In the fall of 1967, Princeton’s great quantum physicist, John Archibald Wheeler was giving a lecture on pulsars at a conference where he was arguing that we should consider the possibility that the center of a pulsar is a gravitationally completely collapsed object. He remarked that one couldn’t keep saying “gravitationally completely collapsed object” over and over. That we needed a shorter descriptive phrase. “How about black hole?” asked someone in the audience, giving birth to the name of one of the most paradoxical objects in the universe.
Fast forward to 2020, two teams of astronomers searching for a missing compact object that should have formed in the leftovers from the two-light-year wide blast of Supernova 1987A, leading them to wonder if instead of a neutron star it had collapsed into a black hole. A compelling case in the 33-year-old mystery has been made based on observations of the Atacama Large Millimeter/submillimeter Array (ALMA) and a theoretical follow-up study. The scientists provide new insight for the argument that a neutron star is hiding deep inside the remains of the exploded star –the youngest neutron star known to date.
Because particles known as neutrinos were detected on Earth on 23 February 1987, astronomers expected that a neutron star had formed in the collapsed center of the star. But when scientists could not find any evidence for that star, they started to wonder whether it might be Wheeler’s “gravitationally completely collapsed object.” For decades the scientific community has been eagerly awaiting a signal from this object that has been hiding behind a very thick cloud of dust.
The “Blob” at Core of SN 1987A
Recently, observations from the ALMA radio telescope provided the first indication of the missing neutron star after the explosion. Extremely high-resolution images revealed a hot “blob” in the dusty core of SN 1987A, which is brighter than its surroundings and matches the suspected location of the neutron star.
“We were very surprised to see this warm blob made by a thick cloud of dust in the supernova remnant,” said Mikako Matsuura from Cardiff University and a member of the team that found the blob with ALMA. “There has to be something in the cloud that has heated up the dust and which makes it shine. That’s why we suggested that there is a neutron star hiding inside the dust cloud.”
Extremely high-resolution ALMA images above revealed the hot “blob” 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 at radio wavelengths with ALMA. The green and blue hues reveal where the expanding shock wave from the exploded star is colliding with a ring of material around the supernova. The green represents the glow 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. Over subsequent years the ring material has brightened considerably as the explosion’s shock wave slams into it.
Even though Matsuura and her team were excited about this result, they wondered about the brightness of the blob. “We thought that 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 that indicated that the neutron star can indeed be this bright because it is so 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.”
“In spite of the supreme complexity of a supernova explosion and the extreme conditions reigning in the interior of a neutron star, the detection of a warm blob of dust is a confirmation of several predictions,” Page explained in the theoretical study by Page and his team, published today in The Astrophysical Journal, that strongly supports the suggestion made by the ALMA team that a neutron star is powering the dust blob.
The Predictions –Location and Temperature
These predictions were the location and the temperature of the neutron star. According to supernova computer models, the explosion has “kicked away” the neutron star from its birthplace with a speed of hundreds of kilometers per second (tens of times faster than the fastest rocket). The blob is exactly at the place where astronomers think the neutron star would be today. And the temperature of the neutron star, which was predicted to be around 5 million degrees Celsius, provides enough energy to explain the brightness of the blob.
“Likely Not a Pulsar”
“A pulsar’s power depends on how fast it spins and on its magnetic field strength, both of which would need to have very finely tuned values to match the observations, while the thermal energy emitted by the hot surface of the young neutron star naturally fits the data, said Page, suggesting that contrary to common expectations, the neutron star –a 25 km wide, extremely hot ball of ultra-dense matter.–is likely not a pulsar. A teaspoon of its material would weigh more than all the buildings within New York City combined. Because it can only be 33 years old, it would be the youngest neutron star ever found. The second youngest neutron star that we know of is located in the supernova remnant Cassiopeia A and is 330 years old.
“The neutron star behaves exactly like we expected,” added James Lattimer of Stony Brook University in New York, and a member of Page’s research team. Lattimer has also followed SN 1987A closely, having published prior to SN 1987A predictions of a supernova’s neutrino signal that subsequently matched the observations. “Those neutrinos suggested that a black hole never formed, and moreover it seems difficult for a black hole to explain the observed brightness of the blob. We compared all possibilities and concluded that a hot neutron star is the most likely explanation.”
Waiting for the Dust to Settle
Only a direct picture of the neutron star would give definite proof that it exists, but for that astronomers may need to wait a few more decades until the dust and gas in the supernova remnant become more transparent.
Even though many telescopes have made images of SN 1987A, none of them have been able to observe its core with such high precision as ALMA. Earlier (3-D) observations with ALMA already showed the types of molecules found in the supernova remnant and confirmed that it produced massive amounts of dust.
“This discovery builds upon years of ALMA observations, showing the core of the supernova in more and more detail thanks to the continuing improvements to the telescope and data processing,” said Remy Indebetouw of the National Radio Astronomy Observatory and the University of Virginia, who has been a part of the ALMA imaging team.
Sources: ALMA observation of the “blob”: “High Angular Resolution ALMA Images of Dust and Molecules in the SN 1987A Ejecta”, by P. Cigan et al., The Astrophysical Journal. https://doi.org/10.3847/1538-4357/ab4b46
Theoretical study favoring 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: Chandra X-Ray Observatory top of page and inset ALMA (ESO/NAOJ/NRAO), P. Cigan and R. Indebetouw; NRAO/AUI/NSF, B. Saxton; NASA/ESA