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| - Active nucleus in a post-starburst galaxy: KUG 1259+280
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| - 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 ΓX∼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.
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