LOOP LENGTH AND MAGNETIC FIELD ESTIMATES FROM OSCILLATIONS DETECTED DURING AN X-RAY FLARE ON AT MIC

AbstractWe analyse oscillations observed in the X-ray lightcurve of the late-type star AT Mic. The oscillations oc-curred during flare maximum. We interpret these oscilla-tions as density perturbations in the flare loop. Applyingvarious models derived for the Sun, the loop length andthe magnetic field of the flare can be estimated. We finda period of 740 s, and that the models give similar results(within a factor of 2) for the loop length (∼ 5.4 10 10 cm)and the magnetic field (∼ 100 G). For the first time, anoscillation of a stellar X-ray flare has been observed andresults thus obtained for otherwise unobservable physicalparameters.Key words:Stars:coronae– Stars: flare– Stars:individual:AT Mic – Stars: late-type – Stars: magnetic fields – Stars:oscillations – X-rays: stars1. IntroductionOscillations in the solar corona have been observed formany years. Wavelength regimes ranging from hard X-rayright down to radio have been investigated to search forevidence of waves. Periods have been found ranging from0.02 to 1000 s. Table 1 in Aschwanden et al. (1999) pro-vides an excellent summary of the different periods thathave been found, and an explanation for their existence.Most of these waves have been explained by MHD oscil-lations in coronal loops. Roberts (2000) has provided anexcellent review of waves and oscillations in the corona.Many of the observations of waves have been deter-mined from variations in intensity brightness. Recently,however, a huge step forward has been achieved in solarcoronal physics due to the high spatial resolution avail-able with the Transition Region and Coronal Explorer(TRACE). The first spatial displacement oscillations havebeen observed in coronal loops (Aschwanden et al. 1999).It was suggested that these oscillations were triggered bya disturbance from the core flare site. Various MHD waveswere investigated and it was found that a fast kink modewave provides the best agreement with the observed pe-riod of 280±30s.One of the most exciting aspects of observing wavesin this fashion is that it potentially provides us with thecapability of determining the magnetic field in the corona.It is notoriously difficult to measure the magnetic field inthe corona. Techniques using the near-infrared emissionlines have been successful, but have poor spatial resolu-tion. Most frequently, indirect methods are used such asthe extrapolations of the coronal magnetic field from thephotospheric magnetic field that can be measured usingZeeman splitting. Nakariakov & Ofman (2001) made useof the flare-related spatial oscillations sometimes observedwhen a flare occurs, to determine the magnetic field. Theyassume that the oscillation is due to a global standing kinkwave, and hence the magnetic field is related to the periodof the oscillation, the density of the loop, and the length ofthe loop. In the case of Nakariakov & Ofman (2001) theyfound the magnetic field to be 13±9G. The errors on thisare large due to errors on the determination of density,loop length and period. Schrijver & Brown (2000) suggestthat the oscillations are due to a sudden displacement ofthe magnetic field at the surface which causes an oscilla-toryrelaxationofthe field. RecentworkbyNakariakov et al. (2004)suggests that the oscillations are second standing harmon-ics of acoustic waves tied to the loop length. The modelgives a relationship between the oscillation period, theloop length and the average plasma temperature, all ofwhich can be independently observed on the Sun.Waves are observed across the electromagnetic spec-trum on the Sun. However, observations on other stars arerare. One reason for this is that on the Sun, we have thespatial resolution to zoom in on small regions. For exam-ple, the transverse TRACE loops that have been observedare away from the main flare site, and hence have loweremission measure. Analysing an X-ray light-curve, for ex-ample, would be unlikely to provide evidence of waves ifthe emission was averaged over the whole Sun. Oscilla-tions have been observed for optical stellar flares (e.g.,Mathioudakis et al. 2003andMullan et al. 1992). Mathioudakis etfound a period of 220 s in the decay phase of a white-lightflare on the RS CVn binary II Peg. Using this they de-termine a magnetic field of 1200 G. Mullan et al. (1992)again found oscillations in X-ray active red dwarfs. Theycompared their observations to the possibility that theywere observing p-modes. On the Sun p-modes give rise to5-minute oscillations on the surface. They concluded thatp-modes could not produce the amplitudes they observed