Hard Nonthermal Continua and Cosmic Ray Acceleration

It has long been thought that the shocks in supernova remnants are the primary sites of Galactic cosmic ray acceleration. This hypothesis is supported observationally by the presence of nonthermal radio emission from SNR shells arising from electrons accelerated to GeV energy. Theoretical models of diffusive shock acceleration (1st order Fermi acceleration) indicate that efficient acceleration can occur within SNR shocks. The signature of cosmic ray acceleration is subtle. Both electrons and protons have signatures in the TeV band, but this has been largely inaccessible, and is only now becoming open to sensitive observations. Synchrotron radiation from electrons accelerated to TeV energy falls in the X-ray band. Most bright SNRs have strong thermal emission from the candidate acceleration regions, and so finding the signature of TeV electrons required instruments with good angular resolution and a broad spectral band. It also required the existence of a fortuitous object, SN 1006.

SN 1006, the remnant of the brightest historical supernova, is located at high galactic latitude, in a region of apparently low ambient density. It is one of two (and possibly three) historical remnants of Type Ia SNe. The first broad band X-ray spectral observations showed a spectrum dominated by continuum, with no Fe K lines, in contrast to other young SNRs [11]. This led to competing interpretations, one requiring shock accelerated electrons [129], the other invoking continuum thermal emission from fully ionized carbon [45]. The first imaging data showed the remnant to have bright NE and SW rims, whose spectrum is considerably harder than elsewhere in the remnant. The hardness and brightness of the rims were problematic for a thermal model because one would expect brightness to correlate with density squared and temperature (hardness) to correlate inversely with density.

The required observational breakthrough came from ASCA, which showed that the emission from the bright rims was essentially free of line emission, while the interior radiated line emission from elements characteristic of Type Ia ejecta. The only plausible explanation for the hard continuum emission from the bright rims is synchrotron emission from electrons shock accelerated to TeV energy [89].

Chandra and XMM-Newton observations have provided clearer views of the SN 1006 rims (Fig. 17.10). The Chandra observations show that the hard, nonther-mal rims follow the radio emission extremely closely indicating a single emission mechanism is responsible for both [96].

Fig. 17.10 Multicolor ACIS image of SN 1006 dramatically contrasts the hard, nonthermal emission from the NE and SW rims with the thermal emission from the bulk of the remnant. The nonthermal X-ray filaments correlate strongly with the radio filaments, indicating they arise from the same emission measure. The projected interior thermal emission shows plume-like structures like those in Tycho, the other known historical Type Ia remnant 17.2. The remnant is —30arcmin across, correpsonding to a physical diameter of 18 pc at a distance of 2.1 kpc (Figure courtesy of Chandra X-ray Center and J. P. Hughes.)

Fig. 17.10 Multicolor ACIS image of SN 1006 dramatically contrasts the hard, nonthermal emission from the NE and SW rims with the thermal emission from the bulk of the remnant. The nonthermal X-ray filaments correlate strongly with the radio filaments, indicating they arise from the same emission measure. The projected interior thermal emission shows plume-like structures like those in Tycho, the other known historical Type Ia remnant 17.2. The remnant is —30arcmin across, correpsonding to a physical diameter of 18 pc at a distance of 2.1 kpc (Figure courtesy of Chandra X-ray Center and J. P. Hughes.)

Since the SN 1006 discovery, nonthermal emission has been discovered from a number of SNRs. These remnants fall into two groups with different emission characteristics. The first group comprises the majority of the historical remnants, including Cas A, Tycho, and Kepler. The X-ray emission is dominated by line and continuum from the forward and reverse shock-heated thermal plasma. Nonthermal emission has been detected in two ways. First, observations at energies above the band dominated by thermal emission has revealed hard tails. Hard tails extending to 10keV can be identified in ASCA spectra, but the spectra do not have sufficient band pass or signal-to-noise to establish whether they are thermal or nonthermal. The RXTE spectra in the 10-20 keV band can be fit by power laws but are subject to multiple interpretations, as no morphological information is available [114]. Figure 17.11 shows a collection of RXTE spectra of young SNRs. Subsequently, Chandra and XMM-Newton resolved thin, hard outer rims, the spectra of which show evidence for a nonthermal continuum component. The isolation of the hard emission within these outer rims is a strong argument in favor of the shock acceleration interpretation.

In Cas A, there is a substantial contribution to the hard emission from the projected interior. This is likely to arise from nonthermal bremsstrahlung, as the interior magnetic fields are sufficiently high to reduce the maximum electron energy well

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Tycho --RCW 86

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