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Astron. Astrophys. 362, L29-L32 (2000)

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3. Discussion

The compact hard X-ray features may be interpreted in terms of the interaction between the remnant shock and a molecular cloud. Bykov et al. (2000) have presented a model of non-thermal emission from an evolved SNR interacting with a molecular cloud, but the predicted diffuse hard X-ray emission in the MECS bandwidth is below the sensitivity threshold. On the other hand, several clumps have been observed in the molecular cloud near IC443. The spatial scales of the molecular clumps from IR line observations are in the range from 150" down to 1". The non-thermal X-rays from clumps were modelled by Bykov et al. (2000), who predicted a hard spectrum with a photon index below 2.0. The emission is originated from electrons of energy below 1 GeV. The predicted flux density is [FORMULA] keV cm-2 s-1 keV-1 at 10 keV for a 30 [FORMULA] shock in a clump of half parsec radius and density of 10[FORMULA]. Using a power law with [FORMULA], this corresponds to [FORMULA] MECS cnt s-1 in the 4.0-10.5 keV energy range and [FORMULA] PDS cnt s-1 in the 15-30 keV. The predicted count-rates are consistent with the observed rate of Src B (Table 4), having in mind that [FORMULA] cm-3 density is some representative number and other parameters could easily account for the difference. Because of hard spectrum and heavy photoelectric absorption, localized spots of a scale of a few arcmin would be seen only in hard X-rays. Thus, the compact Src B correlated with bright spot of molecular hydrogen emission (Fig. 1) can be shocked molecular clump. The soft X-ray brightening having apparent shift from hard X-ray features towards the SNR interior, observed in the case of Src B (Fig. 4), can be attributed to the effects of the shocked clump edges. The morphology and the spectrum of the Src A are somewhat different from that of Src B.

As already discussed by K97, a low-luminosity pulsar nebula is a plausible explanation for isolated hard X-ray sources (especially for the slightly extended Src A), but we do not expect to see two of them in one SNR 1. High resolution Chandra or XMM data are needed to distinguish between pulsar nebula and shocked clump interpretation. We have also performed a timing analysis of MECS events collected inside a circle of [FORMULA] radius in the 4.0-10.5 energy range for both Src A and B, and we found no pulsations at 99% confidence level, with an upper limit of the pulsed fraction of a sinusoidal signal in the [FORMULA] Hz frequency range of 5% and 6%, respectively, more stringent than the one derived by K97 using ROSAT data.

We have also estimated the Src A and B EGRET [FORMULA]-ray [FORMULA] MeV flux contribution, using the best-fit models reported in Table 3, and they are [FORMULA] and [FORMULA] photons cm-2 s-1. The expected [FORMULA]-ray flux of Src B therefore is compatible with the value of [FORMULA] photons cm-2 s-1 observed by Esposito et al. (1996). Gamma-ray observations with forthcoming INTEGRAL mission with expecting angular resolution about 12´ could better constrain the spectrum of the sources above 30 keV.

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© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000