X-RAY DIPS IN THE SEYFERT GALAXY FAIRALL 9: COMPTON-THICK 'COMETS' OR A FAILED RADIO GALAXY?

摘要

We investigate the spectral variability of the Seyfert galaxy Fairall 9 using almost 6 years of monitoring with the Rossi X-ray Timing Explorer with an approximate time resolution of 4 days.We discover the existence of pronounced and sharp dips in the X-ray flux,with a rapid decline of the 2-20 keV flux of a factor of two or more followed by a recovery to pre-dip fluxes after ～10 days.These dips skew the flux distribution away from the commonly observed lognormal distribution. Dips may result from the eclipse of the central X-ray source by broad-line region clouds,as has recently been found in NGC 1365 and Mrk 766. Unlike these other examples,however,the clouds in Fairall 9 would need to be Compton-thick,and the non-dip state is remarkably free of any absorption features.A particularly intriguing alternative is that the accretion disk is undergoing the same cycle of disruption/ejection as seen in the accretion disks of broad-line radio galaxies such as 3C120 but,for some reason,fails to create a relativistic jet.This suggests that a detailed comparison of Fairall 9 and 3C120 with future high-quality data may hold the key to understanding the formation of relativistic jets in active galactic nucleus.