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

We report the discovery of an active nucleus in a post-starburst galaxy, KU G 1259+280, based on its X-ray and optical characteristics. The X-ray sourc e 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 f rom KUG 1259+280 is highly variable; an episode in which X-ray intensity ch anged by a factor similar to 2.5 within similar to 1300 s has been detected . ROSAT PSPC spectra of this galaxy are found to be well represented by a s teep power law of photon index Gamma(X) similar to 4.25, and a change in th e absorbing column within similar to 1 d is indicated. The rest frame intri nsic X-ray luminosity of KUG 1259+280 is found to be similar to 3.6-4.7 x 1 0(42) erg s(-1), similar to that of low-luminosity Seyfert galaxies. The ma ss of the central massive object within KUG 1259+280 is estimated to be in the range 10(5)-10(7) M.. The optical spectrum of the nuclear region of the galaxy is complex and shows Balmer absorption and collisionally excited li nes 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 Balm er 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] lambda 6300 usually present in LINER spectra is not detected in the sp ectrum of KUG 1259+280. X-ray characteristics - variability, point-like app earance, luminosity and steepness of spectrum - indicate that the nucleus i n KUG 1259+280 is active and perhaps like that of narrow-line Seyfert type 1 galaxies.