Issue |
A&A
Volume 505, Number 1, October I 2009
|
|
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Page(s) | 169 - 176 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200911948 | |
Published online | 24 July 2009 |
Cosmic ray acceleration parameters from multi-wavelength observations. The case of SN 1006
E. G. Berezhko1 - L. T. Ksenofontov1 - H. J. Völk2
1 - Yu. G. Shafer Institute of Cosmophysical Research and Aeronomy, 31 Lenin Ave., 677980 Yakutsk, Russia
2 -
Max-Planck-Institut für Kernphysik, Postfach 103980, 69029 Heidelberg, Germany
Received 25 February 2009 / Accepted 11 June 2009
Abstract
Aims. The properties of the Galactic supernova remnant (SNR) SN 1006 are theoretically reanalysed.
Methods. Nonlinear kinetic theory is used to determine the acceleration efficiency of cosmic rays (CRs) in the supernova remnant SN 1006. The known range of astronomical parameters and the existing measurements of nonthermal emission are examined in order to define the values of the relevant physical parameters that determine the CR acceleration efficiency.
Results. It is shown that the parameter values - proton injection rate, electron to proton ratio and downstream magnetic field strength -are determined with the appropriate accuracy. In particular the observed azimuthal variations in the -ray morphology also agree with the theoretical expectation. These parameter values, together with the reduction of the
-ray flux relative to a spherically symmetric acceleration geometry, allow a good fit to the existing data, including the recently detected TeV emission by H.E.S.S.
Conclusions. SN 1006 represents the first example where a high efficiency of nuclear CR production, required for the Galactic CR sources, is consistently established.
Key words: ISM: cosmic rays - acceleration of particles - shock waves - stars: supernovae: individual: SN 1006 - radiation mechanisms: nonthermal - gamma rays: theory
1 Introduction
Cosmic rays (CRs) below an energy of 1017 eV are believed to be accelerated in the shell-type supernova remnants (SNRs) of our Galaxy (e.g. Berezhko & Völk 2007) by means of the diffusive shock acceleration process. However, this proposition still has only a limited observational/theoretical basis. In addition, SNRs may not only act individually but possibly also more collectively in the environment of interacting stellar winds and supernova explosions in OB associations (e.g. Parizot et al. 2004; Binns et al. 2007). To ensure that Galactic SNRs are indeed efficient sources of the Galactic CRs, a number of SNRs is required with clearly determined astronomical parameters, like the type of supernova explosion, the SNR age, the distance, and the properties of the circumstellar medium. Applying an appropriate model which consistently describes the dynamics of the SNR to such SNRs, one can then predict the properties of the accelerated particles and the dynamical and radiative effects which they produce, like the shock modification and the multi-wavelength nonthermal emission.
The success of the theoretical model can be judged by comparison with the experimentally determined overall broad band spectrum and with the morphological characteristics of the SNR, like its filament structures and the general radial and azimuthal variations of the emission pattern, as well as the internal dynamics characterized by the contact discontinuity between ejected and swept-up mass. From the point of view of Galactic CR origin, the key quantity is the efficiency of CR production.
In practice such a program is most often hampered by the limited amount and detail of relevant observations. First of all, the astronomical parameters of SNRs are as a rule poorly known. Even though the SNR age is known for several historical SNRs, the distance is usually quite uncertain. In this regard, SN 1006 is an exception: the distance was determined using optical measurements with relatively high precision (Winkler et al. 2003).
The second problem is that even the presently most advanced nonlinear kinetic
theory of CR acceleration in SNRs (Berezhko & Völk 1997; Berezhko et al. 1996) contains physical parameters that cannot yet be theoretically
calculated with the necessary precision (see Völk 2004, for a
review). This concerns the magnitude and the spatial distribution of
the injection rates of ions and of electrons into the diffusive shock
acceleration process as well as the extent of magnetic field amplification in
this process. Fortunately, the values of these parameters can be inferred from
the observed radio and X-ray synchrotron spectra if they are measured in
sufficient detail (see Berezhko 2008,2005, for reviews). In such a case, this
theory provides a consistent if still approximate model of both SNR and CR
dynamics, and of the properties of the emission produced by the accelerated
particles. In particular, the theory predicts the high-energy
-ray spectrum.
Up to now SN 1006 has been the only example for which all astronomical parameters are quite well known (e.g. Cassam-Chenaï et al. 2008). In addition, the nonthermal emission in the radio and X-ray bands has by now been measured quite accurately with Chandra (Allen et al. 2008,2004) and Suzaku (Bamba et al. 2008). Beyond that, the VHE emission of SN 1006 has been recently detected with H.E.S.S., both regarding its flux and its morphology (Naumann-Godo et al. 2009). This makes SN 1006 uniquely suitable for theoretical study and for a detailed comparison with the experimental data.
We have already applied this nonlinear theory to the case of SN 1006 before
(Ksenofontov et al. 2005; Berezhko et al. 2002; Völk et al. 2003,2008b; Berezhko et al. 2003; Völk et al. 2005). In the following, a brief review
of this previous work is given. The corresponding model assumes spherical
symmetry and a quasi-parallel shock. However it was argued that efficient
nuclear particle injection/acceleration should arise only within two polar cap
regions, where the SN shock is quasi-parallel with respect to the ambient ISM
magnetic field which in turn is approximately perpendicular to the line of
sight (Völk et al. 2003). The calculated size of the efficient nuclear CR production
regions, which amounts to about 20% of the shock surface, corresponds very
well to the observed sizes of the bright X-ray synchrotron emission
regions. Over the remaining 80% of the surface, the shock is
quasi-perpendicular and this leads to a depression of the injection of nuclear
particles into the diffusive shock acceleration process. This means that
efficient nuclear CR production - that occurs in a spherically symmetric model
uniformly across the whole shock surface - in the actual SNR takes place only
within the polar regions. It is therefore consistent with a correction for the
spherically symmetric solution for nuclear CR production and the associated -ray emission by a renormalization factor
(for details,
see the Appendix; to emphasize the three-dimensional character of the
configuration a description in terms of magnetic flux tubes is given.). In the
first study (Berezhko et al. 2002), it was demonstrated that the observed properties of
the nonthermal emission can be consistently understood assuming a considerably
amplified magnetic field
G within the nuclear
injection/acceleration region. Such a strong magnetic field is consistent with
theoretical expectations and was confirmed by the Chandra detection
(Long et al. 2003; Bamba et al. 2003) of filament structures in the nonthermal X-ray emission
of the rims of the SNR shell (see Berezhko et al. 2003, for an interpretation). A
filamentary structure of this character is indeed expected in young SNRs, where
the magnetic field is strongly amplified, as the result of the strong
synchrotron losses of the X-ray emitting CR electrons. In fact, since there
generally exists a simple relation between the filament thickness and the
interior magnetic field strength (Berezhko & Völk 2004a), the measurement of the filament
thickness represents a new and independent method for the determination of the
magnetic field strength inside young SNRs. It is important to note that the
interior magnetic field strength, determined in such a way for all known
Galactic SNRs, agrees very well with its value derived from a fit of the shape
of the overall synchrotron spectra, wherever both methods can be applied
(Völk et al. 2005). It was also shown (Berezhko et al. 2002) that nonlinear kinetic theory
of CR acceleration is consistent with the TeV-emission detected by CANGAROO in
1998 for a value
cm-3 of the ambient interstellar
medium ISM density from the range
cm-3existing in the literature. However, SN 1006 was not confirmed as a TeV source
by the H.E.S.S. experiment in a total of 18.2h (in 2003) and 6.3h (in 2004)
``livetime'' of ON source observations with SN 1006 in the field of view
(Aharonian et al. 2005). The H.E.S.S. upper limit was roughly one order of
magnitude lower than the published CANGAROO flux. In a subsequent paper by the
present authors (Ksenofontov et al. 2005) it was demonstrated that this H.E.S.S. upper
limit does not invalidate the theoretical picture on which the previous
calculation of the
-ray emission spectrum had been based. As it was shown, it is
rather the value of the external astronomical parameter
that
strongly influences the hadronic
-ray flux: the hadronic
-ray flux is very
sensitive to the ambient gas density
and the H.E.S.S. upper
limit requires
cm-3. It is important that only a
rather strongly amplified magnetic field, produced nonlinearly by efficiently
accelerated nuclear CRs, is compatible with such a low
-ray flux, which is
therefore expected to be predominantly of hadronic origin. Otherwise the
overproduction of accelerated electrons would lead to an unacceptably high
inverse Compton (IC)
-ray emission (Berezhko et al. 2002). This requires a strong
component of accelerated nuclear particles whose energy density substantially
exceeds that of the synchrotron electrons. The opposite simplest IC scenario,
which is based on the assumption that the entire nonthermal SNR emission is
produced by accelerated electrons without any substantial nuclear CR production
and magnetic field amplification, substantially overpredicts the existing
-ray upper limits for all known type Ia Galactic SNRs (Völk et al. 2008b).
Since TeV -ray emission of SN 1006 was recently detected (Naumann-Godo et al. 2009) it
is worthwhile to reconsider SN 1006 on the basis of this nonlinear
theory. Compared with the previous consideration (Ksenofontov et al. 2005) this makes it
possible to determine the ambient ISM density within narrow limits. In
addition, we shall demonstrate that the values of the relevant physical
parameters - proton injection rate, electron to proton ratio and downstream
magnetic field strength - are determined for SN 1006 with appropriate
accuracy from the measured synchrotron spectrum. The
-ray morphology agrees
with the theoretical expectations regarding the morphology of ion injection
and the corresponding morphology of magnetic field amplification
(Völk et al. 2003). It is therefore indeed consistent with a correction for the
total nuclear particle pressure and the total
-ray flux obtained in the
spherically symmetric solution by a renormalization factor
.
The correction implies the full value of particle pressure and
-ray flux, as calculated in the spherically symmetric model, in the
quasi-parallel part of the magnetic flux tubes that
thread the polar cap regions, but a value of zero elsewhere. The above
parameter values plus the mentioned morphological aspects allow a physical
explanation of all existing multi-wavelength data, including the
H.E.S.S. measurement of the TeV spectrum. There is possibly one exception to
this conclusion. It concerns the radius of the contact discontinuity between
ejected and swept-up mass relative to the radius of the SNR blast wave,
cf. recent data discussed by Cassam-Chenaï et al. (2008) which indicate a large value -
apparently quite close to unity - of this ratio. However another very recent
analysis (Miceli et al. 2009) claims values
0.9 for this ratio which are
rather in line with quantitative expectations for a CR-modified shock in an
object of comparable evolutionary phase like Tycho's SNR
(Warren et al. 2005; Völk et al. 2008a; Cassam-Chenaï et al. 2007). Future work will have to assess these
differences.
2 Results and discussion
Since SN 1006 is a type Ia supernova it presumably expands into a uniform ISM,
ejecting roughly a Chandrasekhar mass
.
Since the
gas density indeed varies only mildly across the SNR (Acero et al. 2007), it
appears reasonable to assume also the circumstellar magnetic field to be
uniform. The ISM mass density
,
which is
usually characterized by the hydrogen number density
,
is an
important parameter which strongly influences the expected SNR dynamics and
nonthermal emission.
As in our earlier study (Ksenofontov et al. 2005) we solve here the
coupled set of nonlinear equations. mentioned before, in order to find the
optimum set of physical parameters of SN 1006 which gives a consistent
description of the observed overall dynamics and of the nonthermal emission
together with its morphology. The theory includes all the important physical
factors that influence CR acceleration and SNR dynamics: shock modification by
the CR backreaction, MHD wave damping within the shock transition, a consistently
determined CR spectrum, and the spatial distributions in each evolutionary
phase. In addition, it includes synchrotron losses of CR electrons and a
determination of all nonthermal emission processes produced in SNRs by the
accelerated CRs. It had also been shown that the values of
the key parameters (proton injection rate ,
electron to proton ratio
and interior (downstream) magnetic field strength
)
which cannot be predicted theoretically with the required
accuracy, can be determined from a fit of the observed synchrotron emission
data. It is of importance here that the parameter values for SN 1006,
determined in this way, were very well confirmed by the Chandra measurements of
the fine structure of the nonthermal X-ray emission (Long et al. 2003; Bamba et al. 2003), as
analyzed by Berezhko et al. (2003).
Compared with the previous study (Ksenofontov et al. 2005), the analysis includes the
most accurate X-ray data from Chandra (Allen et al. 2004) and Suzaku
(Bamba et al. 2008) that make it possible to quite precisely determine the
acceptable range of these parameter values. Also a time-dependent,
amplified upstream magnetic field
![]() |
(1) |
is used - in the spherically symmetric calculation over the entire shock surface - where








![]() |
Figure 1:
Gas dynamical evolution for the two values of the total explosion
energy
|
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Since the properties of accelerated CR nuclear and electron spectra and their dependence on the relevent physical parameters, as well as the dynamical properties of the system, were described in detail in previous papers (e.g Ksenofontov et al. 2005; Berezhko et al. 2002), they will not be discussed here (see however the Appendix for a summary).
The values of the SN explosion energy
erg and
erg are taken to fit the
observed shock size
and shock speed
(Moffett et al. 1993) at the current epoch
yr (see Fig. 1) for the
ISM hydrogen number densities
and
,
respectively. These densities are
consistent with the observed level of the VHE emission (see below). Note that
the calculation presented in Fig. 1 corresponds to the best fit value of the
amplified upstream magnetic field strength B0=30
G, even though
the shock size
and shock speed
are quite
insensitive to B0. The resulting current total shock compression ratios
for
and
are
and 4.7, respectively,
whereas the subshock compression ratios
are both close to
.
As the most reliable estimate for the distance the value d=2.2 kpc was taken (Winkler et al. 2003).
All these results stem mainly from the gas dynamics part of the governing equations.
![]() |
Figure 2: Overall (=spatially integrated) spectra of energetic particles. Solid and dashed lines correspond to protons (p) and electrons (e), respectively. |
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Figure 2 shows the volume-integrated momentumg distributions of energetic protons
(representative for the nuclear component) and electrons. The proton cutoff
energy
is above 1015 eV. The electron
spectral index
equals that of the protons below the onset of
synchrotron losses at the energy
TeV. The electron
cutoff energy
is near 1014 eV.
![]() |
Figure 3:
Differential synchrotron radio emission flux as a function of
frequency, calculated for the ISM hydrogen number density
|
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Figures 3-5 illustrate the
consistency of the synchrotron spectrum, calculated for the ISM number density
with the best set of parameters (
,
,
B0=30
G), with the
observed spatially integrated spectra. Note that we use the differential
synchrotron fluxes in the radio (
)
and in the X-ray
(d
/d
)
range in a slightly different form. They
are however simply connected by the relation
d
/d
.
It should be noted that the values
of the differential radio
spectral index
/d
,
as observed in young SNRs,
require a curved electron spectrum that hardens to higher energies as
predicted by nonlinear shock acceleration models, as originally emphasized by
Reynolds & Ellison (1992). To have
in the radio range, as observed for
SN 1006, one needs efficient CR acceleration with a proton injection rate
which leads to the required shock modification, and a
high interior magnetic field
G is required
(Ksenofontov et al. 2005; Berezhko et al. 2002; Völk et al. 2005). The hardening of the observed radio synchrotron
spectrum for SN 1006 has been recently demonstrated by Allen et al. (2008).
Detailed X-ray synchrotron spectral measurements are, however, required to
find the optimum value of the magnetic field strength
(Ksenofontov et al. 2005) since for a given fit of the calculated synchrotron spectrum in
the radio range, the X-ray synchrotron amplitude is very sensitive to
.
The reason is the following: in the polar regions, where the
scattering mean free path decreases down to the particle gyroradius (Bohm
limit), the spectrum of accelerated electrons at the shock front is quite
insensitive to the magnetic field strength
if all other
relevant parameters are fixed; the exception is their maximal energy
which is due to synchrotron losses (e.g. Berezhko & Völk 2004b). Since all energy of
the electrons at high energies
,
where synchrotron losses are significant, is rapidly and
completely transformed into the synchrotron emission, the same is true for
the high frequency (
)
part of the synchrotron spectrum
including its cutoff part, because the cutoff frequency
does not depend on
.
(In the case B0=30
G and
G we have
eV and
Hz.) However, varying the
magnetic field strength
changes the amplitude of the overall
calculated synchrotron spectrum at high frequencies
in
order to keep the fit of the measured radio spectrum at low frequencies.
Therefore the high frequency part of the spectrum
changes due to
the variation of the breaking frequency
.
Since below
the synchrotron spectrum has
the form
with
,
we conclude
that the high frequency part of the synchrotron spectrum depends on
as
.
The
amplitude
is therefore the main determinant of the
value of
.
Together with the observed synchrotron amplitude in
the radio frequency region, where radiative losses play no role yet, the
amplitude of the nonthermal electron distribution and thus the
electron/proton ratio ratio
also is determined. Note that
the synchrotron spectrum is almost insensitive to the ISM density
(e.g. Ksenofontov et al. 2005).
![]() |
Figure 4:
X-ray synchrotron emission flux as a function of photon energy for
the same case as in Fig. 1 (solid line). Calculations for B0=27 |
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To obtain the fit to the data shown in Fig. 3 and Fig. 4, it
is assumed that only the highest-energy part of the observed global X-ray
spectrum, corresponding to
keV, is of predominantly nonthermal
origin. Towards lower energies
keV, the contribution of thermal
emission to this X-ray spectrum progressively increases, as clearly indicated
by the Suzaku spectrum. For these considerations, the X-ray Chandra flux from a
small region of the bright north eastern rim of SN 1006 (Allen et al. 2004) is
used, where the contributions from thermal X-rays are presumably minimal
(Cassam-Chenaï et al. 2008; Allen et al. 2004). In order to apply it to the whole remnant, this
X-ray flux was normalized to be consistent at energies
keV with
the global X-ray spectrum as observed by Suzaku (Bamba et al. 2008). This
consistency in fact exists. For quantitative purposes, the normalised Chandra
spectrum above 2 keV is used, under the assumption that it corresponds to pure
nonthermal emission. Comparison with the experimental X-ray data shows in
Figs. 3 and 4 that the optimum magnetic field value is
B0=30
G, and corresponds to a downstream field
G. This is in agreement with the field amplification that is implied
by the filamentary structures in hard X-rays (Völk et al. 2005).
Figure 5 shows the contours of
characterizing the quality of
fit of the radio spectrum, and of the X-ray spectrum for
keV,
for different values of
and
.
One can see that the
quality of the fit is rather good, allowing only a rather small range of these
parameters.
The quality of fit of the radio spectrum, and of the X-ray spectra for
keV, for B0=30
G is characterised by the value
.
An increase
of
(see Fig. 4) implies a change of the magnetic field strength value by
only 10%.
![]() |
Figure 5:
Contours of equal deviations
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The -ray morphology, as found in the H.E.S.S. measurements
(Naumann-Godo et al. 2009), is consistent with the prediction of a polar cap geometry
by Völk et al. (2003). Such a geometry has been also found experimentally from an
analysis of the synchrotron morphology in hard X-rays by Rothenflug et al. (2004) and
Cassam-Chenaï et al. (2008). This means that the
-ray emission calculated in the
spherically symmetric model must be renormalized (reduced) by a factor
,
as in Ksenofontov et al. (2005). The
renormalization factor is applied here.
This morphology is also the key argument for the existence of
an energetically dominant nuclear CR component in SN 1006, because only such a
component can amplify the magnetic field to the observed degree. If on the
contrary the existing energetic electron component would have to drive the
magnetic field amplification all by itself then we would, as a minimum, require
,
where
is the pressure of CR
electrons at the shock front. Clearly
,
where
is the pressure of CR
protons and
is the pressure of the nuclear component. Since
we have
for
and
km s-1. Therefore accelerated electrons are not able to amplify the
magnetic field to the required level by a large margin. On the other hand, the
effectively accelerated nuclear component has the pressure
and can therefore readily amplify the field from a purely
energetic point of view.
The amplified field must also be the reason for the corresponding polar
cap-type morphology of the synchrotron emission in hard X-rays. Even though the
weak radio synchrotron emission, essentially all around the periphery of
SN 1006 (Cassam-Chenaï et al. 2008; Rothenflug et al. 2004), demonstrates that electrons are at least to some
extent injected everywhere into the acceleration process over the shock, they
reach the multi-TeV energies for X-ray synchrotron emission only in the polar
caps, where the amplified field allows their acceleration to these energies. In
the equatorial region the upstream magnetic field strength corresponds to its
ISM value, at the height of SN 1006 above the Galactic plane probably not more
than G. Particle scattering is comparatively weak and the maximum
electron energy is expected to lie below the limit where the acceleration rate
equals the synchrotron loss rate. Then the maximum electron energy is - like
that of the nuclear particles - determined by the system geometry
(Berezhko 1996); an explicit calculation can be found in Berezhko & Völk (2004b). Assuming
then for simplicity that the spatial diffusion coefficient has the form
,
with a constant factor
,
requires
even for this low field strength in order to reduce the
equatorial maximum synchrotron frequency
to
Hz, for an azimuthally uniform shock velocity
km s-1. Such a plausible reduction of
is consistent with the findings of Rothenflug et al. (2004); see also
Miceli et al. (2009).
The only important parameter that cannot be determined from the analysis of
the synchrotron emission data is the external density .
Therefore we have performed the calculations for the pair of values
and
which
seems to bracket the density range consistent with the H.E.S.S.
-ray measurements.
Figure 6 shows the total (-decay plus inverse Compton (IC)),
and seperately the
-decay and the IC
-ray energy spectra of
the remnant, calculated for
and
.
H.E.S.S. has reported the measured value of the energy flux
erg/(cm2 s) of
-rays with energies
,
where
TeV and
TeV. It is related to the differential
-ray energy spectrum according to the expression

In a selfconsistent picture, the theoretically predicted form of the differential spectrum d



Note that the peak in the -ray energy spectrum at
TeV and the quasi-exponential cutoff at higher energies is due to
the substantial contribution of the IC component: at TeV energies the IC
component contributes about half of the total
-ray flux for
,
whereas for
it is already two thirds. Since the
maximal proton energy, reached at some time during the evolution, is about
eV, the corresponding
-decay component has a power law form
with
up to the cutoff energy
eV. The cutoff energy is defined here as that energy where the
spectral energy density has dropped by a factor 1/e from its maximum value,
which is given by log
.
![]() |
Figure 6:
Total ( |
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According to Fig. 6, the H.E.S.S. data are consistent with an ISM
number density from quite a narrow interval
,
since for the theoretically derived
-ray spectrum we have
erg/(cm2 s) and
erg/(cm2 s) for
and
respectively. The corresponding explosion
energy
erg is close to the upper end
of the typical range of type Ia SN explosion energies that vary by a factor of
about two (Blinnikov et al. 2006; Gamezo et al. 2005).
Acero et al. (2007) find the value
on the
basis of X-ray measurements. The above interval is consistent with their
result.
In Figure 6, the -ray spectrum produced by the nuclear CRs is very close to the
IC emission spectrum produced by CR electrons alone. Since the differential
energy spectrum of freshly accelerated nuclear particles and electrons is
rather close to a spectrum
,
and since the
electrons with energies
TeV
subsequently undergo significant synchrotron cooling in the interior, leading
to the spectrum
,
not only the amplitude but
also the shape of these two components are very similar within the energy
interval
eV. Therefore the VHE
-ray spectrum
alone is not able to discriminate between the hadronic
-decay and the
leptonic IC
-ray components. However, it was already shown by Ksenofontov et al. (2005) that
such a low VHE emission flux, with a highly depressed IC
-ray flux, is only
possible if the nuclear CR component is efficiently produced with accompanying
strong magnetic field amplification. In the framework of the interpretation
developed in this paper, the most direct evidence for the energetic dominance of
a nuclear energetic particle component is the observed
-ray morphology. It
corresponds to the theoretical prediction and is consistent with all other
measurements.
A last point concerns the radial extent
of the contact
discontinuity between ejected and swept-up mass relative to the radius
of the SNR blast wave, cf. recent data presented and discussed
by Cassam-Chenaï et al. (2008). The ratio
is given as
outside the synchrotron rims, and essentially as 1 in the region
within the synchrotron rims. In the case of Tycho's SNR we have discussed in
quantitative detail (e.g. Völk et al. 2008a) the reduction of this ratio compared
to a pure gas shock as a result of the considerable shock modification produced
by accelerated nuclear CRs, which leads to the increase of the shock
compression. Qualitatively such considerations agree with the experiment for
SN 1006: the ratio
is larger in the equatorial
region, where CR injection/acceleration is inefficient, and it is
smaller within the polar region, where CRs are efficiently produced. While for
Tycho's SNR particle acceleration gives a good theoretical explanation of the
relatively small ratio within our model, this is clearly impossible
quantitatively for the above numbers in the case of SN 1006. Cassam-Chenaï et al. (2008)
believe that the value of the contact discontinuity radius
was
overestimated ``... since our measurements are likely to be affected by
projection and other effects, ...''. On the other hand, very recently
Miceli et al. (2009) have found
of the order of
1.1, which is in the expected range. Future work will have to resolve this
difference.
3 Summary
Since the relevant astronomical parameters as well as the synchrotron spectrum
of SN 1006 are measured in impressive detail, it is possible to determine the
values of the relevant physical parameters with the appropriate accuracy for
this SNR: the proton injection rate
,
the
electron to proton ratio
,
and the
downstream magnetic field strength
G.
As a result, the flux of TeV emission detected by H.E.S.S. is consistent with
the ISM number density
.
The
corresponding hydrodynamic SN explosion energy
erg is close to the upper end
erg
of the typical range of type Ia SN explosion energies that vary by a factor of
about two. Also the magnetic field amplification properties of this SNR are
well understandable as the result of azimuthal variations of ion injection over
the projected SNR circumference and corresponding acceleration which lead to a
polar cap-type morphology for the X-ray synchrotron as well as the
-ray emission. As a consequence, the recent H.E.S.S. measurement of a dipolar
morphology also of the TeV
-ray emission is considered as the most significant
empirical evidence for the existence of an energetically dominant nonthermal
nuclear component.
Thus, SN 1006 appears to represent the first example where a high efficiency of nuclear CR production, required for the Galactic CR sources, is consistently established.
Acknowledgements
This work has been supported in part by the Russian Foundation for Basic Research (grants 06-02-96008, 07-02-0221). The authors are grateful to Drs. Aya Bamba and Glen Allen who provided them with the most recent X-ray data from the Chandra and Suzaku satellite observatories. The authors also thank Dr. Vladimir Zirakashvili for discussions regarding electron acceleration in SN 1006. EGB acknowledges the hospitality of the Max-Planck-Institut für Kernphysik, where part of this work was carried out.
Appendix A
The relation between the model calculations in spherical symmetry (1D) and the physical characteristics of the system in the presence of a symmetry-breaking magnetic field in the circumstellar medium were discussed earlier (Völk et al. 2003). In order to clarify the necessary corrections to the 1D-calculation for SN 1006, a qualitative discussion of the physical situation is given here.
A key feature is the empirical fact that the force density of the magnetic
field in the diffusive shock acceleration process can be neglected compared to
the gas ram pressure, the thermal pressure and the CR pressure. The average
magnetic field configuration can thus in principle be calculated purely
kinematically from the induction equation. However, the magnetic field
direction influences the injection at least of heavy ions in a significant
way. In a quasi-parallel shock, where the angle
between
the local mean field and the shock normal is small compared to
,
the required velocity of suprathermal ions from the downstream region, required
to be able to cross the shock into the upstream region, is much smaller than in
a quasi-perpendicular shock, where
is close to
.
For a steeply falling velocity distribution of the shocked
downstream gas, this leads to effective suppression of ion injection in
quasi-perpendicular shocks (Malkov & Völk 1995; Ellison et al. 1995). The acceleration of
injected particles, on the other hand, is efficient for all
for non-relativistic shocks, as long as
,
where
and c denote the
shock velocity and the speed of light, respectively (Drury 1983). The
injection of electrons is not yet well understood. From the observed azimuthal
distribution of the radio synchrotron emission in SN 1006
(e.g. Cassam-Chenaï et al. 2008), electron injection seems to operate for all angles
,
although apparently less efficiently in the
quasi-perpendicular region. This is the least restrictive assumption and it is
made here in the sequel.
A second point is that in the instantaneously quasi-parallel part of the
magnetic flux tubes in which ions are injected, the accelerated particles drive
a current against the flow of the upstream gas that generates strong magnetic
fluctuations and amplifies the magnetic field, as referred to in the main
text. This enhances the acceleration and leads to a pressure
of
the energetic nuclear particles that is comparable to the gas ram pressure for
strong shocks, e.g. in young SNRs, as well as to heating of the gas by wave
dissipation, both of the non-resonant and of the resonant Alfvénic modes
(McKenzie & Völk 1982; Pelletier et al. 2006; Zirakashvili et al. 2008; Bell & Lucek 2001; Bell 2004; Lucek & Bell 2000). Especially the
formation of density fluctuations and corresponding secondary shocks is viewed
here as a reason for strong wave dissipation (Völk et al. 2008a; Berezhko 2008). The pressure
of accelerated electrons is negligible in comparison. This implies a strong
hadronic
-ray emission from that part of these flux tubes, which at some time
became quasi-parallel. This hadronic flux is negligible elsewhere because there
are few energetic nuclei there. A certain amount of cross-field diffusion will,
however, also populate neighboring flux tubes and somewhat smear out the
acceleration boundaries (Völk et al. 2003). Accelerated electrons will emit strong
synchrotron radiation in the enhanced magnetic field regions, but may also
yield a more moderate synchrotron flux elsewhere. The reason is that by
assumption they are injected and accelerated to some extent ``everywhere'' on
the shock surface, albeit to considerably lower maximum energies in the
quasi-perpendicular regions. This is the result of the lack of self-excited
magnetic field fluctuations and of field amplification (see Sect. 2).
The third point regards differences in overall dynamics of the quasi-parallel and quasi-perpendicular regions. Conservation of the overall mass, momentum and energy fluxes allows only a different partition within the momentum and energy fluxes between their kinetic, thermal and nonthermal components, but the sums of these fluxes always remain the same. However, the compressibility of the relativistic CR ``gas'' is higher than that of the nonrelativistic thermal gas, and therefore this different partition implies dynamical differences between the quasi-parallel and the quasi-perpendicular regions.
When primarily the cumulative effect of the higher compressibility counts, such as in the relative distance between the outer SNR blast wave and the following contact discontinuity, then the different partitions in a CR-modified shock and a pure gas shock play the determining role.
However, in the present context, where the main characteristics of the
radiation effects and of their spatial distributions are to be evaluated, such
dynamical differences can to a first approximation be neglected. In practice,
for SN 1006, the overall shock compression ratio changes from
without CR production to
with efficient CR production, given
the assumed strong gas heating in the shock precursor.
Thus, for the evaluation of the hadronic -ray emission, only those flux tubes
from the 1D gas dynamic configuration are considered, which
connect to the part of the present phase shock surface where ion injection is
efficient. This requires the correction factor
.
Since the
particle transport perpendicular to the magnetic field direction is primarily corrective, adiabatic
expansion of the thermal gas leads to adiabatic losses of the energetic
particles and weakens the interior particle distribution. In addition, almost
all flux tubes originally have been quasi-perpendicular. The corresponding parts
of these flux tubes are now in the deeper interior of the SNR and therefore, to
a first approximation, lack accelerated nuclei there. For electrons the
radiative losses also increase towards the interior where the ``older'' particles
reside. These arguments show that both the synchrotron and the
-ray emission are
concentrated near the shock surface rather than in the interior. As long as the
ejected mass is not negligible compared to the swept-up mass, the ejecta volume
in the interior with its extremely small magnetic field strength will appear as
a region with particularly little radiation. For SN 1006 with its supposedly
simple magnetic field geometry, the evaluation of the ion injection fraction
of the shock surface has been given by Völk et al. (2003).
The electron spectrum also is calculated in spherical symmetry from Eq. (3) of
Berezhko et al. (2002). However, the actual evolutionary phase of SN 1006 appears to be
still rather in the transition from the sweep-up phase to what one can loosely
call a Sedov phase. Being pushed away by the ejecta, the density of the shocked
circumstellar medium has remained rather uniform. In the quasi-parallel flux
tubes, downstream of the shock, with massively injected nuclear particles, the
field
should therefore be strong and roughly uniform if field
dissipation is negligible. In the quasi-perpendicular regions on the other
hand, the field is only about four times larger than in the ambient
circumstellar medium. Finally, in the ejecta mass, the magnetic field strength
is presumably very small at the present time. Taken together, the overall
magnetic field strength in the SNR is far from uniform. Yet what counts
primarily for the integrated synchrotron emission are the quasi-parallel
regions.
It had indeed been assumed in past work (Ksenofontov et al. 2005; Berezhko et al. 2002) that to a first
approximation only those magnetic flux tubes play a role in the spatially
integrated emission that end at the forward shock in the polar caps. Therefore
this integral already contains the correction factor
implicitly and can be directly compared to the observed integral flux. The key
consideration in all of this is that the SNR consists of a strongly CR-modified
part with strong magnetic field amplification and a part that is essentially
unmodified. The correction factor changes only the amplitude but not the form of the calculated spectrum.
Adding a small population of electrons accelerated in the unmodified shock of the quasi-perpendicular regions would smear out the specifically nonlinear properties of the synchrotron spectrum. Therefore, the observed synchrotron spectrum exhibits the minimum nonlinear modifications which in reality should be even stronger. This means that the derived ion injection rate, magnetic field amplification and electron/proton ratio are lower limits for the quasi-parallel regions.
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Footnotes
- ...
) - These papers contain the basic equations used, in the form of a transport equation for energetic charged particles in a shock environment and its coupling to the gas dynamical equations for the thermal plasma. Considering seperately the energetic electron component, a corresponding transport equation is added that includes radiative energy losses (Berezhko et al. 2002). Also the particles' diffusion properties in the form of the Bohm limit, wherever nuclear particles are effectively injected into strong shocks, as well as the assumed heating rate of the thermal plasma by wave dissipation are detailed in these papers.
All Figures
![]() |
Figure 1:
Gas dynamical evolution for the two values of the total explosion
energy
|
Open with DEXTER | |
In the text |
![]() |
Figure 2: Overall (=spatially integrated) spectra of energetic particles. Solid and dashed lines correspond to protons (p) and electrons (e), respectively. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Differential synchrotron radio emission flux as a function of
frequency, calculated for the ISM hydrogen number density
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
X-ray synchrotron emission flux as a function of photon energy for
the same case as in Fig. 1 (solid line). Calculations for B0=27 |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Contours of equal deviations
|
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Total ( |
Open with DEXTER | |
In the text |
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