Tuesday, May 27, 2014

Gamma Ray Burst NOT Detected in Andromeda Galaxy

MAJORLY IMPORTANT UPDATE: The Swift X-ray Telescope team has taken a look at the X-ray data, and has shown that there was a mistake that was made in the analysis. As Robert Rutledge (@rerutled) has been saying on Twitter, this analysis error gave this source an X-ray brightness 300 times higher than what it should have been. Repeat, NO GRB IN M31. Disappointing I know, but it's our job as scientists to propagate correct information, so there it is. I will discuss what went exactly went wrong shortly. My post on the topic is here, and Phil Evan's (vastly superior and more comprehensive) post is here.

At 21:24:27 UT (4:24:27 PM EST), the Swift Gamma Ray Burst Telescope detected a sudden emission of gamma rays (REALLY high energy photons, you know, the ones that turned Bruce Banner into The Hulk) from our nearest galactic neighbor, M31, better known as the Andromeda Galaxy. This is by far the closest gamma ray burst (assuming it's a gamma ray burst, still unconfirmed) Swift has ever seen, and will provide a fantastic opportunity for astronomers to follow up by observing with other telescopes. The Andromeda Galaxy is roughly 2.5 million light years away, which is nearly 40 times closer than the next closest GRB we have detected. Hell, if it wasn't mostly cloudy and thundering tonight, I'd be outside with a telescope as soon as it got dark! Not that I'd be likely to see much yet, especially with a small telescope, but one can dream.

NASA Swift ultraviolet image of the Andromeda Galaxy.
The Swift Telescope is an awesome little piece of work, and probably deserves a blog post of its own, but that's for another time. In short, Swift has a wide-field gamma ray detector that it uses to find gamma ray bursts. Then it slews to point its X-ray and ultraviolet telescopes at the target that was first detected in gamma rays to get observations of the object (whatever it happens to be) at different wavelengths of light.

Gamma ray bursts (GRBs) are a curious type of object (with a fun detection history) that we're only recently coming to understand, but that's probably another cool post for a different time. Long story short, there are two different types of gamma ray bursts we see. Short GRBs spit out gamma rays for less than two seconds, while long GRBs spit out gamma rays for longer than two seconds. While that initially seems like a strangely arbitrary distinction, it turns out that those two populations are actually rather different. (This always seemed weird to me, too, but hey, the universe is under no obligation to make sense to us!)

Theoretically, short and long GRBs have two completely different mechanisms. Long GRBs come from massive stars exploding as particularly large supernovae. Usually, when a star goes supernova, it leaves behind a neutron star that used to be the star's core. However, in some cases (generally with more massive stars) the supernova leaves behind a black hole instead, giving off a burst of gamma rays in the process. Short GRBs come from neutron stars in a binary system merging with (ok, slamming into) one another. This happens when neutron stars that happened to end up in a close binary system lose orbital energy due to gravitational radiation. This effect was measured in 1974 by Hulse and Taylor in a binary pulsar system they discovered with the Arecibo Observatory. They received the Nobel Prize in Physics for 1993 for their experimental confirmation of the existence of gravitational radiation (a prediction of Einstein's General Relativity).

At the moment, most astronomers are expecting the event Swift has detected in the Andromeda Galaxy to be a gamma ray burst. However, the possibility of the source being an Ultraluminous X-ray Source (how's that for a creative name?) is being thrown around on Twitter. Steinn Sigur├░sson (@steinly0) has pointed out that the object could be a 50 solar mass black hole gobbling up a lot of matter really freaking fast if the radiation is beamed. This isn't a particularly far-fetched idea, as accreting black holes generally (probably) do emit radiation in polar beams.

To me a bit more specific, the source of the event is currently a globular cluster associated with the Andromeda Galaxy (Robert Rutledge @rerutled reports >99.9% certainty of the location). Globular clusters are a collection of closely-bound mostly old, small stars that can be found in galactic halos. The Milky Way actually has a number of its own globular clusters and they make for some really pretty pictures! Because globular clusters don't have any more big stars (they died out billions of years ago), there are no stars within the clusters that can go supernova, so we're expecting this particular event to be a short GRB.

What many of us are waiting for now is the potential detection of neutrinos from the GRB. If the source truly is a GRB, then we expect that neutrino detectors such as IceCube should be able to detect them.

I will continue to update this post as more information becomes available.

Edit 1: Goddard Space Flight Center's Internet is back online (it was taken out by a storm), which means we can finally take a look at the raw data taken by Swift. The spectra haven't been processed yet, but here's the raw gamma ray light curve from Swift's Burst Alert Telescope.
Raw gamma ray light curve from the Swift BAT. Image courtesy of NASA Goddard Space Flight Center: http://gcn.gsfc.nasa.gov/

Edit #2: Sounds like a number of telescopes are going to be busy observing Andromeda tonight (rumor has it that the VLA will be among them), and that a gravity wave detector may have been online during the event. The next few days are going to be REALLY interesting for astronomy. I'll update this post with other observations and the reduced data from Swift when it becomes available. Green Bank will be taking a look as well, according to their Twitter feed.

Also, my post received a shout-out from Mika McKinnon on her far more comprehensive post over at io9. Definitely give it a look!

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