Jun 17 2022Reviewed by Alex Smith
Using data from the VLA Sky Survey (VLASS), astronomers have identified one of the youngest neutron stars known — the superdense relic of a huge star that burst like a supernova.
Top Left: A giant blue star, much more massive than our Sun, has consumed, through nuclear fusion at its center, all its hydrogen, helium, and heavier elements up to iron. It now has a small iron core (red dot) at its center. Unlike the earlier stages of fusion, the fusion of iron atoms absorbs, rather than releases, energy. The fusion-released energy that has held up the star against its own weight now is gone, and the star will quickly collapse, triggering a supernova explosion. Top Right: The collapse has begun, producing a superdense neutron star with a strong magnetic field at its center (inset). The neutron star, though containing about 1.5 times the mass of the Sun, is only about the size of Manhattan. Bottom Left: The supernova explosion has ejected a fast-moving shell of debris outward into interstellar space. At this stage, the debris shell is dense enough to shroud from view any radio waves coming from the region of the neutron star. 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 inside can escape. This allowed observations by the VLA Sky Survey to detect bright radio emission created as the rapidly spinning neutron star's powerful magnetic field sweeps through the surrounding space, accelerating charged particles. This phenomenon is called a pulsar wind nebula. Image Credit: Melissa Weiss, NRAO/AUI/NSF
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. Its initial appearance was in January 2018 in a VLASS photograph. It did not appear in a 1998 photograph of the same location obtained by the VLA’s FIRST Survey. Further VLASS observations in 2018, 2019, 2020, and 2022 confirmed this.
What we are most likely seeing is a pulsar wind nebula.
Dillon Dong, Graduate Student, California Institute of Technology
Dillon Dong will begin a Jansky Postdoctoral Fellowship at the National Radio Astronomy Observatory (NRAO) later this year.
When the intense magnetic field of a rapidly rotating 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.
Gregg Hallinan, Professor, Astronomy, California Institute of Technology
Gregg Hallinan is Dong’s Ph.D. advisor at Caltech.
The scientists presented their findings at a conference of the American Astronomical Society in Pasadena, California. The object was identified by Dong and Hallinan in data from VLASS, an NRAO effort that began in 2017 to study the entire sky viewable from the VLA—around 80% of the sky.
VLASS will scan the sky three times over the course of seven years, with one of the goals being to discover transitory objects. In the first VLASS survey from 2018, the scientists 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 which may be linked with known galaxies.
Dong stated, “This one stood out because its galaxy is experiencing a burst of star formation, and also because of the characteristics of its radio emission.”
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 colossal than the Sun burst like 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 emissions.
The radio emission was first obscured by the shell of explosion debris. That shell grew less thick as it expanded, allowing radio waves from the pulsar wind nebula to flow through.
Hallinan added, “This happened between the FIRST observation in 1998 and the VLASS observation in 2018.”
The Crab Nebula in the constellation Taurus, which was formed by a supernova that exploded brilliantly in 1054, is perhaps the most renowned example of a pulsar wind nebula. Small telescopes can 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. It likely is an emerging ‘super Crab’,” further added Dong.
While Dong and Hallinan feel 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.
Dong outlined, “In that case, this would be the first magnetar caught in the act of appearing, and that, too, is extremely exciting.”
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.
Dong concluded, “Our discovery of a very similar source switching on suggests that the radio sources associated with FRBs also may be luminous pulsar wind nebulae.”
Scientists intend to continue their studies to understand more about the object and to track its progress over time.
Source: https://public.nrao.edu/odi/pride/