High-Energy Stellar, Protostellar, and Protoplanetary Physics
Magnetic Flares: Prototypes of Energy Lifecycles and Release in the Universe
Simulated XMS spectra in the region of the He-like Fe and Ar complexes for a 25-ks observation of AD Leo. The simulations assumed that the plasma was heated by continuous flaring with upflows in the range of 0-200 km s-1. Understanding flares occurring under these conditions (much more extreme than in the Sun) will help bridge the gap to the conditions around black hole accretion disks.
Throughout the Universe – in stars, accretion disks, interstellar and intergalactic media, for example – magnetic fields store energy imparted by mechanical turbulent or convective plasma motions. This energy can be released again through magnetic reconnection events, whereby the plasma is spontaneously heated and accelerated. Our Sun provides dramatic examples of this process through solar flares. Solar flares and CMEs pose a hazard to both manned and unmanned spacecraft, and understanding them is important for predictive and preventative measures. However, magnetic reconnection, flares and coronal mass ejections remain quite poorly understood: What we know is based on what has been learned from the Sun, yet even current models fail to match some salient aspects of solar flares (Doschek 1990, Feldman 1990). In particular, blushifted emission expected from upflowing evaporation of the chromosphere heated by protons and electrons accelerated in the reconnection event is inadequate to explain the flare emission and sometimes appears prematurely at flare onset. The temporal relationship between hard X-rays (due to the accelerated particles) and soft X-rays (due to chromospheric evaporation) is often not satisfied either, in that soft X-rays can commence before the hard X-ray burst.
Time series analyses of EUV and X-ray observations of active stars have provided evidence that plasma at temperatures >= 4 × 106 K arises purely from flares, analogous to the idea of "nanoflare" theories of solar coronal heating (Parker 1988, Guedel 1997, Audard et al. 2000, Kashyap et al. 2002). Constellation-X will provide a sensitive test of flare heating through both Doppler shifts and photon arrival times. A Constellation-X XMS effective area of 6,000 cm2 at E 3-6.5 keV and resolving power of E/ΔE > 1,000 brings within reach Doppler diagnostics in H-like and He-like S (λ4.73, 5.04), Ar (λ 3.95, 3.73) and Fe (λ1.85). A simulation of a 25 ks XMS observation of the nearby flare star AD Leo (dM3e; d = 4.7pc) is illustrated above.
The top panel shows an eclipse-mapping spatial deconvolution of the coronae of the RS CVn binary AR Lac as derived from a long EXOSAT exposure. The lower panel shows a simulated 20 ks Constellation-X observation of AR Lac, assuming that the exposure was centered on orbital quadrature when the velocity separation of the two stars in this binary system is at its maximum value of 230 km s-1. For simplicity, it is assumed that each star contributes equally to the total X-ray emission. The strong Fe XXV λ 1.85 resonance line is clearly split into two components due to the differential Doppler shifts of the two stars. (Figure courtesy of Steve Drake.)
Solar coronal loops undergoing transverse oscillations observed by TRACE (Schrijver et al. 2002). Analysis of loop oscillations by (Wang & Solanki 2004) indicates X-ray intensity variations have a pulse fraction as high as 13%.
Another major Constellation-X breakthrough in the study of stellar flares will be the enormous improvement in photometric precision of flare light curves and spectra, allowing direct measurement of coronal loop resonant frequencies themselves. (Mitra-Kraev et al. 2005) have recently detected oscillations in the X-ray emission from M-dwarf AT Mic by XMM-Newton. Oscillations in solar loops are seen routinely (Schrijver et al. 2002, Wang et al. 2003, Mariska 2005) and arise from a resonance in the flare loop whereby the loop plasma is successively compressed and rarified. The observed period and its decay can be used to determine the magnetic loop length and magnetic field. Loop "wobble" velocities on the Sun have reached up to 200 km s-1. Constellation-X detections of loop oscillations, both spectroscopically and photometrically, could provide unique measurements of these quantities in a wide range of stars, from accreting T Tauri stars to evolved giants. Resolving powers of 1000 are needed to make firm detections of line-of-sight velocity components of 100 km s-1.
Detection of hard X-rays in stellar flares would define a major breakthrough for stellar physics. This emission is unequivocally related to impulsively accelerated electrons and ions that do not suffer from magnetic trapping (as radio-emitting electrons do). In the case of the Sun, hard X-rays and gamma rays have been the prime source for the study of energy release physics, particle acceleration in magnetic fields, and coronal heating. The different, and probably more extreme, magnetic configurations in magnetically active stars could lead to quite different acceleration histories and heating efficiencies in large flare events. Detection of hard X-ray components would thus open an entirely new avenue in the study of the energetics of hot, magnetized coronal plasma. For the Constellation-X HXT area of a few 103 cm2, bright flares on nearby stars can be detected in only 100 s.
An XMM-Newton lightcurve of a flare on AT Mic decomposed into three frequency bands: low (P > 1200 s) showing gross flare structure; medium (500 s < P < 1200 s) illustrating a coherent flare loop oscillation; and the high frequency (10 s < P < 500 s) noise component. The sum of these add up to the original data (lower panel). Loop oscillations provide direct diagnostics of magnetic field strength and loop length (Mitra-Kraev et al. 2005).
References
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