Heavens above! What made the cosmic flash that lit Earth today?
Analysis for The Conversation by Dr Alan Duffy, Swinburne University of Technology
A titanic eruption in our neighbouring galaxy, Andromeda, has sent shockwaves through the astronomical community here on Earth.
NASA’s Swift satellite detected a flood of gamma rays at 21:15 UTC yesterday (7:15 AEST today), triggering telescopes across the globe within minutes to hunt for the afterglow of the explosion. Not long after, the news had raced around the world thanks to Twitter.
While the cause of the explosion is a mystery at the moment, its implications could be huge.
There are many potential causes of such a flood of gamma rays, but the one that tops astronomers' wish-list is a short-duration gamma ray burst. The cause is still unknown but current models suggest it’s an enormous collision (or inspiral) of two neutron stars. These are the dead cores of massive stars with as much mass as the sun crushed into a region no larger than a city.
A neutron star compared to the island of Manhattan.NASA, CC BY
During their inspiral and ultimate collision into a black hole, an enormous amount of energy is blasted out which we see as light of all wavelengths (from gamma rays, X-rays, visible/optical light and even into the radio).
The energies are so large that were such an explosion to occur in our galaxy it might lead to mass extinctions on Earth. Don’t worry though – the event in Andromeda is 2.5 million light years away making it harmless to us (and that this “breaking” story happened while humanity was barely in the Stone Age).
Such events will also set spacetime itself rippling, offering the hope of directly detecting Einstein’s final prediction of gravitational waves.
The (un)usual suspects
The most discussed cause for the explosion is a gamma ray burst, a collision between neutron stars. These are rare, with perhaps only one every million years expected in a galaxy such as ours or Andromeda.
The first 500 gamma ray bursts seen by NASA’s Swift satellite.
Other suggested causes could be a “belch” from a feeding black hole in what’s called a Low-Mass X-ray Binary. This is unlikely as the energies are hundreds of time greater than what can normally be produced.
A similar idea, given a catch-all title of Ultraluminous X-ray (ULX) object, is that it’s a much larger black hole feeding messily, although likely we should have seen this before now.
Another possibility, and the one hardest to discriminate right now, is a flare from a magnetar, similar to the eruptions from our sun that cause the Northern and Southern Lights on Earth, but hugely more energetic.
These magnetars are super-magnetised neutron stars, with a magnetic field strong enough to wipe credit cards from half a million kilometres away. While plausible, the gamma ray signal comes from a clump of ancient stars, called a Globular Cluster, which would be unlikely to house a still powered-up magnetar.
A new sense for humanity
Everything we know about the world around us is via light or electromagnetic waves. Even when we touch objects the atoms never meet – instead we are stopped by electromagnetic fields between our hand and the object.
Yet if this explosion is a collision between two neutron stars then it would have opened up a fundamentally new sense for humanity, that of gravitational waves, entirely separate to electromagnetic waves.
Although indirect detections have been claimed, today could have been the first direct confirmation in the lab.
Such a detection would be a potential Nobel Prize winning discovery, which is why it’s particularly unlucky that the Laser Interferometer Gravitational Wave Observatory (LIGO) facility built to detect these gamma ray bursts is currently shutdown for an upgrade.
After the excitement
Over the next few days the manner in which the afterglow fades will tell us if it’s the hoped-for gamma ray burst or less extreme but no less exotic magnetar flares or feeding black holes.
In addition to telescopes, particle detectors such as the IceCube facility in Antarctica will be searching for any hints of ghost-like neutrino particles that are often associated with high energy events in the sky, although recent efforts show gamma ray bursts unusually might be an exception.
Finally, operating gravitational wave detectors, such as GEO600, will analyse their data for a tell-tale high frequency “chirp” as the two neutron stars orbited each other ever faster until finally colliding.
Whatever the cause of the explosion ultimately is, the events of today have shown that even in a science that measures time in billions of years, things can still move fast in astronomy.
Update: an announcement from the NASA Swift team was that this completely unexpected signal from Andromeda was a false alert after all.
The signal from an existing X-ray source, added to the normal random extra X-rays from distant objects, pushed the NASA satellite over its threshold and triggered an automatic email to astronomers worldwide. Some excellent reanalysis by the team discovered the mistake within a day.
In science, just because you want a result doesn’t mean nature will oblige, but one thing is for sure – with a cosmic explosion once a day it’s only a matter of time before we get to see the most violent event in the universe close up.