The Youngest Known X-Ray Binary: Circinus X-1 and Its Natal Supernova Remnant

Abstract

Because supernova remnants are short lived, studies of neutron star X-ray binaries within supernova remnants probe the earliest stages in the life of accreting neutron stars. However, such objects are exceedingly rare: none were known to exist in our Galaxy. We report the discovery of the natal supernova remnant of the accreting neutron star Circinus X-1, which places an upper limit of t < 4, 600 years on its age, making it the youngest known X-ray binary and a unique tool to study accretion, neutron star evolution, and core collapse supernovae. This discovery is based on a deep 2009 Chandra X-ray observation and new radio observations of Circinus X-1. Circinus X-1 produces type I X-ray bursts on the surface of the neutron star, indicating that the magnetic field of the neutron star is small. Thus, the young age implies either that neutron stars can be born with low magnetic fields or that they can rapidly become de-magnetized by accretion. Circinus X-1 is a microquasar, creating relativistic jets which were thought to power the arcminute scale radio nebula surrounding the source. Instead, this nebula can now be attributed to non-thermal synchrotron emission from the forward shock of the supernova remnant. The young age is consistent with the observed rapid orbital evolution and the highly eccentric orbit of the system and offers the chance to test the physics of post-supernova orbital evolution in X-ray binaries in detail for the first time.11 The unambiguous identification of Circinus X-1 as a neutron star is based on the detection of type I X-ray bursts from the source (Tennant et al. 1986;Linares et al. 2010)

The nature of the companion star in Circinus X-1

“…The distance to the source was recently determined from an X-ray light echo to be 9.4 +0.8 −1.0 kpc (Heinz et al 2015), a distance consistent with previous best estimates of around 8 kpc (see Heinz et al 2013 for a discussion of the various suggested distances to the source).…”

Shell-shocked: the interstellar medium near Cygnus X-1

“…two systems are embedded in their natal supernova remnants, making them unusually young XRBs (a few ×10 3 yr for Circinus X-1 - Heinz et al 2013 and ∼ 2 × 10 4 yr for SS 433 -e.g. Goodall et al 2011).…”

“…for SS 433 (Panferov & Fabrika 1997) and 3 × 10 35 -2 × 10 37 erg s −1 for Circinus X-1 (Sell et al 2010) but could be as high as ∼ 10 39 erg s −1 (based on what could be a protrusion of the jet on the south side of the supernova remnant; Heinz et al 2013). …”

“…In its very low state, the system does exhibit a steady jet (Calvelo et al 2012a,b). The fact that it is not easily classified with the rest of the neutron star binary population can now be attributed to the fact that it is in a unique evolutionary phase as the youngest neutron star XRB in the galaxy (Heinz et al 2013). These qualitative differences can bring about considerable quantitative systematic uncertainties in ǫ and α, and, therefore, κjet (these could change by an order of magnitude or more between states because the variability is so strong).…”

The coupled effect of tides and stellar winds on the evolution of compact binaries

“…One possibility is that tides are more efficient than the ones in our model, by a factor of ≈ 3 when taking a non-rotating star. Alternatively, the observed orbital decay is either induced by the mass-transfer itself, or an additional spin-orbit coupling is responsible for it, as suggested by Heinz et al 2013. 6.3 Remarks on our results on the circularization timescale and their consequences (i) There is a variety of papers in the literature which base their description of the evolution of compact binaries on timescale considerations (see for example, Kalogera 1999, Yungelson et al 2006, Pylyser & Savonije 1988, Ma & Li 2009). Since citing all of them does not seem like a sensible choice, we rather show an example of how timescale considerations could fail.…”