Caution: fast radio burst from space on the way!


You see, all those objects are … well, actual objects. Tangible, real objects. One of them—the Moon—has even been visited by several humans, if now nearly half-a-century ago. FRB 180916.J0158+65 might be such an object too, but the thing is, we don’t know. All we do know right now is that it is a source of radio waves. Not your ordinary BBC radio broadcasts that you struggle to catch on your shortwave set (remember those?), either. Instead, this is one more in a growing catalogue of short, intense bursts of energy in the radio spectrum—in three words, fast radio bursts (FRB). Two astronomers, Duncan Lorimer and David Narkevic, found the first FRB we know about (the so-called “Lorimer Burst”) in 2007, though it had actually been recorded in 2001; they found it by poring through archival data from an Australian telescope. Since then, we’ve registered about 40 more FRBs.

How short are they? A few milliseconds. How intense? Well, for those few milliseconds, there may be no more bountiful source of energy—any energy—in the entire universe. These FRBs we detect speak of energy sometimes equivalent to several hundred million Suns. Or put it like this: in that tiny fraction of a second, an FRB can put out as much energy as the Sun does in 10,000 years.

If you can even imagine a flare-up of that magnitude in that short a time, I envy you—because I cannot wrap my head around what that must mean.

And yet, what does it mean?

Well, let’s understand some things about FRBs. First of all, as powerful as FRBs are, it’s not as if they are knocking us over when we find them. Instead, the energy from an FRB that we actually detect here on Earth is, it has been pointed out, weaker than the signal we’d sense from a cellphone left on the Moon. That should give you an idea of just how enormous a place our universe is. Even the unimaginably violent punch of several hundred million Suns subsides, as it travels several hundred million light years through space, to a mere blip. And even so, the blip stands out enough from the usual background radiation of the universe that we know this must be an unusual, remarkable event. Naturally, astronomers want to find out more.

Except that, second, the earliest several FRBs we detected were all one-time events, happening seemingly at random. Before FRBs, astronomers usually attributed bursts like these to pulsars, stars that rotate and so are not unlike lighthouse beams in how their radiation sweeps past the Earth. But pulsars are regular like clockwork. FRBs were not. That is, they did not repeat their performance, nor did they follow any clear pattern. This made them nearly impossible to study and learn from. How do you pinpoint where an FRB originates, and what made it erupt at all, if all you have to go on is one very brief blip in radio static?

No wonder a 2016 article in Nature (go.nature.com/2SpRGfr) described the phenomenon of FRBs as “one of the most perplexing mysteries in astronomy.” It also noted that astronomers had offered explanations for them like “evaporating black holes, colliding neutron stars and enormous magnetic eruptions.” Yet none of these satisfactorily explained everything in the FRB data. (It’s worth pointing out that some early observations, assumed to be FRBs, turned out to be from a microwave oven that scientists were opening at lunchtime).

Third, one feature of FRBs was telling: lower-frequency waves in the data from them were lagging behind higher-frequency ones. That is, if you separate the observed data into different frequency bands, the bursts show up earlier in higher frequencies than in lower ones. Now whatever causes the burst sends out waves at all those frequencies at the same instant. What delays the lower frequencies on their way to us?

Astronomers had an answer to that. As waves travel through clouds of gas that float out there in the cosmos, we know that electrons in the clouds interact more with lower-frequency waves. This slows down the waves and actually further lowers their frequency, effectively “stretching” them out. Much like with the Doppler effect, astronomers can measure this delay and stretch and deduce how far the waves have travelled. The Lorimer Burst’s delay, for example, was so large that they knew its waves must have travelled from somewhere well beyond the limits of our Milky Way galaxy, and through a lot of gas clouds indeed. This is how we know that the sources of FRBs are an enormous distance away.

Fourth, in 2016, the Arecibo radio telescope in Puerto Rico finally discovered an FRB that repeated. Again, this was one that had been recorded in November 2012 —so it is called FRB 121102—but not noticed till some years later. This was a series of bursts that were sometimes separated only by minutes. The same source sent out 16 more bursts in 2015, and then, in 2017, 15 more in a period of 24 minutes.

The repetition was significant because a single burst implies some kind of vast destruction—a gargantuan explosion perhaps, or objects colliding. But a burst that repeats? A Cornell University team of astronomers led by Shami Chatterjee studied the bursts from FRB 121102, concluding that their work “unambiguously identifies FRB 121102 as repeating and demonstrates that its source survives the energetic events that cause the bursts.” (Nature, 2 March 2016).

Chatterjee’s team then pinpointed this source: a “dwarf galaxy”, much smaller than the Milky Way, about 2.5 billion light years away. This was startling to some astronomers, because a dwarf is much less likely than a more “normal” galaxy to contain whatever it is that causes FRBs. But still, there it is. Yet, even if FRB 121102 repeats, it does so sporadically rather than regularly. What we’ve detected so far is a cluster of bursts, followed by a long silence, followed by a cluster of a different number of bursts. At least so far, astronomers have found no apparent pattern to the repetition.

Again, that makes it harder to explain just what is going on with FRBs.

But hold on. We now know of 11 repeating FRBs. Of them, one is particularly interesting, because it’s the only one that does indeed display a pattern to its repetition. That’s FRB 180916.J0158+65.

A team at the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Collaboration in Canada detected this FRB in September 2018, and then observed it for 409 days, until October 2019. They found multiple bursts in that time. Applying different statistical techniques to their data suggested consistently what they reported in a recent paper (“Periodic activity from a fast radio burst source”, bit.ly/3bCj9Sz). This FRB repeats in a cycle that lasts just over 16 days: four days with a few bursts or maybe none, then 12 quiet days, then repeat. “[T]he periodicity is obvious with all the approaches we have tried”, write the authors, “and does not exist in any of the control samples.” What does this tell them? “We conclude that the periodicity of FRB 180916.J0158+65 is significant and astrophysical in origin” —that is, there is some vast stellar or galactic process that emits these bursts. Where is it happening? The CHIME team traced FRB 180916.J0158+65 to the edge of a spiral galaxy that’s about 500 million light years away.

There’s more. Of the 11 repeaters, astronomers originally thought one was a once-only burst. Except that the repeating signals were so faint that the instrument that found that FRB failed to detect them. So with the use of more powerful telescopes, it’s entirely possible other once-only FRBs will turn out to be not just repeaters, but ones whose bursts follow a cycle.

Just like FRB 180916.J0158+65.

But it’s not just more powerful telescopes at work here. It’s also the poring through piles of data for tiny anomalies, making deductions from them, finding explanations that will fit, searching for corroboration. All of which applies to the short history of FRBs. And just like that, we have an entirely new direction to explore in astronomy.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun





Follow:Source link

Leave a Reply

Your email address will not be published. Required fields are marked *