Detailed Plasma and Fluorescence Diagnostics of a Stellar X-Ray Flare

Detailed Plasma and Detailed Plasma and Fluorescence Diagnostics Fluorescence Diagnostics of aof a Stellar X-Ray FlareStellar X-Ray Flare

Paola Testa (1)

Fabio Reale(2), Jeremy Drake(3), Barbara Ercolano(3), David Huenemoerder(1), David Garcia-Alvarez(3,4)

1 MIT, 2 Universita’ di Palermo (Italy), 3 SAO, 4 Imperial College

July 11 2007 - CXC X-ray spectroscopy workshop

Rationale

• X-ray activity in evolved intermediate mass stars

• Coronal structuring • Physics of stellar flares

• Plasma diagnostics, and geometry diagnostics: – hydrodynamic loop modeling– fluorescence emission


HETGS observation of the G1 III giant HR 9024 Hertzsprung gap giant, 3M, 13R, peak Lx~1032ergs/s

Diagnostic Tools:

Comparison of results from the two independent analyses

Motivations:

HETGS Spectrum


HR 9024: Hertzsprung gap giant, 3M, 13R, peak Lx~1032ergs/s

Light curve and hardness ratio


HR 9024: Hertzsprung gap giant, 3M, 13R, peak Lx~1032ergs/s

X-ray activity of intermediate-mass giants

HR 9024: Hertzsprung gap giant, 3M, 13R, Lx~1032ergs/s

Evolved intermediate mass star: what are the characteristics of the X-ray production mechanisms? How is the corona structured (typical size, filling factors, …)? (e.g., Ayres et al. 1998, Ayres et al. 2007)

• These giants are thought to develop a dynamo when they enter the convective region of the H-R diagram, also given their typical fast rotation in MS (when they are non-coronal late-B/early-A dwarfs)

• Variability: flares are very unusual in these massive evolved giants


Hydrodynamic Modeling

For the hydrodynamic modeling we use information mainly from the continuum that is strong, and it probes the hot flaring plasma:we derive T and EM by fitting the continuum in line-free regions (according to both APED and CHIANTI)

Loop Model:

1. start with an educated guess for the parameters

2. synthesize the HETG spectrum of the solution

3. repeat the analysis carried out on the observed spectrum and compare the same quantities

4. refine the model if needed


MEG cts/sMEG cts/s TT

EMEM T vs. nT vs. n


Model parameters:

• loop semi-length L = 5· 1011 cm ~ R/2, as in normal coronae

• cross-section radius r ~ 4.5· 1010 cm, i.e. aspect ratio r/L~0.1 as in typical solar loops

• impulsive heating (15 ks; starting 8 ks before the beginning of the observation) at the footpoints; volumetric heating ~ 4 erg/cm3/s, heating rate ~ 1033 erg/s



Cross-check of results:

• light curves in strong spectral features:

Fe XXV,

Si XIV, Mg XII




• EM(T) (flare)




• cross-section radius - we derive estimates from the normalization of different light curves:

(a) integrated MEG counts (r ~ 4.9· 1010 cm),(b) EM from continuum (r ~ 4.3 · 1010 cm),(c) single spectral features (r ~ 4.8 · 1010 cm);

they all agree with each other within a 15%



Geometry Diagnostics from Fluorescence


HEGHEG

In the HEG spectrum there is evidence of Fe K fluorescence (1.94Å, 6.4keV) that provides an independent diagnostic for the coronal geometry



• the solid angle subtended by the cold material as seen by the X-ray source h

• the inclination angle at which the reflecting surface is viewed by the observer

• the Fe abundance of the cold material

the fluorescence efficiency depends on (e.g., Bai 1979):



Measured value (1)

Prediction from hd model

• the X-ray characteristics of HR9024 are typical of normal coronae but scaled up to the larger stellar radius

• HETGS allows for the first time to test HD models predictions for single spectral features: we find very good agreement at least for the hottest feature of Fe XXV

• the HEG spectrum shows evidence of Fe K fluorescence emission, and its analysis provides an independent check of the results of the hydrodynamic modeling:

the fluorescence efficiency predicted from the HD model is in agreement with the observed one within

the uncertainties

Results


Thank you!




• light curves in strong spectral features:

Fe XXV, Si XIV, Mg XII

tt=0-10ks=0-10ks

tt=10-15ks=10-15ks

tt=15-30ks=15-30ks

tt=30-45ks=30-45ks


Comparison of MEG observed spectrum with predictions of loop model


Comparison of MEG observed spectrum with predictions of loop model

t=0-10kst=0-10ks

t=10-15kst=10-15ks

t=15-30kst=15-30ks

t=30-45kst=30-45ks


Apex TApex T Apex nApex nee

Apex pApex p max vmax v


Detailed Plasma and Fluorescence Diagnostics of a Stellar X-Ray Flare

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