by Using the James Webb Space Telescope (JWST), astronomers have ended a nearly decade-long celestial game of hide and seek after discovering a neutron star in the midst of a stellar explosion.
Supernova 1987A represents the remnants of an exploded star that once had a mass of about 8 to 10 times that of the Sun. It is located about 170,000 light years away in the Large Magellanic Cloud, a dwarf galaxy neighbor of the Milky Way. Supernova 1987A was first observed by astronomers 37 years ago in 1987, hence the numerical aspect of its name. As it exploded, Supernova 1987A showered the Earth for the first time with ghostly particles called neutrinos and then became visible in bright light. This was the closest and brightest supernova seen in the night sky on Earth for around 400 years.
Supernova explosions like this are responsible for seeding the cosmos with elements like carbon, oxygen, silicon and iron. Ultimately, these elements become the building blocks for the next generation of stars and planets, and can even form molecules that may one day become an integral part of life as we know it. These explosions also produce dense stellar remnants in the form of neutron stars or black holes; For the past 37 years, astronomers have not known which of these could be the core of Supernova 1987A.
“For a long time, we have been searching for evidence for a neutron star in the gas and dust of Supernova 1987A,” Mike Barlow, professor emeritus of physics and astronomy and part of the team behind this discovery, told Space.com. “Finally, we have the evidence we are looking for.”
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How does a neutron star hide for 4 decades?
Neutron stars are born when massive stars exhaust their supplies of fuel needed for nuclear fusion to occur at their cores. This reduces the outward energy that flows from the cores of these stars which protects them from collapsing under their own gravity.
As a stellar core collapses, massive supernova explosions rip through the star’s outer layers, exploding outward. This leaves behind a “dead” star as wide as the average city here on Earth, but with a mass about one to two times that of the sun; the star ends up consisting of a fluid of neutron particles, which is the densest matter known in the universe.
Neutron stars are supported against total collapse, however, by quantum effects that occur between neutrons in their interior. These effects prevent the neutrons from cramming together. The so-called “neutron degeneracy pressure” can be overcome if a stellar core has enough mass – or if a neutron star has more mass after it is formed. This would result in a black hole (if the minimum mass is not reached, however, it will not happen.)
Scientists were fairly certain that the object in Supernova 1987A was a neutron star, but they could not rule out that this recently extinct star, as we see it at least 170,000 or so years ago, had not gather its mass to transform itself into a black hole.
“Another possibility was that the infalling material could have accreted onto the neutron star and caused it to collapse into a black hole. So, a black hole was another possible scenario,” Barlow said. “However, the spectrum produced by infalling matter is not the right type of spectrum to explain the emission we see.”
You’re getting hotter…
The newly identified neutron star eluded detection for 37 years because, as a newborn, it was still surrounded by a thick mantle of gas and dust launched during the supernova explosion that signaled the death of its progeny star.
“Its detection was hindered because the supernova condensed about half a solar mass of dust in the years after the explosion,” Barlow said. “This dust acted as a screen obscuring radon from the center of Supernova 1987A.”
Dust is much less effective at blocking infrared light than it is at blocking visible light. So, to see through this shadow of death and into the core of Supernova 1987A, Barlow and colleagues turned to the JWST’s highly sensitive infrared eye, specifically the telescope’s Mid-Infrared Instrument and the Near-Infrared Spectrometer.
The smoking gun evidence for this hidden neutron star involved emissions from the elements argon and sulfur from the center of Supernova 1987A. These elements are ionized, meaning that electrons have been removed from their atoms. Barlow said that this ionization could only occur due to radiation emitted by a neutron star.
The emissions allowed the team to limit the brightness or luminosity of the once-hidden neutron star. They found it to be about one-tenth the brightness of the sun.
The team may have determined that a neutron star was born in Supernova 1987A, but all the mysteries of this neutron star have not yet been solved.
That’s because the ionization of their smoking gun of argon and sulfur could lead to a neutron star in one of two ways. Winds of charged particles that were dragged along and accelerated to near the speed of light by a rapidly rotating neutron star could have interacted with the surrounding supernova material, causing the ionization. Alternatively, ultraviolet and X-ray light emitted from the million-degree surface of the hot neutron star may have stripped electrons from atoms at the heart of this starburst.
If the first scenario is correct, then the neutron star at the heart of Supernova 1987A is a pulsar surrounded by a pulsating wind nebula. Pulsars are very spinning neutron stars. If the latter case is the correct recipe for these emissions, however, this close supernova rendered a “bare” or “exposed” neutron star, exposing its surface directly to space.
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Barlow suggested that by taking additional infrared observations of the core of Supernova 1987A with JWST’s NIRSpec instrument, researchers might be able to distinguish between a bare neutron star and a star with a pulsar wind nebula.
“We have a program that is collecting data now, which will be getting data that will have 3 or 4 times the resolution in the near infrared,” he said. “Thus, by obtaining these new data, we may be able to distinguish between the 2 models proposed to explain the neutron star-driven emission.”
The team’s research was published Thursday (February 22) in the journal Science.
