non-thermal hard x-ray emission from stellar coronae

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


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


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


Ohki & Hudson, 1975

Non-thermal radiation from the flaring Sun

• Observed simultaneously during large flares

SYNCHROTRON

NON-THERMALBremsstrahlung


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


Example of more complex structures


Time scales and the Neupert effect

Güdel et al. 1996


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


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)


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


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) ???


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?


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


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


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)


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


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


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 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


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


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

nt r

ate

Em

issi

on M

easu

re

Reale et al. 2003, A&A

Tem

pera

ture


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




The case of GT Mus




Simbol-X vs. SUZAKU

non-thermal hard x-ray emission from stellar coronae

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