An artist’s impression of a very bad day…(Credit: ESO/A. Roquette).
Sure, we’ve all seen the movies with the impending death by asteroid or comet. You might have even heard of the havoc that can be wrecked by the Sun or an errant black hole, but have you ever heard of death by… gamma ray burst? Very much outside of public consciousness, this was but one of the more exotic ways humanity could have an official bad day that was outlined in Phil Plait’s outstanding Death from the Skies! But what are these exotic beasties, and just how likely are they?
First, the bad news. A gamma-ray burst is observed somewhere in the universe, nearly every few days! Back in the 1970’s, when the cat and mouse game known as the Cold War was in vogue, world super-powers were highly interested in monitoring the goings on behind various “curtains” both Iron and Bamboo. To this end, satellites such as Vela were launched with the intent of monitoring gamma ray emissions from atmospheric nuclear testing. These observations also detected something astonishing; cosmic sources of gamma rays from points unknown.
Astronomers scrambled to discern the source of these high energy events. Early missions lacked the resolution, and the best we could say is that a burst is happening somewhere in a large portion of sky. One thing, however, soon became apparent; the sources were scattered throughout the sky, and hence was unlikely to be emanating from our own solar system or galaxy. At extra-galactic distances, these bursts had to be massive, especially to sustain such high energies over great distances.
Compton flies high, freshly deployed from the shuttle cargo bay. (Credit: STS-37 & NASA).
Later missions, such as NASA’s Compton, Swift and the Fermi spacecraft launched in 2008 can swiftly spin into action, and in the case of Fermi has discovered over 500 gamma-ray bursts to date. Another new weapon in the arsenal of astronomers is the VERITAS array of telescopes, an array of gamma-ray scintillators watching for flashes of Cerenkov radiation high in the Santa Rita Mountains.
But just what are gamma-ray bursts? Studies have revealed two basic classes of Gamma-Ray Burst events (or GRBs for short): Long period and short period. Long period bursters last for over two seconds in duration and are now known to occur when a massive hyper nova-style explosion occurs, driving a massive in fall of material into a newly formed magnetar or black hole and spewing out a burst of gamma rays along the narrow axis of a pair of opposing relativistic jets. The progenitors of these bursts are thought to be what is known as Wolf-Rayet stars, or suns equal to 20 times more massive or greater than that of our own. For these jets to be visible from Earth, we need to lie within 20 degrees of the burst axis; most jets in the universe come and go without us ever detecting them. When large jets occur, telescopes around the world and in space swing into action, as the name of the game is to catch a signature afterglow and perhaps an optical extra-galactic counterpart.
Short bursts of less than two seconds duration are less well understood. These are thought to be generated when a very rare and exotic event occurs; the merging of two neutron stars. Of course, a system with two pulsars is itself unlikely; it will also produce copious amounts of gravity waves as the massive pulsars smash together. These would be detected by exotic observatories already in place such as LIGO, the Laser Interferometry Gravitational wave Observatory or the proposed LISA, the Laser Interferometry Space borne Antenna mission, which could fly in the next decade.
LIGO Livingston; the frontline in the search for gravity waves. (Photo by Author).
Now, for the wow factor. GRBs unleash in a few seconds as much energy as our Sun does in its whole 10 billion year odd lifetime. One of the most distant bursts thus far recorded was GRB 090423 on April 23rd, 2009 at a redshift of 8.2 which occurred when the universe was a tender young age of 630,000 years old. GRB’s have been observed to briefly ionize the upper atmosphere on occasion, and one, GRB 080319B was observed to peak at visual magnitude +5.4, which would be visible to the naked eye! I’m waiting for the first chance catch by a lucky amateur astronomer of such an event; be sure to scan those wide field images before deleting them…
So, how close is too close? Well, a galactic source GRB within 1,000 light years or so could spell a bad day for the Earth. The all important factor is the orientation of the ejection poles. Think of the GRB source as a spinning top, putting most of its energy out along its axis. If the burst is tipped away from us ever so slightly, we see a supernova, detect lots of gravity waves and neutrinos and get a pretty light show; if the axis is pointed right at our solar system; we get a bath of lethal cosmic radiation. Keep in mind, however, that while a GRB in our galaxy may not be a good thing, they are extremely rare, perhaps once in a million year events… in fact, seeing so many occur at great distances in the universe may show us just how common they were in dense star forming regions in the early universe, and just how rare they are in more relatively laid back galaxies like we reside in today. Some theories even suggest they may have been the trigger for certain mass extinctions, but perhaps the current lack of nearby GRBs may be the reason we’re here today.
What could a nearby GRB be capable of? True, a disaster of this type has little precedence, but it’s fun (in a dark sort of way) to pretend “what if”. The threat from a gamma-ray burst would be two-fold. Visually, a galactic source GRB might appear as nothing more innocuous than a pretty blue glow about the size of a Full Moon in the night sky. That’s the Cerenkov radiation you’re looking at, by the way. Most of the GRB’s energy would be absorbed by the upper atmosphere, to shower down in a cascade of particles. Gravity wave and Neutrino detectors such as Ice Cube might sound the first alarm, and they detect signatures moving at the speed of light.
Where the pulsars are…(Credit: NASA/Fermi/LAT Collaboration)
The burst of gamma-rays would, however damage our ozone layer and strip away our stratospheric protection, which will complicate things further, as we’ll see…
Within a few days, the really bad stuff in the form of cosmic rays and highly energetic nuclei arrive. This is the DNA busting stuff that can zip right through our atmosphere and cause wide-spread damage on a cellular level. At best, a near-miss burst might cause a subtle rise in cancer rates over the years; at worst, it would be a global mass-extinction event. Any space-borne astronauts would be in deep trouble and would either have to evacuate the International Space Station or seek refuge in its central core structure as a last resort. If we’re lucky, the burst would occur deep in one celestial hemisphere or another, so the bulk of the planet would shield a portion of humanity from the month long blast of radiation. A GRB near the celestial equator would be the worst case scenario, slowly roasting the Earth in a comic ray oven as it turns on its axis. The poles may or may not be a safe bet, has the cosmic radiation gets funneled into our magnetic field and provides us with a beautiful but deadly light show in the form of the aurora borealis…
But ultimately, the worst may be yet to come. Research by Princeton Universities’ Stephen Thorsett sites that a substantial amount of our upper atmosphere could be converted into nitric oxides, a smog type aerosol that would wreak havoc on the life cycle on the Earth. The destruction of our ozone layer could spell doom for those primary producers that we all depend on, plants and algae. This effect could linger for decades before our atmosphere fully recovers.
So, are there any smoking guns out there? Unfortunately, yes. And we are not talking about the recent stir about the star Betelgeuse, which may or may not go supernova today or a million years from now. At a suspected mass of 18-19 suns, Betelgeuse is just below the hyper-nova threshold, although not comfortably so. What is known is that its rotational axis is tipped safely away from us; at around 650 light years, a supernova explosion such as Betelgeuse would just give us a good show and astronomers a chance to study such an event in unprecedented detail.
Two more suspects are more troubling; Eta Carinae, 100-150 solar mass star 7,500-8,000 light years distant in the constellation Carina, and WR 104 a massive Wolf-Rayet star 8,000 light years distant in the constellation Sagittarius. Eta Carinae is deep in the southern hemisphere and it precise orientation is not clearly known; WR 104 lies towards the core of our galaxy and is a little more of a threat. Images show a spiral nature, which would suggest that we are looking down one of the poles of this massive beast; estimates point to an axial tilt of around 18 degrees, just the edge of the kill zone.
WR 104; A loaded star with our name on it? (Credit: The Keck Observatory).
As for short burst candidates, pulsar pairs PSR B1534+12, PSR B2303+46, and PSR J1518+4904 all lay with 10,000 light years of our solar system; but again, no one has caught one of these exotic pairs in the act for close up observation, and these could occur anytime in the next few millions of years.
A selection of Gamma-Ray Bursts observed by BATSE. (Credit: Daniel Perley and the BATSE Archive).
So, what can we do? Certainly, there is not much that we can do by way of proactive defense; we are where we are in the galaxy, and a burst either is or isn’t pointed at us. Certainly, they are worth studying, if nothing else so that threats could be better understood and predictions such as “tomorrow or a million years from now” could be narrowed down a bit. Perhaps the best bet would be the preservation of some of humanity deep underground, although one wonders what the life of the few survivors would be like. Ideas like the Norway seed vault are step in the right direction, as replanting and repopulating the biosphere would be of paramount importance.
In closing, a gamma-ray burst would be one of the more exotic ways to “do humanity in,” but I wouldn’t lose any sleep over it. Yes, Virginia, the universe is indeed out to kill us, but we are also protected by the Law of Mediocrity, which states that the reason that we evolved in this little niche of the universe is that it is, in fact, a pretty boring place. Certainly, we should seek to understand how events in the cosmos can and do impact us, but I wouldn’t think of refurbishing that backyard fallout shelter just yet. Besides, it’s far more likely that our biggest enemy may be ourselves and our own short sightedness, as we glibly trash the planet…will we choose to become a forward thinking culture that can exist for millions of years to even witness a nearby gamma-ray burst? The jury’s still out on that!
A gamma-ray fall-out shelter? (Credit: James Vaughan X-Ray Delta One under a Creative Commons license).