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Astron. Astrophys. 359, 337-346 (2000)

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5. Gamma ray production in SNR

SNR shocks are able to accelerate particles to TeV energies. Gamma-rays are produced in secondary reactions between these high energy particles and ambient matter and radiation fields. The decay of [FORMULA], produced in ion-ion collisions, is the prime hadronic source of gamma-rays in SNR. Electrons accelerated to multi-TeV energies in SNR give rise to bremsstrahlung and inverse Compton (IC) scattering [FORMULA]-ray production processes in SNR (see e.g. Mastichiadis & de Jager 1996, Gaisser et al. 1998). The cosmic microwave background (CMB) is usually considered the dominant soft photon source with contributions ([FORMULA]10%) from the infrared background as seed photons for the IC process. The detailed model of Baring et al. (1999) uses these processes collectively (including synchrotron and bremsstrahlung radiation) to account for observations from radio to TeV gamma-ray energies for the northern SNR, IC443.

The models of Naito & Takahara (1994) and Drury et al. (1994) give predictions of the TeV gamma ray flux due to [FORMULA] decay. As a first attempt at explaining the particle acceleration processes in W28, we make use of the Naito & Takahara model and compare its predictions of the [FORMULA] channel to our results. By considering only the [FORMULA] production channel, a lower limit on the predicted TeV gamma flux can be estimated. A proton parent spectrum with differential index -2.1 and exponential cutoff at 100 TeV has been assumed ([FORMULA], i.e. consistent with shock acceleration expected in a SNR. The predicted TeV flux will scale according to:


where [FORMULA] is the energy available for cosmic ray acceleration, n is the particle number density of the ISM (cm-3) and d is the distance to the SNR (kpc). [FORMULA] is some fraction, typically [FORMULA]10% of the total energy of the SNR (canonically 1051 erg). In fact, the total SNR energy for W28 has been estimated by Rho et al. (1996) at 4[FORMULA] erg from ROSAT X-ray data.

The most interesting question concerns the possibly of enhanced TeV gamma-ray emission from regions of high ISM density (Aharonian et al. 1994), where there is a greater chance for the interaction of cosmic rays from the SNR. The molecular clouds along the northeast and northern remnant boundary have been mapped in detail by Arikawa et al. (1999) at the CO J=1-0 and J=3-2 lines. The shocked (region undergone passage and compression by the SNR shock) component of the clouds is distributed along the SNR/clouds boundary, has a mean density of [FORMULA] cm-3, and mass [FORMULA][FORMULA]. The other 4[FORMULA][FORMULA] of the unshocked gas has a density of [FORMULA] cm-3 and is displaced radially outward from the shocked gas by [FORMULA]1 arcmin. Clearly, any TeV gamma-ray flux from [FORMULA] decay would be dominated by the shocked gas regions, given that the unshocked regions of the cloud would have a much lower density and lower energy available for CR production, and the mean matter density for regions excluding the molecular cloud is only [FORMULA]1.3 cm-3 (Esposito et al. 1996). Arikawa et al. (1999) has derived the energy deposited into the shocked gas at [FORMULA] ergs, a value consistent with the Rho et al. (1998) estimate for the total SNR energy when considering the volume filling factor V, between the clouds and the SNR. V is simplistically estimated at [FORMULA], by taking the mass and density of the shocked gas (given above), assuming the cloud consists of H2, and a value of 10 pc for the SNR radius. We also assume that [FORMULA]10% of the available SNR energy goes into cosmic ray production, a reasonable value for a Sedov-phase SNR, and also consistent with the measured energetics for the SNR and clouds. Thus we adopt values of 3[FORMULA] ergs for [FORMULA], and [FORMULA] cm-3 for n in Eq. (5). A working band on our flux prediction is obtained if we assume a range of values for d from 1.8 to 3.3 kpc, as discussed in Sect. 1. In Fig. 5, we compare the model predictions based on the above scalings to our upper limit obtained from an extended source of radius 0.25o encompassing the clouds from 1994 data (the highest of our upper limits). The flux from 2EG J1801-2312 (essentially the same as 3EG J1800-2338) and its straight extrapolation to TeV energies is also included as any [FORMULA] gamma-ray flux will be limited by the EGRET measurement.

[FIGURE] Fig. 5. Comparison with our upper limit and the EGRET flux of 3EG J1800-2338 (Merck et al. 1996) with a model predicting the TeV gamma-ray flux due to [FORMULA] decay (hashed area, Naito & Takahara 1994). The hashed region is bounded by limits on the predicted flux when assuming published ranges of values for various scaling parameters (Eq. (5)) defined for the region of shocked gas surrounding W28 (Arikawa et al. 1999). This region is expected to dominate those that would produce a TeV gamma-ray flux from the decay of [FORMULA]. Our upper is for an extended source containing the molecular clouds (see text). The model is practically upper-bounded by the flux from 2EG J1801-2312, and parent-proton energies are derived from a power-law of index -2.1 (differential) with an exponential cutoff at 1014 eV.

Our upper limit lies an order of magnitude below the straight extrapolation of the flux from EGRET (dashed line of Fig. 5), and is able to place some constraint on the prediction of a [FORMULA] gamma-ray flux from the shocked gas region. In order to accommodate our upper limit however a cutoff and/or slighter steeper parent spectrum than -2.1 appears necessary. In assuming an exponential cutoff at 100 TeV for the parent spectrum of accelerated hadrons, we assume that W28 follows the `standard' picture of particle acceleration in SNR, with particle energies limited by radiative losses, finite age of the shock and particle escape. When the cutoff for electrons is due only to radiative losses however, it can be expected that the hadron spectrum will continue (Reynolds & Keohane 1999). Apart from the long-established `knee' at [FORMULA] eV in the all-particle cosmic ray spectrum there is now direct experimental evidence pointing to a continuation of the proton/helium spectra up to at least [FORMULA] TeV (Asakimori et al. 1998), implying that strong cutoffs may not be required. Our result here is consistent however, with previous comparisons of upper limits (from other SNRs) to hadron-induced gamma-ray models (e.g. Buckley et al. 1998, Allen et al. 1995 and Prosch et al. 1996) which seem to require some cutoff below the knee energy/and or spectra steeper than -2.1. Further constraints may arise if electronic components are considered. Particularly, electronic bremsstrahlung may dominate over the inverse Compton component due to the very high density of target matter in the clouds.

The above discussion aside, the location of 3EG J1800-2338 by itself, makes it's interpretation difficult in terms of simple CR-matter interaction, or as the result of a pulsar-powered process (proposed by Merck et al. 1996). Many of the promising sites within W28 for gamma-ray production are now outside the 95% error circle, leaving just the filled X-ray centre, and southern/western portions of the radio shell. At best, we are in a position to rule out the interpretation of the EGRET source as resulting totally from [FORMULA] decay gamma-rays with an unlimited parent spectrum, and to place limits on the parent spectral index/cutoff energy combination.

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Online publication: June 30, 2000