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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. Non-thermal hard X-ray emission from stellar coronae.

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Non-thermal hard X-ray emission from stellar coronae

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  1. 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 Non-thermal hard X-ray emission from stellar coronae

  2. 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

  3. 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 multi-wavelength photometry and time-resolved spectroscopy

  4. Non-thermal radiation from the flaring Sun • Observed simultaneously during large flares SYNCHROTRON NON-THERMAL Bremsstrahlung Ohki & Hudson, 1975

  5. Flaring X-ray emission sites:the “Masuda flare” prototype • 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) Masuda et al. 1994

  6. Hard X-ray imaging of the solar corona with RHESSI Sui & Holman, 2004 Anzer & Pneuman, 1982

  7. Example of more complex structures

  8. Time scales and the Neupert effect Güdel et al. 1996

  9. 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-Xrange Positron and Nuclear Gamma-Ray lines Courtesy H. Hudson

  10. High-energy tails in solar microflares • X-ray luminosities 1024 – 1025 erg/s • Characteristics similar to large flares: thermal component + broken power-law • Lower break energies and steeper slopes RHESSI spectra(Krucker & Lin 2005)

  11. Reference phenomenological model • Magnetic field reconnection event • Particle acceleration (electron beam) • Gyrosynchrotron emission from mildly relativistic electrons with a power-law energy distribution • Thick-target non-thermal bremsstrahlung (hard X-ray emission from loop footpoints) • Chromospheric plasma heating and evaporation • Optically-thin thermal soft X-ray emission

  12. From the Sun to the stars ???

  13. Evidence of non-thermal processes in active stars • Steady, quiescent emission with rather flat spectra • Non-thermal gyrosynchron + gyroresonance components • Interpretation: mildly relativistic electrons in 100G fields with power-law indices 2-4 • Open question: continuous acceleration? Güdel 2002

  14. Stellar soft X-ray vs. radio emission • Correlation over 8 dex, including full range of solar flares • Thermal and non-thermal emission appear linked • Are stellar coronae heated by continuous flaring activity? Benz & Güdel 1994

  15. Extreme stellar flares: the case of AB Doradus • 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 Maggio et al. 2000

  16. Pallavicini et al. 2001 AB Dor flares: X-ray light curves LECS (0.1-5 keV) MECS (2-10 keV) HPGSPC (4-20 keV) PDS (15-50 keV)

  17. 300 MK ! Ne(E)E-2.5 AB Dor hard X-ray spectrum • Different evolutionary phases but similar LX • Very similar coronal thermal structure 3-T model (left) and 2-T + power law model (right) yield spectral fits of similar quality

  18. Osten et al. 2007 The case of II Peg • 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

  19. Thermal vs. non-thermal emission:scaling from solar flares GOES 1.55-12.4 keV flux vs RHESSI 20-40 keV flux • 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 F(20-40) ~ 107FG1.37 (Isola et al. 2007, see poster)

  20. 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 - Fe K line ratios - Fluorescence or collisional ionization Fe lines solar flares (Argiroffi et al. 2007, see poster)

  21. 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

  22. Variability studies II: Proxima Cen Count rate • 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). • Analogy with intense solar flares Emission Measure Temperature Reale et al. 2003, A&A

  23. 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

  24. The case of GT Mus • Different evolutionary phases but similar LX • Very similar coronal thermal structure

  25. Simbol-X vs. SUZAKU • Different evolutionary phases but similar LX • Very similar coronal thermal structure

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