Long ultra violet monitoring IUE observation of MRK 478

Abstract

Our universe comprises of many different kinds of objects such as planets, comets, asteroids, minor bodies, stars, galaxies, clusters of galaxies and super cluster of galaxies with their sizes and masses spread over several magnitudes. Amongst them, galaxies are considered as the building blocks of the universe in its large-scale structure perspective. Largely our universe consists of so-called “normal galaxies”, the bolometric luminosity of which is the sum of the luminosity of the constituent stars. The stars generate energy through nuclear fusion reactions. In sharp contrast, a small fraction (<img src='/image/spc_char/wave.gif' border=0> 1-10 %) of all these normal galaxies are distinguished by their overwhelmingly bright compact nuclei, which emits copious amount of radiation. These highly luminous (<em>L</em>_Bol > 10⁴² erg/s) galaxies are called as “active galaxies” or “active galactic nuclei” (AGNs). International ultraviolet satellite (IUE) launched in the year 1978 has made low resolution (~ 6-8 Å) UV spectroscopic observations of about 500 active galaxies until September, 1996 and has enabled us to understand the UV spectral properties. In this paper, we present the results of our variability studies of a narrow line Seyfert 1 galaxy MRK 478. We have found small amplitude continuum flux variability for observations spread over nearly 17 years. The amplitude of variability at shorter wavelengths is found to be higher than the amplitude at longer wavelengths for nearly simultaneous observations, consistent with the predictions of general “photoionization models” of active galaxies.