Are FRBs and GRBs related?

Title: Restrictions on Fast Radio Burst-like counterparts to gamma-ray bursts with CHIME/FRB

Authors: Alice P Curtin, Shriharsh P Tendulkar, Alexander Josephy, et al.

Institution of first author: McGill University

Status: archive [open access]

Astronomers are bursting with excitement to unravel the mystery of Fast Radio Bursts (FRBs). FRBs are a new astronomical phenomenon discovered in the last decade. They are high-energy pulses of radio waves, a few milliseconds long, that were first detected by the Parkes Radio Telescope in 2007. Several hundred FRBs have been discovered since then, but their origin remains a mystery. Today’s authors are investigating whether FRBs could be related to another type of burst phenomenon – gamma ray bursts (GRBs).

What are Gamma Ray Bursts?

As the name suggests, GRBs are very energetic gamma-ray bursts that can last from a few milliseconds to around a hundred seconds. The energy of gamma-ray photons can be up to several GeV (for comparison: the energy of X-rays is ~1-100 keV, the energy of optical photons is ~1 eV, the energy of FRB radio photons is ~ueV). GRBs have been studied extensively over the past three decades. GRBs come in two flavors – long and short GRBs, which are characterized by their duration. Long GRBs (LGRBs) have a duration greater than a few seconds, while short GRBs (SGRBs) have a duration ranging from a few milliseconds to a few seconds. Most LGRBs are believed to be formed when a rapidly rotating massive star explodes as a core-collapse supernova. SGRBs are believed to be formed when a neutron star (NS) merges with another neutron star or a black hole (BH).

What do GRBs have to do with FRBs?

Since GRB and FRB photons are separated by about 15 orders of magnitude in energy, it is natural to wonder if we expect them to be related. It turns out there are several theoretical models linking SGRBs to FRBs (see this bite for an example). Models predict that just before the merger (i.e., before the SGRB) of an NS-NS or an NS-BH merger, an FRB-like outburst can be generated by winds blown off the surface of the NS or by interaction of the magnetospheres of the two neutron stars or due to an induced electric field due to the movement of a NS around a NS or BH. Even after the merger (i.e., after the SGRB), when a new, strongly magnetized NS is formed, a pulsar-like emission that is on the energy scale of FRBs can be produced. On the contrary, there are not many theoretical models linking FRBs to LGRBs. The main reason for this is that a core-collapse supernova ejects a lot of material, which would render the region opaque to FRB-like radio emissions.

Despite the myriad models linking FRBs to SGRBs, there has been no search to test whether these two are actually related.

Astronomers SOUNDED!

The authors of today’s article wanted to investigate whether any of the FRBs we know of match a GRB we know of. For their analysis, they selected about 500 FRBs discovered by the CHIME radio telescope in 2018–2019. In addition to detecting the FRBs, CHIME can pinpoint them to a spot in the sky where the burst most likely came from. Typically, the size of this patch is ~0.27 degrees. The authors then collected a sample of 81 GRBs discovered by multiple space telescopes including Fermi, Swift, INTEGRAL and Konus-Wind over the same period. Similar to CHIME, these telescopes can also locate the GRBs. The authors limited their sample to those GRBs that were located better than 1 square degree. (The angular area of ​​the moon is about 0.5 square degrees).

They then look for spatial and temporal matches between the FRBs and GRBs. For spatial coincidence they require that the 3 sigma error (i.e. ~0.81 square degrees, see the figure mentioned in the previous paragraph) in the FRB position overlaps with the 3 sigma error in the GRB position. For temporal coincidence, they require FRB and GRB to occur within seven days of each other. You won’t find any FRBs that coincide with a GRB in both time and space. However, they find two GRBs that only spatially coincide with two FRBs from their sample. The two GRBs are separated from their FRBs by 10 and 273 days, respectively.

However, due to the large number of GRBs and FRBs detected daily, it is possible for an FRB and a GRB to coincide by chance over a period of one year. The authors run a simulation and estimate that the probability of two FRBs coincidentally with 2 GRBs is ~50% – very high! They conclude that the FRB-GRB association is thus not statistically significant.

Since the authors find no FRB-GRB associations, they do the next best thing! For GRBs in their sample, they use CHIME data around the time of observation to set upper bounds on any FRB-like emission from the GRB. They are able to confine the radio emission from 10 GRBs in their sample because those 10 happened to be in CHIME’s field of view when they exploded. Figure 1 shows an example of the kind of constraints you can derive.

Going forward, the authors plan to continue searching for FRB and GRB coincidences using techniques similar to those in this article. They also stress the importance of doing these searches in real-time so that the next FR/GR burst can be examined in detail!

Edited by: William Balmer

Credit for selected images: James Josephides/Swinburne

About Viraj Karambelkar

I am a second year graduate student at Caltech. My research focuses on infrared time domain astronomy. I study dust explosions and dust-shrouded variable stars with optical and infrared telescopes. I mainly work with data from the Zwicky Transient Facility and the Palomar Gattini IR telescopes. I enjoy watching movies and plays, playing badminton and trying hard to improve my chess and crossword skills.