Simbol-X Workshop, Bologna, May 2007
Non-thermal hard X-ray emission from stellar coronae
A. MaggioINAF Osservatorio Astronomico di Palermo G.S. Vaiana
with contributions byC. Argiroffi, F. Reale
Dip. Scienze Fisiche e Astronomiche – Università di PalermoG. 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 multi-
wavelength photometry and time-resolved spectroscopy
Simbol-X Workshop, Bologna, May 2007
Ohki & Hudson, 1975
Non-thermal radiation from the flaring Sun
• Observed simultaneously during large flares
SYNCHROTRON
NON-THERMALBremsstrahlung
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
Simbol-X Workshop, Bologna, May 2007Sui & Holman, 2004
Anzer & Pneuman, 1982
Hard X-ray imaging of the solar corona with RHESSI
Simbol-X Workshop, Bologna, May 2007
π0 Decay
Non-thermal Bremsstrahlung
Thermal Emission
Large solar flares: X-ray and -ray spectrum
Positron and NuclearGamma-Ray lines
T = 20 MK
T = 40 MK
Fe and Ni K lines
Simbol-X range
Courtesy H. Hudson
Simbol-X Workshop, Bologna, May 2007
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)
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
Simbol-X Workshop, Bologna, May 2007
From the Sun to the starsSun Active stars
X-ray luminosities
Lx/Lbol ~ 10-6 (quiescent)
Lx/Lbol ~ 10-5 (large flares)
Lx/Lbol ~ 10-3 (quiescent)
Lx/Lbol ~ 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 hours up to a few days
Coronal plasma temperatures
106 K (quiencent)
107 K (flaring)
107 K (quiencent)
108 K (flaring) ???
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?
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
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 108 K
Hard X-ray emission detected up to 50 keV with the PDS detector
Maggio et al. 2000
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)
Simbol-X Workshop, Bologna, May 2007
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
300 MK ! Ne(E) E-2.5
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 Ne(E) E-3
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) ~
107 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
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
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
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
Cou
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ate
Em
issi
on M
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Reale et al. 2003, A&A
Tem
pera
ture
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
Different evolutionary phases but similar LX
Very similar coronal thermal structure
The case of GT Mus