What Are Fast Radio Bursts and How Do We Find More?
Often in the past new objects and phenomena were found as technology progressed. Now is no different, and for many it feels like the bounds are endless. Here is one such new class of study, and we are fortunate to be around as it starts to grow. Read on to learn more and be sure to note the scientific processes at play.
It was not until 2007 that the first fast radio burst (FRB) signal was detected. Duncan Lorimer (West Virginia University) along with undergrad David Narkevic were looking at archived pulsar data from the 64-meter-wide Parkes Observatory as they were hunting for evidence of gravitational waves when some weird data from 2001 was spotted. A pulse of radio waves (later named FRB 010724 in the convention of Year/Month/Day, or FRB YYMMDD but unofficially known as the Lorimer Burst) was seen that were not only the brightest ever seen (the same energy the Sun releases in a month, but in this case over a 5 millisecond period) but was also from billions of light years away and lasted for milliseconds. It was definitely from outside our galactic neighborhood based off the dispersion measure (or how much interaction the burst had with interstellar plasma) of 375 parsecs per cubic centimeter plus the shorter wavelengths arriving before the longer ones (implying interaction with the interstellar medium), but what is it? After all, pulsars get their name from their periodic nature, something that a FRB is not -typically (Yvette 24, McKee, Popov, Lorimer 44).
Scientists realized that if such a burst was seen in a small section of the sky (in fast, 40 degrees south of the Milky Way disc), then more eyes would be needed to see even more. Lorimer decides to enlist some help, so he brought in Matthew Bailes (Swinburne University of Technology in Melbourne), while Maura McLaughlin developed software to hunt for the radio waves. You see, its not as easy as pointing a dish in the sky. One thing affecting observations is that radio waves can be as small as 1 millimeter in wavelength and as large as hundreds of meters, meaning a lot of ground has to be covered. Effects can goof up the signal such as phase dispersion, caused by free electrons in the Universe delaying the signal by decreasing the frequency (which actually offers us a way to indirectly measure the mass of the Universe, for the delay in the signal indicates the electron count it passed through). Random noise was also an issue, but the software was able to help filter these effects. Now that they knew what to look for, a new search was on over a 6 year period. And strangely, more were found but only at Parkes. Those 4 were detailed in a July 5 issue of Science by Dan Thorton (University of Manchester), who postulated based on the spread of the bursts seen that one could happen every 10 seconds in the Universe. Based again on those dispersion readings, the closest was 5.5 billion light years away while the furthest was 10.4 billion light years away. To see such an event at that distance would require more energy than the sun puts out in 3000 years. But doubters were out there. After all, if only one instrument is finding something new while other comparable ones haven’t, then something is usually up and it isn’t a new finding (Yvette 25-6, McKee, Billings, Champion, Kruesi, Lorimer 44-5, Macdonald "Astronomers," Cendes "Cosmic" 22).
In April 2014 the Arecibo Observatory in Puerto Rico saw a FRB, ending the speculation, but it too was in archived data. But luckily, scientists didn’t have to wait long for a live sighting. May 14, 2014 saw our buddies at Parkes spot FRB 140514, located about 5.5 billion light-years away, and was able to give a heads up to 12 other telescopes so they too could spot it and look at the source in infrared, ultraviolet, X-ray, and visible light. No afterglow was spotted, a big plus for the FRB model. And for the first time, a curious feature was revealed: the burst had a circular polarization of both electric and magnetic fields, something very uncommon. It points to the magnetar theory, which will be discussed in more detail in the Hyperflare section. Since then, FRB 010125 and FRB 131104 were found in archival data and helped scientists realize that the indicated rate of FRBs possible was wrong. When scientists looked at these locations for months, no more FRBs were found. It is worth noting, however, that these were in mid latitude (-120 to 30 degrees), so perhaps FRBs do have an orientation component no one is aware of (Yvette 25-6, Hall, Champion, White, Cendes "View" 24-5).
And our good old buddy, the Parkes telescope, along with the Effelsberg telescope (a 100-meter beast) found 5 more FRBs over a 4-year period: FRB 090625, FRB 121002, FRB 130626, FRB 130628, and FRB 130729. They were found in the southern latitudes after the two telescopes, both partners in the High Time Resolution Universe (HTRU) array, looked at 33,500 objects for a total of 270 seconds per object at 1.3 GHz with a bandwidth of 340 MHz. After running the data through special programs that looked for FRB like signals, the 4 were discovered. After looking at the spread of the sky that was looked at for all known FRBs at that time (41253 square degrees), by comparing that data collection rate to the rotation of the Earth presented scientists with a substantially lowered rate of possible FRB detection: around 35 seconds between events. Another amazing find was FRB 120102, for it had two peaks in its FRB. That supports the idea of FRBs originating from supermassive stars collapsing into black holes, with rotation of the star and distance from us affecting the timing between peaks. It does deal a blow to the hyperflare theory, for two peaks requires that either two flares happened close by (but too close based on the known periods of these stars) or that the individual flare had multiple structures to it (of which no evidence suggest this is possible) (Champion).
Now confirmed for sure, scientists began to speculate as the possible causes. Could it be just a flare? Active magnetars? A neutron star collision? Black hole evaporation? Alfven waves? Cosmic string vibrations? Pinpointing the source has proven to be a challenge, for no prior glow nor afterglow have been seen. Also, many radio telescopes have low angular resolution (usually just a quarter of a degree) because of the range of radio waves, meaning that determining a particular galaxy for the FRB is nearly impossible. But as more data came rolling in, some options were eliminated (Yvette 25-6, McKee, Cotroneo, Bilings, Champion, Cendes "Cosmic" 23, Choi).
Sadly, FRBs are too bright for them to be the aftermath of a supermassive black hole evaporating. And because they happen more frequently than neutron star collisions, those are off the table as well. And the May 14, 2014 FRB had no lingering afterglow spotted despite so many eyes staring at it, eliminating Type Ia supernova for they definitely do have those (Billings, Hall).
Evan Keane and his team, along with the Square Kilometer Array and good ol’Parkes, finally found the location of one of the bursts the next year. FRB 150418 was found not only to have an afterglow up to 6 days later, but that it was in an elliptical galaxy about 6 billion light years away. Both further hurt the supernova argument, for they have an afterglow lasting for weeks and not too many supernovas happen in old elliptical galaxies. More likely is a neutron star collision producing the burst as they merge. And the awesome part about the discovery of 150418 was that since the host object was found, by comparing the bursts peak luminosity to the expectation, scientists can determine the matter density between us and the galaxy, which can help resolve models of the Universe. All of this sounds great, right? Just one problem: scientists got 150418 all wrong (Plait, Haynes, Macdonald "Astronomers").
Edo Berger and Peter Williams (both from Harvard) looked a bit harder at the afterglow. It had been determined from the roughly 90 and 190 days post-FRB inspection of the host galaxy that the energy output differed significantly from the merger of neutron stars but lines up well with an active galactic nucleus, or AGN, because the supposed afterglow kept happening well after the FRB (something that a collision would not do). In fact, observations from February 27th and 28th show that the afterglow had gotten brighter. What gives? At the initial study, some data points were taken within a week of each other and could have been mistaken for star activity because of their proximity to each other. However, AGN have a periodic nature to them and not a hit and run nature of FRB. Further data demonstrates a reoccuring radio emmision at 150418, so was it for real? At this point, likely a no. Instead, 150418 was just a big burp from a feeding galaxy's black hole or an active pulsar. Because of the uncertainty in the region (200 times that which is likely), the problem becomes arithmetic (Williams, Drake, Haynes, Redd, Harvard).
But some big scientific pay dirt was shortly around the corner. When Paul Scholz (a McGill University grad student) did a follow-up study of FRB 121102 (found by Laura Spitler in 2012 and based on the dispersion measure found by the Arecibo Radio Telescope indicates an extragalactic source), they were surprised to find that 15 new bursts came from the same location in the sky with the same dispersion measure! That is huge, because it points to FRBs as not a one off event but something continuous, a reoccurring event. Suddenly, options such as active neutron stars are back in play while neutron star collisions and black holes are out, at least for this FRB. Averaging 11 bursts measured and using VLBI gives a location of right ascension of 5h, 31m, 58s and a declination of +33d, 8m, 4s with an uncertainty of the dispersion measure of about 0.002. Also worthy of note was that more double peaks were observed in followups by VLA and that over the 1.214-1.537 GHz scientists looked at, many bursts had their peak intensity at different portions of that spectrum. Some wondered if diffraction may be the cause, but no elements of typical interactions were seen. After this spike, 6 more bursts were seen from the same location and some were very short (as small as 30 microseconds), helping scientists pinpoint the location of the FRBs since such changes could only happen in a small space: a dwarf galaxy 2.5 billion light-years away in the constellation Auriga with a mass content that was 20,000 times less than the Milky Way (Spitler, Chipello, Crockett, MacDonald "6", Klesman "Astronomers", Moskvitch, Lorimer 46, Timmer "Arecibo", Cendes "Cosmic" 22, Timmer "Whatever").
But the big question of what causes FRBs remains a mystery. Let us now explore some possibilities in a bit more depth.
Hyperflares and Magnetars
Scientists in 2013 decided to look more into the Lorimer burst in hopes of seeing some clues as to what a FRB could be. Based on the aforementioned dispersion measure, scientists looked for a host galaxy that would line up at a distance greater than 1.956 billion light years away. Based on that hypothetical distance, the FRB was an event that would have been an energy burst of about 1033 Joules and would have hit a temperature of about 1034 Kelvin. Based on prior data, such energy level bursts happen about 90 times per year per gigaparsec (y*Gpc), which is way less than the approximately 1000 supernova events that happen per y*Gpc but more than the 4 gamma ray bursts per y*Gpc. Also of note was the lack of gamma rays at the time of the burst, meaning that they are not related phenomena. One star formation that does seem to line up nicely are magnetars, or highly polarized pulsars. A new one forms in our galaxy roughly every 1000 years and hyperflares from their formation would theoretically match the energy output like the one witnessed in the Lorimer burst, so looking for young pulsars would be a start (Popov, Lorimer 47).
So what would be happening with this hyperflare? A tearing mode instability, a form of plasma disruption, can occur in a magnetar’s magnetosphere. When it snaps, a max of 10 milliseconds can occur for a radio burst. Now, since magnetar formation is reliant on having a neutron star to begin with, they arise from short lived stars and thus we need a high concentration if we were to have the number of flares witnessed. Unfortunately, dust frequently obscures active sites and hyperflares are already a rare enough event to witness. The hunt will be difficult, but data from the Spitler burst indicates that it may be a candidate for such a magnetar. It displayed a prominent Faraday rotation that would only arise from an extreme condition such as formation or a black hole. 121102 had something twist its FRB with a Faraday rotation and radio data indicated a nearby object, so maybe it was this. The higher frequencies for 121102 showed polarization associated with young neutron stars before they become magnetars Other magnetar possibilities include a magnetar-SMBH interaction, a magnetar trapped in a cloud of debris from a supernova, or even a collision of neutron stars (Popov, Moskvitch Lorimer 47, Klesman "FRB," Timmer "Whatever," Spitler).
With all of this in mind, a potential model was developed in 2019 by Brian Metzger, Ben Margalit, and Lorenzo Sironi based off those repeater FRBs. With something that is powerful enough to provide a huge outflow of charged particles in a flare and polarized surroundings (like a magnetar), the out-flowing debris makes contact with old material around the star. Electrons become excited and as a result of the polarized conditions start to rotate about magnetic field lines, generating radio waves. This happens as the wave of material makes more and more impacts, which causes the shock wave to slow down. This is where things get interesting, for the slowing down of the material causes a Doppler shift in our radio waves, lowering their frequency to what we end up seeing. This results in a main burst follows by several minor ones, as many data sets have shown (Sokol, Klesman "Second").
In a different theory first postulated by Heino Falcke (from the Radboud University Nijmegen in the Netherlands) and Luciano Rezzolla (from the Max Planck Institute for Gravitational Physics in Postdam), this theory involves another type of neutron star known as a blitzar. These push the mass boundary to the point where they are nearly able to collapse into black holes and have a huge spin associated with them. But as time goes on, their spin decreases and it will no longer be able to fight the pull of gravity. Magnetic field lines break apart and as the star becomes a black hole the energy released is a FRB – or so theory goes. An attractive feature of this method is that gamma rays will be absorbed by the black hole, meaning that none will be seen, just like what has been observed. A big downside is that most neutron stars would need to be blitzars if this mechanism is correct, something that is highly unlikely (Billings).
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© 2016 Leonard Kelley