front side of the star (directed towards the observer). Such kind of rotationally modulated emission was observed in the TTS SR 12 in [FORMULA] Oph by Damiani et al. (1994).

We propose that the untypical shape of the X-ray lightcurves we present is due to a flare event modulated by the rotation of the star. Skinner et al. (1997) suggested rotational occultation of an X-ray flare to explain the broad maximum and slow decay of a flare on V773 Tau observed by ASCA . While they model their data by fitting a sine function to the lightcurve without allowing for an exponential decay phase (similar to Damiani et al. 1994), we start out from a decaying flare and modify it by a time varying volume factor. By this approach we take into consideration that the flare might be occulted by the star during part of the observation and we are able to estimate the decay timescale of the lightcurve [FORMULA] and the size of the emitting loop. Such a model was first suggested by Casanova (1994), and Montmerle (1997) classified the corresponding flare event as `anomalous'. A rotationally modulated flare was also mentioned as possible interpretation of a flare-like event in P1724 (Neuhäuser et al. 1998), shown in our Fig. 6, who advertised the more detailed and quantitative treatment that we present in this paper.

The outline of our presentation is as follows: In Sect. 2 we introduce the X-ray observations that we chose in view of the untypical broad maximum of their lightcurves. A model that describes modulations of X-ray flares by the rotation of the star is presented in Sect. 3. In Sect. 4 we explain the structure of the lightcurves from the observations introduced in Sect. 2 by applying our model, and we summarize the results in Sect. 5.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999