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Title: Searching for Broadband Pulsed Beacons from 1883 Stars Using Neural Networks
Authors: Vishal Gajjar et al.
First Author’s Institution: Breakthrough Listen, University of California Berkeley
Status: Published in ApJ
The search for extraterrestrial intelligence (SETI) is perhaps humankind’s most ambitious and forward-thinking endeavor. We’ve been asking ourselves the fundamental question of “Are we alone?” since the dawn of written history, but technological advancements in the last 100 years have allowed us to take our first steps toward finding an answer. Today’s article describes a reimagination of one of the most common search techniques to look for signatures of extraterrestrials (ETs), and while we haven’t found any alien signals just yet, our search capabilities only continue to get better!
The easiest way to find ETs would be to look for their technosignatures — the light waves emitted by the technology they use (check out this Astrobite for more on technosignatures). In particular, if an alien civilization wanted to be found by other intelligent life, they would want to send out a signal that wouldn’t be deflected or absorbed by the space between us, would travel as fast as possible, and would require the least amount of energy to produce. For these and other reasons, most SETI searches have involved searching for artificial radio signals coming from the vicinity of nearby stars.
But what specific kinds of signals should we search for? Will they be transmitted over a narrow frequency range, or will they be “broadband” signals covering a large range of frequencies? Will the signal be continuously transmitted, or will it pulse on and off at specific intervals to clearly demonstrate it’s made by intelligent life? There is no one satisfactory answer to these questions, and most previous searches have looked for narrowband signals that are always being emitted, since we would need less time to detect a signal of that type than other types of signals.
However, the authors of today’s article were able to prove that for a civilization generating these signals, it would cost less energy to produce broadband pulsed signals, as long as those signals were being sent out for longer than a few hundred seconds. They made the reasonable assumption that ETs will try for longer than a few minutes to get our attention and went about searching for broadband pulsed signals in radio data from the Green Bank Telescope.
A Very Small Needle in a Turbulent Haystack
The Breakthrough Listen collaboration, of which many authors of this article are a part, chose 1,883 stars (explained in this article) as targets for their observations. They chose every star within 5 parsecs (a little more than 16 light-years) of Earth — so that the distance between us would not attenuate the signals too much — as well as all stars within 5–50 parsecs (163 light-years) that fall on the main sequence or the early part of the giant branch. Stars on these earlier segments of the stellar evolutionary track are less volatile and, if they have planets orbiting them, create environments that are the most likely to aid life to grow. The authors took 233 total hours of observations, broken up into 5-minute segments, since that is approximately the observational length for which a 0.3-millisecond long broadband pulse would take less power to send than a continuous narrowband signal.
Luckily, we have lots of experience searching for repeating broadband radio pulses in the form of radio pulsars and fast radio bursts! Pulsars are useful physical tools for a wide range of astronomical applications (for more, see the astrobites here, here, here, here, here, here, here, and here), but today, we can use our experience in analyzing transient radio signals to predict how a broadband signal sent by ETs would be affected by the interstellar medium between us. Radio waves are scattered and dispersed by the interstellar medium, and broadband radio signals undergo a dispersion delay, where the lower-frequency part of a pulse will be delayed relative to the higher-frequency part due to the ionized medium it travels through. The authors of today’s article focus on this dispersion delay.
Figure 1: The received signal from a dispersed broadband pulse, as a function of frequency and time. Note that the higher-frequency parts of the signal arrive before the lower-frequency parts. [Gajjar et al. 2022]
Most importantly, the dispersion delay time between two frequencies always scales as the inverse of the frequency difference, squared. The authors of today’s article suggest that if an alien civilization were to send us a signal, the best strategy would be to artificially arrange it in some way so that we would not see a normal dispersion trend; rather, we would see some other pattern that does not occur in nature, proving that it comes from other sentient life.
These other types of artificial dispersion are shown in Figure 2. The authors searched for dispersed pulses from their original dataset, and they also created artificial datasets by flipping the frequency and time axes, both independently and simultaneously. By doing this, each type of artificially dispersed pulse shown in Figure 2 would look to the single-pulse-search software as a normally dispersed pulse, allowing the team to run the same search code on all four datasets. Searching all of these datasets resulted in a staggering 133,393 candidates!
Figure 2: The three types of artificially dispersed signals that the authors searched for. From left to right, they are made by flipping the time axis, the frequency axis, and both simultaneously, to make artificially dispersed broadband pulses that are not seen in nature. [Gajjar et al. 2022]
How to Analyze 133,000 Candidates This Century
Of course, having a human sit down and examine that many candidates would be beyond unreasonable — thankfully, machine learning and graphics processing units allow us to quickly filter out many bad options. The authors filtered out candidates that looked too much like human-made radio frequency interference or didn’t show enough of a difference between their on-pulse and off-pulse energy distributions. Many other filters were used to weed out unpromising candidates, leading to a shortlist of only 2,948 candidates.
The best candidates in each class of artificial dispersion were examined more closely, but similar-looking signals were found in other 5-minute-long pointings in completely unrelated areas of the sky. It’s not easy for us to prove definitively that these signals actually come from the region of the stars we’re pointing at, rather than human-made radio frequency interference; it’s much more reasonable to conclude that these “signals” are bright human-made signals that have made their way into the telescope, rather than two extremely distant alien civilizations sending us the exact same signal.
The authors used these non-detections to place limits on the maximum signal strength any civilization in those areas could be sending, finding some signals as weak as a few hundred times stronger than our strongest airplane radar. That doesn’t sound like much of a limit, but that’s a signal we’d be detecting from a whole other star system — and each new search is another step towards better technology and better search methods to make a possible discovery!
Original astrobite edited by Lili Alderson.
About the author, Evan Lewis:
Evan is a third-year graduate student in astronomy at West Virginia University. His research focuses on transient radio sources, including pulsars, magnetars, and fast radio bursts. Outside of research, he enjoys playing percussion, hugging dogs, baking, and playing video games!