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Astron. Astrophys. 343, 23-32 (1999)

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1. Introduction

X-ray observations, beginning with the Einstein   Observatory (Giacconi et al. 1979), have demonstrated that normal early-type galaxies are X-ray emitters, with 0.2-4 keV luminosities ranging from [FORMULA] to [FORMULA] erg s-1 (Fabbiano 1989; Fabbiano et al. 1992). The X-ray luminosity [FORMULA] is found to correlate with the blue luminosity [FORMULA] ([FORMULA]), but there is a large scatter of roughly two orders of magnitude in [FORMULA]  at any fixed [FORMULA][FORMULA][FORMULA]. The analysis of the Einstein   spectral data already revealed that in the X-ray brightest objects the X-ray radiation comes from thermal emission from a hot, optically thin gas, at a temperature of [FORMULA] keV (Canizares et al. 1987, hereafter CFT). It also revealed that the X-ray emission temperature increases with decreasing [FORMULA]/[FORMULA], until the dominant contribution to the total emission comes from a hard thermal component, similar to that dominating the emission in spiral galaxies (Kim et al. 1992). Since a population of low mass X-ray binaries (LMXB) can explain the X-ray emission of the bulge of M31, and that of bulge-dominated spirals, it is likely that in early-type galaxies an increasing fraction of the X-ray emission comes from stellar sources as [FORMULA]/[FORMULA]  decreases. Better quality spectra of low and medium [FORMULA]/[FORMULA]  galaxies (i.e., belonging to Group 1 or Group 2 in the analysis of Kim et al. 1992, or with [FORMULA]/[FORMULA][FORMULA] in the Einstein band) have been made available recently for several galaxies, thanks to ROSAT and ASCA pointed observations.

The analysis of ROSAT data confirmed the findings based on the Einstein data for several low and medium [FORMULA]/[FORMULA]  galaxies (see, e.g., the sample of 61 early-type galaxies observed with the ROSAT PSPC built by Irwin & Sarazin 1998). The emerging picture was the existence of at least two spectral components, a soft one likely due to hot gas or stellar sources, and a hard one, whose temperature is not well constrained, due to the lack of sensitivity of the PSPC above 2 keV.

Sensitive over (0.5-10) keV, ASCA pointed [FORMULA] early-type galaxies, among which there are a handful with low or medium [FORMULA] (Matsushita et al. 1994; Kim et al. 1996; Matsumoto et al. 1998; Buote & Fabian 1998, hereafter BF). In all the 12 E/S0s of their sample Matsumoto et al. find a variable amount of soft thermal emission, of temperature ranging from 0.3 to 1 keV, coupled to hard emission. The amount of this hard emission roughly scales as the optical luminosity of the galaxies, and is consistent with that of bulge-dominated spirals, such as M31. Using ASCA data over the energy range 0.5-5 keV, also BF find a hard component of [FORMULA] keV coupled with a soft component, in the low or medium [FORMULA] galaxies of their sample.

The precise knowledge of the contribution of stellar sources to the total X-ray emission is important to better constrain the properties of the hot gas (i.e., temperature, abundance and luminosity), or to identify peculiarities such as the presence of a mini-AGN, another possible contributor to the hard emission (e.g., Colbert & Mushotzky 1998). So, stronger constraints on the gas flow phase and the galaxy properties can be derived. The contribution of stellar sources can be best evaluated with observations over a large bandwidth. In this paper we report on the X-ray emission from the E4 galaxy NGC 3923, as detected by the BeppoSAX satellite over (0.5-10) keV. Our purpose is to investigate in detail the nature of the X-ray emission in this medium [FORMULA]/[FORMULA]  galaxy (Einstein observations showed that log[FORMULA] for it, Fabbiano et al. 1992). The BeppoSAX observation of NGC 3923, exploiting a moderate spatial resolution coupled with the large energy band and the good spectral resolution of the satellite, permits the separation of the soft and the hard emission components eventually present. The spectral properties of NGC 3923 over 0.5-5 keV have been already studied by BF, using ASCA data referring to a radius of 2 arcmin (note that the radius encircling 80% of the photons of a point source is 3 arcmin for the ASCA -XRT). The BeppoSAX observation of NGC 3923 presents some advantages over that performed by ASCA : a sharper PSF above 2 keV with respect to that of the ASCA -XRT, which is also asymmetrical 1; a lower instrumental background for the MECS with respect to the GIS, which favors weak sources. Finally, a larger covering in energy is presented here with respect to that analysed by BF (i.e., 0.5-10 keV versus 0.5-5 keV), a spatial analysis is attempted over (1.7-10) keV, and a larger extraction region for the spectral data is used. NGC 3923 has been pointed also by the ROSAT PSPC and HRI, which allowed a detailed investigation of the spatial distribution of the X-ray emission (Buote Canizares 1998, hereafter BC).

This paper is organized as follows: we present in Sect. 2 the main properties of NGC 3923, in Sect. 3 the results of the data analysis, in Sect. 4 the possible origins for the X-ray emissions of NGC 3923 are discussed in light of the present results, and more in general the possible origins of the large scatter in the X-ray emission shown by galaxies of similar [FORMULA]  are also reviewed; in Sect. 5 we summarize the conclusions.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999