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Non-thermal hard X-ray emission from stellar coronae A. Maggio INAF Osservatorio Astronomico di Palermo G.S. Vaiana with contributions by C. Argiroffi, F. Reale Dip. Scienze Fisiche e Astronomiche – Università di Palermo G. Micela INAF Osservatorio Astronomico di Palermo G.S. Vaiana Simbol-X Workshop, Bologna, May 2007 Why bother with hard X-rays from stellar coronae Scientific issues : Physics of plasma heating in magnetized astrophysical environments How magnetic energy is converted in kinetic and thermal energy Particle acceleration, thermalization, and energy dissipation Birth, evolution, and dynamics of stellar coronae Influence of high-energy emission on the circumstellar environment Ionization of protoplanetary disks and ISM “Space weather” effects on planetary systems Simbol-X Workshop, Bologna, May 2007 Why non-thermal hard X-rays Non-maxwellian (supra-thermal) particle populations How are they generated? How do they depend on the stellar magnetic activity level? How efficiently are they trapped in stellar magnetospheres? What fraction does escape to the outer space? Multi-wavelength issues Soft (thermal) and hard (non-thermal) X-ray scaling Relation with synchrotron radio emission Probing energy release mechanism(s) by means of multiwavelength photometry and time-resolved spectroscopy Simbol-X Workshop, Bologna, May 2007 Non-thermal radiation from the flaring Sun • Observed simultaneously during large flares SYNCHROTRON NON-THERMAL Bremsstrahlung Ohki & Hudson, 1975 Simbol-X Workshop, Bologna, May 2007 Flaring X-ray emission sites: the “Masuda flare” prototype Masuda et al. 1994 Simbol-X Workshop, Bologna, May 2007 Simple geometry Localized hard X-ray emission (15-90 keV, in 3 sites) Extended soft X-ray emission (1-3 keV) Cusp-like magnetic field configuration (inferred) Hard X-ray imaging of the solar corona with RHESSI Anzer & Pneuman, Simbol-X Workshop, Bologna, May1982 2007 Sui & Holman, 2004 Example of more complex structures Simbol-X Workshop, Bologna, May 2007 Time scales and the Neupert effect Güdel et al. 1996 Simbol-X Workshop, Bologna, May 2007 Large solar flares: X-ray and -ray spectrum Thermal Emission T = 20 MK T = 40 MK Fe and Ni K lines Non-thermal Bremsstrahlung π0 Decay Simbol-X range Courtesy H. Hudson Simbol-X Workshop, Bologna, May 2007 Positron and Nuclear Gamma-Ray lines High-energy tails in solar microflares ● ● ● RHESSI spectra (Krucker & Lin 2005) Simbol-X Workshop, Bologna, May 2007 X-ray luminosities 1024 – 1025 erg/s Characteristics similar to large flares: thermal component + broken power-law Lower break energies and steeper slopes Reference phenomenological model 1. Magnetic field reconnection event 2. Particle acceleration (electron beam) 3. Gyrosynchrotron emission from mildly relativistic electrons with a power-law energy distribution 4. Thick-target non-thermal bremsstrahlung (hard X-ray emission from loop footpoints) 5. Chromospheric plasma heating and evaporation 6. Optically-thin thermal soft X-ray emission Simbol-X Workshop, Bologna, May 2007 From the Sun to the stars Sun X-ray luminosities Active stars Lx/Lbol ~ 10-6 (quiescent) Lx/Lbol ~ 10-3 (quiescent) Lx/Lbol ~ 10-5 (large flares) Lx/Lbol ~ 10-1 (large flares) Occurrence of 1 every 10 days large flares (at max of solar cycle) A few per day (no magnetic cycle?) Flare time scales up to a few hours up to a few days 106 K (quiencent) 107 K (quiencent) 107 K (flaring) 108 K (flaring) Coronal plasma temperatures Simbol-X Workshop, Bologna, May 2007 ??? Evidence of non-thermal processes in active stars ● ● ● ● Güdel 2002 Simbol-X Workshop, Bologna, May 2007 Steady, quiescent emission with rather flat spectra Non-thermal gyrosynchron + gyroresonance components Interpretation: mildly relativistic electrons in 100G fields with powerlaw indices 2-4 Open question: continuous acceleration? Stellar soft X-ray vs. radio emission ● ● ● Benz & Güdel 1994 Simbol-X Workshop, Bologna, May 2007 Correlation over 8 dex, including full range of solar flares Thermal and nonthermal emission appear linked Are stellar coronae heated by continuous flaring activity? Extreme stellar flares: the case of AB Doradus Maggio et al. 2000 Simbol-X Workshop, Bologna, May 2007 Young active K1V star observed with BeppoSAX 100-fold increase of X-ray emission Peak temperatures 108 K Hard X-ray emission detected up to 50 keV with the PDS detector AB Dor flares: X-ray light curves LECS (0.1-5 keV) MECS (2-10 keV) HPGSPC (4-20 keV) PDS (15-50 keV) Pallavicini et al. 2001 Simbol-X Workshop, Bologna, May 2007 AB Dor hard X-ray spectrum Different evolutionary phases but similar LX 300 MK ! Very similar coronal thermal structure Ne(E) E-2.5 3-T model (left) and 2-T + power law model (right) yield spectral fits of similar quality Simbol-X Workshop, Bologna, May 2007 The case of II Peg Osten et al. 2007 Simbol-X Workshop, Bologna, May 2007 Flare detected by Swift/BAT, followed for 3 orbits with XRT Emission up to 80 keV lasting 2 hours Alternative interpretations: - 300 MK thermal emission (rejected) OR - thick-target bremsstrhlung with Ne(E) E-3 Thermal vs. non-thermal emission: scaling from solar flares GOES 1.55-12.4 keV flux vs RHESSI 20-40 keV flux (Isola et al. 2007, see poster) Simbol-X Workshop, Bologna, May 2007 Soft and hard X-ray emission at flare peak are correlated Extreme stellar flares follow the solar scaling We can predict what Simbol-X would see Two caveats: - Extreme flares are rare AND - hot thermal components may contribute significantly to the hard X-ray emission Simbol-X spectral diagnostics of Non-Thermal emission Simulations of NT components in typical stellar flares NT recognized when unphysical thermal components are found (T > 300 MK) Required > 20 total counts in the 20-40 keV band Other constraints - Neupert effect - thermalization and energy loss time scales (Argiroffi et al. 2007, see poster) Simbol-X Workshop, Bologna, May 2007 - Fe K line ratios - Fluorescence or collisional ionization Fe lines Conclusions Simbol-X will allow us to explore hard-X emission from stellar coronae in a regime not reached by past observatories The best targets to search for non-thermal emission components are nearby active stars known to exhibit frequent, moderately hot flares Spectral fitting + timing analysis + physical time scales arguments will allow to infer non-thermal components if > 20 total counts are collected in the 20-40 keV band Simbol-X Workshop, Bologna, May 2007 dM5.5e flare star GO, Aug 2001 (PI: Güdel) Hydrodynamic modeling Evidence of triggered impulsive events Contraints on primary and secondary heating pulse duration (~10 min), and heating decay time scale (~ 1 h). Temperature Emission Measure Count rate Variability studies II: Proxima Cen Analogy with intense solar flares Reale et al. 2003, A&A Simbol-X Workshop, Bologna, May 2007 Prox Cen vs. Sun Analogy with class X6 “Bastille day” solar flare Striking difference of spatial scales and energy budget, but similar morphology and time evolution Simbol-X Workshop, Bologna, May 2007 The case of GT Mus Simbol-X Workshop, Bologna, May 2007 Different evolutionary phases but similar LX Very similar coronal thermal structure Simbol-X vs. SUZAKU Simbol-X Workshop, Bologna, May 2007 Different evolutionary phases but similar LX Very similar coronal thermal structure