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Astron. Astrophys. 331, L77-L79 (1998)

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1. Introduction

The radio source G0.9+0.1 was first recognized as a supernova remnant by Kesteven (1968) and later studied in detail with the VLA and the Molonglo Observatory Synthesis Telescope. These observations clearly show that G0.9+0.1 consists of a steep-spectrum radio shell of [FORMULA] diameter surrounding a core component with a flatter spectrum and significant polarization (Helfand & Becker 1987; Gray 1994). Thus G0.9+0.1 belongs to the class of "composite" supernova remnants, that, in addition to the radio/X-ray shell formed by the expanding ejecta, show the signature of a central neutron star powering non-thermal emission through the loss of rotational energy (see Weiler & Sramek (1988) for a review of the classification of SNRs into shell-type, plerionic, and composite).

Considering that the formation of a neutron star is expected in most supernovae (Branch 1990), it has been pointed out that plerionic and composite SNR should represent a larger fraction of the known population of SNR than currently observed (Helfand et al.1989). It is not clear whether selection effects, that, especially in the radio band, favour the classification as SNR of shell sources, can entirely account for the observed discrepancy. X-ray observations are playing a crucial role in solving this problem, both through the detection of new remnants (Pfeffermann et al. 1991, Greiner et al. 1994, Busser et al. 1996) and by showing that not all the neutron stars associated with SNR manifest themselves in the radio band as synchrotron nebulae and/or radio pulsars (Mereghetti, Bignami & Caraveo 1996; Petre, Becker & Winkler 1996; Vasisht & Gotthelf 1997).

Until recently, most X-ray observations of SNR have been conducted below a few keV, where interstellar absorption is severe and hampers the study of distant objects in the galactic plane, such as G0.9+0.1. Though the distance of G0.9+0.1 is very uncertain, both the standard [FORMULA] -D analysis (Downes 1971, Green 1991), and its location near the galactic center direction, indicate a distance of [FORMULA] 10 kpc or greater. Despite the high absorption along this line of sight ([FORMULA]  cm-2), the Italian-Dutch BeppoSAX satellite, providing imaging capabilities with arcminute angular resolution and good sensitivity also above 4 keV, has allowed a clear detection of the X-ray emission from G0.9+0.1.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998