Detection of Excess Hard X-Ray Emission from the Optical Jet Galaxy NGC 1097

Abstract

Using the ASCA observatory, X-ray observations were made of the spiral galaxy NGC 1097, which exhibits four optical jets. Hard X-ray emission from a point-like source at the nucleus was detected at a 2–10 keV luminosity of 1× 1041 erg s−1 for an assumed distance of 24 Mpc. The 2–10 keV spectrum was well fitted with a power-law of photon index ∼ 1.8. Although no X-ray variability was detected during a ∼ 40 ks pointing, the observed X-ray to optical luminosity ratio of NGC 1097 is too high for the X-ray emission to be explained by an assembly of discrete X-ray sources. From our results and optical activity, the existence of a low-luminosity active galactic nucleus (LLAGN) is strongly suggested. In the soft X-ray band (0.5−2 keV) a thin thermal emission with a temperature kT ∼ 0.6 keV was also detected, which is understood to be emission from hot gas associated with starburst activity.