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Astron. Astrophys. 360, 671-682 (2000)

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1. Introduction

The supernova remnant RCW86 (also known as G315.4-2.3 and MSH14-63) is a complete shell in radio (Kesteven & Caswell 1987), optical (Smith 1997) and X-rays (Pisarski et al. 1984), with a nearly circular shape and a 40´ diameter. It has received substantial attention because of the longstanding issue of its correlation with SN185, the first historical galactic supernova. However, this connection is based on circumstantial evidence (Clark & Stephenson 1977), and a recent reinterpretation of the Chinese records has even raised some doubt whether the events described really refer to a supernova, rather than a comet (Chin & Huang 1994).

Related to the issue is the distance to the remnant. For RCW86 to be the remnant of AD 185, it has to be closer than 2 kpc (Pisarski et al. 1984) with a distance around 1 kpc favored (e.g., Long & Blair 1990). However, most distance estimates either assume that RCW86 is SN185, or imply a distance much larger than 2 kpc. This larger distance dates back to Westerlund (1969), who suggested a possible connection between RCW86 and an OB association at 2.5 kpc. This association is consistent with the distance based on the [FORMULA] relation 1 ([FORMULA] kpc) and a recent determination based on the systematic velocity of the nebula as measured in [FORMULA], which implies a distance of 2.8 kpc (Rosado et al. 1996). However, a distance around 3 kpc is not universally accepted, since the [FORMULA] method is unreliable (Green 1991; Strom 1994), and using the systematic velocity to measure the distance relies upon the assumption that RCW86 is the result of a Type II explosion (Rosado et al. 1996).

A number of features make RCW86 a very interesting remnant in its own right. It displays a large contrast in densities (Leibowitz & Danziger 1983; Pisarski et al. 1984; Petruk 1999) and recently Vink et al. (1997), using ASCA data, showed that two very distinct X-ray spectra are present. One type of spectra is soft and is associated mostly with the radiative shocks making up the knee of RCW86 (c.f. Smith 1997). The hard spectra are associated with the rest of the remnant. The spatial separation of the two components implies that the morphology of RCW86 changes dramatically when going from photon energies around 1 keV to energies above [FORMULA]3 keV. Some of the peculiar features of this remnant were interpreted by Vink et al. (1997) as due to a partial interaction with a cavity wall. In this view, the non-radiative shocks are still moving within the cavity whereas the bright, radiative shocks are due to encounters between the remnant and the cavity wall.

Vink et al. (1997) also pointed out that the analysis of the ASCA hard spectra of RCW86 yielded strong under-abundances of several elements ([FORMULA] solar) at several locations of the shell. However, it is not clear if these abundances represent the true abundances, since they are not consistent either with models of grain destructions behind the shock, or with the presence of reverse shocks in the ejecta.

In this paper we describe the BeppoSAX observations of RCW86. Like ASCA, BeppoSAX, is also capable of spatially resolved spectroscopy. The imaging instruments onboard BeppoSAX have a spatial resolution better than those on ASCA and, moreover, span a wider energy range. The BeppoSAX observations also cover the Northwestern part of the remnant, which was not observed by ASCA. In addition, we analyze archival ROSAT Posistion Sensitive Proportional Counter (PSPC) data, and provide new estimates of the metal abundances in RCW86. We will also attempt to interpret the data in the light of a shock expanding in a not homogeneous medium, finding evidence of shocked ejecta in the Southwestern (SW) part of the shell. We will use the Sedov model in conjunction with the X-ray emission of the Northern (N) rim to derive a new and independent estimate of the distance towards this remnant.

In Sect. 2 we describe the data. In Sect. 3, we show the results of spatial and spectral analysis of the SW and N rim of RCW86, while in Sect. 4 we discuss the interpretation of the results. Sect. 5 summarizes our findings.

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000