Top left: A giant blue star, much more massive than our Sun, has consumed, by nuclear fusion at its center, all of its hydrogen, helium and heavier elements down to iron. It now has a small iron core (red dot) in its center. Unlike the early stages of fusion, the fusion of iron atoms absorbs rather than releases energy. The energy released by the fusion that held the star against its own weight is now gone, and the star will rapidly collapse, triggering a supernova explosion.Top right: The collapse has begun, producing a super dense neutron star with a strong magnetic field at its center (inset). The neutron star, although containing about 1.5 times the mass of the Sun, is only about the size of Manhattan. Bottom left: The supernova explosion ejected a rapidly outward-moving shell of debris into interstellar space. At this point, the debris shell is dense enough to mask any radio waves coming from the neutron star region. Bottom right: As the shell of explosion debris expands over a few decades, it becomes less dense and eventually becomes thin enough that radio waves from within can escape. This allowed VLA Sky Survey observations to detect the bright radio emissions created when the rapidly rotating neutron star’s strong magnetic field sweeps through surrounding space, accelerating charged particles. This phenomenon is called a pulsar wind nebula. Credit: Melissa Weiss, NRAO/AUI/NSF
Astronomers analyzing data from the VLA Sky Survey (VLASS) have discovered one of the youngest known neutron stars, the superdense remnant of a massive star that exploded as a supernova. Images from the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) indicate that the bright radio emission powered by the spinning pulsar’s magnetic field only recently emerged from behind a dense envelope of debris from the explosion. of the supernova.
The object, called VT 1137-0337, is in a dwarf galaxy 395 million light-years from Earth. It first appeared in a VLASS image made in January 2018. It does not appear in an image of the same region made by the VLA’s FIRST Survey in 1998. It continued to appear in subsequent VLASS observations in 2018 , 2019, 2020 and 2022. .
“What we are most likely seeing is a pulsar wind nebula,” said Caltech graduate Dillon Dong, who will begin a Jansky Postdoctoral Fellowship at the National Radio Astronomy Observatory (NRAO) later this year. A Pulsar Wind Nebula is created when the powerful magnetic field of a rapidly rotating neutron star accelerates the surrounding charged particles to almost the speed of light.
“Based on its characteristics, this is a very young pulsar – possibly as young as 14, but no older than 60 to 80,” said Gregg Hallinan, Dong’s PhD advisor at Caltech. .
The scientists reported their findings at the American Astronomical Society meeting in Pasadena, California.
Dong and Hallinan discovered the object in data from VLASS, an NRAO project that began in 2017 to study all of the sky visible from the VLA, or about 80% of the sky. Over a period of seven years, VLASS performs a full scan of the sky three times, with one goal of finding transient objects. Astronomers found VT 1137-0337 in the first VLASS scan of 2018.
Comparing this VLASS scan with data from a previous VLA sky survey called FIRST revealed 20 particularly bright transient objects that may be associated with known galaxies.
“This one stood out because its galaxy is experiencing a burst of star formation, and also because of the characteristics of its radio said Dong. The galaxy, called SDSS J113706.18-033737.1, is a dwarf galaxy containing about 100 million times the mass of the Sun.
While studying the characteristics of VT 1137-0337, astronomers considered several possible explanations, including a supernova, a gamma-ray burst, or a tidal disruption event in which a star is shredded by a supermassive black hole. They concluded that the best explanation is a pulsar wind nebula.
VLA images of the location of VT 1137-0337 in 1998, left, and 2018, right. The object became visible to the VLA between these two dates. Credit: Dong & Hallinan, NRAO/AUI/NSF
In this scenario, a star much more massive than the Sun exploded in a supernova, leaving behind a neutron star. Most of the original star’s mass was thrown outward as a shell of debris. The neutron star spins rapidly, and as its powerful magnetic field sweeps through the surrounding space, it accelerates charged particles, causing a strong radio emission.
Initially, the radio emission was masked by the explosion debris shell. As this shell expanded, it gradually became less dense until radio waves from the Pulsar Wind Nebula could finally pass through.
“This happened between the FIRST sighting in 1998 and the VLASS sighting in 2018,” Hallinan said.
Probably the most famous example of a pulsar wind nebula is the Crab Nebula in the constellation Taurus, the result of a supernova that shone brightly in the year 1054. The Crab is easily visible today. today in small telescopes.
“The object we found appears to be about 10,000 times more energetic than the crab, with a stronger magnetic field,” Dong said. “It’s probably an emerging ‘super crab’,” he added.
While Dong and Hallinan consider VT 1137-0337 to be most likely a pulsar wind nebula, it’s also possible that its magnetic field is strong enough for the neutron star to qualify as a magnetar, a class of super-magnetic objects. Magnetars are one of the prime candidates for causing the mysterious fast radio bursts (FRBs) that are currently being extensively studied.
“In this case, it would be the first Magnetar caught in the act of appearing, and that too is extremely exciting,” Dong said.
Indeed, some fast radio bursts have been found to be associated with persistent radio sources, the nature of which is also a mystery. They show a strong resemblance in their properties to VT 1137-0337, but showed no evidence of high variability.
“Our finding of a very similar source that lights up suggests that the radio sources associated with FRBs could also be bright pulsar wind nebulae,” Dong said.
Astronomers plan to conduct further observations to learn more about the object and monitor its behavior over time.
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