Lord of the Rings: A Kinematic Distance to Circinus X-1 From a Giant X-Ray Light Echo

Abstract

Circinus X-1 exhibited a bright X-ray flare in late 2013. Follow-up observations with Chandra and XMM-Newton from 40 to 80 days after the flare reveal a bright X-ray light echo in the form of four welldefined rings with radii from 5 to 13 arcminutes, growing in radius with time. The large fluence of the flare and the large column density of interstellar dust towards Circinus X-1 make this the largest and brightest set of rings from an X-ray light echo observed to date. By deconvolving the radial intensity profile of the echo with the MAXI X-ray lightcurve of the flare we reconstruct the dust distribution towards Circinus X-1 into four distinct dust concentrations. By comparing the peak in scattering intensity with the peak intensity in CO maps of molecular clouds from the Mopra Southern Galactic Plane CO Survey we identify the two innermost rings with clouds at radial velocity ∼ −74 km s −1 and ∼ −81 km s −1 , respectively. We identify a prominent band of foreground photoelectric absorption with a lane of CO gas at ∼ −32 km s −1 . From the association of the rings with individual CO clouds we determine the kinematic distance to Circinus X-1 to be D CirX−1 = 9.4 +0.8 −1.0 kpc. This distance rules out earlier claims of a distance around 4 kpc, implies that Circinus X-1 is a frequent super-Eddington source, and places a lower limit of Γ ∼ > 22 on the Lorentz factor and an upper limit of θ jet ∼ < 3 • on the jet viewing angle.

A Joint Chandra and Swift View of the 2015 X-Ray Dust-Scattering Echo of V404 Cygni

“…As was found in Heinz et al (2015), rings from dustscattering echoes can show significant deviations from axisymmetry due to variations in the column density of the scattering dust concentrations. Asymmetries have also been found in a number of dust-scattering halos by Seward & Smith (2013), Valencic & Smith (2015), and McCollough et al (2013).…”

“…The geometry of dust-scattering echoes is described in detail by, e.g., Vianello et al (2007), Tiengo et al (2010), and Heinz et al (2015). Here, we will briefly summarize the basic geometric features of light-echo rings and define the quantities used throughout the paper.…”

“…Observing X-ray light echoes can be used to study properties of the source as well as the intervening dust. For example, if the distance to the source is not known, one may use light echoes in combination with kinematic information from molecular gas to constrain the source distance (Predehl et al 2000;Heinz et al 2015).…”

“…Because no blank background files exist for HETG data, and because the presence of the HETG significantly affects the background rates, we were not able to derive accurate Heinz et al (2015), using 1000 logarithmically spaced radial bins between 3 and 20 arcmin from the location of V404 Cyg.…”

“…The azimuthal intensity peaks in rings In Section 3.5, we will employ the same radial-deconvolution technique of the ring profile used in Heinz et al (2015) to recover the dust distribution along the LOS. That is, we use the fact that for a thin scattering sheet of dust the intensity profile of the light echo is described by Equation (3) for a single x and use Equation (2) to relate Δt to θ.…”