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Mysterious Fast Radio Bursts Are Coming From Neutron Star Orbiting A Black Hole

[Estimated read time: 4 minutes]
  • The Fast Radio Bursts, named 121102, have been identified repeatedly over the last 10 years, but still remains one of the biggest mysteries in radio astronomy.  
  • The new study suggests that these incredibly short bursts are coming from a neutron star orbiting dominant black hole of the dwarf galaxy. 

The first Fast Radio Burst (FRB) was discovered in 2007 when Duncan Lorimer and his student were looking through archival pulsar data. Since then, numerous repeating FRBs have been found, and astronomers have been debating what might be causing the Spitler burst.

So far, the biggest mystery in radio astronomy is how these powerful and short-duration bursts are emitted. No one actually knows the source – from where they are coming. Only a few FRBs have ever been detected as one-and-off events. Some of them are explainable, but the one named, FRB 121102, identified repeatedly over the last 10 years, still remains a puzzle.

The latest study suggests that FRBs are coming from a ridiculously strong magnetic field environment. This field twists radio waves into spirals, leaving a signature mark. Researchers at Puerto Rico have used Arecibo Observatory (a radio telescope) to demonstrate that the burst is emitting from an intensively strong magnetic field, about 200 times stronger than the Milky Way’s average magnetic field.

Fast Radio Burst

The polarized radio bursts indicate that they have originated from a source surrounded by a powerful magnetic field. They are bright, broadband and short-duration (millisecond) flashes found outside the Milky Way.

Unlike other radio sources, these bursts appear as a single spike of energy with no variation in strength over time, and they last for a few milliseconds. They aren’t concentrated on the plane of the Milky Way and their locations are biased by the sky parts that the observatories can image.

In August 2017, researchers detected 15 additional repeating FRB 121102 at 5 to 8 Gigahertz, using Green Band Telescope data. The waves are highly polarized, which could have formed when passing through hot plasma with powerful magnetic field. Furthermore, these bursts are nearly 500 times more polarized (twisted) than other FRBs.

New Observations

Recently, the astronomers examined the radio waves from 16 different bursts over three 2-hour inspectional runs spanning months. They discovered these bursts were incredibly short (as short as 30 microseconds), indicating that the source should not be wider than 10 kilometers.

A small region is required to emit such short bursts. That’s why compact bodies like neutron stars are favored by this result.

The individual radio bursts were comprised of smaller sub-bursts, complicating the scenario even further. It is possible that these sub-bursts are a result of the waves passing through plasma blobs, or they are intrinsic to the objects that forms them. 

All waves were polarized, oriented in the single direction. However, they were rotating in corkscrews all the way from dwarf galaxy to our planet.

Source of FRBs

One of the main theories that explain the behavior of the burst is that they are coming from energetic, young neutron stars known as magnetar, which is sitting inside a giant shell of magnetized gas. The neutron star emits radio waves and the shell is responsible for rotating those waves.

A very young magnetar (just a few decades old) could emit massive amounts of radio waves at an extremely fast speed.

However, the supernova remnant would have to be million times brighter than the Crab Nebula (Milky Way’s brightest remnant) in order to drive such powerful magnetic fields. Instead, radio bursts could come from magnetar orbiting dominant black hole of the dwarf galaxy. This neutron star could have mass between 10,000 to 1 million times the mass of our Sun.

Reference: Nature | doi: 10.1038/nature25149

Those gigantic black holes are known for their powerful magnetic fields and ability to make polarized wave rotate. And a magnetar orbiting the supermassive black hole at Milky Way’s center is a plausible setup.

Perhaps the remarkably higher activity level of FRB 121102 compared to other FRBs is mostly a consequence of its environment, for instance, because these magnetized structures can also enhance the detectability of the bursts through plasma lensing. .

What’s Next?

It is still unclear whether all FRBs (including non-repeating ones) come from such environment. Are there two separate classes with different properties, or it is just a single class of FRBs exhibiting different properties? We don’t know yet.

Read: How Strong Are Black Holes? | Precise Measurement of Magnetic Field

The researchers plan to observe FRB 121102 more closely before moving to other FRB sources. They want to examine waves at higher frequency (up to 12 Gigahertz) vs the current Green Bank Observation in the 5-to-8 Gigahertz. This could help us narrow down the possible explanations for these enigmatic events.

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