Ray Flaring

Flares are a very common type of variability observed on stars. During flares intensity outbursts with timescales of minutes up to several days go along with substantial increases of the temperature of the emitting plasma. Stellar flares are thought to be signatures of magnetic field reconnection. However, the picture for the origin of flares contains many unknowns. In one of the most favored scenarios, magnetic energy builds up as subphotospheric convective motions shuffle the footpoints of the magnetic loops around, thereby exerting stresses on the field lines. The energy stored in the magnetic field is then transported up into the corona and finally liberated in reconnection events [15, see e.g., for a review on solar and stellar flares.]

By their very nature, flares are restricted to magnetic stars, and therefore they are a characteristic feature of stars harboring a dynamo. Indeed, X-ray flaring is observed for almost all types of cool stars. Although flares on F and G stars are relatively rare,they are frequently observed on M dwarfs, RS CVn systems, and similar i i & i i i i i | i i i i i i i i i | i i i i i i

Fig. 10.2 left: Reconstructed spatial distribution of the flaring region on Algol B. All of the flaring plasma was assumed to be located at a fixed longitude of 0 = 70°. The solid circle represents the limb of the K star, units are in solar radius; dashed circles indicate heights in steps of 0.1 stellar radii. Fig. 4 from [44]; right: Yohkoh image of a solar limb flare on November 2, 1992 available under http://www.lmsal.com/SXT/. Limb flares on the Sun allow to study the geometry of the X-ray emitting plasma; the loop footpoints and the hot loop top are particularly well visible

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Fig. 10.2 left: Reconstructed spatial distribution of the flaring region on Algol B. All of the flaring plasma was assumed to be located at a fixed longitude of 0 = 70°. The solid circle represents the limb of the K star, units are in solar radius; dashed circles indicate heights in steps of 0.1 stellar radii. Fig. 4 from [44]; right: Yohkoh image of a solar limb flare on November 2, 1992 available under http://www.lmsal.com/SXT/. Limb flares on the Sun allow to study the geometry of the X-ray emitting plasma; the loop footpoints and the hot loop top are particularly well visible active stars. The range of observed stellar flare energetics extends over at least eight orders of magnitude with the largest reported events exceeding 1037 erg in released X-ray energy, i.e, larger than the strongest solar flares by five orders of magnitude.

One of the largest stellar flares ever observed was studied with the BeppoSAX satellite on the eclipsing binary Algol [8]. In Fig. 10.3, we show the BeppoSAX lightcurve of Algol in terms of the orbital phase of the binary. The 2-10 keV lightcurve recorded with the MECS instrument on board BeppoSAX is dominated by a huge flare starting at phase 0 ~ 1.0. This phase corresponds to the instance when the cool secondary is seen in front of the primary B star obscuring it, i.e., at secondary eclipse. The flare rise took about 8.3 h, then the lightcurve first decayed rather rapidly until, at 0 ~ 1.25, a basically exponential decay began. This light curve morphology is in fact quite typical for the decay of solar long duration flares (such as the flare on November 2,1992), which, however, do not extend over 2 days. Around 0 = 1.5 the presumably X-ray dark primary is in front of the X-ray emitting secondary and thus the dip in the lightcurve observed at the same phase must clearly be interpreted as an eclipse of the flaring plasma by the early-type primary. A clear flare-related signal is also seen in the PDS detectors and spectral analysis shows evidence for X-ray emission out to 80 keV. This is a record for the highest energy photons received from stellar X-ray sources. Interestingly, spectral analysis shows full consistency of the recorded spectrum with a thermal source and no evidence for nonthermal X-ray emission. Contrary, large solar flares are always accompanied by

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nonthermal hard X-ray and y-ray emission. By analogy, one expects the same for stellar flares. However, so far no convincing examples of nonthermal emission from stellar flares have been presented at X-ray wavelengths, although nonthermal radio emission has been observed.

Flares come in a wide variety of sizes and shapes [see, e.g., 13 for a summary]. Light curves as the one of EQ Peg shown in Fig. 10.4 are typical for flares on dMe stars. This event was observed simultaneously in soft X-rays and in the optical B band. The observed optical emission in such flares is considered to be a proxy of the nonthermal particles presumably generated in the first phases of the flare. These accelerated particles penetrate into the deeper photospheric layers, leading to the observed impulsive phase emission. This is followed by thermal soft X-ray emission produced by photospheric and chromospheric material heated and evaporated into the corona. Typical delays between optical and soft X-ray peaks are of the order of a few minutes.

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