Line Variability in the High‐Resolution X‐Ray Spectrum of MCG −6‐30‐15

Abstract

The recent 540 ks Chandra HETGS spectrum of the well-studied, variable active galactic nucleus (AGN) MCG -6-30-15 shows strong 1s-2p absorption lines from many ions. The spectrum was obtained over a period of about 10 days, and the large number of counts in the spectrum makes it ideal for testing variability on short timescales. We apply quantitative tests for line variability to the 1s-2p absorption lines of H- and He-like Ne, Mg, Si, and S. We find significant correlations and anticorrelations between lines as a function of time, much as we would expect if ionization levels in the absorber were varying. We also find evidence for variation in at least one 1s-2p resonance absorption line as a function of luminosity. We consider several possibilities to explain the line variation. First we consider factors that could change ionization levels in the absorber: radial motion, density variation, luminosity variation, and continuum shape variation. None of these individually can explain the line variation, although we cannot completely constrain continuum shape variation without simultaneous knowledge of the ultraviolet (UV) continuum. Other factors, considered individually, are also unable to explain all the variation: multiple changing continuum components, variable obscuration, and changes in velocity dispersion. Changes in line emission are an unlikely cause of significant variation in absorption-line measurements, but we are unable to fully constrain them. Variability could be due to a changing line of sight through a structured absorber. Modeling such scenarios should produce useful constraints on continuum emission mechanisms and absorber structure.