NGC 7582: The Prototype Narrow-Line X-Ray Galaxy

Abstract

NGC 7582 is a candidate prototype of the narrow line X-ray galaxies (NLXGs) found in deep X-ray surveys. An ASCA observation shows the hard (>3 keV) X-ray continuum of NGC 7582 drops 40% in ∼6 ks, implying an active galactic nucleus (AGN), while the soft band (<3 keV) does not drop in concert with the hard continuum, requiring a separate component. The X-ray spectrum of NGC 7582 also shows a clear 0.5-2 keV soft (kT = 0.8-0.3+0.9 keV or Γ = 2.4 ± 0.6; LX = 6 × 1040 ergs s-1) low-energy component, in addition to a heavily absorbed [NH = (6 ± 2) × 1022 cm-2] and variable 2-10 keV power law [Γ = 0.7-0.4+0.3; LX = (1.7-2.3) × 1042 ergs s-1]. This is one of the flattest 2-10 keV slopes in any AGN observed with ASCA. (The ROSAT HRI image of NGC 7582 further suggests extent to the SE.) These observations make it clear that the hard X-ray emission of NGC 7582, the most "narrow-line" of the NLXGs, is associated with an AGN. The strong suggestion is that all NLXGs are obscured AGNs, as hypothesized to explain the X-ray background spectral paradox. The separate soft X-ray component makes NGC 7582 (and by extension other NLXGs) detectable as a ROSAT source. © 1993. The American Astronomical Society. All rights reserved.