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Sep 18th 2019

Distant Superluminous Supernovae Light Up the Radio Sky

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Over the past few years astronomers have been observing — and seeking to explain — brief, powerful pulses of radio noise emanating from distant regions of the cosmos. The pulses, known as Fast Radio Bursts or FRBs, are so sudden and brief (in the millisecond range) that they vanish almost at the moment we detect them, making them hard to study.

Recently, though, a team of astronomers caught an FRB in a repeat performance. This allowed them to pinpoint its location, which in turn could link FRBs to some of the most spectacular events in the universe: superluminous supernovae.

When Stars Blow Up

Supernovae are exploding stars, and a typical supernova may outshine all the rest of the stars in a moderate-sized galaxy for a few weeks. In recent decades, though, astronomers came to realize that some supernova explosions, called superluminous supernovae, are up to a hundred times brighter, briefly outshining all the stars in a large galaxy.

One such superluminous supernova, called PTF10hgi, is believed to have triggered the repeat FRB. “It was so bright that if it had taken place in the Andromeda Galaxy [a mere 2 million light-years away] it would have been visible to the naked eye,” notes Scientific American. It caused no stir at the time (7.5 years ago, according to Sky & Telescope) only because it blew up in a much more distant galaxy, a billion and a half light-years away.

What we see as a supernova is actually a collapsing star; the expanding fireball we observe is roughly equivalent to the cloud of dust rising from a demolished building. The interior of the star may collapse entirely, forming a black hole, or it may form a neutron star, a stellar cinder a few miles across but roughly as massive as the sun.

Spinning Up an FRB

In some cases, the newly formed neutron star may be spinning so fast — thousands of rotations per second — that its magnetic field transfers the spinning star’s enormous torque to the surrounding nebula formed by the star’s former outer layers.

Such a rapidly spinning neutron star is called a magnetar because it produces immensely powerful magnetic fields in space. And, says Sky & Telescope, the magnetic transfer of energy from the magnetar to its surrounding nebula can further heat and brighten the nebula — enough to turn an ordinary supernova into a superluminous supernova.

Most of this energy transfer happens in the first weeks after the explosion, but the rotating magnetic fields in space continue to pump energy into the nebula. Calculations show that this energy transfer would be just the thing to produce FRB pulses, even decades after the initial explosion.

These calculations also indicate that the magnetar’s energy should keep the nebula explosion glowing, resulting in a persistent radio signal as well as the FRB bursts.

The Power of Prediction

All of these indicators came together for a research team including Harvard astronomer Tarraneh Eftekhari, lead author of their findings report.

The repeat FRB, known as FRB 121102, took place 3 billion light-years away in a distinctive type of galaxy, one with low metallicity and a high rate of star formation — a pattern associated with superluminous supernovae.

And while the superluminous supernova PTF10hgi was in a different galaxy only half as far away, examination with the Very Large Array radio telescope showed that it is surrounded by a persistent radio source. A similar persistent source coincides with the location of the repeat FRB.

All of this is circumstantial evidence. But as AAS Nova reports, the research team has one more powerful weapon at its disposal. The researchers were able to predict, based on their detailed observations, how FRB 121102 should behave over the next few years. This will allow what AAS Nova describes as a robust test of the magnetar theory of FRBs.

Meanwhile, a recently discovered second repeating FRB may add further insights to our understanding of the links between superluminous supernovae and FRBs.

The jury is still out on our understanding of FRBs and superluminous supernovae — but the evidence is starting to pile up as new investigative tools are becoming available.

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