“Extragalactic Transients in the Era of Wide-Field Radio Surveys”

Over the past few months I’ve been working on a paper with my adviser Edo Berger and Brian Metzger of Columbia, and as of today it’s submitted to ApJ and out on Arxiv! It’s entitled Extragalactic Transients in the Era of Wide-Field Radio Surveys. I. Detection Rates and Light Curve Characteristics, and as the title might suggest it connects more to my doctoral research on radio transients than my more recent work on magnetism in cool stars and brown dwarfs.

The term “radio transient” generally refers to any astronomical source of radio emission that appears unexpectedly. Since the definition involves something new appearing, radio transients are generally associated with events rather than objects. Lots of astrophysical events are known — or predicted — to result in radio emission, so the set of radio transients includes many different kinds of phenomena: it is an astronomical definition rather than an astrophysical one.

But there’s a reason we lump many kind of events into this overarching category. Up until the past decade or so, it’s been difficult to discover radio transients of any kind. There are several reasons for this, but one of the key ones is that fairly basic physical and technical tradeoffs have historically driven the best-performing radio telescopes to have very small “fields of view,” meaning that they can only observe small patches of the sky at once. And if you’re interested in unexpected events that could occur anywhere in the sky, you have to get pretty lucky to find one when you’re only ever looking at a tiny little piece of it.

You can’t change the laws of physics, but some of the same technologies that have driven the digital revolution have also significantly changed the tradeoffs involved in building radio telescopes. (For more on this, see Chapter 1 of my dissertation.) It’s now possible to build sensitive telescopes that can see much more of the sky at once than before, making searches for radio transients much more feasible. In astronomy, new ways of looking at the sky have almost always been associated with exciting new discoveries, so this new capability of studying the “time domain” radio sky has brought a lot of excitement to see what’s out there waiting to be found. That’s why there are a variety of ongoing and upcoming searches for radio transients such as VAST, VLASS (partially), the LOFAR RSM, and whatever acronym will eventually be given to the SKA transient surveys; and hence the “Era of Wide-Field Radio Surveys” in the title of our paper.

That’s the background. What our paper does is predict what these surveys might actually find. Our work is the first comprehensive end-to-end simulation of the search process, modeling the rates and distributions of various events, their radio emission, and the detection methods that surveys will likely use.

Science! Radio light curves of the various events we consider at 1.3 GHz. A big part of the work in the paper was to find and implement the best-available models for radio emission from a wide variety of astrophysical events.
Science! Radio light curves of the various events we consider at 1.3 GHz. A big part of the work in the paper was to find and implement the best-available models for radio emission from a wide variety of astrophysical events.

To keep things tractable, we focused on a particular kind of potential radio transients — extragalactic synchrotron events. The “extragalactic” means that the events come from outside the Milky Way, which is usually the genre that people are most interested in. The “synchrotron” refers to the radio emission mechanism. For the general group of surveys we consider, all known radio transients are synchrotron emitters, and I’d argue that it’s hard to concoct plausible events in which synchrotron will not be the primary emission mechanism. This is important, because one of the things we show in the paper is that the synchrotron process brings certain basic restrictions to the kind of emission you can get. In particular, brighter events generally evolve on slower timescales, so that something bright enough to be seen from another galaxy cannot get significantly brighter or dimmer in less than about 100 days. That means that if you’re looking for radio transients, it’s not helpful to check the same part of the sky more frequently than this pace.

Various other papers have predicted detection rates for these sorts of events, but many of them have done so in a back-of-the-envelope kind of way. But we tried to do things right: take into account cosmological effects like non-Euclidean luminosity distances and K-corrections, use best-available radio emission models, and model the actual execution of a given survey. Doing this brought home a point that I’d say has been insufficiently appreciated: if you observe more frequently than the 100-day timescale mentioned above, typical back-of-the-envelope calculations will overestimate your survey’s efficacy. (If you do this you can recover some power by averaging together multiple visits, but in most cases it’s better to cover more sky rather than to revisit the same spot.)

Overall, we predict that few (dozens) radio transients will be found until the advent of the Square Kilometer Array. Several of the choices that lead to this result are on the conservative side, but we feel that that’s justified — historically, radio transient surveys have turned up more false positives than real events, and you’re going to need a robust detection to actually extract useful information from an event. This is particularly true because radio emission generally lags that in other bands, so if you discover something in the radio, you have poor chances of being able to learn much about it from, say, optical or X-ray emission. This is unfortunate because it can be quite hard to learn much from radio observations without the context established from data at shorter wavelengths. We’ll pursue this idea in a follow-up paper.

There’s quite a lot more to the paper (… as you’d hope …) but this post is awfully long as it is. Overall, I’m very happy with our work — people have treated this topic handwavily for a while, and it’s finally getting the detailed treatment it deserves. The process of writing the paper was also rewarding: this happens to be the first one I’ve been on in which more than one person has had a heavy hand in the manuscript. (I’d call it a coincidence, but in my prior experiences the writing has been more-or-less entirely one person’s responsibility.) The mechanics of collaborative writing are still awkward — hence upstart websites like Overleaf or ShareLatex — but that aspect made it feel like more of a team project than other work I’ve done so far. I’ll be spearheading the follow-up paper, so there should be more of this feeling in my future!