In 1987, astronomers witnessed a spectacular event when they spotted a titanic supernova 168,000 light-years away in the Hydra constellation. Designated 1987A (since it was the first supernova detected that year), the explosion was one of the brightest supernova seen from Earth in more than 400 years. The last time was Kepler’s Supernova, which was visible to Earth-bound observers back in 1604 (hence the designation SN 1604).
Since then, astronomers have tried in vain to find the company object they believed to be at the heart of the nebula that resulted from the explosion. Thanks to recent observations and a follow-up study by two international teams of astronomers, new evidence has been provided that support the theory that there is a neutron star at the heart of SN 1604 – which would make it the youngest neutron star known to date.
The studies that describe their respective findings were both published in The Astrophysical Journal. The first, “High Angular Resolution ALMA Images of Dust and Molecules in the SN 1987A Ejecta,” appeared in the November 19th, 2019, issue while the second, “NS 1987A in SN 1987A,” was published in the July 30th, 2020 issue. Both studies represent the culmination of thirty years of research and waiting by astronomers.
When light first reached Earth from SN 1987A on February 23rd, 1987, astronomers also detected neutrinos, leading many to suspect that a neutron star formed with the collapse of the star. But when scientists couldn’t find any evidence of that star, they began to wonder if the supernova had resulted in a black hole instead.
As a result, the astronomical community has been waiting for decades to get a peek at what lies behind the very thick cloud of dust and gas that is SN 1987A. In 2019, a team led by Research Associate Dr. Phil Cigan of Cardiff University did exactly that using the Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope.
Using ALMA, the team obtained high-resolution images of a hot “blob” in the core of SN 1987A that was brighter than its surroundings and coincided with where astronomers anticipated a neutron star would be. While many telescopes have taken images of SN 1987A over the years, none were able to observe the core with the same precision. From their observations, they concluded that a neutron star (NS 1987A) lies at the heart of the supernova.
As Dr. Mikako Matsuura, a research associate and STFC Ernest Rutherford Fellow from Cardiff University (was also part of the discovery team) said in a recent NRAO press release:
“We were very surprised to see this warm blob made by a thick cloud of dust in the supernova remnant. 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.
However, while Matsuura and her fellow team members were excited by the result, they were curious about the blob’s brightness – which seemed too bright to exist. This is where Dany Page and his colleagues entered the fray with some fresh insights. Page, an astrophysicist with the Instituto de Astronomía at the National Autonomous University of Mexico, has been studying SN 1987A for years.
With his colleagues from Stony Brook University, the University of Ohio, and the Max Planck Institute for Astrophysics, Page conducted a theoretical study about SN 1987A’s brightness. Their conclusion, the neutron star is particularly bright because it’s so young. As he explained:
“I was halfway through my PhD when the supernova happened, 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.”
These predictions include the location and the temperature of the progenitor star, which their study indicated is precisely where astronomers believe it should be after undergoing collapse and exploding. According to the supernova models Page and his team created, the explosion forced the neutron star away from its point of origin at relativistic speeds (a fraction of the speed of light).
Meanwhile, their model predicted that the temperature of the neutron star would be around 5 million °C (9 million °F), which is enough to explain the brightness of the blob. However, their findings also contradicted the common expectation that the neutron star would be a pulsar. In short, the power of a pulsar depends on its spin rate and magnetic field strength, which would have to be very finely tuned to match the team’s observations.
James Lattimer, a professor of astronomy from Stony Brook University and a member of Page’s research team, has also been following SN 1987A since it first appeared. Prior to this, he published research in which he predicted the kind of neutrino signal a supernova would produce, which subsequently matched the observations. As he put it:
“The neutron star behaves exactly like we expected. 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.”
For the time being, these findings bolster the prediction that a neutron star (not a pulsar, and not a black hole) lies at the center of 1987A. However, nothing short of a direct image of the star would prove that it exists at this point. For that to happen, astronomers will have to wait a few decades until the dust and gas dissipates and becomes more transparent. In the meantime, though, this represents a big step towards that eventual goal.
Further Reading: NRAO, The Astrophysical Journal, TAJ (2)