More Clues to the Environment in Which FRBs Originate?

Title: A bright, high rotation-measure FRB that skewers the M33 halo

Authors: Liam Connor, Joeri van Leeuwen, et. al.

First Author’s Institution: Anton Pannekoek Institute, University of Amsterdam, Amsterdam, The Netherlands

Status: Submitted to MNRAS, open access on arXiv 

Fast radio bursts (FRBs) are one of the hottest topics in astronomy right now. First discovered by Dr. Duncan Lorimer in 2007, these intense millisecond-long bursts of radio emission have continued to captivate scientists across the planet because they keep defying our expectations with discoveries like the repeater. Now, with the discovery of an interesting property of a new FRB just outside a major galaxy, we may be getting one step closer to finally solving one of the many puzzles of FRBs.

More Questions Than Answers

Our questions about FRBs seem to fall into two categories: What causes the bursts? And how can they be put to use? Each time the community moves toward an answer on one of these questions, a new discovery throws a wrench in it.  For example, astronomers thought FRBs were single events but a discovery in 2016 showed that they can actually repeat. This opens new questions, like whether the repeaters and non-repeaters come from the same mechanism. In another case,  we thought FRBs only came from dwarf galaxies until one was localized to a massive spiral galaxy.  This finding opened more questions about the types of environments that could produce FRBs in very different galaxies. The authors of today’s article present a newly discovered FRB with a very high rotation measure that may give clues to the kind of environment FRBs originate from.

Understanding Rotation Measures

When light passes through a medium, it gets Faraday rotated, which is a rotation of the orientation of the light by a magnetic field. How much that light gets rotated as it travels is called its rotation measure (RM). Rotation measure depends on the strength of the magnetic field between us and the source, the density of the material that the light is going through, and the distance to the source. RMs can be used to understand the environment an FRB traveled through. Most of the RMs from FRBs are -100 rad m-2 to -120 rad m-2 (a negative RM simply denotes the fact that the magnetic field is pointing away from the observer) but newly discovered FRB 191108 has an RM of almost 500 rad m-2.

Caught Between a Galaxy…and Some Interstellar Medium

FRB 191108 was detected with the Apertif Radio Transient System on the Westerbork Synthesis Radio Telescope in the Netherlands. It lies close to the halo of the galaxy M33 (see Figure 1) and is within the intergalactic medium of the Andromeda Galaxy. The total observed rotation measure is a combination of the RM from the Milky Way, the RM from the medium between galaxies, and the RM of the host itself. Each of these different components has its own electron density, magnetic field, and thickness so they all contribute to the RM differently. What the authors find from the RM is that it points to an extragalactic contribution of 525 rad m-2, which would require the magnetic field between galaxies to be 1000x greater than they are. It’s possible that ionized material surrounding the two galaxies could be the cause but because other sources around M33 have RMs of <100 rad m-2, it is not likely (see Figure 2 for a comparison). Therefore, the RM has to come from somewhere else.

Figure 1. The location of FRB 191108. The blue cross denotes the most likely location, the red line indicates where the authors believe with 90% certainty the FRB came from, and the circles are the beam size.

Dense Plasma Environment?

One of the only explanations the authors find plausible is that the high RM is due to magnetized plasma in the host galaxy. It is possible that the burst originated from an area of very dense, magnetized material. FRBs have been seen to exist in many different environments and some of the RMs of FRBs that have been found point to a high contribution of material from the host galaxy that the burst goes through. FRB 121102 (the repeating FRB, “the repeater”) has an RM that is 100 times  greater than FRB 191108 and it has been localized to an environment that is extreme and dynamic. It also has a persistent radio source counterpart to it. The fact that the authors don’t find a radio counterpart to this FRB and don’t find it to repeat means it’s formation environment is likely different from the repeater. 

Figure 2. The RM of sources near M31 and M33. The x-axis shows how far away they are from the galaxies and the y-axis shows the RM. The fact that the RM is so different from the others around the galaxies show that it cannot be caused by plasma around the galaxies (if it was, others along that line of sight would see it).

Has this discovery pointed to an answer to the question of what kind of environment FRBs originate from, or does the differences between FRB 191108 and FRB 121102 only raise more questions? Only by finding more FRBs will we get closer to answering these questions!

About Haley Wahl

I'm a third year grad student at West Virginia University and my main research area is pulsars. I'm currently working with the NANOGrav collaboration (a collaboration which is part of a worldwide effort to detect gravitational waves with pulsars) on polarization calibration. In my set of 45 millisecond pulsars, I'm looking at how the rotation measure (how much the light from the star is rotated by the interstellar medium on its way to us) changes over time, which can tell us about the variation of the galactic magnetic field. I'm mainly interested in pulsar emission and the weird things we see pulsars do! In addition to doing research, I'm also a huge fan of running, baking, reading, watching movies, and I LOVE dogs!

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