1. Simbol-X Workshop, Bologna, May 2007 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

2. 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

3. 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 multi- wavelength photometry and time-resolved spectroscopy

4. Simbol-X Workshop, Bologna, May 2007 Ohki & Hudson, 1975 Non-thermal radiation from the flaring Sun Observed simultaneously during large flares SYNCHROTRON NON-THERMAL Bremsstrahlung

5. Simbol-X Workshop, Bologna, May 2007 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. Simbol-X Workshop, Bologna, May 2007 Sui & Holman, 2004 Anzer & Pneuman, 1982 Hard X-ray imaging of the solar corona with RHESSI

7. Simbol-X Workshop, Bologna, May 2007 Example of more complex structures

8. Simbol-X Workshop, Bologna, May 2007 Time scales and the Neupert effect Güdel et al. 1996

9. Simbol-X Workshop, Bologna, May 2007 π 0 Decay Non-thermal Bremsstrahlung Thermal Emission Large solar flares: X-ray and -ray spectrum Positron and Nuclear Gamma-Ray lines T = 20 MK T = 40 MK Fe and Ni K lines Simbol-X range Courtesy H. Hudson

10. Simbol-X Workshop, Bologna, May 2007 High-energy tails in solar microflares ● X-ray luminosities 10 24 – 10 25 erg/s ● Characteristics similar to large flares: thermal component + broken power-law ● Lower break energies and steeper slopes RHESSI spectra (Krucker & Lin 2005)

11. Simbol-X Workshop, Bologna, May 2007 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

12. Simbol-X Workshop, Bologna, May 2007 From the Sun to the stars SunActive stars X-ray luminosities L x /L bol ~ 10 -6 (quiescent) L x /L bol ~ 10 -5 (large flares) L x /L bol ~ 10 -3 (quiescent) L x /L bol ~ 10 -1 (large flares) Occurrence of large flares 1 every 10 days (at max of solar cycle) A few per day (no magnetic cycle?) Flare time scales up to a few hoursup to a few days Coronal plasma temperatures  10 6 K (quiencent)  10 7 K (flaring)  10 7 K (quiencent)  10 8 K (flaring) ???

13. Simbol-X Workshop, Bologna, May 2007 Güdel 2002 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?

14. Simbol-X Workshop, Bologna, May 2007 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. Simbol-X Workshop, Bologna, May 2007 Extreme stellar flares: the case of AB Doradus  Young active K1V star observed with BeppoSAX  100-fold increase of X-ray emission  Peak temperatures  10 8 K  Hard X-ray emission detected up to 50 keV with the PDS detector Maggio et al. 2000

16. Simbol-X Workshop, Bologna, May 2007 AB Dor flares: X-ray light curves Pallavicini et al. 2001 LECS (0.1-5 keV) MECS (2-10 keV) HPGSPC (4-20 keV) PDS (15-50 keV)

17. Simbol-X Workshop, Bologna, May 2007 AB Dor hard X-ray spectrum  Different evolutionary phases but similar L X  Very similar coronal thermal structure 3-T model (left) and 2-T + power law model (right) yield spectral fits of similar quality 300 MK ! N e (E) E -2.5

18. Simbol-X Workshop, Bologna, May 2007 The case of II Peg Osten et al. 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 N e (E)  E -3

19. Simbol-X Workshop, Bologna, May 2007 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) F (20-40) ~ 10 7 F G 1.37  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

20. Simbol-X Workshop, Bologna, May 2007 Simbol-X spectral diagnostics of Non-Thermal emission (Argiroffi et al. 2007, see poster)  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

21. Simbol-X Workshop, Bologna, May 2007 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. Simbol-X Workshop, Bologna, May 2007 Variability studies II: Proxima Cen  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 Count rate Emission Measure Reale et al. 2003, A&A Temperature

23. 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

24. Simbol-X Workshop, Bologna, May 2007  Different evolutionary phases but similar L X  Very similar coronal thermal structure The case of GT Mus

25. Simbol-X Workshop, Bologna, May 2007  Different evolutionary phases but similar L X  Very similar coronal thermal structure Simbol-X vs. SUZAKU