The Variable Warm Absorber in Circinus X-1

Abstract

We observed Circinus X-1 twice during a newly reached low-flux phase near zero orbital phase using the High-Energy Transmission Grating Spectrometer (HETGS) onboard Chandra. In both observations the source did not show the P Cygni lines we observed during the high-flux phases of the source in 2000 and 2001. During pre-zero phase the source did not exhibit significant variability and exhibited an emission-line spectrum rich in H- and He-like lines from high Z elements such as Si, S, Ar, and Ca. We analyzed all high resolution X-ray spectra by fitting photoionization and absorption models from the most recent version of the XSTAR code. The pre-zero phase spectrum could be fully modeled with a very hot photoionized plasma with an ionization parameter of log xi = 3.0. Post-zero phase episodes feature absorbers with variable high columns, ionization parameter, and luminosity. While cold absorption remains at levels quite similar to the one observed in previous years, the new observations show unprecedented levels of variable warm absorption. The line emissivities also indicate that the observed low source luminosity is inconsistent with a static hot accretion disk corona (ADC), an effect that seems common to other near edge-on ADC sources as well. We conclude that unless there exists some means of coronal heating other than X-rays, the true source luminosity is likely much higher and we observe obscuration in analogy to the extragalactic Seyfert II sources. We discuss possible consequences and relate cold, luke-warm, warm, and hot absorbers to dynamic accretion scenarios.