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Astron. Astrophys. 322, 719-729 (1997)

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1. Introduction

NGC 4051 is a spiral galaxy of morphological type SAB with a redshift of z = 0.0023. It hosts a low-luminosity Seyfert 1 nucleus (Seyfert 1943). The object is well known for its rapid X-ray variability which was discovered by Marshall et al. (1983) with the Einstein  observatory. Subsequently, NGC 4051 has been extensively studied in the X-ray spectral region with all majour X-ray missions (e.g. Lawrence et al. 1985, with EXOSAT ; Matsuoka et al. 1990, with Ginga ; Pounds et al. 1994, with ROSAT; Mihara et al. 1994, with ASCA).

The soft X-ray spectrum of NGC 4051 shows several components. A black-body-like soft excess is seen in source high-states (e.g. Pounds et al. 1994, Mihara et al. 1994). In addition, there is occasional evidence for cold absorption in excess of the Galactic value (Walter et al. 1994). Around 0.8 keV the spectrum exhibits absorption features, interpreted and modeled as the signature of absorption by ionized material along the line of sight (Pounds et al. 1994, with ROSAT survey data; McHardy et al. 1995, with ROSAT PSPC pointed data; Mihara et al. 1994, with ASCA data; another study of ASCA data by Guainazzi et al. is underway). The existence of a warm absorber in NGC 4051 was firstly proposed by Fiore et al. (1992) as a possible explanation for the observed spectral variability pattern in Ginga  data. Netzer et al. (1994) found evidence for a warm absorber in NGC 4051 by introducing and applying X-ray colour diagrams.

The overall spectral variability of NGC 4051 is complex with different behaviour at different epochs, and no consistent description of the data has emerged yet. In general, there is a trend for the source to be softer when brighter (Lawrence et al. 1985, Papadakis & Lawrence 1995, with EXOSAT  data; Matsuoka et al. 1990, with Ginga  data), traced back by Papadakis & Lawrence (1995) to high flux peaks in the soft (0.05 - 2 keV) energy region only. Matsuoka et al. (1990) favour a correlation of powerlaw index with the 2 - 10 keV luminosity, which Torricelli-Ciamponi & Courvoisier (1995) explain in terms of inverse Compton scattering of thermal UV photons by a population of nonthermal relativistic electrons. Kunieda et al. (1992) and Fiore et al. (1992) describe part or the total of another Ginga  observation in terms of partial covering of the central source emitting a powerlaw with approximately constant slope.

The optical spectrum of NGC 4051 has been classified as either Seyfert 1.8 (e.g. Rosenblatt et al. 1992) or narrow-line Seyfert 1 (NLSy1 hereafter; e.g. Malkan 1986). The line width (FWHM) of H [FORMULA] is 990 km/s (Osterbrock & Shuder 1982), broader than the forbidden lines with a FWHM of [OIII] [FORMULA] 5007 of 330 km/s (De Robertis & Osterbrock 1984). An additional weak broad component is seen in H [FORMULA] (Veilleux 1991, Rosenblatt et al. 1994). High-ionization coronal lines are present in the optical and IR spectrum, like [FeXI] [FORMULA] 7892 (De Robertis & Osterbrock 1984) and [SiVI]1.96µm (Giannuzzo et al. 1995).

The present X-ray observation was performed with the ROSAT PSPC (Trümper 1983; Pfeffermann et al. 1987) in order to study in detail the warm absorption feature, to discuss the time variability of the spectral components and to assess the possibility of a warm-absorber contribution to one of the emission-line components seen in the IR to UV spectral region.

The paper is organized as follows: In Sect. 2 we present the observations. In Sects. 3 and 4 the spectral and temporal analysis of the data is described and properties of the warm absorber are derived. Sect. 5 is concerned with the discussion of the data; constraints on the unobserved EUV continuum are provided, the properties of the warm gas are discussed and further constrained, the absorber-intrinsic optical-UV emission and absorption is investigated, the influence of dust is assessed, the thermal stability of the warm material is examined, and the NLSy1 character of NGC 4051 is commented on. In Sect. 6 we provide a summary and the conclusions.

A distance of 14 Mpc is adopted for NGC 4051, resulting from a Hubble constant of [FORMULA] = 50 km/s/Mpc and the assumption that the galaxy follows the Hubble flow, i.e. any peculiar velocity component is small.

If not stated otherwise, cgs units are used throughout.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998