In the fall of 1967, Princeton’s great quantum physicist John Archibald Wheeler lectured on pulsars at a lecture arguing that we should consider the possibility that the center of a pulsar is a completely gravitational collapse object. He commented that you couldn’t keep saying “completely gravitationally collapsed object” over and over again. That we needed a shorter descriptive phrase. “How about a black hole?” someone in the audience asked, giving birth to the name of one of the most paradoxical objects in the universe.
Fast forward to 2020, two teams of astronomers looking for a lost compact object that should have formed in the leftovers of the 1987 Supernova explosion over two light years, prompting them to ask whether instead of a neutron star, had collapsed in a dungeon. A compelling case in the 33 year mystery has been made based on observations from the Atacama Large Millimeter / submillimeter Array (ALMA) and a follow-up desk study. Scientists provide a new perspective for the argument that a neutron star hides deep within the remains of the exploded star, the youngest known neutron star to date.
Missing evidence
Because the particles known as neutrinos were detected on Earth on February 23, 1987, astronomers hoped that a neutron star would have formed at the collapsed center of the star. But when scientists couldn’t find any evidence for that star, they began to wonder if it could be Wheeler’s “gravity-collapsed object”. For decades, the scientific community has been anxiously waiting for a signal from this object that has hidden behind a very thick dust cloud.
The “Blob” at the core of SN 1987A
Recently, observations from the ALMA radio telescope provided the first indication of the missing neutron star after the explosion. Very high resolution images revealed a hot “drop” in SN 1987A’s dusty core, which is brighter than its surroundings and matches the suspicious location of the neutron star.
“We were very surprised to see this warm bubble made by a thick dust cloud in the supernova remnant,” said Mikako Matsuura of Cardiff University and a team member who found the bubble with ALMA. “There has to be something in the cloud that has heated the dust and made it glow. That is why we suggest that there is a neutron star hidden within the dust cloud. “
The extremely high resolution ALMA images shown above revealed the hot “drop” in the dusty core of Supernova 1987A (inset), which could be the location of the missing neutron star. The red color shows dust and cold gas in the center of the supernova remnant, taken in radio waves with ALMA. The shades of green and blue reveal where the blasting blast wave from the exploded star collides with a ring of material around the supernova. Green represents the glare of visible light, captured by NASA’s Hubble Space Telescope. The blue color reveals the hottest gas and is based on data from NASA’s Chandra X-ray Observatory. The ring was initially made to glow by the flash of light from the original explosion. In the following years, the ring material has lit up considerably as the shock wave from the explosion crashes into it.
Although Matsuura and his team were excited about this result, they wondered about the brightness of the bubble. “We thought the neutron star might be too bright to exist, but then Dany Page [an astrophysicist at the National Autonomous University of Mexico] and his team published a study indicating that the neutron star can be so bright because it is very young, “said Matsuura.
“I was halfway through my PhD when the supernova happened,” Page said, “it was one of the biggest events in my life that made me change the course of my career to try to solve this mystery. It was like a modern holy grail. “
“Despite the supreme complexity of a supernova explosion and the extreme conditions that reign inside a neutron star, the detection of a drop of hot dust is a confirmation of several predictions,” Page explained in the theoretical study of Page and his team, published today in The Astrophysical Journal, which strongly supports the ALMA team’s suggestion that a neutron star is feeding the mass of dust.
Predictions: location and temperature
These predictions were the location and temperature of the neutron star. According to supernova computer models, the explosion has “ejected” the neutron star from its birthplace at a speed of hundreds of kilometers per second (tens of times faster than the fastest rocket). The drop is exactly where astronomers think the neutron star would be today. And the neutron star’s temperature, which is forecast to be around 5 million degrees Celsius, provides enough energy to explain the brightness of the bubble.
“Probably not a Pulsar”
“The power of a pulsar depends on how fast it rotates and the intensity of its magnetic field, which would need to have very well adjusted values to match the observations, while the thermal energy emitted by the hot surface of the young star The neutron star – naturally, the data, said Page, contrary to common expectations, suggests that the neutron star – an extremely hot ball of ultra-dense matter 25 km wide – is probably not a pulsar. It would weigh more than all New York City buildings combined. Because it can only be 33 years old, it would be the youngest neutron star ever found. The second youngest neutron star we know of is found in the remnant of Cassiopeia A supernova and is 330 years old.
“The neutron star behaves exactly as we expected,” added James Lattimer of Stony Brook University in New York, and a member of the Page research team. Lattimer has also closely followed SN 1987A, having published pre-SN 1987A predictions of the neutrino signal from a supernova that subsequently matched the observations. “Those neutrinos suggested that a black hole never formed and, furthermore, it seems difficult for a black hole to explain the observed brightness of the bubble. We compare all the possibilities and conclude that a hot neutron star is the most likely explanation. “
Waiting for the dust to settle
Only a direct image of the neutron star would give definitive proof that it exists, but astronomers may need to wait a few more decades for the dust and gas in the supernova remnant to become more transparent.
Although many telescopes have imaged SN 1987A, none have been able to observe its nucleus as accurately as ALMA. Previous (3-D) observations with ALMA have already shown the types of molecules found in the supernova remnant and confirmed that it produced massive amounts of dust.
“This discovery is based on years of ALMA observations, showing the supernova nucleus in more and more detail thanks to continuous improvements in the telescope and data processing,” said Remy Indebetouw of the National Radio Astronomy Observatory and the University of Virginia, who has been part of the ALMA imaging team.
Sources: ALMA observation of “drop”: “High-resolution angular ALMA images of dust and molecules in SN 1987A Ejecta”, by P. Cigan et al., The Astrophysical Journal. https://doi.org/10.3847/1538-4357/ab4b46
Theoretical study in favor of a neutron star: “NS 1987A in SN 1987A”, by D. Page et al., The Astrophysical Journal. https://doi.org/10.3847/1538-4357/ab93c2
The Daily Galaxy, Max Goldberg, via NRAO
Image credits: top of Chandra X-ray Observatory page and ALMA box (ESO / NAOJ / NRAO), P. Cigan and R. Indebetouw; NRAO / AUI / NSF, B. Saxton; NASA / ESA
