Abstract Details
| Name: Susmita Chakravorty Affiliation: Indian Institute of Science Conference ID: ASI2018_1349 Title : Changing look quasars - BLR gone with the stable wind Authors and Co-Authors : Susmita Chakravorty, Martin Elvis, Gary Ferland Abstract Type : Poster Abstract Category : Extragalactic astronomy Abstract : With the unprecedented sky monitoring provided by Sloan Digital Sky Survey (SDSS), it has been possible to discover quasars which "change their look". The broad emission lines (BELs, that are hallmarks of active super-massive black holes) that were present in 10 year old SDSS spectra have been found to diminish or disappear in the latest SDSS spectra (DR12) in at least 3 quasars. There was at least, one more known detection of such an effect since 1976. Given the duty cycle of Active Galactic Nuclei (AGN), any such variability can be detected only with decade long (or longer) systematic surveys. Such surveying was not present before SDSS and hence the convincing results are emerging only now. We propose a hypothesis to explain the changing strength of the BELs in AGN. We have found that broad emission line regions (BELR) exist for high metallicity gas under stringent conditions of pressure equilibrium with warmer outflowing X-ray absorbing gas. The pressure equilibrium works for only a narrow range of gas density. Further, the conditions of the pressure equilibrium is sensitively dependant on the metallicity of the gas and the spectral energy distribution (SED) of the ionizing light. Among the SED parameters, alpha_OX - the optical to X-ray flux ratio, is one of the crucial parameters that destroys the pressure equilibrium if alpha_OX > 1.6! Our systematic analysis will let us design intelligent observational strategies to look for these rare “changing look AGNâ€. |