New X-ray Constraints on Starburst and Seyfert Activity in the Barred Spiral Galaxy NGC 1672

Abstract

The nearby barred spiral galaxy NGC 1672 shows dramatic starburst activity and may also host a Seyfert 2 nucleus. We present new X-ray observations that set constraints on starburst and Seyfert activity in NGC 1672. Two ROSAT HRI exposures, taken in 1992 and 1997, are used to investigate long-term variability of the known X-ray sources and to search for new sources of X-ray emission. We find large-amplitude (about 69%) variability from X-3, one of the off-nuclear sources located near an end of the galactic bar. X-3 has a peak observed 0.2-2.0 keV luminosity of about 2.5 x 10^{39} erg/s, and it is probably a luminous X-ray binary or young supernova remnant. We do not observe variability of the nuclear source X-1 or the strong off-nuclear source X-2. Our analyses also reveal two new off-nuclear sources, one of which is associated with a bright region along a spiral arm, and we find evidence for large-scale diffuse X-ray emission throughout part of the disk of NGC 1672. Furthermore, we use ASCA data taken in 1995 to constrain the hard X-ray properties of NGC 1672. While the nuclear source X-1 is the dominant soft X-ray source in NGC 1672, we find that the bulk of the 2-10 keV and 5-10 keV emission is spatially coincident with the off-nuclear source X-3, giving it an apparent 0.2-8 keV luminosity of about 6 x 10^{39} erg/s. A power-law plus Raymond-Smith model provides an acceptable fit to the full-band ASCA spectra. We do not find any evidence for a luminous but absorbed nuclear X-ray source. If there is a luminous Seyfert 2 nucleus in NGC 1672, it must be obscured by a `Compton-thick' torus with a column density of > 2 x 10^{24} cm^{-2}.