Andrew Siemion, who heads up the Breakthrough Listen initiative and is director of the Berkeley SETI Research Center, sent out a message to astronomers on August 29 noting recent activity from the radio source FRB 121102. The heightened activity had been noted by Breakthrough Listen postdoctoral researcher Vishal Gajjar. You’ll recall that Fast Radio Bursts (FRBs) are powerful but extremely short-duration radio pulses whose sources generally remain unknown.
What tags FRB 121102 as especially interesting is that it is the only FRB known to repeat. In fact, more than 150 bursts have been observed coming from the dwarf galaxy 3 billion light years from Earth that is thought to be its place of origin. And now we have heightened activity in the form of 15 new bursts, as the Astronomer’s Telegram notes:
These are the highest frequency and widest bandwidth detections of bursts from FRB 121102 obtained to-date. Additional fully calibrated full-Stokes analysis employing coherent dedispersion on raw voltage data is ongoing. These observations may indicate FRB 121102 is currently in a heightened activity state [Law et al. arXiv:1705.07553], and follow-on observations are encouraged, particularly at higher radio frequencies.
Image: A sequence of 14 of the 15 detected bursts illustrate their dispersed spectrum and extreme variability. The streaks across the colored energy plot are the bursts appearing at different times and different energies because of dispersion caused by 3 billion years of travel through intergalactic space. In the top frequency spectrum, the dispersion has been removed to show the 300 microsecond pulse spike. Capturing this diverse set of bursts was made possible by the broad bandwidth that can be processed by the Breakthrough Listen backend at the Green Bank Telescope. Credit: Breakthrough Listen.
Do FRBs mark the catastrophic end of some kind of astronomical object? If so, the fact that FRB 121102 repeats sets it apart from the rest. A rotating neutron star with an extremely strong magnetic field is also a possibility. There is also a SETI possibility: High-energy laser bursts from a distant civilization could be observable by us. Harvard’s Avi Loeb, working with Manasvi Lingam, discussed the prospects in a recent paper, working out their parameters in terms of engineering requirements if FRBs were indeed produced by an extraterrestrial civilization (see Fast Radio Bursts: Signature of Distant Technology?)
Vishal Gajjar used the Breakthrough Listen backend instrument at the Green Bank Telescope in West Virginia to observe the recent activity of FRB 121102. Says Gajjar:
“The extraordinary capabilities of the backend receiver, which is able to record several gigahertz of bandwidth at a time, split into billions of individual channels, enable a new view of the frequency spectrum of FRBs, and should shed additional light on the processes giving rise to FRB emission.”
Indeed. According to this UC Berkeley news release, the backend instrument accumulated 400 terabytes of data in a five hour period on Saturday, August 26, with observations extending across the 4 to 8 GHz frequency band. What the team was looking for were the signatures of short pulses with a dispersion — a delay as a function of the frequency — that is caused by gas between the source and the Earth. Remember that these beams have been traveling for a long time. As Berkeley astronomer Steve Croft notes, they left their host galaxy when life on our planet consisted of nothing more than single-celled organisms.
The current active state of FRB 121102 could prove quite useful, allowing us to measure this source of FRBs at the highest precision yet. The 15 new pulses Gajjar observed show emissions at higher frequencies than have previously been seen. The brightest FRB 121102 emission occurred around 7 GHz. We’re a long way from being able to figure out what causes FRBs, but the Breakthrough Listen instrumentation is giving us the best look at the frequency spectrum of this powerful source yet.
The results of this work will be presented in an upcoming paper. The paper exploring artificial origins for FRBs is Loeb and Lingam, “Fast Radio Bursts from Extragalactic Light Sails,” Astrophysical Journal Letters Volume 837, Number 2 (8 March 2017). Preprint) available.