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Astron. Astrophys. 349, L49-L52 (1999)

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5. Discussion

Our observation of the SgrA complex confirms the presence of a hot plasma with multiple temperatures and/or in non-equilibrium ionization, as already found with the ASCA instruments (Koyama et al. 1996). The increase in the 6.4 keV line equivalent width that we find in the North-East sector, is in agreement with the more detailed maps of this line obtained with ASCA that show a correlation with the molecular clouds. The limited spectral resolution of the MECS, compared with that of the ASCA solid state detectors, does not allow to study in more detail the energy profile and spatial distribution of the individual lines. On the other hand, the regular point spread function provided by the BeppoSAX mirrors has allowed us to produce unbiased maps in wide energy bands that demonstrate a clear difference in the spatial distribution of the softer and harder X-ray emission. Since also our spectral data could be well described by the sum of two thermal models with kT[FORMULA]0.6 and 8 keV, it is tempting to give an interpretation in terms of two plasma components at different temperatures and with different spatial distributions (although in reality the situation is certainly much more complex, with, e.g., a distribution of temperatures). In this interpretation, it is remarkable that the lower temperature plasma is well correlated with the SgrA East triangular radio halo (Fig. 4).

[FIGURE] Fig. 4. Contour levels of the 2-5 keV emission overlayed on the 90 cm radio contours (dashed line, adapted from Anantharamaiah et al. 1991).

The presence of unresolved point sources could affect the apparent distribution of the diffuse emission. We note however that previous observations, e.g. with Einstein and ROSAT (Watson et al. 1981, Predehl & Trümper 1994) do not show the presence of strong sources distributed in such a way to reproduce the triangular shape visible in our low energy map. In particular, the straight contours of the soft emission corresponding to the SW side of the triangular radio halo seem hardly explainable by a distribution of sources. Although we cannot exclude that the apparent shape of the softer X-rays is due to an absorption effect, we favour the interpretation of the lower temperature plasma as a component physically related to the halo of SgrA East. In the following we will adopt this working hypothesis and assume a distance of 8.5 kpc.

We derive for the soft component an emission measure EM=(1.2[FORMULA] cm-5, which assuming emission from a spherical region with radius 10 pc and an electron filling factor f, corresponds to ne [FORMULA] [FORMULA] cm-3 and to a total mass [FORMULA][FORMULA].

The average thermal pressure in the SgrA East halo, [FORMULA] ergs cm-3, is consistent with the pressure [FORMULA][FORMULA] ergs cm-3 derived for a SNR in a Sedov phase, where [FORMULA] is the explosion energy of the SNR and R is the shell radius in cm. Indeed, if we assume [FORMULA] ergs s-1 and R=10 pc, we find [FORMULA] ergs cm-3.

The X-ray luminosity (2-10 keV) of the soft component is [FORMULA] ergs s-1. If we assume that this thermal emission is mostly produced by the SgrA East halo, its X-ray luminosity, pressure, density, temperature of the emitting gas (0.6 keV) and size ([FORMULA]20 pc), match well with a supernova remnant origin in which thermal line emission is produced when the expanding supernova ejecta heats the ISM to X-ray temperatures.

Another test of the SNR hypothesis can be done by comparing the X-ray and radio surface brightnesses: indeed a strong correlation [FORMULA] between the radio [FORMULA] (at 1 GHz) and the X-ray surface brightness [FORMULA] (0.15-4.5 keV) was found by Berkhuijsen (1986). From the radio spectrum reported by Pedlar et al. (1989) we derived [FORMULA] W m- 2 Hz-1 sr-1. The [FORMULA] measured from our X-ray data and converted to the 0.15-4.5 energy band is [FORMULA] ergs s-1 pc-2. These values fall well inside the region defined by other typical SNRs.

In conclusion, all the physical quantities derived from the analysis of the MECS are consistent with a SNR origin for the SgrA East halo.

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999