Astronomers Discover Lonely Neutron Star with a Curious Past

New observations have helped astronomers identify the source of a 2,000-year-old supernova explosion. But this blast, and the neutron star that created it, seem to have a curious history that defies explanation.

Scientists discovered remnants of the supernova dubbed E0102 in the 1980s, located in the Small Magellanic Cloud more than 200,000 light years from Earth. Now, data from the Chandra X-ray Observatory and the European Southern Observatory's Very Large Telescope (VLT) in Chile have enabled astronomers to confirm the presence of a neutron star inside its own visible-light-emitting wreath of gas.

Supernova Remnant E0102

This image of the supernova remnant dubbed E0102 shows both X-rays (blue) and visible light (red and green). Vogt and colleagues argue that the faint X-ray source at the center of the red ring is a neutron star, the progenitor of the supernova. The larger X-ray ring had been spotted previously, but new data from the Very Large Telescope led to the discovery of the smaller ring, which emits visible light at a narrow range of red wavelengths. The green-emitting gas are the outer chunks of the star ejected during the supernova explosion.
X-ray: (NASA / CXC / ESO / F.Vogt et al); Optical: (ESO / VLT / MUSE & NASA / STScI)

This neutron star doesn’t have a stellar companion. Even more intriguingly, the crushed stellar remnant doesn’t give off radio pulsations or high-energy X-rays, which means it likely has a weak magnetic field, making it the only neutron star with these characteristics to be discovered outside the Milky Way so far.

The neutron star is off-center with respect to the giant X-ray wave that surrounds it, sent propagating outward through space by the supernova explosion. Astronomers have long understood that supernova blasts can be asymmetric. “In such cases, the neutron star can get ‘kicked’ in the opposite direction,” says Frédéric Vogt (European Southern Observatory, Chile). In E0102, such a kick could have displaced the neutron star from its central position.

“It’s a bit like fireworks,” Vogt says. “If you take one of those big exploding ones, and place it on the floor, when the black powder explodes, the surrounding cardboard flies in the opposite direction from the blow out.”

But when Vogt and colleagues took a closer look at the remnant in visible light using the VLT, they saw an off-center ring of visible-light-emitting gas. That prompted another look at numerous X-ray observations of E0102, totaling almost four days’ worth of exposure time, which had previously revealed an X-ray source at the center of the red ring. Vogt and colleagues argue in the April 2nd online issue of Nature Astronomy (read the article preprint here) that this source is in fact the neutron star that was created in the supernova explosion.

If the neutron star had been kicked out of its central position in the supernova remnant, then where did the red ring of gas come from? The neutron star’s gravitational field cannot account for its bull’s-eye location — its gravity is too weak to hold on to the gaseous debris, which has been propagating outward and is already 3 light-years away.

Another possible explanation is that the ring has been moving along with the neutron star since the supernova. However, such a scenario is extremely unlikely, as it would require the ring and the neutron star to be moving at the same speed and in the same direction over the course of millennia.

“It’s hard to explain the location and nature of this ring of gas if the neutron star is moving through space at several hundred kilometer per seconds after being kicked during the supernova explosion,” says Vogt.

A more likely scenario is that the neutron star is now where the explosion happened; VLT data show that neither it nor the red ring has moved sideways significantly; the gas inside the red ring of emission is simply expanding radially over time. This explanation, however, would not account for the neutron star’s off-center location in the X-ray ring.

The problem can be resolved by figuring out where the explosion actually happened. In order to do this, scientists would have to play the explosion backwards. Another team is doing so, using archived images from the Hubble Space Telescope to track the motion of the debris over time. Researchers are also collecting new data that will enable them to figure out the exact location of the supernova. If it turns out that the supernova took place at the neutron star’s current location, the scientists will have the key to solving the puzzle.

6 thoughts on “Astronomers Discover Lonely Neutron Star with a Curious Past

  1. Roger-Keith

    The discussion seems to be about proper motion. Perhaps the neutron star is traveling nearly along our line of sight, with little proper motion.

    1. Monica YoungMonica Young

      Good suggestion! Looking at the paper, it seems that Hubble observations over an 8-year baseline collected proper motion data of the larger ring. The more recent VLT observations of the smaller ring show velocity along the line of sight.

  2. John-Murrell

    I can’t find the position of the Neutron Star but there are several Gaia DR2 objects in the area. Hopefully Gaia will be able to provide a full 6D proper motion set though in DR2 Neutron Stars will not have a radial velocity due to the need to have known emission lines to measure.

    I can’t find a reference to the visual magnitude of the Neutron star – the GaiaDR2 objects are mag 16 to 19 (G)

    1. Monica YoungMonica Young

      The neutron star was detected as an X-ray point source, and I don’t believe it was detected in visible light — the VLT images in the paper show only the ring and some diffuse material. The authors say in the paper that the X-ray point source is located at R.A: 01h04m02.7s, Dec: -72d02m000.2s [J2000]. The uncertainty in this position is 1.2 arcseconds.

  3. Howard RitterHoward Ritter

    Here’s my take:

    The red ring is far smaller than the larger, off-center ring. Obviously, the inner shell was ejected later than the outer shell was. It must have been done so either with a much lower velocity at the end of the SN event, or much later than the outer material, or both. Is there a known mechanism for either of these cases? In either case, the flying neutron-star remnant must have been clear of the debris cloud when the red shell was emitted in order for it to have retained its spherical symmetry.

    So the neutron star was already in flight at the time the inner-shell material was ejected, since the phase of a SN explosion that accelerates the great majority of the star’s mass asymmetrically, imparting momentum to the neutron-star remnant, lasts only for an astronomical eyeblink. Having been produced by the neutron star already in flight, the inner shell remains centered on it as the two continue to co-move through space relative to the earlier shell.

    1. Monica YoungMonica Young

      Hi Howard: The idea that the neutron star had already been kicked when it emitted a second shell of material seems like a plausible explanation! The only problem (challenge!) is, how would a neutron star eject a second shell after its formation in the supernova? It’s an intriguing question!

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