ChandraX‐Ray Observations of NGC 4151

Abstract

We present Chandra X-ray observations of the nearby Seyfert 1.5 galaxy NGC 4151. The images show the extended soft X-ray emission on the several hundreds of parsec scale with better sensitivity than previously obtained. We show that the hard X-ray component (greater than 2 keV) is spatially unresolved. The spectrum of the unresolved nuclear source may be described by a heavily absorbed (NH 3 × 1022 cm-3), hard power law (Γ 0.3) plus soft emission from either a power-law (Γ 2.6) or a thermal (kT 0.6 keV) component. The flux of the high-energy component has decreased from that observed by ASCA in 1993, and the spectrum is much harder (Γ ~ 0.3 in 2000 vs. Γ 1.5 in 1993). The large difference between the soft and hard spectral shapes does not favor the partial covering or scattering model of the soft excess. Instead, it is likely that the hard and soft nuclear components represent intrinsically different X-ray sources. The stronger nuclear emission lines of those seen by the Chandra HETGS spectrum are detected. Spectra of the extended emission to almost 1 kpc northeast and southwest of the nucleus have also been obtained. The spectra of these regions may be described by either thermal bremsstrahlung (kT 0.4-0.7 keV) or power-law (Γ 2.5-3.2) continua plus three emission lines. There is an excellent correlation between the extended X-ray and [O III] λ5007 line emissions. We discuss the nature of the extended X-ray emission. Because there is no extended electron-scattered hard X-ray emission, an upper limit to the electron scattering column can be obtained. This upper limit is much too low for the soft X-rays to be electron-scattered nuclear radiation, unless the nucleus radiates soft X-rays much more strongly toward the extended regions than toward Earth, a situation we consider unlikely. We favor a picture in which the extended X-ray-emitting gas is heated in situ by the nuclear radiation. Some of the X-rays may originate from a hot phase that confines the warm ionized gas seen optically, although X-ray emission produced via photoionization by the nucleus is also likely. A faint, probably background, compact X-ray source lies 22 from the nucleus to the southwest, approximately along an extension of the extended southwest X-ray and [O III] emission.