Fast Radio Bursts as Cosmological Probes

One of the brightest Fast Radio Bursts seen since the phenomena were first detected in 2001 has been observed by the Parkes radio telescope in New South Wales. Maybe it should come as no surprise that Parkes was involved, given that most of the 18 FRBs that have so far been detected have been found there, including the so-called ‘Lorimer’ burst of 2001, which launched researchers’ interest in these mysterious processes. This one is thought to be particularly helpful in constraining magnetic fields and gases in intergalactic space, for observed distortions produced by an FRB’s travel yield data about the medium.

Ryan Shannon (ICRAR-Curtin University), a co-author of the paper, refers to the region between the galaxies as the ‘cosmic web,’ a region of all but invisible gases and plasma particles that is extremely hard to map. FRBs are short but intense pulses of radio waves — each lasts about a millisecond — that are usually discovered by accident, and no two look the same. Radio pulse FRB 150807, however, may be uniquely useful because its travel path can be traced back to an area in space that contains only a small number of stars and galaxies.

“This FRB, like others detected, is thought to originate from outside of Earth’s own Milky Way galaxy,” says Shannon, “which means their signal has travelled over many hundreds of millions of light years, through a medium that – while invisible to our eyes – can be turbulent and affected by magnetic fields. It is amazing how these very few milliseconds of data can tell how weak the magnetic field is along the travelled path and how the medium is as turbulent as predicted.”

Image: The radio pulse FRB 150807. The colour shows the frequency of the waves, which is like the colour of light. The brightness varies with frequency due to a process termed “scintillation”, which is caused by the twinkling of the burst in the cosmic web. This scintillation is the fingerprint of turbulence in the cosmic web and tells us that web is very placid. Credit: Dr Vikram Ravi/Caltech and Dr Ryan Shannon/ICRAR-Curtin/CSIRO.

So we have a probe of sorts of the intergalactic medium, one that was detected in 2015 and has now made its way into the literature. But just how deeply can we probe this region with FRBs? The paper argues that the bursts thus far studied may revolutionize cosmology, a bold claim, but one based on the information within FRBs that may be obtained in no other way.

From the paper:

Besides probing a heretofore-unknown astrophysical phenomenon, the bursts potentially carry imprints of propagation through inhomogeneous, magnetized plasma in the ionized interstellar media of other galaxies, and the diffuse intergalactic medium (IGM). Simultaneous measurements of redshifts and line-of-sight free electron column densities for FRBs can constrain the cosmological mass-density and ionization history of baryons.

Using FRBs as cosmological probes has been made difficult by the uncertainty about their origins, which is why FRB 150807 is so helpful — we can reconstruct its path. The archival images the team is using show three stars and six galaxies that are possible sites (see image below). The brightest galaxy is between 1 and 2 gigaparsecs away — roughly between 3.3 and 6.6 billion light years. The other galaxies are fainter than this object by factors of 6 and more, and all are thought to be more than 500 Mpc (1.6 billion light years) distant. This assumes, of course, that we can associate the burst with a star or a galaxy.

Image: The location of the FRB 150807. The yellow circle shows the typical location of an FRB. There are thousands of stars and galaxies in this direction. Because the burst was very bright researchers were able to locate it to a small region near the edge of that circle, shown as the pink banana-shaped region in the inset. In this region there are only 6 detected galaxies. The position of the most likely host galaxy, VHS7, is highlighted on the plot. Credit: Dr Vikram Ravi/Caltech and Dr Ryan Shannon/ICRAR-Curtin/CSIRO.

A supercomputing group led by Matthew Bailes (Swinburne University of Technology) produced the software that has been used in the analysis of the burst. The hope among the researchers is that technologies like Parkes’ multibeam receiver, the Murchison Widefield Array (MWA) in Western Australia, and the upgraded Molonglo Observatory Synthesis Telescope near Canberra can be used to detect and study future FRBs.

“Ultimately, FRBs that can be traced to their cosmic host galaxies offer a unique way to probe intergalactic space that allow us to count the bulk of the electrons that inhabit our Universe,” said Bailes. “To decode and further understand the information contained in this FRB is an exceptional opportunity to explore the physical forces and the extreme environment out in space.”

And get this: Lead author Vikram Ravi (Caltech) believes that there are between 2,000 and 10,000 FRBs occurring in the sky every day, with one in 10 being as bright as FRB 150807. We’ll take a step forward in finding the locations of these bursts when the Deep Synoptic Array also comes online. This array of 10 dishes at the Owens Valley Radio Observatory in California will pinpoint individual galaxies, allowing astronomers to use distance measurements and FRB analysis to further probe this deepest of all deep space.

The paper is Ravi et al., “The magnetic field and turbulence of the cosmic web measured using a brilliant fast radio burst,” published online in Science 17 November 2016 (abstract).