A&A 446, 1157-1163 (2006)
DOI: 10.1051/0004-6361:20053672
E. P. Kontar1 - A. L. MacKinnon2 - R. A. Schwartz3 - J. C. Brown1
1 - Department of Physics and Astronomy, University of Glasgow,
G12 8QQ, UK
2 -
DACE, University of Glasgow, G3 6NH, UK
3 -
SSAI, Science Systems & Applications, Inc., Code 612.1, NASA/GSFC, Greenbelt, MD 20771, USA
Received 21 June 2005 / Accepted 30 September 2005
Abstract
The observed hard X-ray (HXR) flux spectrum
from
solar flares is a combination of primary bremsstrahlung photons
with a spectrally modified component from
photospheric Compton backscatter of downward primary emission. The
latter can be significant, distorting or hiding the true features
of the primary spectrum which are key diagnostics for acceleration
and propagation of high energy electrons and of their energy
budget. For the first time in solar physics, we use a Green's
function approach to the backscatter spectral deconvolution
problem, constructing a Green's matrix including photoelectric
absorption. This approach allows spectrum-independent extraction
of the primary spectrum for several HXR flares observed by the
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We
show that the observed and primary spectra differ very
substantially for flares with hard spectra close to the disk
centre. We show in particular that the energy dependent photon
spectral index
is
very different for
and for
and that
inferred mean source electron spectra
differ
greatly. Even for a forward fitting of a parametric
to the data, a clear low-energy cutoff required to fit
essentially disappears when the fit is to
- i.e. when albedo correction is included. The
self-consistent correction for backscattered photons is thus shown
to be crucial in determining the energy spectra of flare
accelerated electrons, and hence their total number and energy.
Key words: Sun: flares - X-rays: general - methods: data analysis - techniques: spectroscopic - scattering
Comptonisation of prescribed primary HXRs can be straightforwardly studied using Monte-Carlo simulations (e.g. Bai & Ramaty 1978). This technique is ideal for obtaining the reflected, and hence the total observed, photon spectrum if one knows or assumes the form of the primary X-ray spectrum (Alexander & Brown 2002). However, the primary spectrum is unknown and unlikely to be an exact power-law, as is usually assumed. Even with a power-law distribution of primary electrons a number of physical processes exist which will cause the resultant photon distribution to deviate from pure power-law form e.g. effects of nonuniform ionization (Brown 1973; Kontar et al. 2003), beam return current losses (Zharkova & Gordovskyy 2005) and others. Results obtained from Monte Carlo simulation for power-law primary photon spectra cannot be straightforwardly applied to such situations. More sophisticated parametric modelling of the primary photon spectrum, with more theoretical uncertainties and free parameters, would become necessary. Therefore, an approach independent of the primary spectrum is required.
Understanding and modelling backscatter has become even more
important with the advent of high quality X-ray spectra from the
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) with
spectral resolution as high as keV, in combination with
uncertainties as low as a few percent (for strong flares) allowing
detailed spectral analysis over a broad X-ray range (Lin et al.
2002). A correct treatment of Compton backscatter by a cold medium
has also been important for understanding spectra from cosmic
X-ray sources (reviewed e.g. by Svensson 1996). This led to
construction (Magdziarz & Zdziarski 1995) of an analytic
(piecewise continuous), angle dependent, Green's function
for the probability that a photon, of
initial energy
and drawn from a distribution
isotropic in the downward hemisphere, incident on a plane-parallel
atmosphere, is re-emitted in direction
(
)
with energy
.
This was obtained by fitting
the results of a large set of Monte Carlo calculations, and its
form is guided, particularly in various limits, by analytical
results such as those of Illarionov et al. (1979), Lightman et al.
(1981), White et al. (1988). Representing as it does the results
of Monte Carlo calculations, no assumptions of single scattering
or on the form of primary spectrum are involved. The only major
assumptions in the Green's function formulation are those already
mentioned: plane geometry of the cold matter - this is well
satisfied for the photospheric conditions and downward isotropy of
the primary photon distribution.
Combined with a treatment of absorption (Morrison & McCammon 1983) with standard solar photosphere element abundances (Anders & Grevesse 1989), the Green's function formulation of Compton backscatter provides us with a powerful tool for interpreting flare hard X-ray spectra. Firstly, it allows us to calculate easily, without time consuming Monte Carlo simulations, the total observable photon spectrum for any specified primary spectrum, not just the power-law cases treated by Bai & Ramaty (1978). More importantly, because the Green's function may be used to express the observed spectrum in terms of any primary spectrum, we may use regularized inversion techniques (Craig & Brown 1986; Kontar et al. 2004, and references therein) to deconvolve the primary photon spectrum from observations. Features of this primary spectrum arise only from features of the HXR source electron distribution, not of the energy-dependence of backscattering, so it may be safely inverted (Kontar et al. 2004) to yield a meaningful source-averaged electron distribution in the sense of (Brown et al. 2003). Although the Green's function makes such model-independent inversion possible, this step is carried out here for the first time.
Sections 2, 3 describe the Green's function formulation and the incorporation of absorption respectively. Section 4 describes how to recover the primary spectrum from data, and Sect. 5 applies this to real RHESSI data. In Sect. 6 we discuss applications and implications of our work.
As discussed in Sect. 1, downward-emitted photons will be either absorbed or scattered, and some scattered toward the observer, adding to the total flux detected. The backscattered photons will be emitted from an extended region of the photosphere, out to the horizon distance but with greatest intensity in an area of extent comparable to the altitude of the primary source (Brown et al. 1975; Bai & Ramaty 1978; Schmahl & Hurford 2004). The Green's function implemented here ignores spatial structure, however, simply summing over all photons that emerge in the correct direction. This makes it ideal for dealing with spatially unresolved spectra.
While the Green's function (Magdziarz & Zdziarski 1995) was obtained for use in studying cosmic X-ray sources, it applies to any plane-stratified slab of "cold'' material (i.e. the scattering electrons are non-relativistic). Scattering takes place only on electrons, whether free or atomic. To account for elements heavier than hydrogen the Compton cross-section is multiplied by an effective mean atomic number z=1.2. The detailed density structure of the medium is irrelevant (Tomblin 1972).
Absorption, on the other hand, does depend strongly on chemical composition, and the best estimate of photospheric abundances should be included when adding absorption to the Green's function treatment. We use standard abundances given by Anders & Grevesse (1989). The heavy elements Fe/Ni play the most important role in the range from 10 keV up to 30 keV, while lighter elements contribute below 10 keV (Morrison & McCammon 1983).
The total observed spectrum
is given by the sum of
and
,
i.e. using (1)
In practice measurements yield discrete quantities and the
integral Eq. (2) is presented in matrix form
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Figure 1:
Green's functions
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The shape of the Green's function depends on the maximum energy of
a primary photon. For primary photons with low energies
keV, the Green's function has a rather simple structure
close to a Dirac's delta function (Fig. 1) showing
that backscattering is dominated by the first scattering, while
the contribution from higher orders of scattering is small. In the
range below
keV photoelectric absorption dominates
scattering and Compton scattering can be treated as diffuse
monochromatic radiation transfer. The Green's function then can be
replaced by
(Chandrasekhar 1960,
where the form of
is given),
,
,
are the total Compton
and absorption cross-sections respectively.
gives the
single-scattering albedo modified by absorption and completely
controls the influence of absorption. Application to the solar
case simply requires evaluation of
with solar
photospheric abundances. In this energy range below 10 keV the
total Compton scattering cross-section can be approximated by the
Thompson cross-section
.
The absorption
cross-section is based on the element abundances from Anders &
Grevesse (1989). We use the coefficients of a piecewise polynomial
fit to the numerical results given by Morrison & McCammon (1983)
in the range below 10 keV and extrapolate them as
above 10 keV. Due to
rapid decrease of the absorption cross-section
around 12 keV. The generated Green's matrices
(4) have uncertainty around 6% (due to the approximations
used) and 1 keV energy resolution.
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Figure 2:
Upper panel: primary (solid line), reflected (dotted
line) and total (dashed line) photon spectra calculated assuming a
primary spectrum
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The absorption is photo-electric in nature and below 10 keV has a complex structure with multiple absorption edges due to elements heavier than hydrogen. In consequence it is also strongly influenced by elemental abundances. Figure 2 shows the two absorption edges of Fe at 7.1 keV and Ni at 8.3 keV in the reflected component. The spectrum shows features in agreement with the study of the reflection spectra in intergalactic nuclei by Ross & Fabian (1993). Here we ignored the fluorescence of the lines which is beyond the scope of our paper.
Note that Bai & Ramaty (1978) and Zhang & Huang (2004) assumed a different absorption approximation from us based on earlier photoelectric absorption cross-sections by Fireman (1974) and photospheric abundances by Withbroe (1971). However, we should note that the difference is rather modest accounting for around 30% of reflectivity at low energies and becoming smaller at higher energies, where absorption is less important (Fig. 2). The reflectivity, and thus the contribution of Compton back-scattered photons into observed spectra is spectrum-dependent (Fig. 3). Therefore, the albedo spectrum depends on the shape of the primary spectrum (Fig. 3), - previous studies (Bai & Ramaty 1978; Alexander & Brown 2002) considered only the results for prescribed power-law primary spectra. This difference is important when one tries to infer a primary spectrum from an observed one.
We verified that we can reproduce the forward results of Bai &
Ramaty (1978) by numerically evaluating Eq. (1) for the
particular case
and the parameters
described in Fig. 2. Consistency with previous
treatments being established in this way, we now turn to the
inverse problem - deduction of the primary spectrum from
observations of the total spectrum.
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Figure 3:
Spectrum dependency of the reflected flux. Upper panel:
primary (dashed line), reflected (solid line) and total (dotted
line) photon spectra calculated for a modelled flare spectrum
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In the absence of data noise we could immediately invert
Eq. (2) to obtain :
For a given instrument such as RHESSI, the count flux
is the linear composition of instrument response matrix
and the photon flux
coming from the
Sun (Schwartz et al. 2002)
This approach has already been incorporated into the SPEX software (Schwartz et al. 2002) including the object oriented version (OSPEX). The software is publicly available as a part of Solar SoftWare (SSW).
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Figure 4: Temporal variation in four energy channels (4 seconds cadence) of the count rates summed over seven front RHESSI detectors for the September 17, 2002 solar flare. The vertical lines show the accumulation interval selected 5:50:48-5:51:36 UT for further spectral analysis. |
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We consider the spectrum around flare maximum (5:50:48-5:51:36 UT) when we have enough counts over the range 3 keV up to >100 keV. We used 7 out of 9 front segments, excluding detectors 2 and 7 due to their low energy resolution at the time of observation (Smith et al. 2002). The time interval is shown in Fig. 4.
After background subtraction, spectral results are shown in
Fig. 5 for both the total observed spectrum, and the
primary spectrum
determined via the method of the previous
section and using
(isotropic emission). We note how
removal of backscattered photons, as expected, reduces the primary
flux and steepens the spectrum in the 20-50 keV photon energy
range and slightly flattens the spectrum at energies above the
albedo maximum.
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Figure 5: Spectrum of the September 17, 2002 solar flare in the time interval 5:50:48-5:51:36 UT. The solid line shows the observed spectrum; the dash line is the primary spectrum (corrected for Compton back-scattering). |
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The albedo changes the spectral indices of photon spectra, both of
power-law fits and of the local energy dependent spectral index
defined by
Figure 6 shows the energy dependent spectral index
for our flare spectrum. Both spectra (with and without albedo
correction) increase toward the ends of the energy interval. At
low energies this is probably due to a thermal component while
at higher energies it is likely connected with softening of a
nonthermal electron spectrum. The spectral index of the observed
data shows a clear minimum around 15 keV, while the spectrum
corrected for albedo has a rather extended minimum in the broad
range 15-30 keV. The albedo correction increases the minimum
value of
from 2.0 to 2.6. The opposite effect can be
seen at higher energies: the spectral index at 40 keV decreases
from 3.5 to 3.2, albedo correction making the primary spectrum
harder. Qualitatively similar results have been obtained for
forward modelled spectra (Bai & Ramaty 1978; Zhang & Huang
2004).
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Figure 6:
Energy dependent photon spectral index
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The primary photon flux is directly connected with the mean source
electron (so-called thin target) spectrum (Brown 1971; Brown
et al. 2003)
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(10) |
Solution of Eq. (9) for
is a
somewhat unstable inverse problem (Craig & Brown 1986; Piana et al. 2003) and so results are strongly affected when the observed
is used instead of the albedo corrected
required for correct solution of Eq. (9).
is important in the study of acceleration
mechanisms and of the flare electron energy budget. Of particular
interest are data such as from the flare discussed above which
exhibit very flat regions which may correspond to
with a low energy cut-off in the mean electron spectrum which
would have major implications for electron acceleration and
propagation, and for the energy budget (Kontar & Brown 2005).
We used standard SPEX software (Schwartz et al. 2002) to fit to
the observed X-ray counts an approximate parametric
comprising an isothermal plus broken power-law form with low
energy cut off. The minimum
fit of the observed spectra
gives emission measure
cm-3,
temperature kT=1.36 keV, spectral indexes
,
with break energy EB=54 keV and a low
energy cut-off at 18 keV very clearly visible above the
isothermal part (Fig. 7).
Next we carried out the same model fit but using the instrument
response matrix corrected for albedo as described above - i.e. we
repeat the process for
instead of I. The result shows the
same tendency as we saw in the analysis of energy dependent photon
spectral index. The primary photon spectrum is softer at low
energies and harder at high energies. Now we find a slightly
bigger emission measure
cm-3 and
slightly smaller temperature kT=1.29 keV. The spectral index of
energetic electrons becomes softer at low energies
and harder at high energies
with the break energy at higher energies EB=59 keV. Most
importantly, in contrast with the fit to the observed spectrum,
the primary spectrum fit yields a much lower low energy cut-off,
around 10 keV, not clearly visible above the large thermal
component there (Fig. 7). Therefore, we cannot
conclude that the true
derived from the primary
photon spectrum has a well defined low-energy cut-off.
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Figure 7: Mean electron flux spectra of the September, 2002 solar flare. The solid/dash lines show the spectrum without and with albedo correction. |
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Figure 8: Inverted mean electron flux of the August 20, 2002 solar flare for the time interval 08:25:20-08:25:40 UT. The dash line shows the spectra with albedo correction. The confidence intervals represent the range of solutions found by allowing the incident photon spectrum to range randomly within the estimated (instrument + shot noise) errors. |
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Figure 8 gives a further example from the flare of
August 20, 2002 (Kasparova et al. 2005). We have obtained
regularized
using the method of Kontar et al, 2005
starting from the total observed spectrum
and the
albedo-corrected primary spectrum
.
Note that a
local maximum in
at
keV, difficult to
accommodate in conventional thick-target models (Kontar & Brown
2005), is no longer required when albedo is properly accounted
for.
We have shown how the Compton backscatter Green's function of
Magdziarz & Zdziarski (1995) may be employed to deduce primary
hard X-ray spectra from observations. We have applied this
procedure to RHESSI data, particularly from a couple of flares
with hard photon spectra. Without a treatment of albedo, spectral
hardening found in some flares at photon energies of
keV
appears to require a local minimum in the mean fast electron
distribution (Piana et al. 2003; Kasparova et al. 2005). These
local minima are particularly interesting, since they might, if
steep, (Kontar & Brown 2005) be inconsistent with the very
widely used collision-dominated thick-target model for X-ray
production (Brown 1971; Lin & Hudson 1976). Here we have seen
that a complete treatment of albedo removes much of the spectral
hardening in this photon energy range, potentially restoring the
viability of the collisional thick target.
The major assumption here is the isotropy of the downward directed
radiation. At the relevant photon energies here 10-100 keV, the
intrinsic bremsstrahlung cross-section polar diagram has a
characteristic width of about
(see figure in Massone et al.
2004). Since the emitting electron angular distribution will be
broadened by pitch-angle scattering (Leach & Petrosian 1981;
MacKinnon & Craig 1991), the resulting hard X-ray flux might be
fairly close to downward isotropic.
Inclusion of the albedo effect reduces the number of energetic
electrons required for the production of the observed spectra. The
total flux of energetic electrons
It should be also stressed here that these are rather conservative (lower limit) estimates of the albedo correction, assuming an isotropic primary X-ray source. In fact, if the electrons are strongly downward directed, the back-scattered photons could produce a several times larger contribution to the observed spectrum. This would have a major effect and clearly one must consider the albedo to get a realistic idea of the flare electron spectrum and energy budget. The capacity to account completely for the effects of albedo also restores some optimism over discussing, in terms of X-ray spectra, issues such as the lowest energies at which electron acceleration operates (see Zhang & Huang 2004), though electron transport close to the thermal speed complicates the interpretation of the X-ray spectrum there (Galloway et al. 2005).
Acknowledgements
We are grateful to Gordon Holman and Jana Kasparova for valuable discussions and to Kim Tolbert for adding our code to OSPEX package. EPK, ALM, and JCB acknowledge the financial support of a PPARC Rolling Grant. RAS is supported by contract NAS5 - 01160.