Do dead stars crash with the light of 500 million suns?

An illustration of the magnetic field lines of a flare (blue) interacting with field lines of an orbital current sheet (yellow), in the course of a binary neutron-star collision.
| Photo Credit: E. Most and A. Philippov

Astrophysicists are trying to wrap their heads around another twist in the story of fast radio bursts (FRBs) – mysterious radio frequency emissions that reach us from distant galaxies. FRBs are extremely powerful, discharging from their source somewhere out in deep space as much energy in milliseconds as the output of 500 million suns. Yet they are so short-lived that astronomers have only been able to track them for milliseconds, as their ghostly pings show up on radio telescope consoles like a ‘now you see, it now you don’t’ trick.

What are fast radio bursts?

Ever since astronomers detected the first FRB in 2007, more than 600 of these celestial flashes have been recorded to date. But we know very little about their exact origins and why they show up as such short-lived spurts. Due to their elusive nature, all these FRBs were detected by happenstance – when astronomers used their radio telescopes to scan the right part of the sky at the right time.

Astronomers have speculated that a type of neutron stars – the incredibly dense remnants of exploding stars – called magnetars could be a likely source of FRBs. Magnetars rotate slowly compared to other neutron stars, so scientists reasoned that it is the objects’ ultra-strong magnetic energy rather than their rotation that probably produces the emission of FRBs. A magnetar’s magnetic field is more than a thousand-times-stronger than that of other neutron stars, and a trillion times that of the earth. Nevertheless, in the absence of evidence, the role of magnetars in engendering FRBs has remained in the realm of speculation – until now.

How are neutron stars involved?

New findings by Elias Most of the California Institute of Technology and Alexander Philippov of the University of Maryland, College Park, provide just the proof scientists have been searching for. In a study published on June 16 by Physical Review Letters, Drs. Most and Philippov suggested that FRBs could be triggered by a collision between two neutron stars, and released just before they crash into each other. The impact, they have said, could set off two different kinds of signals: wrinkles in space-time called gravitational waves (tell-tale signatures of high energy cosmic events) and FRBs.

“Neutron star mergers have been known to be accompanied by electromagnetic counterparts,” Dr. Most told The Hindu via email. This was spectacularly recorded in August 2017, when the Laser Interferometer Gravitational-wave Observatory (LIGO) in the US and the Virgo instrument in Italy identified, for the first time, gravitational waves from two colliding neutron stars. The event was not only ‘heard’ as gravitational waves but also ‘seen’ in visible light by terrestrial and space-based telescopes.

What is a neutron-star merger?

The new study explains how a neutron star binary system behaves when the two bodies collide and coalesce. As a neutron star spins ever faster, the strong magnetic field above its poles causes electrons to speed up too, generating an electron-positron plasma. As the stars get closer to each other, the increasing electromagnetic energy breaches their by now distorted magnetic fields and throws out flares into the orbital plane of the stellar system. This sends out torrents of radio waves just before the actual collision, followed – a second later – by the radiation of gravitational waves from the event. These emissions, the scientists pointed out, are not unlike FRBs emanating from magnetars.

But, Dr. Most added, these cosmic light and sound shows are produced after the stellar collision. “We make a first prediction supported by numerical simulations for how neutron stars could have another – not yet detected – radio counterpart sourced before the merger,” he said. “Detecting such a precursor event would reveal insights into the magnetic field configuration, place constraints on how fast the stars spin before the merger, and potentially allow for improved localisation of the merger site in the sky.”

How do the findings affect gravitational-wave astronomy?

The idea could also explain the intense radio light ‘seen’ in the host galaxies of some FRBs. Some astronomers attribute this radio light to the glow around high energy events, such as a gigantic black hole at the centre of the galaxy devouring stars. Others believe that in active galactic nuclei where magnetars generate FRBs, it is possible that two neutron stars could merge into a single stellar body without actually becoming a black hole. “Yes, that will happen if the stars are not very massive, and each star would have around the same mass as our Sun,” Dr. Most said.

These findings give a leg-up to the study of gravitational waves, which were first observed in 2015 when scientists watched agape as LIGO recorded the signature of two black holes a billion times bigger than the Sun smashing into each other, hurling gravitational radiation out into space. Apart from validating Albert Einstein’s theory of relativity, propounded exactly a hundred years ago in 1915, the event heralded the new era of gravitational-wave astronomy.

Dr. Most said he believes that radio telescopes of the future will work with gravitational-wave observatories to study these high energy events. “Given the frequency band we predict, the Square Kilometer Array (which is expected to come online in 2027) will likely provide the best chance for a detection,” he added.

What is LISA?

For centuries, astronomers have depended on light to observe the heavens, which hamstrung their efforts to study deep space phenomena that are too distant or very dim. Space telescopes and radio telescopes changed this equation dramatically, allowing astronomers to peer farther and deeper into the universe. And then, the advent of gravitational-wave astronomy led to some incredible discoveries, as gravitational waves can pass through space without interruption, allowing scientists to learn more about the universe like never before.

The bar will be raised even higher when NASA’s space-based Laser Interferometer Space Antenna (LISA) becomes operational in the next decade. LISA comprises three spacecraft that will form an equilateral triangle in space, with each side of the triangle a million miles long to tap parts of the spectrum that are inaccessible from the earth. (LIGO is L-shaped and each side is 4 km long, constraining the frequencies it can scan for gravitational waves.) Laser beams will be relayed between the spacecraft and the signals will identify gravitational waves from distortions in space-time.

By ‘listening in’ on the universe using gravitational waves, LISA will explore cosmic evolution and structure more thoroughly than electromagnetic observations ever could.

Prakash Chandra is a freelance science writer.