Active Nucleus in a Poststarburst Galaxy : KUG 1259+280

Abstract

We report the discovery of an active nucleus in a poststarburst galaxy, KUG1259+280. High resolution X-ray imaging observations with ROSAT HRI show that X-ray emission from KUG1259+280 is unresolved. X-ray emission from KUG1259+280 is highly variable, an episode in which X-ray intensity changed by a factor 2.5 within about 1300s has been detected. ROSAT PSPC spectra of this galaxy is found to be well represented by a steep power law of photon index 4.25, and a change in the absorbing column within about a day is indicated. The rest frame intrinsic X-ray luminosity of KUG1259+280 is found to be 3.6-4.7X10^42 erg/s similar to that of low luminosity Seyfert galaxies. Mass of the central massive object within KUG1259+280 is estimated to be in the range of 10^5-10^7 M_sun. Optical spectrum of the nuclear region of the galaxy is complex and shows Balmer absorption and collisionally excited lines of [OIII], and [NII]. The presence of forbidden emission lines and the absence of Balmer emission lines in spectrum of KUG1259+280 could be due to photoionization by a diluted power-law continuum modified by enhanced stellar absorption due to a poststarburst event. Estimated Balmer line strengths free of stellar absorptions and forbidden line strengths indicate the nucleus of KUG1259+280 to be LINER-like in nature. However, the low-ionization forbidden line [OI]6300 usually present in LINER spectra, is not detected in the spectrum of KUG1259+280. X-ray characteristics -- variability, point-like appearance, luminosity and steepness of spectrum indicate that nucleus in KUG1259+280 is active and perhaps like that of narrow-line Seyfert type 1 galaxies.