Title: A repeating fast radio burst source in a globular cluster
Authors: F. Kirsten, B. Marcote, K. Nimmo, J. W. T. Hessels, M. Bhardwaj, S. P. Tendulkar, A. Keimpema, J. Yang, M. P. Snelders, P. Scholz, A. B. Pearlman, C. J. Law, W. M. Peters, M. Giroletti, D. M. Hewitt, U. Bach, V. Bezukovs, M. Burgay, S. T. Buttaccio, J. E. Conway, A. Corongiu, R. Feiler, O. Forssén, M. P. Gawroński, R. Karuppusamy, M. A. Kharinov, M. Lindqvist, G. Maccaferri, A. Melnikov, O. S. Ould-Boukattine, Z. Paragi, A. Possenti, G. Surcis, N. Wang, J. Yuan, K. Aggarwal, R. Anna-Thomas, G. C. Bower, R. Blaauw, S. Burke-Spolaor, T. Cassanelli, T. E. Clarke, E. Fonseca, B. M. Gaensler, A. Gopinath, V. M. Kaspi, N. Kassim, T. J. W. Lazio, C. Leung, D. Li, H. H. Lin, K. W. Masui, R. Mckinven, D. Michill, A. Mikhailov, C. Ng, A. Orbidans, U. L. Pen, E. Petroff, M. Rahman, S. M. Ransom, K. Shin, K. M. Smith, I. H. Stairs, and W. Vlemmings
First Author’s Institution: Department of Space, Earth and Environment at CHALMERS University of Technology, Göteborg, Sweden
Fast Radio Bursts (FRBs) have been the talk of the town for the last few years. While their name gives it away, these are really fast (on the order of milliseconds) bursts of energy that have been detected by radio telescopes. Fast radio bursts are really interesting for many reasons. First, we don’t know what produces them! While the fast radio burst within our own galaxy came from a magnetar, the burst was a bit weaker than those seen from other galaxies. Additionally, while we detected one FRB within our own galaxy, the rest of the bursts have come from other galaxies. This makes FRBs an extremely energetic phenomena!
Recently, astronomers at the Canadian Hydrogen Mapping Experiment (CHIME) Fast Radio Burst Experiment (CHIME/FRB) detected a nearby fast radio burst known as FRB 20200120E. The authors of the detection noted that the fast radio burst was along the line of sight of the galaxy M81, and might even be coming from this galaxy due to its low dispersion measure. (Dispersion measures are often used as a proxy for distance, as they trace out the amount of stuff between us and a given source.) M81 is a huge spiral galaxy and is one of our closest neighbor galaxies. Thus, if this FRB does lie in M81, it would make it the closest extragalactic FRB.
Today’s authors perform more observations of this FRB (which happens to repeat) to determine a more precise position for it.
The detections (and lack of detections)
The authors follow-up on this FRB using two different telescopes. First, they use the European VLBI Network (EVN). The EVN is a part of a very long baseline interferometry (VLBI) program the authors have been deploying to follow-up FRBs detected by CHIME/FRB. VLBI is often used to get very precise positions of astronomical objects. It works by correlating signals from multiple telescopes to image a given source.
The authors detect four bursts from FRB 20200120E in their EVN data and use the bursts to improve the localization of the FRB. They find that the FRB is at the same location as a known globular cluster of M81. As seen in the below Fig. 1, the FRB is coincident with the center of the globular cluster (within error regions). But there is still the possibility that this is a fluke coincidence, so the authors calculate the chance probability of the FRB and the globular cluster overlapping. The probability is 1 in 10000 – a very, very small chance.
The authors also looked for emission from FRB 20200120E using the Very Large Array (VLA), but didn’t find anything.
In addition to these two radio telescopes, the authors also comb through some old, higher energy data. They look at optical data from the Subaru telescope, x-ray data from Chandra X-ray Observatory, and gamma-ray data from the Large Area Telescope on Fermi. Unfortunately, they do not find any significant x-ray emission or any known gamma-ray sources at the location of the FRB.
So what does this mean for this FRB, and others?
It is a bit surprising to find this FRB in a globular cluster, since it appears to be a rather old, metal-poor globular cluster. One of the leading theories for the sources of FRBs involves magnetars formed from core-collapse supernovae. Yet, globular clusters are known for having older stellar populations, and so any magnetar formed through a core-collapse supernovae would be extremely old, and likely unable to produce FRBs. Thus, another mechanism is needed to explain this FRB. The authors discuss a variety of other mechanisms such as two white dwarfs that merge to form a magnetar, a magnetar produced by the collapse of a white dwarf due to accretion, a giant pulse from a recycled millisecond pulsar, a compact binary system, or a low mass x-ray binary system as the possible source of this FRB. Note that almost all of the methods still involve neutron stars.
This discovery leads to a major unexpected conclusion: if all FRBs are produced by the same source (which is still highly debated), then this source likely can’t be a magnetar formed from a core-collapse supernovae.
Astrobite edited by Sabina Sagynbayeva
Featured image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
I have often wondered about using those dispersion measures as a proxy measure of distance. Suppose, for example, that FRBs are related to tidal disruptions of stars. We know those are surrounded by a lot of ionized gas (there was an Astrobites on this recently). The _local_ DM could be rather large, thus mimicking a source at a larger distance. (Of course, this object has a low DM, but there could be many reasons why _some_ FRBs don’t have much gas, or have holes in their gas.)