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H.E.S.S. and the origin of cosmic rays
Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 157–163 www.elsevier.com/locate/npbps
H.E.S.S. and the origin of cosmic rays Dieter Hornsa , for the H.E.S.S. Collaboration∗ a
Department of Physics, University Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
Galactic cosmic rays up to hundreds of TeV are very likely of Galactic origin, however the available observational data from measurements of energy spectra and chemical composition of cosmic rays provide only an indirect link to the originating particle accelerators. Very high energy (VHE: E > 100 GeV) gamma-rays are ideal tracers of ongoing acceleration as well as propagation of cosmic rays in the interstellar medium. Imaging air Cherenkov telescopes (IACTs) have achieved sufficient sensitivity to detect a number of cosmic-ray accelerators. In this contribution, we focus on shell-type and mixed-morphology supernova remnants (SNRs) as well as the starburst galaxy NGC 253 that have been observed with the H.E.S.S. (high energy stereoscopic system) array of IACTs.
1. Introduction The origin of Galactic cosmic-rays is linked to astrophysical accelerators which provide sufficient (acceleration) power (LCR ≈ 1040 ergs/s) to balance the escape losses and sustain an energy density of uCR ≈ 1 eV/cm3 in the volume of the Galaxy. The sum of all shell-type SNR in the Galaxy heat the inter-stellar medium with roughly 1041 ergs/s. Assuming that ≈ 10 % of the kinetic energy is not thermalized but instead is dissipated in non-thermal particle population, SNR provide sufficient power to sustain the Galactic cosmic ray population. The acceleration of particles at the forward shock of a shelltype SNR could very well explain the measured power-law energy spectrum of Galactic cosmicrays. This common paradigm of the origin of Galactic cosmic rays leads to two predictions observable at gamma-rays: (1) The injection of Galactic cosmic-rays should follow the distribution of shell-type SNR and (2) shell-type SNR (and their environment) should be gamma-ray bright. Ad (1): The recent analysis of gamma-ray data from the Galactic anti-centre region indicates that the distribution of sources required to match the data differs from the distribution of SNR or pulsars [1]. However, the approach using a pos∗ For a complete list of authors: hd.mpg.de/hfm/hess
http://www.mpi-
0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.03.022
sible biased measurement of the SNR distribution and comparing it with a complicated, multicomponent model of diffuse gamma-ray emission which again is difficult to measure may not be very reliable. Ad (2): With the advent of powerful VHE spectrometer and imager, it has become possible to detect individual shell-type SNR over a wide range of energies and to resolve their morphology with an unrivaled accuracy in the high energy band. Here, we summarize the observations and the main results obtained with the H.E.S.S. array of IACTs. 2. Instrument and observations The experimental technique of air Cherenkov detection and its historical development is discussed in the contribution by A. Watson in this volume. The latest generation of imaging air Cherenkov detectors has been established with the H.E.S.S. system of four individual telescopes located in the Khomas highlands in Namibia (−23.27◦ south, 16.5◦ east) at 1 800 m altitude. Each telescope consists of a tessellated mirror surface of 107 m2 and a photo-multiplier tube (PMT) camera (960 individual PMT, each sub-tending 0.16◦ in the sky) viewing in total a patch in the sky with 5◦ diameter [2]. The stereoscopic imaging and reconstruction of air showers has been developed further over the past years achieving
158
D. Horns / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 157–163
an angular resolution per event of 0.06◦ (68 % containment radius) and a relative energy resolution better than 15 % over the entire energy range [3]. A recently developed precision pointing model using two CCD cameras in the mirror dish has improved the astrometry to be better than 10 for bright sources [4]. The large field of view and wide spacing of the telescopes provides an unrivaled large dynamical range from a threshold of 100–150 GeV (varying with the ageing of the camera and mirrors) up to 100 TeV, where event statistics limits the sensitivity. A point-like source with 1 % of the flux from the Crab nebula is detected within an observation time of 25 hrs [2]. This corresponds to a minimum luminosity of Lmin ≈ 3 × 1031 ergs s−1 (d/1 kpc)2 for a source at 1 kpc distance. The observations of shell-type and mixed morphology SNR with H.E.S.S. are summarized in Table 1. In the following section, the sources are discussed individually. 3. Gamma-ray emission from SNR The following sections summarizes the properties of currently known gamma-ray emitting SNR. The objects are ordered in a subjective sequence where the best-understood (from astrophysical and theoretical arguments) are followed by objects with larger uncertainties on important parameters like distance and age. 3.1. SN 1006 The historical supernova type Ia remnant SN 1006 is a recent addition to the list of HESSdetected SNR (see Table 1). The object is among the faintest Galactic sources detected with HESS. A considerable amount of observation time (130 hrs) has been spent on this object after the initial data-set did not reveal a signal with an upper limit of 5 % of the Crab flux [15]. The main motivation for a deeper observation is the rather low (and as it turned out accurate) flux predictions from detailed modelling of the evolution and properties of the SNR using realistic values of the gas density [16]. Even though the flux of SN 1006 is only at the level of 1 % of the Crab [11], it is possible to distinguish two main
emission regions which line up perfectly with the non-thermal X-ray emission detected from the NE and SW rim of the SNR [17] (see Fig. 1a). The energy spectra measured with HESS between 200 GeV and 10 TeV from the two rims are within the errors consistent with each other: both energy spectra follow a power-law with dNγ /dE ∝ E −Γ with ΓNE = 2.35(14) and ΓSW = 2.29(18). The combined Gamma-ray luminosity is Lγ = 7.8 × 1032 ergs/s integrated between 1 and 10 TeV. If the gamma-ray emission is produced through inelastic scattering of (mainly) protons on the ambient gas, the gamma-ray luminosity can be converted into lower limit2 for the energy in protons (Wp (10 − 100 TeV) ≈ τpp Lγ ≈ 4 × 1049 ergs with τpp = 1.7 × 109 yrs (n/0.0085 cm−3 )−1 . The ambient medium density at the location of SN 1006 600 pc above the Galactic plane has been estimated using different methods to be n = 0.085. The measured gamma-ray luminosity is consistent with efficient cosmic-ray acceleration in the NE and SW rim of SN 1006. Taken at face value there appears to be tension between the observed photon index of ≈ 2.3 and the expected index < 2 in the non-linear acceleration regime (see e.g. [18]). However, the error on the photon index of σΓ ≈ 0.2 (statistical only at 68 % c.l) is still too large to claim a significant deviation from the expectation. 3.2. RCW 86 In comparison to SN 1006, RCW 86 has evolved in a complex inhomogeneous environment including a wind-swept bubble and high density shell as well as a high density HII region to the south. VHE gamma-rays have been detected from the direction of RCW 86 [10]. The observed intensity map is generally in agreement with the angular extent of the radio as well as X-ray emission, however the gamma-ray emission does not vary significantly along the outer rim whereas the Xray emission is strongly peaked towards the HII region in the south (see Fig. 1b). This part of the shell is about a factor of 5 brighter than any other region along the shell. The published gamma-ray 2 depending on the extrapolation of the gamma-ray spectrum to smaller energies the total energy in protons could be more than one order larger
D. Horns / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 157–163
data do not provide sufficient statistics to distinguish between a shell like or centrally-filled morphology. The energy spectrum of RCW 86 is slightly softer (well-fit by a power-law with Γ = 2.41(16)) than for SN1006 with a gamma-ray luminosity almost one order of magnitude larger than in the case of SN 1006. The resulting energy contained in protons is a factor of 2-4 larger than the value found for SN 1006 (for an ambient medium density between 0.3−0.6 cm−3 ). Given the soft spectrum, the (extrapolated) total energy of cosmicrays above 1 GeV becomes quite close to the explosion energy, providing additional evidence for efficient shock acceleration. However, the measured photon index is significantly softer than expected in the case of non-linear shock acceleration.
3.3. RX J1713.7-3946 and RX J0852.04622 Two well-known, but complex VHE emitting shell-type SNR have been observed in great detail with the HESS instruments: RX J1713.7-3946 [5– 7] and RX J0852.0-4622 [8,9]. Both objects are not associated with historical events and therefore, estimates on the distance suffer from considerable uncertainties. Observations with HESS have revealed very similar gamma-ray spectra Γ ≈ 2 from these two objects with a clear indication for curvature in the spectrum of RX J1713.73946 which is best described by a smooth roll-over at 3-6 TeV. The energy spectrum of RX J0852.04622 shows a similar behavior. However, the curvature is not detected at a significant level. Even though the objects appear from their energy spectrum almost like twins, the morphology is significantly different. In the case of RX J1713.73946, the emission originates from a thick shell with ΔR/R ≈ 0.5 while for RX J0852.0-4622, the ratio of shell-thickness to shell-radius ΔR/R ≈ 0.2. In both cases, the gamma-ray emission and non-thermal emission are significantly correlated. Given the large uncertainties on the distance and age of these two objects, it is difficult to discuss the energetics.
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3.4. HESS J1731-347 This recently identified SNR had initially been discovered as a counterpart-less object in the HESS Galactic plane survey [19] and was later found to coincide with a shell-like SNR candidate visible at 1.4 GHz [20]. Deeper observations with the HESS telescopes and additional multiwavelength observations with XMM-Newton and Suzaku provide evidence for a shell-like VHE morphology, well correlated with the X-ray as well as the radio morphology [12]. The distance of the object has been estimated using absorption features in the 9 to -24 km/s band to be > 3.2 kpc [20,12]. Taking the lower limit on the distance into account, HESS J1731-347 is the largest (28 pc in diameter) and also the oldest (27 kyrs [20]) shell-type SNR in the sample discussed here (see Fig. 3). The energy spectrum of the object is consistent with the one measured from younger SNR (Γ = 2.3(1)) without evidence for a break or a cut-off below 10 TeV. The VHE emission from the shell of HESS J1731-347 does not show azimuthal modulation.
3.5. Mixed-morphology SNR: W28 The mixed-morphology system W28 and its environment has been detected to host up to four VHE sources (see Fig. 2) [13]. The northern source HESS J1801-233 lines up with the X-ray bright NE part of the radio shell. The presence of Maser emission indicates that this part of the shock is interacting with dense molecular gas which is also traced in the CO molecular transition line emission (see Fig. 2a). The VHE gamma-ray emission coincides with a region of high gas density indicates strong evidence for gamma-ray emission from protons/nuclei interacting with molecular gas. This picture is confirmed by the detection of lower-energy gammarays with the Fermi-LAT [21]. The three sources to the south of W28 show considerable overlap with dense molecular gas. Under the assumption, that cosmic-rays accelerated and released from W28 illuminate these clouds, the cosmicray energy density in this particular volume is roughly an order of magnitude larger than in the solar environment.
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3.6. Starburst galaxy NGC 253 Even though the gamma-ray emission in the individual objects discussed above can not be clearly assigned to either protons/nuclei or electrons, the observations imply an upper limit on the energy stored in hadronic cosmic-rays which in turn constrain the maximum acceleration efficiency. The results summarized here indicate that not more than 10% of the bulk kinetic energy of the supernova remnant is converted to accelerated cosmic-rays. This efficiency is sufficient (by a small margin) to explain the origin of galactic cosmic-rays with particle acceleration at supernova-shocks (even more so, if the efficiency remains high even for older SNR in the post-sedov evolutionary phases like HESS J1731347). A complementary approach to test the origin of cosmic rays is to observe starburst galaxies. These galaxies are characterized by an increased star formation rate either traced through radio or infra-red observations. The classical, nearby starburst galaxies are M82 (in the northern hemisphere) and NGC 253 (in the southern hemisphere). Both objects have been discovered to emit VHE gamma-rays (NGC 253 with HESS [22] and M82 with VERITAS [23]). In both cases, a similar cosmic-ray energy density has been inferred. The average cosmic-ray energy density at TeV energies is ≈ 6.4 eV/cm3 . Even though the gas density is quite large at 600 cm−3 with a correspondingly small value of τpp ≈ 105 yrs, the observed gamma-ray luminosity implies that only 5% of the cosmic-rays interact with the gas producing gamma-rays before leaving the star burst region. Very likely, convective motion of the heated gas is responsible for the fast escape of cosmic-rays from the high density starburst core [22]. 4. Conclusions Observations of young shell-type supernova remnants with HESS have revealed a rich morphology with thin as well as thick shells, partial and complete shells. With the exception of RCW 86, X-ray, radio, and VHE morphologies show a close resemblance, indicating that at least a fraction of the observed gamma-rays
can be plausibly produced by energetic electrons. Consistent signatures for efficient acceleration of energetic protons/nuclei has not been detected in the VHE spectra and the overall gamma-ray luminosity of SNR, in line with the most recent self-consistent modeling of thermal emission from cosmic-ray induced heating [24]3 . However, the discovery of the VHE supernova remnant HESS J1731-347 which is already in its postSedov evolutionary phase indicates that SNR may accelerate particles for more than 10 000 yrs and therefore, the overall efficiency of converting bulk kinetic energy of the shock into cosmic-rays may be higher than originally anticipated. Observations of gamma-ray emission from regions of high gas density in the environment of evolved SNR (e.g. W28) provides important clues on the history of particle acceleration and release into the ISM. Finally, the detection of gamma-rays and consequently large (three orders of magnitude) cosmic-ray density in starburst galaxies establishes a direct link between star-formation history and cosmic-ray production. Acknowledgements The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staying in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. We thank the organisers for the invitation to this fruitful and vivid seminar. 3 Collisional heating should lead to strong X-ray line emission at keV energies, which is not seen
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Figure 1. (a) Excess VHE gamma-ray counts from SN 1006 smoothed with a top-hat kernel with 0.05◦ radius. Overlaid are isophotes from the XMM-Newton map (integrated between 2–4.5 keV and smoothed with the HESS point-spread function, see inset). (b) Excess-rate VHE gamma-ray contours (0.55, 0.8, and 1.05 γ min−1 from RCW 86 overlaid on XMM-Newton excess map (3–6 keV). The two pictures are to scale (10’ corresponds to 6 pc at 2.3 kpc distance).
Figure 2. W28: (a) The density of molecular gas traced through the CO (J = 1 → 0) line integrated for velocities between 10 and 20 km/s [13]. Overlaid as continuous light-gray contours are the significance levels of 4, 5, and 6 σ from the H.E.S.S. observations. (b) The 90 cm continuum emission (VLA) is compared to the observed gamma-ray excess.
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Figure 3. (HESS J1731-347: (a) The preliminary gamma-ray skymap covering 1.5◦ × 1.5◦ (N is up, E left) shows a closed shell morphology (HESS J1731-347A) with a second, more compact component to the north-west (HESS J1731-347B). (b) For HESS J1731-347A, the radial profile (gray filled circles) measured at VHE gamma-rays is very similar to the profile measured at 1.4 GHz (squares). The observed profile is not consistent with a homogeneously emitting sphere. Table 1 Summary of observations and results of SNR observations (in order of discovery, CCO: compact central object). Name
RX J1713.7-3946 RX J0852.0-4622 RCW 86 SN 1006 HESS J1731-347 W28 Kepler’s SNR
distance
Lγ (1-10 TeV)
age
[kpc]
[1033 ergs/s]
[kyrs]
1 0.33 2.8 2.2 3.2 1.9 4.8
5.7 0.6 5.5 0.8 16.7 0.5 < 2.4
1.6 0.66 1.8 1 27 33 0.4
REFERENCES 1. Fermi LAT Collaboration, preprint arXiv1011.0816 (2010) 2. Aharonian, F. et al. (HESS coll.), A&A 457, 899 (2006) 3. de Naurois, M. and L. Rolland, Astrop. Phys 32, 231 (2009) 4. Acero, F. et al. (HESS coll.), MNRAS 402,
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7 (2005) 9. Aharonian, F. et al. (HESS coll.), ApJ 661, 236 (2007) 10. Aharonian, F. et al. (HESS coll.), ApJ 692, 1500 (2009) 11. Acero, F. et al. (HESS coll.), A&A 516, 62 (2010) 12. Acero, F. et al. (HESS coll.), presented at the 38th COSPAR (2010), session E19 (see http://www.physics.umanitoba.ca/˜samar/COSPAR2010E19 ) 13. Aharonian, F. et al. (HESS coll.), A&A 481, 401 (2008) 14. Aharonian, F. et al. (HESS coll.), A&A 488, 219 (2008) 15. Aharonian, F. et al. (HESS coll.), A&A 437, 135 (2005) 16. Ksenofontov, L.T., Berezhko, E.G., V¨ olk, H.J., A&A 443, 973 (2005) 17. Koyama, K. et al., Nature 378, 255 (1995) 18. Berezhko, E.G. et al., A&A 505, 169 (2009) 19. Aharonian, F. et al. (HESS coll.), A&A 477, 342 (2008) 20. Tian, W. W. et al., ApJ 679, 85 (2008) 21. Fermi LAT Collaboration, preprint arXiv1005.4474 (2010) 22. Aharonian, F. et al., Science 326, 1080 (2009) 23. VERITAS Collaboration, Acciari, V. A. et al., Nature 462, 770 (2009) 24. Ellison, D. et al., ApJ 712, 287 (2010)
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H.E.S.S. and the origin of cosmic rays Dieter Hornsa , for the H.E.S.S. Collaboration∗ a
Department of Physics, University Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
Galactic cosmic rays up to hundreds of TeV are very likely of Galactic origin, however the available observational data from measurements of energy spectra and chemical composition of cosmic rays provide only an indirect link to the originating particle accelerators. Very high energy (VHE: E > 100 GeV) gamma-rays are ideal tracers of ongoing acceleration as well as propagation of cosmic rays in the interstellar medium. Imaging air Cherenkov telescopes (IACTs) have achieved sufficient sensitivity to detect a number of cosmic-ray accelerators. In this contribution, we focus on shell-type and mixed-morphology supernova remnants (SNRs) as well as the starburst galaxy NGC 253 that have been observed with the H.E.S.S. (high energy stereoscopic system) array of IACTs.
1. Introduction The origin of Galactic cosmic-rays is linked to astrophysical accelerators which provide sufficient (acceleration) power (LCR ≈ 1040 ergs/s) to balance the escape losses and sustain an energy density of uCR ≈ 1 eV/cm3 in the volume of the Galaxy. The sum of all shell-type SNR in the Galaxy heat the inter-stellar medium with roughly 1041 ergs/s. Assuming that ≈ 10 % of the kinetic energy is not thermalized but instead is dissipated in non-thermal particle population, SNR provide sufficient power to sustain the Galactic cosmic ray population. The acceleration of particles at the forward shock of a shelltype SNR could very well explain the measured power-law energy spectrum of Galactic cosmicrays. This common paradigm of the origin of Galactic cosmic rays leads to two predictions observable at gamma-rays: (1) The injection of Galactic cosmic-rays should follow the distribution of shell-type SNR and (2) shell-type SNR (and their environment) should be gamma-ray bright. Ad (1): The recent analysis of gamma-ray data from the Galactic anti-centre region indicates that the distribution of sources required to match the data differs from the distribution of SNR or pulsars [1]. However, the approach using a pos∗ For a complete list of authors: hd.mpg.de/hfm/hess
http://www.mpi-
0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.03.022
sible biased measurement of the SNR distribution and comparing it with a complicated, multicomponent model of diffuse gamma-ray emission which again is difficult to measure may not be very reliable. Ad (2): With the advent of powerful VHE spectrometer and imager, it has become possible to detect individual shell-type SNR over a wide range of energies and to resolve their morphology with an unrivaled accuracy in the high energy band. Here, we summarize the observations and the main results obtained with the H.E.S.S. array of IACTs. 2. Instrument and observations The experimental technique of air Cherenkov detection and its historical development is discussed in the contribution by A. Watson in this volume. The latest generation of imaging air Cherenkov detectors has been established with the H.E.S.S. system of four individual telescopes located in the Khomas highlands in Namibia (−23.27◦ south, 16.5◦ east) at 1 800 m altitude. Each telescope consists of a tessellated mirror surface of 107 m2 and a photo-multiplier tube (PMT) camera (960 individual PMT, each sub-tending 0.16◦ in the sky) viewing in total a patch in the sky with 5◦ diameter [2]. The stereoscopic imaging and reconstruction of air showers has been developed further over the past years achieving
158
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an angular resolution per event of 0.06◦ (68 % containment radius) and a relative energy resolution better than 15 % over the entire energy range [3]. A recently developed precision pointing model using two CCD cameras in the mirror dish has improved the astrometry to be better than 10 for bright sources [4]. The large field of view and wide spacing of the telescopes provides an unrivaled large dynamical range from a threshold of 100–150 GeV (varying with the ageing of the camera and mirrors) up to 100 TeV, where event statistics limits the sensitivity. A point-like source with 1 % of the flux from the Crab nebula is detected within an observation time of 25 hrs [2]. This corresponds to a minimum luminosity of Lmin ≈ 3 × 1031 ergs s−1 (d/1 kpc)2 for a source at 1 kpc distance. The observations of shell-type and mixed morphology SNR with H.E.S.S. are summarized in Table 1. In the following section, the sources are discussed individually. 3. Gamma-ray emission from SNR The following sections summarizes the properties of currently known gamma-ray emitting SNR. The objects are ordered in a subjective sequence where the best-understood (from astrophysical and theoretical arguments) are followed by objects with larger uncertainties on important parameters like distance and age. 3.1. SN 1006 The historical supernova type Ia remnant SN 1006 is a recent addition to the list of HESSdetected SNR (see Table 1). The object is among the faintest Galactic sources detected with HESS. A considerable amount of observation time (130 hrs) has been spent on this object after the initial data-set did not reveal a signal with an upper limit of 5 % of the Crab flux [15]. The main motivation for a deeper observation is the rather low (and as it turned out accurate) flux predictions from detailed modelling of the evolution and properties of the SNR using realistic values of the gas density [16]. Even though the flux of SN 1006 is only at the level of 1 % of the Crab [11], it is possible to distinguish two main
emission regions which line up perfectly with the non-thermal X-ray emission detected from the NE and SW rim of the SNR [17] (see Fig. 1a). The energy spectra measured with HESS between 200 GeV and 10 TeV from the two rims are within the errors consistent with each other: both energy spectra follow a power-law with dNγ /dE ∝ E −Γ with ΓNE = 2.35(14) and ΓSW = 2.29(18). The combined Gamma-ray luminosity is Lγ = 7.8 × 1032 ergs/s integrated between 1 and 10 TeV. If the gamma-ray emission is produced through inelastic scattering of (mainly) protons on the ambient gas, the gamma-ray luminosity can be converted into lower limit2 for the energy in protons (Wp (10 − 100 TeV) ≈ τpp Lγ ≈ 4 × 1049 ergs with τpp = 1.7 × 109 yrs (n/0.0085 cm−3 )−1 . The ambient medium density at the location of SN 1006 600 pc above the Galactic plane has been estimated using different methods to be n = 0.085. The measured gamma-ray luminosity is consistent with efficient cosmic-ray acceleration in the NE and SW rim of SN 1006. Taken at face value there appears to be tension between the observed photon index of ≈ 2.3 and the expected index < 2 in the non-linear acceleration regime (see e.g. [18]). However, the error on the photon index of σΓ ≈ 0.2 (statistical only at 68 % c.l) is still too large to claim a significant deviation from the expectation. 3.2. RCW 86 In comparison to SN 1006, RCW 86 has evolved in a complex inhomogeneous environment including a wind-swept bubble and high density shell as well as a high density HII region to the south. VHE gamma-rays have been detected from the direction of RCW 86 [10]. The observed intensity map is generally in agreement with the angular extent of the radio as well as X-ray emission, however the gamma-ray emission does not vary significantly along the outer rim whereas the Xray emission is strongly peaked towards the HII region in the south (see Fig. 1b). This part of the shell is about a factor of 5 brighter than any other region along the shell. The published gamma-ray 2 depending on the extrapolation of the gamma-ray spectrum to smaller energies the total energy in protons could be more than one order larger
D. Horns / Nuclear Physics B (Proc. Suppl.) 212–213 (2011) 157–163
data do not provide sufficient statistics to distinguish between a shell like or centrally-filled morphology. The energy spectrum of RCW 86 is slightly softer (well-fit by a power-law with Γ = 2.41(16)) than for SN1006 with a gamma-ray luminosity almost one order of magnitude larger than in the case of SN 1006. The resulting energy contained in protons is a factor of 2-4 larger than the value found for SN 1006 (for an ambient medium density between 0.3−0.6 cm−3 ). Given the soft spectrum, the (extrapolated) total energy of cosmicrays above 1 GeV becomes quite close to the explosion energy, providing additional evidence for efficient shock acceleration. However, the measured photon index is significantly softer than expected in the case of non-linear shock acceleration.
3.3. RX J1713.7-3946 and RX J0852.04622 Two well-known, but complex VHE emitting shell-type SNR have been observed in great detail with the HESS instruments: RX J1713.7-3946 [5– 7] and RX J0852.0-4622 [8,9]. Both objects are not associated with historical events and therefore, estimates on the distance suffer from considerable uncertainties. Observations with HESS have revealed very similar gamma-ray spectra Γ ≈ 2 from these two objects with a clear indication for curvature in the spectrum of RX J1713.73946 which is best described by a smooth roll-over at 3-6 TeV. The energy spectrum of RX J0852.04622 shows a similar behavior. However, the curvature is not detected at a significant level. Even though the objects appear from their energy spectrum almost like twins, the morphology is significantly different. In the case of RX J1713.73946, the emission originates from a thick shell with ΔR/R ≈ 0.5 while for RX J0852.0-4622, the ratio of shell-thickness to shell-radius ΔR/R ≈ 0.2. In both cases, the gamma-ray emission and non-thermal emission are significantly correlated. Given the large uncertainties on the distance and age of these two objects, it is difficult to discuss the energetics.
159
3.4. HESS J1731-347 This recently identified SNR had initially been discovered as a counterpart-less object in the HESS Galactic plane survey [19] and was later found to coincide with a shell-like SNR candidate visible at 1.4 GHz [20]. Deeper observations with the HESS telescopes and additional multiwavelength observations with XMM-Newton and Suzaku provide evidence for a shell-like VHE morphology, well correlated with the X-ray as well as the radio morphology [12]. The distance of the object has been estimated using absorption features in the 9 to -24 km/s band to be > 3.2 kpc [20,12]. Taking the lower limit on the distance into account, HESS J1731-347 is the largest (28 pc in diameter) and also the oldest (27 kyrs [20]) shell-type SNR in the sample discussed here (see Fig. 3). The energy spectrum of the object is consistent with the one measured from younger SNR (Γ = 2.3(1)) without evidence for a break or a cut-off below 10 TeV. The VHE emission from the shell of HESS J1731-347 does not show azimuthal modulation.
3.5. Mixed-morphology SNR: W28 The mixed-morphology system W28 and its environment has been detected to host up to four VHE sources (see Fig. 2) [13]. The northern source HESS J1801-233 lines up with the X-ray bright NE part of the radio shell. The presence of Maser emission indicates that this part of the shock is interacting with dense molecular gas which is also traced in the CO molecular transition line emission (see Fig. 2a). The VHE gamma-ray emission coincides with a region of high gas density indicates strong evidence for gamma-ray emission from protons/nuclei interacting with molecular gas. This picture is confirmed by the detection of lower-energy gammarays with the Fermi-LAT [21]. The three sources to the south of W28 show considerable overlap with dense molecular gas. Under the assumption, that cosmic-rays accelerated and released from W28 illuminate these clouds, the cosmicray energy density in this particular volume is roughly an order of magnitude larger than in the solar environment.
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3.6. Starburst galaxy NGC 253 Even though the gamma-ray emission in the individual objects discussed above can not be clearly assigned to either protons/nuclei or electrons, the observations imply an upper limit on the energy stored in hadronic cosmic-rays which in turn constrain the maximum acceleration efficiency. The results summarized here indicate that not more than 10% of the bulk kinetic energy of the supernova remnant is converted to accelerated cosmic-rays. This efficiency is sufficient (by a small margin) to explain the origin of galactic cosmic-rays with particle acceleration at supernova-shocks (even more so, if the efficiency remains high even for older SNR in the post-sedov evolutionary phases like HESS J1731347). A complementary approach to test the origin of cosmic rays is to observe starburst galaxies. These galaxies are characterized by an increased star formation rate either traced through radio or infra-red observations. The classical, nearby starburst galaxies are M82 (in the northern hemisphere) and NGC 253 (in the southern hemisphere). Both objects have been discovered to emit VHE gamma-rays (NGC 253 with HESS [22] and M82 with VERITAS [23]). In both cases, a similar cosmic-ray energy density has been inferred. The average cosmic-ray energy density at TeV energies is ≈ 6.4 eV/cm3 . Even though the gas density is quite large at 600 cm−3 with a correspondingly small value of τpp ≈ 105 yrs, the observed gamma-ray luminosity implies that only 5% of the cosmic-rays interact with the gas producing gamma-rays before leaving the star burst region. Very likely, convective motion of the heated gas is responsible for the fast escape of cosmic-rays from the high density starburst core [22]. 4. Conclusions Observations of young shell-type supernova remnants with HESS have revealed a rich morphology with thin as well as thick shells, partial and complete shells. With the exception of RCW 86, X-ray, radio, and VHE morphologies show a close resemblance, indicating that at least a fraction of the observed gamma-rays
can be plausibly produced by energetic electrons. Consistent signatures for efficient acceleration of energetic protons/nuclei has not been detected in the VHE spectra and the overall gamma-ray luminosity of SNR, in line with the most recent self-consistent modeling of thermal emission from cosmic-ray induced heating [24]3 . However, the discovery of the VHE supernova remnant HESS J1731-347 which is already in its postSedov evolutionary phase indicates that SNR may accelerate particles for more than 10 000 yrs and therefore, the overall efficiency of converting bulk kinetic energy of the shock into cosmic-rays may be higher than originally anticipated. Observations of gamma-ray emission from regions of high gas density in the environment of evolved SNR (e.g. W28) provides important clues on the history of particle acceleration and release into the ISM. Finally, the detection of gamma-rays and consequently large (three orders of magnitude) cosmic-ray density in starburst galaxies establishes a direct link between star-formation history and cosmic-ray production. Acknowledgements The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staying in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. We thank the organisers for the invitation to this fruitful and vivid seminar. 3 Collisional heating should lead to strong X-ray line emission at keV energies, which is not seen
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Figure 1. (a) Excess VHE gamma-ray counts from SN 1006 smoothed with a top-hat kernel with 0.05◦ radius. Overlaid are isophotes from the XMM-Newton map (integrated between 2–4.5 keV and smoothed with the HESS point-spread function, see inset). (b) Excess-rate VHE gamma-ray contours (0.55, 0.8, and 1.05 γ min−1 from RCW 86 overlaid on XMM-Newton excess map (3–6 keV). The two pictures are to scale (10’ corresponds to 6 pc at 2.3 kpc distance).
Figure 2. W28: (a) The density of molecular gas traced through the CO (J = 1 → 0) line integrated for velocities between 10 and 20 km/s [13]. Overlaid as continuous light-gray contours are the significance levels of 4, 5, and 6 σ from the H.E.S.S. observations. (b) The 90 cm continuum emission (VLA) is compared to the observed gamma-ray excess.
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Figure 3. (HESS J1731-347: (a) The preliminary gamma-ray skymap covering 1.5◦ × 1.5◦ (N is up, E left) shows a closed shell morphology (HESS J1731-347A) with a second, more compact component to the north-west (HESS J1731-347B). (b) For HESS J1731-347A, the radial profile (gray filled circles) measured at VHE gamma-rays is very similar to the profile measured at 1.4 GHz (squares). The observed profile is not consistent with a homogeneously emitting sphere. Table 1 Summary of observations and results of SNR observations (in order of discovery, CCO: compact central object). Name
RX J1713.7-3946 RX J0852.0-4622 RCW 86 SN 1006 HESS J1731-347 W28 Kepler’s SNR
distance
Lγ (1-10 TeV)
age
[kpc]
[1033 ergs/s]
[kyrs]
1 0.33 2.8 2.2 3.2 1.9 4.8
5.7 0.6 5.5 0.8 16.7 0.5 < 2.4
1.6 0.66 1.8 1 27 33 0.4
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