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

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 :
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Physics of plasma heating in magnetized astrophysical
environments
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How magnetic energy is converted in kinetic and thermal energy
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Particle acceleration, thermalization, and energy dissipation
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Birth, evolution, and dynamics of stellar coronae
Influence of high-energy emission on the circumstellar
environment
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Ionization of protoplanetary disks and ISM
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“Space weather” effects on planetary systems
Simbol-X Workshop, Bologna, May 2007
Why non-thermal hard X-rays
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Non-maxwellian (supra-thermal) particle populations
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How are they generated?
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How do they depend on the stellar magnetic activity level?
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How efficiently are they trapped in stellar magnetospheres?
What fraction does escape to the outer space?
Multi-wavelength issues
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Soft (thermal) and hard (non-thermal) X-ray scaling
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Relation with synchrotron radio emission
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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
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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
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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
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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
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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
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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
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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
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(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
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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
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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
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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
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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
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Different evolutionary phases but similar LX
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Very similar coronal thermal structure
Simbol-X vs. SUZAKU
Simbol-X Workshop, Bologna, May 2007
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Different evolutionary phases but similar LX
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Very similar coronal thermal structure