Astronomers using data from the VLA Sky Survey (VLASS) have identified one of the youngest neutron stars known – the superdense relic of a huge star that burst as a supernova. Bright radio emission generated by the spinning pulsar's magnetic field has just recently emerged from behind a thick shell of debris from the supernova explosion, according to images from the National Science Foundation's Karl G. Jansky Very Large Array (VLA).
The object, known as VT 1137-0337, is located 395 million light-years from Earth in a dwarf galaxy. In January of 2018, it initially appeared in a VLASS photograph. It was not seen in a 1998 photograph of the same location obtained by the VLA's FIRST Survey. Later VLASS observations in 2018, 2019, 2020, and 2022 confirmed this.
“What we’re most likely seeing is a pulsar wind nebula,” said Dillon Dong, a Caltech graduate student who will start a Jansky Postdoctoral Fellowship at the National Radio Astronomy Observatory (NRAO) later this year. When the intense magnetic field of a rapidly spinning neutron star accelerates nearby charged particles to almost the speed of light, a pulsar wind nebula is formed.
“Based on its characteristics, this is a very young pulsar — possibly as young as only 14 years, but no older than 60 to 80 years,” Dong's Caltech Ph.D adviser Gregg Hallinan stated.
At the American Astronomical Society conference in Pasadena, California, the scientists presented their findings.
The object was identified in VLASS data, an NRAO initiative that began in 2017 to survey the entire sky viewable from the VLA — around 80% of the sky. VLASS will survey the sky three times over the course of seven years, one of the goals of which is to discover transitory objects. In the first VLASS survey from 2018, astronomers discovered VT 1137-0337.
When that VLASS scan was compared to data from the FIRST VLA sky survey, researchers discovered 20 unusually bright transient objects that may be related 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 emission,” Dong explained. SDSS J113706.18-033737.1 is a dwarf galaxy with a mass of approximately 100 million times that of the Sun.
The researchers explored numerous possible explanations for the properties of VT 1137-0337, including a supernova, gamma ray burst, or tidal disruption event, in which a star is torn by a supermassive black hole. They came to the conclusion that a pulsar wind nebula is the best explanation.
A star far more massive than the Sun exploded as a supernova, leaving behind a neutron star in this scenario. The majority of the mass of the original star was flung outward as a shell of debris. The neutron star spins quickly, and its enormous magnetic field accelerates charged particles in the surrounding space, resulting in significant radio emission.
The radio emission was first obscured by the shell of explosion debris. That shell grew ever less thick as it expanded, allowing radio waves from the pulsar wind nebula to flow through.
“This happened between the FIRST observation in 1998 and the VLASS observation in 2018,” according to Hallinan.
The Crab Nebula in the constellation Taurus, which was formed by a supernova that exploded brilliantly in the year 1054, is perhaps the most renowned example of a pulsar wind nebula. Small telescopes may easily see the Crab today.
“The object we have found appears to be approximately 10,000 times more energetic than the Crab, with a stronger magnetic field,” Dong said. “It likely is an emerging ‘super Crab’,” he continued.
While Dong and Hallinan believe VT 1137-0337 is most likely a pulsar wind nebula, the neutron star's magnetic field might be powerful enough to define it as a magnetar, a type of super-magnetic phenomenon. Magnetars are a leading contender for the enigmatic Fast Radio Bursts (FRBs) that are now being investigated.
“In that case, this would be the first magnetar caught in the act of appearing, and that, too, is extremely exciting,” Dong added.
While Dong and Hallinan believe VT 1137-0337 is most likely a pulsar wind nebula, the neutron star's magnetic field might be powerful enough to define it as a magnetar, a type of super-magnetic phenomenon. Magnetars are a leading contender for the enigmatic Fast Radio Bursts (FRBs) that are now being investigated.
“Our discovery of a very similar source switching on suggests that the radio sources associated with FRBs also may be luminous pulsar wind nebulae,” Dong continued.
In fact, several Fast Radio Bursts have been linked to persistent radio sources, the nature of which is likewise unknown. They exhibit features that are very similar to VT 1137-0337, but no indication of high variability.
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