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

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Detailed Plasma and Fluorescence Diagnostics of a
Stellar 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
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Rationale
HETGS observation of the G1 III giant HR 9024
Hertzsprung gap giant, 3M?, 13R?, peak
Lx1032ergs/s
(see also Ayres et al 2007, Nordon Behar 2006)
Motivations

• X-ray activity in evolved intermediate mass stars
• Coronal structuring
• Physics of stellar flares
• Plasma diagnostics, and geometry diagnostics
• hydrodynamic loop modeling (Testa et al. 2007,
  ApJ, 663)
• fluorescence emission

Diagnostic Tools
Comparison of results from the two independent
analyses
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HETGS Spectrum
HR 9024 Hertzsprung gap giant, 3M?, 13R?, peak
Lx1032ergs/s
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Light curve and hardness ratio
HR 9024 Hertzsprung gap giant, 3M?, 13R?, peak
Lx1032ergs/s
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X-ray activity of intermediate-mass giants

• HR 9024 Hertzsprung gap giant, 3M?, 13R?,
  Lx1032ergs/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

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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
• start with an educated guess for the parameters
• synthesize the HETG spectrum of the solution
• repeat the analysis carried out on the observed
  spectrum and compare the same quantities
• refine the model if needed

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Hydrodynamic Modeling
MEG cts/s
T
T vs. n
EM
(Testa et al. 2007, ApJ, 663, 1232)
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Hydrodynamic Modeling

• 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/L0.1 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

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Hydrodynamic Modeling

• Cross-check of results
• light curves in strong spectral features
• Fe XXV,
• Si XIV,
• Mg XII

(Testa et al. 2007, ApJ)
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Hydrodynamic Modeling

• Cross-check of results
• EM(T)
• (flare)

(Testa et al. 2007, ApJ)
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Hydrodynamic Modeling

• Cross-check of results
• 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

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

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

• 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

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Geometry Diagnostics from Fluorescence
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Results

• 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

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Thank you!
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Hydrodynamic Modeling

• Cross-check of results
• light curves in strong spectral features
• Fe XXV, Si XIV, Mg XII

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Hydrodynamic Modeling
t0-10ks
Comparison of MEG observed spectrum with
predictions of loop model
t10-15ks
t15-30ks
t30-45ks
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Hydrodynamic Modeling
t0-10ks
Comparison of MEG observed spectrum with
predictions of loop model
t10-15ks
t15-30ks
t30-45ks
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Hydrodynamic Modeling
Apex T
Apex ne
max v
Apex p
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Hydrodynamic Modeling