They are fast. Faster than you can believe. Don’t turn your back. Don’t look away.
—THE DOCTOR, “BLINK” EPISODE OF DOCTOR WHO
Nikhil Sarin’s work began where decades of hair-pulling frustration had ended.
“Most of the observations I interpret come from the NASA Swift Observatory or the Chandra X-ray Observatory,” he explained. “Swift can see a good chunk of the sky at the same time. Once it detects a gamma ray burst, it automatically starts pointing its X-ray detector towards the source. This typically takes a few seconds.”
“A few seconds,” I thought in amazement. It has taken me longer to find the ~ key on my computer than it takes for this orbiting observatory to zero in on a cosmic flash. But I knew that things hadn’t always been this straightforward.
His career just beginning when I spoke with him, Sarin was standing on the shoulders of giants like astronomer Kevin Hurley, who spent a good portion of his career chasing down the energetic fireflies first detected by the Vela satellites in the late 1960s. Hurley was just finishing his PhD when these gamma ray bursts (GRBs) burst onto the scene. Attracted by the high-energy unknown, he and others aimed to figure out what they could about flashes of gamma radiation lasting anywhere from milliseconds to tens of seconds.
The biggest problem then was localizing the bursts in the sky. GRBs pop off like paparazzi flashbulbs, never repeating.* Trying to determine exactly where they are is like trying to catch a firefly that flashes only once. Worse, an individual gamma ray detector at the time was good only for letting astronomers know when it had picked up a signal. If they wanted to know where an event had occurred, they needed several detectors spread out with as much space between them as possible.
So, in the late 1970s, Hurley spearheaded a plan to piggyback gamma ray detectors on spacecraft, the first of an armada of experiments that would become the Interplanetary Network (IPN). Soviet probes Venera 11 and 12, sent to explore the harsh environment of Venus, each took a French gamma ray experiment and trekked off toward our sister planet tens of millions of kilometers away. Closer to home was the Prognoz satellite, another Soviet spacecraft fitted with a French gamma ray detector. There was eventually a solar observer orbiting the Sun, a cometary explorer far above Earth’s surface, and a mission to Mars. Basically, any spacecraft that could accommodate a gamma ray sensor got one.
The vast distances between the spacecraft made the IPN a powerful tool in narrowing down the direction of the bursts. Just as a clap of thunder isn’t heard by everyone at the same time, a gamma ray burst isn’t detected by every station simultaneously. If a spacecraft orbiting Earth picked up the signal first and one near Mars caught it a few minutes later, then scientists knew that the basic geometry was “source, then Earth, then Mars.” Adding more spacecraft helped localize the source even better. This multinational ensemble gave astronomers a fairly good idea which direction a burst came from, even if there was no way to triangulate the distance.
The information was far from instantaneous, though. It took a day or more to get the information that gamma rays exceeding a predetermined threshold had been detected by spacecraft at various locations in the solar system. By the time astronomers sorted out the general patch of sky that had hosted the flash, the event was long gone. But that shouldn’t matter, they figured. Surely, something would remain, and all they would have to do is point an optical telescope at the location of the burst, find something obvious, and, voila! The mystery would be solved, and they could move on to the next puzzle.
When we spoke, Hurley recalled, “We naïvely thought that the sources would be there for a long time, and you could just go back and look a month or two later, and you would see something.”
When the hunt for GRBs began, everyone’s money was on a straightforward phenomenon, perhaps the neutron star version of a nova. A companion star would overflow its territory, and then its material would race down the steep gravitational pit in spacetime where, well, um . . .
The exact mechanics of producing an epic burst of gamma rays from a neutron star siphoning matter from its partner were unclear, but that was something that could be sorted out once an optical follow-up observation had been made. “But as we accumulated more and more of these positions, we kept turning up—” Hurley paused. “Absolutely nothing.”
No remnants. No lingering glows. No further clues.
He continued, “Then we thought maybe we just needed to work faster, and the race was on to build instruments that could get rapid locations.” That race turned from a sprint into a marathon, continuing into the 1990s and beyond as astronomers fought to catch GRBs in the act.
It was absolutely imperative that they get at least approximate locations, down to a few degrees at worst, and then plot the GRBs on a map of the sky. Even though it wouldn’t be perfect, it would at least help astronomers determine whether the bursts were coming from inside our Milky Way Galaxy or beyond. If the GRBs tended to fall along the plane of the Milky Way Galaxy, that cloudy ribbon of sky observable far from the glow of city lights, then they were most likely relatively nearby, which, to an astronomer, is just thousands to tens of thousands of light-years away. If the bursts happened all over the sky, then there were two wildly different possibilities: They were either very close to the Sun, scattered less than a few hundred light-years away, or they were in distant galaxies perhaps billions of light-years away.
The arguments in favor of local sources were the same ones that had been trotted out for a century. Nearby sources, while still pumping out the equivalent of a year of sunshine in seconds, would require the mysterious objects to generate energies that made sense. Accepting that GRBs were coming from well outside our own galaxy would mean accepting that something in the universe could spit out a foe of energy in the snap of a finger. In gamma rays, no less. The sheer power required for such a feat turned many astronomers away from the idea that they could be outside our Galaxy, but until more definitive data came in, there was no real evidence either way.
By the early 1980s, with over 100 discoveries and 46 general locations ascertained, astronomers announced that GRBs seemed to be happening randomly all over the sky. There was still a glimmer of hope that they were extremely close, a conclusion that suited 95% of GRB researchers just fine.
Hurley recalled the uncertainty. “I thought every Monday, Wednesday, and Friday that gamma ray bursts would turn out to be some sort of Galactic phenomenon related to neutron stars, and then on Tuesdays and Thursdays, I thought, wow, what if they were extragalactic? Wouldn’t that be cool?”
“What about weekends?” I asked.
Hurley laughed. “On Saturdays and Sundays, I just didn’t think about it at all because it was so confusing.”
Amid all the confounding cosmic paparazzi, the last thing they needed was GRB 790305. Following the naming convention of GRB for a gamma ray burst and then year-month-day, it was affectionately known as MF.
For March Fifth, of course.
It probably wasn’t even a true GRB. The tsunami of gamma rays that washed over the solar system on 5 March 1979 was an event that stood out even among the standouts. It was announced in May of that year through the International Astronomical Union’s Central Bureau for Astronomical Telegrams (CBAT), the same channel that had announced countless novae and supernovae. First, the Soviet probes Venera 11 and 12 had spiked, and within minutes, ten other missions in the Interplanetary Network found their detectors saturated. With such a brief blast and so many detections, astronomers were able to pin down the source not just to within a couple of degrees in the sky, but to a region only one-sixtieth of a degree across. Within that box, deep in the Large Magellanic Cloud, was a relatively young supernova remnant discovered by radio astronomers. It was a toddler, only a few thousand years old.
There was no way, scientists said, that this was just a coincidence. But there was also no way that they could account for so much energy emitted by an object 180,000 light-years away. Within a tenth of a second, it had radiated the same amount of light that the Sun produces in a millennium.
It wasn’t finished either. Less intense gamma rays pulsed every eight seconds for over a minute after the initial blast, and as the years passed, the object that had produced GRB 790305 occasionally and without warning belched out lower-energy gamma rays. The periodic gamma rays following the initial intense burst were fine, astronomers conceded, but anything beyond that was simply unacceptable. GRBs might be enigmatic and inexplicable, but with a growing catalog of known events, astronomers were certain that they were one-and-done.
And maybe GRB 790305 was actually done, at least as a gamma ray burst. The signals that came in later fell in a regime known as “soft gamma rays.” X-rays, really, at the low-energy portion of the gamma ray spectrum. The March burst itself not only was more intense, but also consisted of higher-energy “hard gamma ray” photons, not the later soft ones. Don’t let the name fool you, though. Even a soft gamma ray photon is tens of thousands of times more energetic than a visible photon, so there is nothing cuddly about an object that emits a sudden pulse of this type of light.
Conveniently, soft gamma repeaters (SGRs), as they became known, are recurrent. Not on a regular time scale—that would be too convenient—but these fireflies do blink more than once, a property that has allowed astronomers to catch them in an ever more certain jar. Resistance to the idea that the March 1979 event had been as far away as the Large Magellanic Cloud gradually—very gradually—dissolved.
The year 1979 saw the discovery of another soft gamma repeater, one that flashed over 100 times in as many months with varying intensity and absolutely no rhyme or reason to its activity. With so many bursts, SGR 1806-20 practically begged astronomers to pinpoint its location: near a supernova remnant over 40,000 light-years away. After years of chasing these particular fireflies, astronomers were able to definitively associate the three then-known SGRs with neutron stars, but these weren’t run-of-the-mill neutron stars.
Run-of-the-mill neutron stars . . . as if such a thing exists. It seems unfathomable that our explorations into the gamma ray universe had pushed astronomy to the point that neutron stars could ever be considered ordinary. They are stellar masses collapsed to densities beyond imagination, some spinning faster than a kitchen blender. They’re armed with magnetic fields that can shred you and put one of your charged particles on one side of the room and another on the other side of the room.
These things are anything but ordinary.
Soft gamma repeaters, though, implied something even more extreme. The theory was fuzzy, but they seemed to be newborn neutron stars trying to relieve the tension in their immensely powerful and tangled magnetic fields. They are magnetars, and the fact that there are definitely some of these in our Galaxy would eventually become cause for concern.
But what about the other gamma ray monsters? Where are they?
Observers weren’t the only ones trying to wrap their brains around the energetic new twist the universe had thrown our way. Theorists were also scratching their heads as they tried to understand what could produce such a burst of gamma rays at any distance. Frustratingly, other than being bursts of gamma rays, no two GRBs appeared to be the same. The puzzle pieces were scant, but as in the case of Bell Burnell’s original pulsar, timing was everything. Although some GRBs lingered for over a minute—these were cleverly designated “long bursts”—some winked in and out in mere milliseconds. Those were the short bursts, and astronomers figured that their underlying mechanism had to be different from that of their long burst brethren.
Long or short, GRBs showed tiny, even shorter-term variations in their signals, variations that implied that the source was very small. Very small indeed, and capable of generating gamma rays without obviously emitting any of the lower-energy types of light (and certainly not in any lingering fashion, if they did). The obvious suspects were neutron stars and black holes, objects that had been purely hypothetical just a generation before. If anything could pull off extreme events, they could.
The impending merger of the Hulse-Taylor binary neutron star provided another possible explanation. By the early 1990s, astronomers had observed a whopping four pulsars in binary neutron star systems in our Galaxy. Fortunately, the universe is brimming with galaxies, and those galaxies are brimming with stars, and many of those are massive stars in binary systems. Massive stars in binary systems live fast and die hard, and after their explosive deaths, they have a fighting chance of leaving behind a binary neutron star system. Even from this paucity of nearby data points, astronomers estimated that there would be a binary neutron star merging event within a few billion light-years about once every three days. The entire observable universe would host a handful of these mergers each day.
But how to observe such a thing? During a neutron star merging event, the vast majority of the energy—over 100 foes of it—is blasted out as neutrinos and gravitational waves. The gravitational waves would be the smoking gun, but in 1991, gravitational wave astronomy was still a dream. The US Congress had approved funding for two ambitious gravitational wave observatories, but the sites hadn’t even been determined yet. It would be at least a decade before either was up and running, and easily another decade before they were fine-tuned to the point of picking up wiggles in spacetime that are characteristic of two colliding neutron stars.
Gravitational wave detection was clearly out, but what about neutrinos? Surely, SN 1987A had proven that we could catch those. True, but SN 1987A was practically in the same room with us, and we caught only two dozen of its trillion quadrillion quadrillion quadrillion neutrinos. A collision between two neutron stars 10,000 times farther away would give us nothing.
Thankfully, the universe has always been involved in countless cosmic energy laundering schemes, and theorists posited that there was at least one channel that might give us a window on a neutron star merger. If a flood of neutrinos and their antimatter counterparts (antineutrinos) converted to electrons and their antimatter counterparts (positrons), then the electrons and positrons would meet up, annihilate each other, and create gamma rays. This chain of events is not as preposterous as it may seem at first glance. As long as the universal accountant is satisfied, swapping out one type of particle for another or even cashing in particles for photons is not only permissible, but commonplace in the most extreme environments in the universe. A swift burst of gamma rays could indeed be the most easily observed consequence of a neutron star merger. It was a long shot, particularly without any additional way of confirming the event, but it did provide one possible answer.
On the other hand, a different cohort of astronomers argued, supernovae were far more common, so perhaps they were the bangs behind the bursts. Perhaps there was something about the exact geometry of the collapsing core and the subsequent explosion that shot out gamma rays in narrow jets, like cosmic blowtorches. As in the case of pulsar observations, if Earth happened to stare down the barrel of one of these jets, we would see a burst. If not, well, we’d miss out, just as we miss out on countless neutron stars that don’t point their beams of energy our way.
On another hand—you’ll need to start using your neighbor’s hands—maybe these are just bizarre things in the outskirts of our own solar system. On yet another hand, perhaps there is something far more exotic going on, completely beyond the scope of 1990s astrophysical understanding. Or perhaps, more than one answer is right. Maybe the shorter bursts come from one type of source, and the longer bursts come from another.
There were plenty of hypotheses. Legend has it that at one astronomical meeting on GRBs, a speaker claimed that it was harder to find someone who hadn’t proposed an origin for GRBs than to find someone who had.
So much had changed since the appearance of a new dot in the Andromeda Galaxy a century earlier, and yet so much remained the same. Answering the question required more data, and there was a new urgency. They needed some way to instantly follow up on a burst detection.
Getting a detailed picture of the universe in gamma rays was not nearly as straightforward as, say, mapping the visible or radio universe. Astronomers wanting to see in other wavelength regimes need orbiting observatories, and as a rule, those are expensive, painstakingly difficult to construct, and fraught with peril.* A launchpad mishap can turn years of planning and work to debris, and remedying a tiny miscalculation in design can be challenging at best, impossible at worst.
Still, NASA agreed that exploring the multiwavelength universe was the only way to get the answers to the biggest questions, and it committed billions of dollars to its four great observatories to do just that. The first, launched in 1990, was the Hubble Space Telescope, which would—after a servicing mission to correct its slightly astigmatic optics—give us the clearest view yet of cosmic goings-on. Then, just a year after the Hubble Space Telescope was sent into orbit, the Compton Gamma Ray Observatory (CGRO) hitched a ride on the Space Shuttle Atlantis, from which it was gently sent into orbit. The year 1999 saw the launch of the Chandra X-ray Observatory, and in 2003 the Spitzer Space Telescope was launched to reveal the infrared universe in unprecedented detail.
Occupying the eight corners of the CGRO was BATSE—the Burst and Transient Source Experiment—which was able to view the entire gamma ray sky 24/7. Immediately, it began picking up one GRB per day. They were everywhere. Up. Down. Left. Right. After several hundred had been logged and analyzed, astronomer Chryssa Kouveliotou grudgingly reported that “a consensus is slowly forming within the community toward an extragalactic origin.”
There was still no definitive proof, though, no clear evidence that any particular GRB occurred in any particular galaxy. Then in 1996, an Italian-Dutch X-ray and gamma ray observatory called BeppoSAX was launched, and just ten months later, astronomers finally got what they had been after for three decades: the precise location of a gamma ray burst. Astronomers celebrated this milestone, but they were billions of years late to the party. The light from GRB 970228 had been traveling for more than half the age of the universe by the time the blast swept over our instruments.
“When the first one came along that had a pretty secure redshift, that was a really nice moment,” Hurley recalled. “It made it all worthwhile.”
Simon Johnston, a senior research scientist at CSIRO, had a slightly different take on the first definitive host galaxy. “As soon as they had localization, it was game over. Gamma ray bursts were extragalactic.” He paused thoughtfully. “But they’re not actually useful for all that much in terms of cosmology.”
“Useful” was not a word I expected to hear about something billions of light-years away, but it did reveal a fascinating divide in astronomers. Some are excited about the extreme events themselves. I mean, what could be more thrilling than crushing gravity, hyperactive spin rates, immense blasts of energy, and distortions in spacetime? Other astronomers, while still suitably in awe of the objects they study, are more interested in using them as tools to understand the grander goings-on in the universe. I recalled my chat with Suntzeff, who had been using Type Ia supernovae to chase H0 and q0, the numbers revealing how the universe itself is behaving. His interest in the supernovae is secondary to their utility.
And it wasn’t clear if GRBs were anything other than cosmic show-offs.
Useful or not, these beasts can pack a punch. After BeppoSAX’s first localization, which proved that at least some GRBs are indeed at truly cosmological distances, billions of light-years away, astronomers could then begin to sort out the problem of their power source, and it was definitely a problem. If researchers assumed that the same amount of energy went out in all directions, the bursts were clearly out of the question. There simply was no way for the universe to pull off this feat. Full stop. Yes, this seems like a very familiar story, but this time they were really serious.
Something that the universe had revealed to us for decades, though, is its talent for channeling energy and particles into narrow beams or jets. There are pulsars, after all, and that odd tail jutting out of the quasar 3C 273, but there are plenty more. Jets had been first observed in 1918 by Heber Curtis, who noticed a “curious straight ray” in the galaxy M87. That curiosity turned out to be a 5,000-light-year-long stream of high-energy particles racing away from the heart of this otherwise unassuming galaxy. On a much smaller scale, stars in the process of forming—protostars—also kick out jets.
Making a jet is, it turns out, a fairly universal process wherever gravity is involved. Material falling toward a massive object never seems to make a direct hit, but instead forms a hot, turbulent disk that spirals inward. It’s the same reason a pancake of matter forms around a white dwarf as it siphons material off its bloated companion. As that matter jostles toward its gravitational destination, it becomes hotter, and the atoms are stripped of their electrons. With so many charged particles zipping in tighter circles, the disk essentially becomes an enormous and enormously strong electromagnet. Caught in the ever-tightening grip of some of the strongest magnetic fields the universe can muster, particles shoot out in tight columns above and below the disk. Think Saturn, but with blowtorches blasting out of its north and south poles.
With its energy concentrated into two narrow cones, a jet-producing object gives different observers distinctly different views. If you imagine staring down the barrel of one of these jets, it’s easy to see how you might be fooled into thinking that the object is far more powerful than it is. If you observe it from the side, though, it doesn’t seem so impressive. In fact, if an object isn’t firing off its jets in your direction, it might not even be visible. Like the pulsars that never swing their beams past us, these are probably the norm, rather than the exception.
Having netted a host of jetted objects in the universe by the 1990s, it wasn’t much of a leap to blame jets for gamma ray bursts. For one thing, this solved the energy crisis.
“The energies needed are, maybe, 1052 ergs,” Hurley explained nonchalantly.
Oh. “Only tens of foes,” I thought. Whatever.
Sensing my disbelief, he added, “That’s maybe a hypernova, but it isn’t requiring us to invoke any kind of new physics or exotic types of stars.”
Readily accepting this kind of energy output is major progress for a profession that once refused to admit that the Sun’s entire lifetime energy output could be compressed into months.
As more and more of these objects popped off, and as astronomers got better at catching them in the act, two prevailing candidates for GRBs were, if not universally accepted, at least most plausible.* Shorter bursts, which make up only about a third of the total, seemed to be best explained by neutron star merging events, an explanation that was just the starting point for Sarin’s research in the early 2020s. If the two neutron stars are comparative lightweights, the resulting object survives. If not, a short-duration GRB might be the universe’s way of telling astronomers that it has just cooked up a brand-new black hole.
Astronomers generally agree that longer-duration GRBs always herald the formation of a black hole. To produce these bursts, the cores of some of the most-massive stars collapse. Those cores spin faster and faster and faster still, disks whirling at relativistic speeds as the center of the star tries to race down the newly unplugged gravitational drain. From the poles come powerful jets that blast through the outer shell of the star like bullets.
From a visual standpoint, events responsible for the long-duration bursts would also yield some of the brightest supernovae, objects known as superluminous supernovae or hypernovae.* Eta Carinae might very well become one of these, or it might simply collapse. In either event, it will most likely fire out jets along its poles, blasting away much of the core-collapse energy into beams of high-energy radiation and fast-moving particles that, thankfully, will not be pointing in our direction. Astronomers have already seen matter ejected at Eta Carinae’s poles, and they are safely aimed away from Earth.
But what if they’re wrong?