Authors: Alice P. Curtin, Shriharsh P. Tendulkar, Alexander Josephy, et al.
First Author’s Institution: McGill University
Status: arxiv [open access]
Astronomers are bursting with excitement to figure out the mystery of fast radio bursts (FRBs). FRBs are a new astronomical phenomenon discovered in the last decade. They are energetic pulses of radio waves lasting for a few milliseconds that were first discovered by the Parkes Radio Telescope in 2007. Ever since, several hundred FRBs have been discovered, yet their origin remains an enigma. Today’s authors examine whether FRBs could be related to another kind of bursty phenomenon – Gamma Ray Bursts (GRBs).
What are Gamma Ray Bursts?
As the name suggests, GRBs are bursts of very energetic gamma rays that can last for a few milliseconds to about a hundred seconds. The energy of gamma ray photons can be as high as a few GeV (for comparison, the energy of X-rays is ~1-100 keV, the energy of optical photons is ~1eV, energy of FRB radio photons is ~ueV). GRBs have been extensively studied for the last three decades. GRBs come in two flavors – long and short GRBs, characterized by their duration. Long GRBs (LGRBs) have durations exceeding a few seconds, while short GRBs (SGRBs) have durations ranging from a few milliseconds to a few seconds. Most LGRBs are thought to be produced when a rapidly rotating massive star explodes as a core collapse supernova. SGRBs are believed to be produced when a neutron star (NS) merges with another neutron star or a black hole (BH).
What have GRBs got to do with FRBs?
Because GRB and FRB photons are separated by about 15 orders of magnitude in energy, it is natural to wonder whether we expect them to be related. It turns out that there are several theoretical models linking SGRBs to FRBs (See this bite for an example). Models predict that just prior to the merger (so prior to the SGRB) of a NS-NS or an NS-BH merger, an FRB-like burst can be produced from winds driven off the surface of the NS, or interaction of the magnetospheres of the two neutron stars, or due to an induced electric field due to the motion of a NS around a NS or BH. Even after the merger (i.e. after the SGRB), if a new, highly magnetized NS is formed, it can produce a pulsar-like emission that is on the energy-scale of FRBs. On the contrary, there are not many theoretical models linking FRBs to LGRBs. The main reason for this is that in a core-collapse supernova, there is a lot of material ejected, which would make the region opaque to FRB-like radio emission.
Despite the myriad of models connecting FRBs to SGRBs, there have not been any searches to test if these two are actually related to each other.
Astronomers CHIME in!
The authors of today’s paper set out to investigate whether any of the FRBs that we know of was consistent with a GRB that we know of. For their analysis, they chose ~500 FRBs detected by the CHIME radio telescope in 2018-2019. CHIME can not only detect the FRBs, but also localize them to a patch on the sky where the burst most likely came from. Typically, the size of this patch is ~0.27 deg. The authors then collected a sample of 81 GRBs detected during the same period by several space telescopes such as Fermi, Swift, INTEGRAL and Konus-Wind. Similar to CHIME, these telescopes can also localize the GRBs. The authors restricted their sample to only those GRBs that were better localized than 1 sq. deg. (the angular area of the moon is about 0.5 sq. deg).
They then search for spatial and temporal coincidences between the FRBs and GRBs. For spatial coincidence, they require the 3-sigma error (i.e ~0.81 sq.deg, see the number mentioned in the previous paragraph) in the FRB position to overlap with the 3-sigma error in the GRB position. For temporal coincidence, they require the FRB and GRB to occur within seven days of each other. They do not find any FRBs that are both temporally and spatially coincident with a GRB. However, they do find two GRBs that are only spatially coincident with two FRBs from their sample. The two GRBs are separated from their FRBs by 10 and 273 days respectively.
However, because of the large number of GRBs and FRBs that are being detected every day, it is possible that over a period of a year, an FRB and a GRB coincide with each other just by chance. The authors conduct a simulation, and estimate that the probability of getting two FRBs coincident with 2 GRBs just by chance is ~50% – very high! They conclude that the FRB-GRB association is thus not statistically significant.
As the authors do not find any FRB-GRB associations, they do the next best thing! For GRBs in their sample, they use CHIME data around the time of observation to place upper limits on any FRB-like emission from the GRB. They are able to place constraints on the radio emission from 10 GRBs in their sample, as these 10 happened to be in the field-of-view of CHIME when they exploded. Figure 1 shows an example of the kind of constraints they are able to derive.
In the future, the authors plan to continue looking for FRB and GRB coincidences using similar techniques as in this paper. They also emphasize the importance of running these searches in real-time, so that the next FR/GR-Burst can be studied in detail!
Edited by : William Balmer
Featured Image Credit: James Josephides/Swinburne
Disclaimer: Today’s lead author Alice P. Curtin is also an active Astrobites author but was not involved in the publication of today’s bite.