Active nucleus in a post-starburst galaxy: KUG 1259+280

Abstract

We report the discovery of an active nucleus in a post-starburst galaxy, KUG 1259+280, based on its X-ray and optical characteristics. The X-ray source in KUG 1259+280 was detected during a search for ultrasoft sources in the ROSAT pointings. High-resolution X-ray imaging observations with ROSAT HRI show that X-ray emission from KUG 1259+280 is unresolved. X-ray emission from KUG 1259+280 is highly variable; an episode in which X-ray intensity changed by a factor ~2.5 within ~1300 s has been detected. ROSAT PSPC spectra of this galaxy are found to be well represented by a steep power law of photon index Γ_X~4.25, and a change in the absorbing column within ~1 d is indicated. The rest frame intrinsic X-ray luminosity of KUG 1259+280 is found to be ~3.6-4.7×10⁴² erg s⁻¹, similar to that of low-luminosity Seyfert galaxies. The mass of the central massive object within KUG 1259+280 is estimated to be in the range 10⁵−10⁷ M. The optical spectrum of the nuclear region of the galaxy is complex and shows Balmer absorption and collisionally excited lines of [O III] and [N II]. The presence of forbidden emission lines and the absence of Balmer emission lines in the spectrum of KUG 1259+280 could be the result of photoionization by a diluted power-law continuum modified by enhanced stellar absorption owing to a post-starburst event. Estimated Balmer line strengths free of stellar absorptions and forbidden line strengths indicate the nucleus of KUG 1259+280 to be low-ionization nuclear emission region (LINER) like in nature. However, the low-ionization forbidden line [O I] λ6300 usually present in LINER spectra is not detected in the spectrum of KUG 1259+280. X-ray characteristics-variability, point-like appearance, luminosity and steepness of spectrum-indicate that the nucleus in KUG 1259+280 is active and perhaps like that of narrow-line Seyfert type 1 galaxies.