The truth about that mystery ‘gamma ray burst’ in M31
I awoke today to find the astronomical end of Twitter going into meltdown over an event detected overnight. Apparently, a NASA satellite called Swift, which spots gamma ray bursts in the farthest reaches of space, had detected a blast in the Andromeda Galaxy, M31.
No wonder this sounded exciting. Gamma ray bursts, or GRBs, are generally the biggest explosions in the Universe, and are thought to be caused by the collision of neutron stars or black holes.
But the event that NASA’s X-ray telescope had detected and swung round to observe was a heck of a lot closer because M31 is the closest major galaxy to our own Milky Way at a distance of only 2.5 million light-years.
The observation understandably provoked much excitement on Twitter and speculation as to what it might be. News reports were rushed onto online science sites including NBC News, io9, Phys.org and The Conversation which called it “a titanic eruption in our neighbouring galaxy” and added that “its implications could be huge”.
Unfortunately, it turned out to be a false alarm. Swift had simply caught an X-ray source in the galaxy that had previously been discovered and observed and which was certainly not undergoing any sort of outburst.
Dr Phil Evans, a professional astronomer in the UK who spends his life working with and responding to alerts from Swift, spoke to Skymania about what happened. And he noted that the original alert had never claimed the observation was of a GRB.
Phil, of the X-ray and Observational Astronomy Group at the University of Leicester, told us: “I’m sorry to burst the bubble, but this is not exciting at all! The “result” has been completely blown out of proportion by the Twitterati, unfortunately, and in fact, was based on a preliminary analysis which had only limited data. Regrettably, this was not made very clear in the initial circular.
“A reanalysis of the data this morning by myself and a colleague (Kim Page)—who were not online for the original trigger—has just been released.
“Basically, the X-ray object in question is NOT in outburst, it has been observed previously by Swift (the results are in a catalogue which I produced ;) ) and is still at the same brightness as it always has been. The initial trigger was part of an interesting programme to let the Swift Burst Alert Telescope respond to low-significance triggers (i.e. things that may well be noise) if they are in a nearby galaxy, to increase our chances of detecting a faint, nearby GRB.
“Normally when this happens, we find no X-ray source and we realise that the trigger was spurious. In this case, because the trigger was in M31 (which is full of X-ray sources), we found an X-ray source, and a slightly over enthusiastic analysis mistakenly claimed it was in outburst. Once I saw this, and analysed the data that had (for technical reasons) not been available promptly, it was obvious that the source was not in outburst, and my colleague drafted a circular with my analysis results.
“I caution however that this is still preliminary: we only have the limited prompt data available to go on. We are awaiting the full dataset to confirm the results, however (Sod’s law in force) there was a power-cut at the Goddard Space Flight Center in the US last night—Swift data get ingested and circulated from here—so we are having to wait longer than normal to get the full dataset!”
Phil, who has since written his own blog post on the event, added: “While, in this case, this was a spurious result, it highlights the difficulties facing astronomers working in time-domain physics. Swift triggers on something; something faint. An X-ray source is found, and it’s a known one. Is it in outburst? The data are only partially there, but indicate that it may be. Someone says, “Yes it is”. There is no time to lose—*if* this is real, then it may not last long, people *have* to respond. This is the correct thing to do. The last thing one wants to do is miss a once in a century event through excess caution. And yet, if we wait until we’re sure about it, we will miss it.
“Unfortunately in this case, the preliminary analysis was misleading. Even more unfortunately, the Twittersphere has gone mad, and done so in a rather misinformed way: Swift never claimed that this was a GRB (in fact, it was speculated that it was an X-ray binary), but alas in the public eye this has gone out as a “GRB in M31” which it never was, and now definitely isn’t.”
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