Giant Radio Pulses: Now Available in X-Rays

Six and a half thousand light years away is the dense remnant of a supernova. It’s the size of a city but weighs more than the Sun. It is spinning 30 times per second, whipping its surroundings into a twisted morass of magnetism. Its magnetic field gets stretched to its breaking point, then snaps. What follows is called a magnetic reconnection: the magnetic field realigns itself, sending electrons and other charged particles flying. The reconnection releases a humongous burst of radiation right across the spectrum, from low-energy radio to high-energy X-rays. The supernova remnant is the Crab Pulsar, and the burst of radiation is called a giant radio pulse.

That scenario, where a magnetic reconnection triggers a giant radio pulse that includes radiation from across the electromagnetic spectrum, is suggested by a group of astronomers in a paper published in Science this month. They have discovered small increases in X-ray emissions coinciding with giant radio pulses from the Crab Pulsar (situated in the centre of the Crab Nebula and pictured above). This is one step towards solving the mystery of how giant radio pulses are created, but there’s still some way to go to explain exactly what’s going on.

The beams of a neutron star sweep past us, appearing as a pulsar. Image credit: Michael Kramer (JBCA, U. Manchester)

Pulsars are spinning neutron stars — the remains of a supernova — which emit powerful beams of radiation. The beams are highly focused and sweep around as the pulsar spins, much like the light from a lighthouse. We see short flashes of radio and other wavelengths from the pulsar whenever the beams briefly point in our direction. A small number of pulsars emit what are called giant radio pulses, where occasionally one of their regular pulses will peak up to ten times higher than normal.

The astronomers used the NICER telescope on the International Space Station, in combination with two radio telescopes at the Kashima Space Technology Center and the Usuda Deep Space Center, both in Japan, to simultaneously observe the Crab Pulsar in X-ray and radio. Over the course of two years, from 2017 to 2019, they collected data on 3.7 million pulses from the pulsar, finding 26,000 giant radio pulses. They were looking for any increases in X-ray emissions that coincide with the giant radio pulses. Similar increases have already been seen in optical light, so it made sense to also check other parts of the spectrum, too.

NICER, the Neutron Star Interior Composition ExploreR, on the International Space Station. Image credit: NASA

They found that each time there was a giant radio pulse, the pulsar’s X-ray emissions were also slightly higher than usual. The increase was only 4% above normal, so it wasn’t anywhere near as big as the radio pulse itself, but because X-rays are so energetic that small increase represents a pretty huge amount of energy. It means that the total energy released by the giant radio pulse could be hundreds of times larger than previously thought.

Finding simultaneous X-ray emissions answers some questions about what produces giant radio pulses. It shows that whatever causes giant radio pulses also triggers emission in other parts of the spectrum, from low-energy radio to high-energy X-rays (gamma rays are even higher energy, but so far no increase in gamma rays has been found to coincide with giant radio pulses). The amount of X-ray emission compared to the radio fits what you’d expect to see in a magnetic reconnection — when the powerful magnetic fields around the pulsar suddenly rearrange themselves — so that suggests magentic reconnection could play a role in generating giant radio pulses. The exact details of how the pulses are generated still need to be worked out, though. Astronomers aren’t sure yet exactly which types of particles are involved in emitting the bursts, and where exactly those particles are in the pulsar’s surroundings.

Featured image credit: Optical: NASA/HST/ASU/J. Hester et al. X-Ray: NASA/CXC/ASU/J. Hester et al.

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