Neutron star discovered in center of Supernova 1987A

Science:

Editor summary: The nearby supernova SN 1987A was visible to the naked eye, and its evolution has been observed over the ensuing decades. The explosion is thought to have produced a neutron star or black hole, but none has been directly detected. Fransson et al. observed a remnant of SN 1987A using near- and mid-infrared integral field spectroscopy. They identified emission lines of ionized argon that appear only near the center of the remnant. Photoionization models show that the line ratios and velocities can be explained by ionizing radiation from a neutron star illuminating gas from the inner parts of the exploded star

Nature News:

Astronomers used the [JWST] to finally spot signs of an ultradense ‘neutron star’ lurking in the explosion’s core in a galaxy that orbits the Milky Way. Light from the explosion reached Earth 37 years ago this week, in a supernova that revolutionized modern astrophysics by providing an up-close look at how stars die.

JWST did not observe the neutron star directly, because it remains obscured behind a veil of dust from the explosion. But the telescope detected light coming from argon and sulfur atoms that had been ionized, or electrically charged, by radiation blazing from the long-sought neutron star.

Over the years, astronomers watched as rings of gas and dust expanded outwards from the site of the explosion, usually growing dimmer but sometimes brightening when various ejected materials collided.

One outcome of such a supernova is to leave behind a black hole. But early observations of SN 1987A, such as the wave of neutrinos, suggested that it should have given rise to a neutron star, which can be just 20 kilometres across but is so dense that a teaspoonful weighs millions of tonnes.

Space.com:

Using the James Webb Space Telescope (JWST), astronomers have ended a nearly decade-long game of celestial hide-and-seek after they discovered a neutron star in the wreckage of a stellar explosion.

"For a long time, we've been searching for evidence for a neutron star in the gas and dust of Supernova 1987A," Mike Barlow, an emeritus professor of physics and astronomy and part of the team behind this discovery, told Space.com. "Finally, we have the evidence that we've been seeking."

Barlow suggested that researchers may be able to distinguish between a naked neutron star and one clothed by a pulsar-wind nebula by making further infrared observations of the heart of Supernova 1987A with the JWST's NIRSpec instrument.

"We have a program which is gathering data now, which will be getting data with 3 or 4 times the resolution in the near-infrared," he concluded. "So by obtaining these new data, we may be able to distinguish the 2 models that have been proposed to explain the emission powered by a neutron star.

 

On the Astraveo podcast:

• Supernova 1987A Mystery SOLVED! and more! | Astraveo Coast-to-Coast (Feb 25, 2024)
  
• "No Body, No Crime": Supernova Remains Finally Revealed! (Astraveo Coast-to-Coast Highlight)
  

Spherical symmetry in the kilonova AT2017gfo/GW170817

 Nature (via arXiv):

The mergers of neutron stars expel a heavy-element enriched fireball which can be observed as a kilonova. The kilonova's geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. 

We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta, however an additional process seems necessary to make the element distribution uniform.

Kilonovae are one of few objects that allow a relatively direct asymmetry measurement, via the expanding photospheres method. Normally, this method uses information about the expanding photosphere to fix the distance; however, since we can measure the explosion time very precisely from the gravitational wave, and we know the distance fairly accurately by measuring the distance to the host galaxy, we can use this method to work backwards to measure a transverse velocity of the ejecta. Comparing the two velocities gives a direct measurement of the asymmetry of the expansion.

For every epoch we find a line shape that is consistent with a completely spherical expansion to within a few percent. These line-shape constraints are independent of the EPM measurements and verify the spherical nature of the kilonova at early epochs. 

This is pretty interesting. Current GRMHD simulations show that asymmetry is usually favored in explosions like these. Spherical symmetry is of course possible, but it is not a generic attribute, and we would not expect to see it very frequently. Understanding what kind of mechanism could generate spherical symmetry in an explosion that is typically asymmetric is an important problem.

Energy may also be injected in an anisotropic fashion as a relativistic wind from the remnant neutron star or black hole by tapping the rotational energy of the system. Within the first few seconds of the explosion a polar outflow could be launched and produce a rapidly-expanding balloon of high-Ye material with low opacity, dominated by elements like Sr. This polar outflow, outpacing the equatorial ejecta, would expand sideways, covering the low-Ye material and providing a near-spherical kilonova.

ScienceAlert: 

The colossal explosion resulting from a merger between two neutron stars has an unexpectedly perfect shape...The spherical explosion they actually found suggests that our understanding of neutron star mergers is lacking.

"You have two super-compact stars that orbit each other 100 times a second before collapsing. Our intuition, and all previous models, say that the explosion cloud created by the collision must have a flattened and rather asymmetrical shape," Sneppen says.

"The most likely way to make the explosion spherical is if a huge amount of energy blows out from the center of the explosion and smooths out a shape that would otherwise be asymmetrical. So the spherical shape tells us that there is probably a lot of energy in the core of the collision, which was unforeseen."

Phys.org:

However, this theory does not explain another aspect of the researchers' discovery. According to the previous models, while all elements produced are heavier than iron, the extremely heavy elements, such as gold or uranium, should be created in different places in the kilonova than the lighter elements such as strontium or krypton, and they should be expelled in different directions. The researchers, on the other hand, detect only the lighter elements, and they are distributed evenly in space.

They therefore believe that the enigmatic elementary particles, neutrinos, about which much is still unknown, also play a key role in the phenomenon.