Astronomers analyzing data from the VLA Sky Survey (VLASS) have discovered one of the youngest known neutron stars – the super-dense remnant of a massive star that exploded in a supernova. Images from the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) indicate that the bright radio emission driven by the rotating pulse’s magnetic field only recently appeared behind a dense envelope of debris from the supernova explosion.
The object, called VT 1137-0337, is in a dwarf galaxy 395 million light-years from Earth. It first appeared on a VLASS image made in January 2018. It does not appear on an image of the same region made by VLA’s FIRST survey in 1998. It continued to appear in subsequent VLASS observations in 2018, 2019, 2020 and 2022..
“What we most likely see is a pulsar fog,” said Caltech graduate Dillon Dong, who will begin a Jansky Postdoc scholarship at the National Radio Astronomy Observatory (NRAO) later this year. A Pulsar nebula is created when the strong magnetic field of a rapidly spinning neutron star accelerates surrounding charged particles to almost the speed of light.
“Based on its characteristics, it is a very young pulsar – perhaps as young as 14, but not older than 60 to 80,” said Gregg Hallinan, Dong’s PhD advisor at Caltech.
The researchers reported their findings at the meeting of the American Astronomical Society in Pasadena, California.
Dong and Hallinan discovered the object in data from VLASS, an NRAO project that began in 2017 to study the entire sky visible from the VLA – about 80 percent of the sky. Over a period of seven years, VLASS performs a full scan of the sky three times with the sole goal of finding transient objects. Astronomers found VT 1137-0337 in the first VLASS scan in 2018.
Comparison of this VLASS scan with data from a previous VLA sky study called FIRST revealed 20 particularly bright transient objects that may be associated with known galaxies.
“This one stood out because its galaxy is experiencing an eruption of star formation, and also because of the characteristics of its radio emission,” Dong said. 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 outbreak in which a star is shredded by a supermassive black hole. They concluded that the best explanation is a pulsar fog.
In this scenario, a star much more massive than the Sun exploded in a supernova, leaving a neutron star. Most of the original star’s mass was thrown outward like 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 broadcast was masked by the explosion waste grenade. As this one expanded, it gradually became less dense until radio waves from the Pulsar nebula could finally pass through.
“This happened between the FIRST observation in 1998 and the VLASS observation 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 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 a new ‘super crab,'” he added.
While Dong and Hallinan consider VT 1137-0337 to be most likely a pulsar nebula, it is also possible that its magnetic field is strong enough to qualify the neutron star as a magnetar – a class of supermagnetic. Magnetars are one of the primary candidates to cause the mysterious rapid radio eruptions (FRBs) that are currently being thoroughly investigated.
“In this case, it would be the first magnetar that was caught in the process of popping up, and that’s also extremely exciting,” Dong said.
In fact, some rapid radio eruptions have been shown to be associated with persistent radio sources, the nature of which is also a mystery. They show a strong similarity in their properties with VT 1137-0337, but showed no signs of high variability.
“Our finding of a very similar source illuminating suggests that the radio sources associated with FRBs could also be bright pulsar nebulae,” Dong said.
Astronomers plan to perform additional observations to learn more about the object and monitor its behavior over time.
The National Radio Astronomy Observatory is a facility under the National Science Foundation operated under a partnership agreement with the Associated Universities, Inc.