In October, 2018 the team searching for fast radio bursts using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope in the desert of Western Australia delighted us all with a series of 6 papers describing a new large sample of 23 FRBs, increasing the known population (at the time) by 30%. Throughout 2017 and 2018 the CRAFT team (that stands for The Commensal Real-time ASKAP Fast Transients Survey**) has been working hard to make the ASKAP FRB search a success.
Commissioning a new instrument is no small task, particularly when dealing with a system that is brand new in every way -- from dishes to wiring to receiver to computer processor. Dr. Ryan Shannon, one of the survey leaders (based at Swinburne University of Technology), has a deep appreciation for these challenges. "Commissioning the system and keeping it running was definitely a lot of work. It was also very rewarding. I've very much enjoyed working with a new telescope, and getting a better understanding of how it worked. I've come to a great appreciation of its complexity."
And this work definitely paid off. ASKAP uses a new kind of receiver called a phased array feed, or PAF, which is able to make many pixels (or beams) within the field of view of the telescope. For a the dishes of ASKAP this field of view is about 30 square degrees, about 150 times larger than the full moon. This is a huge increase on the field of view of older telescopes that were looking for FRBs. For example, the hugely successful Parkes telescope only searched over half a square degree. The ASKAP dishes are less sensitive because they are smaller, but with enough time on sky the FRBs should start to appear.
In order to increase their field of view even more, the CRAFT team decided to start out their search by using each dish of the array independently in a configuration called "Fly's Eye" where each dish points at a different part of the sky (like the many lenses that make up the visual receptors of flies). In this mode, ASKAP wasn't just looking over 30 square degrees, but instead over something more like 160 square degrees (depending on how many dishes were used in each observation) for new, bright FRBs. The first burst they found was published in 2017 by Bannister et al. FRB 170107 was recorded by ASKAP in the early days of their search in January 2017. I remember Dr. Keith Bannister, one of the leaders of CRAFT (based at CSIRO in Sydney), giving a talk in February 2017 describing their survey saying something along the lines of "we've already taken a lot of data and there's probably an FRB in there, but we haven't had the chance to look through it yet;" and it turns out he was right.
Throughout 2017 and early 2018, ASKAP continued to discover FRBs and with this new suite of papers, they reveal their exciting findings.
Shannon et al. (2018): The dispersion-brightness relation for fast radio bursts from a wide-field survey
The main result of this effort is the Shannon et al. (2018) paper published in Nature detailing the discovery of 19 new FRBs (20 including FRB 170107) in the ASKAP Fly's Eye survey. Since in this mode ASKAP is only sensitive to very bright bursts, they can compare this sample of bright FRBs to the fainter population found with the Parkes telescope. By comparing the distributions of dispersion measures (link) of FRBs from ASKAP and Parkes they find that the brighter ASKAP bursts are coming from more nearby sources, implying that there is a correlation between how bright an FRB is and how far away it is. This correlation is expected for a population of sources distributed throughout the Universe, but not expected if FRBs were all coming from local or nearby galaxies. As Ryan Shannon explains, “If you trace back ASKAP FRBs to larger distances, they well overlap the Parkes population. This would only be the case if the DM was a distance indicator. “
But the FRBs presented in this paper gave the team a lot work with, and the properties of these burst are further explored in 5 supplementary papers.
Macquart et al. (2018): The spectral properties of the bright fast radio burst population
Besides the dispersion measure-brightness relationship of the ASKAP FRBs, one of the most interesting things about them is that they look quite different from the FRBs we’ve seen in the past from more sensitive telescopes like Parkes. Their spectra (or the emission over the observing bandwidth) is very patchy. Most other FRBs have strong, detectable emission over the whole bandwidth but these ones look a little… clumpy. This paper looks more closely at the emission of the ASKAP FRBs and the “spectral modulation” or how variable the emission is over the observing band. They find that the FRBs with lower dispersion measures (more nearby) have more modulation than those with larger DMs (coming from farther away). They conclude that the modulation must come from the material that the burst travels through between the source and the telescope and that bursts coming from further away get this effect smoothed and averaged out over their travel through space, whereas the nearer sources are more affected.
So why don’t we see this spectral modulation in the FRBs from Parkes? In this paper they explain that it might be because the Parkes sample has, on the whole, higher DMs meaning they come from farther away and this effect is less pronounced. Finding more low-DM FRBs with other telescopes might help us answer this!
James et al. (2018a): The slope of the source-count distribution of fast radio bursts
This paper further explores the main result of the primary paper: the relationship between the dispersion measure and brightness for the ASKAP FRBs and how to compare those sources to the ones detected in the past by the Parkes telescope. “The brightness-fluence relation gives us much to chew on,” said Dr. Jean-Pierre Macquart, another primary investigator of the CRAFT team (based at Curtin University). Here they look more in-depth at the statistical analysis of the sample and find that the Parkes and ASKAP samples combined present a consistent picture.
James et al (2018b): The performance and calibration of the CRAFT fly’s eye fast radio burst survey
Just as important as finding FRBs is understanding how your telescope works that’s doing the finding. In this paper, the team dive into the calibration and verification steps behind the CRAFT FRB search. This work gives important information about things like: how do you know the telescope is pointing where you think it’s pointing? What is the conversion factor between signal-to-noise (what you measure) and flux density (the actual brightness of the source)? What kind of radio frequency interference was present in the observations? They may not be the most exciting questions and answers, but they’re vitally important if you want to have confidence in your results!
Mahony et al. (2018): A search for the host galaxy of FRB 171020
In this paper the team focuses in on just one of the FRBs found with ASKAP — FRB 171020. This particular burst is interesting because it has the lowest dispersion measure of the sample (and the second lowest DM of any published FRB). In fact, the DM is so low, meaning that it traveled through so few electrons, that it’s likely coming from a pretty nearby galaxy. In this paper they look at the patch of sky where the burst came from and try to identify the host galaxy. They estimate the maximum distance to the FRB to be about a billion light years, tiny when you consider that some may come from up to 19 billion light years away, but still too far to say with certainty what galaxy in the telescope field of view may have been the host. This further proves that you need a good localization of the FRB to identify a host galaxy.
Sokolowski et al. (2018): No low frequency emission from extremely bright fast radio bursts
It’s all in the title for this paper, but one of the interesting things about the ASKAP FRBs is that sometimes when ASKAP was observing, the same field was being observed by a radio telescope at a much lower frequency called the Murchison Widefield Array (MWA). For some of the brightest FRBs in the ASKAP sample, the team was able to look at the MWA data and see if this telescope also detected the FRB. In all cases, nothing was seen at the MWA which is surprising because if you assume that the same emission seen at ASKAP continues all the way down to low frequencies, the MWA should have been sensitive enough to see it for the brightest bursts. This implies that somewhere between ASKAP frequencies (1300 MHz) and MWA frequencies (200 MHz) the FRB emission must cut off or be substantially weakened in some way. FRBs have still never been seen at low radio frequencies, and this work shows that it might be because there is no emission there.
So what next??
What comes next for ASKAP? Well, finding more FRBs, of course! More specifically, the team has plans to move away from the fly’s eye mode and use all the dishes of the array together to search in a more sensitive way. The combination of all the dishes would also mean that if an FRB is found, it can be localized much better, since the large separation between dishes gives better resolution on the sky. As J-P Macquart says, “We are moving into the real estate business: location, location, location.” If ASKAP can precisely localize FRBs from their discovery pulse, then some exciting measurements can be made about the burst, host galaxy, and environment that haven’t been possible with non-repeating sources before.
Ryan Shannon is equally optimistic: “I'm most looking forward to seeing the results from all the new searches that will be conducted over the next 12 months. I think we are going to have a much better picture of what FRBs are and how they can be used as a cosmological tools.” ASKAP is just one of several facilities starting up their FRB searches in the coming year (more on those in future posts). Like me, Ryan is curious about what we will see: “I'm also looking forward to the unexpected. I'm sure over the next 12 months, with the diverse range of facilities, there will be plenty of surprises!”
**There are going to be a lot of acronyms, buckle up.