Between rainbows, rings, and sharp, hard lines, it’s difficult to not clap my hands in glee while unpacking the levels of awesome crammed in this X-ray image of Circinus X-1. The beautiful bullseye light echoes hint this neutron star is farther, brighter, and more like a black hole than we thought.

I am in love with this X-ray/optical composite image of Circinus X-1 in the constellation Circinus. The main focus is data from the Chandra X-ray Observatory remapped on a colour-scale to be visible to our eyes, with optical sky survey data providing layers of stars for context. The target is a double system of a neutron star and a massive mundane companion star, smothered in the gas and dust. That the neutron star managed to hold on to its companion after the violent supernova explosion that produced the surrounding shrapnel of the dust and debris is just the first of many strange things about Circinus X-1.


Composite image of the Circinus X-1 with X-ray data in red, green, and blue while optical data is in gold. Object is approximately 300 lightyears across. Image credit: NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al/DSS

Space doesn’t come with neatly-labelled distances for us to clearly establish exactly where stars are, but every now and then we get lucky in tackling this notoriously difficult problem. When the neutron star in Circinus X-1 pulsed with a particularly bright flare, it produced a whole series of light echoes, an electromagnetic analogue to sound echoes with light bouncing through clouds of dust and gas instead of off hard walls. Each ring is an echo of light from the original star ricocheting off an intervening cloud.


The closer a cloud is to us, the bigger the ring looks in radio light echoes. Image credit: Univ. of Wisconsin-Madison/S.Heinz.

The echoes of radio light mark the distance from Earth to each cloud, while X-ray data indicated down the distance from the clouds to the binary star system.


The closer a ring is to us, the larger it seems. The outermost ring (A) is from light bouncing around a cloud 41 lightyears away from the Earth, followed by clouds at 49 lightyears (ring B), 55 lightyears (ring C), and 52 lightyears (ring D) away.

By analyzing the echoes, comparing observations from the Mopra radio telescope from before and after the pulse, and doing a bit of fiddly geometry, astronomers determined Circinus X-1 is 30,700 lightyears from Earth. This overturns earlier research placing the system at a much less useful range of 13,000 to 41,000 lightyears away.


This mathematical dance required access to radio data. The Mopra radio telescope in Australia wasn’t specifically tasked to monitor Circinus X-1. Instead, it was mapping carbon monoxide gas distributions in the plane of the galaxy, just like it had been doing for several years.

The voided bars running through the image are visually intriguing, yet the scientifically least interesting part of this image. Measuring the distances requires detecting fine details in faint signals, a feat only possible by Chandra. Alas, the trade off for this sensitivity is a limited field of view. The actual object is a full, glorious smear, but the rings are so much larger than the field of view for the observatory detectors that they get clipped with only partial coverage. This leads to an interesting conundrum: the densest cloud of carbon monoxide was so close that the ring was too large to see at all! The only indication of the cloud’s presence is by it eating X-rays from the rings behind, just enough to launch a new challenge for astrophysicists to try to image the biggest, brightest ring.

Learning the distance to this neutron star by its echoes bring up all sorts of new astrophysical riddles. The object in Circinus X-1 is clearly a neutron star: it has no event horizon (or the fuzzier equivalent), and unlike a black hole, it has a surface, but it’s also weirdly like a black hole.


It’s twice as far away as we thought it was, meaning it’s much brighter. That frequently pushes the system over the Eddington Limit, the threshold for being bright enough that radiation exerts more pressure outwards than gravity pulling gas and dust back in, a trait more common to black holes than neutron star and leading to unexpected flickering as the gas supply keeps getting blown away.

It’s also building something we may as well call an accretion disc, sucking material from its companion star to feed its powerful jets.


Even more confusingly, the jets are ridiculously high energy with particles ejected at extreme velocities, hitting at least 99.9% the speed of light. That’s blazingly fast even for a black hole, and unheard of for a neutron star.

Circinus X-1 produces a high-energy jet, yet lacks an event horizon. Image credit: NASA/CXC/Univ. of Wisconsin-Madison/S.Heintz et al./M.Weiss

We’re still learning about the jets: the star may produce a single, wide jet, or it may be producing pair of highly collimated jets that wobble as the star processes. It may have counter jets out the opposite pole, but the evidence isn’t entirely clear. And because of the crazy flickering produced by occasionally choking out its own gas supply by hovering so close to the Eddington Limit, absolutely nothing about the star is reliably stable. The particular burst that set off these intriguing echoes were first detected in late 2013, arriving over a three-month span.


X-ray observations between January 2013 and 2014. Circinus X-1 is marked with a white circle; the flare is visible between October 17th and December 19th, 2013. The bow-tie patterns are the shadow of the International Space Station. Image credit: Wisconsin-Madison/S.Heinz

We think Circinus X-1 is the youngest X-ray producing binary system, only becoming an X-ray source about 2,500 years ago. This makes pinpointing its three-dimensional location in space even more interesting since now we get to check out the aging process of neutron stars (and if its immaturity explains any of its unusual behaviour).

Read more in The Astrophysical Journal here [open-access preprint on arXiv here].


Top image: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al; Optical: Digitized Sky Survey