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Astron. Astrophys. 364, L80-L84 (2000)

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1. Introduction

The discovery of supermassive black holes ([FORMULA]-[FORMULA]) in the nuclei of several nearby galaxies (Magorrian et al. 1998) has raised the question of why most of them are not active. A possible solution is provided by the so called Advection Dominated Accretion Flows (ADAF; Rees 1982; Narayan & Yi 1995; Fabian & Rees 1995). In this class of accretion solutions, the accreting gas is so tenuous that it cannot cool efficiently, the viscous energy is stored in the protons as thermal energy and eventually advected onto the nuclear compact object. The ADAFs are therefore characterized by small radiative efficiency and accretion rates ([FORMULA]; Narayan & Yi 1995; Rees et al. 1982). At low [FORMULA] the hard X-ray emission is mainly due to bremsstrahlung emission from a population of [FORMULA]100 keV electrons, and is therefore much harder than typically observed in Seyfert galaxies (Nandra et al. 1997; Turner et al. 1997; Matt 2000).

Recently, hard X-ray tails have been discovered in the ASCA spectra of several elliptical galaxies (Allen et al 2000). Photon indices are in the range 0.6-1.5, therefore remarkably consistent with the expectations of the ADAF scenario. Unfortunately, the ASCA energy bandpass is limited to 9-10 keV, and the detection of these hard tails is therefore difficult and somewhat model-dependent.

One of the most intriguing of these ADAF-candidates is the nearby ([FORMULA]) elliptical galaxy NGC 1052. It is a narrow-line LINER (Heckman 1980), with a compact radio core (diameter [FORMULA]0.14 pc) and a radio halo of about 3 kpc diameter. NGC 1052 is the first type 2 LINER where broad lines in spectropolarimetric measurements have been discovered (Barth et al. 1999). The ASCA spectrum (Guainazzi & Antonelli 1999, G99; Weaver et al. 1999) is extremely flat: a formal fit with a simple power law yields a photon index [FORMULA]. This suggests either high photoelectric absorption (column density [FORMULA] cm-2) or a Compton-reflection dominated spectrum (which, of course, would imply an even more strongly absorbed active nucleus). In the former scenario the intrinsic spectral index is still very flat ([FORMULA]). This is intriguingly consistent with the ADAF scenario.

The ASCA results prompted a BeppoSAX program of observations of narrow-line LINERs with broad spectropolarimetric lines, mainly focused to investigate their hard X-ray spectrum ([FORMULA]10 keV). The results of this experiment on NGC 1052 are reported in this paper.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001