Last night, the science social media sphere exploded with the news of a potential … something … in our nearest cosmic neighbor, M31. The Swift mission, which I am privileged to work for, reported the discovery of a potential bright X-ray transient in M31, a sign of a high-energy event. For a while, we had very little to go on — Goddard had an unfortunately timed power outage. Some thought (and some blogs actually reported) that we’d seen a truly extraordinary event — perhaps even a nearby gamma-ray burst. But it turned out to be something more mundane. My friend and colleague Phil Evans has a great explanation:
It started with the Burst Alert Telescope, or BAT, on board Swift. This is designed to look for GRBs, but will ‘trigger’ on any burst of high-energy radiation that comes from an area of the sky not known to emit such rays. But working out if you’ve had such a burst is not straightforward, because of noise in the detector, background radiation etc. So Swift normally only triggers if it’s really sure the burst of radiation is real; for the statisticians among you, we have a 6.5-σ threshold. Even then, we occasionally get false alarms. But we also have a program to try to spot faint GRBs in nearby galaxies. For this we accept lower significance triggers from BAT if they are near a known, nearby galaxy. But these lower significance triggers are much more likely to be spurious. Normally, we can tell that they are spurious because GRBs (almost always) have a glow of X-rays detectable for some time after the initial burst, an ‘afterglow’. The spurious triggers don’t have this, of course.
In this case, it was a bit more complicated There was an X-ray source consistent with the BAT position. The image to the right shows the early X-ray data. The yellow circle shows the BAT error box – that is, the BAT told us it thought it had seen something in that circle. The orange box shows what the XRT could see at the time, and they grey dots are detected X-rays. The little red circle marks where the X-ray source is.
Just because the X-ray object was already known about, and was not something likely to go GRB doesn’t mean it’s boring. If the X-ray object was much brighter than normal, then it is almost certainly what triggered the BAT and is scientifically interesting. Any energetic outburst near to Earth is well worth studying. Normally when the Swift X-ray telescope observes a new source, we get various limited data products sent straight to Earth, and normally some software (written by me!) analyses those data. In this case, there was a problem analysing those data products, specifically the product from which we normally estimate the brightness. So the scientists who were online at the time were forced to use rougher data, and from those it looked like the X-ray object was much brighter than normal. And so, of course, that was announced.
The event occurred at about 6:15 EDT last night. I was feeding kids and putting them to bed but got to work on it after a couple of hours. At about 9:30, my wife asked what I was up to and I told her about a potential event in M31, but was cautious. I said something like: “This might be nothing; but if it is real, it would be huge.” I wish I could say I had some prescience about what the later analysis would show, but this was more my natural pessimism. That skeptical part of my mind kept going on about how unlikely a truly amazing event was (see here).
My role would turn out to be a small one. It turned out that Swift had observed the region before. And while Goddard and its HEASARC data archive were down, friend and fellow UVOT team member Caryl Gronwall reminded me that the MAST archive was not. We had not observed the suspect region of M31 in the same filters that Swift uses for its initial observations. But we knew there was a globular cluster near the position of the even and, by coincidence, I had just finished a proposal on M31’s globular clusters. I could see that the archival measures and the new measure were consistent with a typical globular cluster. Then we got a report from the GTC. Their spectrum only showed the globular cluster.
This didn’t disprove the idea of a transient, of course. Many X-ray transients don’t show a signature in the optical and it might not have been the globular cluster anyway. But it did rule out some of the more exotic explanations. Then the other shoe dropped this morning when the XRT team raced to their computers, probably still in their bathrobes. Their more detailed analysis showed that the bright X-ray source was a known source and had not brightened. So … no gamma-ray burst. No explosive event.
I imagine that, from the outside, this looks rather chaotic and disorganised. And the fact that this got publicity across the web and Twitter certainly adds to that! But in fact this highlights the challenges facing professional astronomers. Transient events are, by their nature, well, transient. Some are long lived, but others not. Indeed, this is why Swift exists, to enable us to respond very quickly to the detection of a GRB and gather X-ray, UV and optical data within minutes of the trigger. And Swift is programmed to send what it can of that data straight to the ground (limited bandwidth stops us from sending everything), and to alert the people on duty immediately. The whole reason for this is to allow us to quickly make some statements about the object in question so people can decide whether to observe it with other facilities. This ability has led to many fascinating discoveries, such as the fact that short GRBs are caused by two neutron stars merging, the detection of a supernova shock breaking out of a star and the most distant star even seen by humans, to name just 3. But it’s tough. We have limited data, limited time and need to say something quick, while the object is still bright. People with access to large telescopes need to make a rapid decision, do they sink some of their limited observing time into this object? This is the challenge that we, as time-domain astronomers, face on a daily basis. Most of this is normally hidden from the world at large because of course we only publish and announce the final results from the cases where the correct decisions were made. In this case, thanks to the power of social media, one of those cases where what proved to be the wrong decision has been brought into the public eye. You’ve been given a brief insight into the decisions and challenges we have to face daily. So while it’s a bit embarrassing to have to show you one of the times where we got it wrong, it’s also good to show you the reality of science. For every exciting news-worthy discovery, there’s a lot of hard graft, effort, false alarms, mistakes, excitement and disappointment. It’s what we live off. It’s science.
People sometimes ask me why I get so passionate about issues like global warming or vaccination or evolution. While the political aspects of these issues are debatable, I get aggravated when people slag the science, especially when it is laced with dark implications of “follow the money” or claims that scientists are putting out “theories” without supporting evidence. Skeptics claims, for example, that scientists only support global warming theory or vaccinations because they would not get grant money for claiming otherwise.
It is true: scientists like to get paid, just like everyone else. We don’t do this for free (mostly). But money won’t drag you out of bed at 4 in the morning to discover a monster gamma-ray burst. Money doesn’t keep you up until the wee hours pounding on a keyboard to figure out what you’ve just seen. Money didn’t bounce my Leicester colleagues out of bed at the crack of dawn to figure out what we were seeing. Money doesn’t sustain you through the years of grad school and the many years of soft-money itinerancy. Hell, most scientists could make more money if they left science. One of the best comments I ever read on this was on an old slash-dot forum: “Doing science for the money is like having sex for the exercise.”
What really motivates scientists is the answer. What really motivates them is finding out something that wasn’t known before. I have been fortunate in my life to have experienced that joy of discovery a few times. There have been moments when I realized that I was literally the only person on Earth to know something, even if that something was comparatively trivial, like the properties of a new dwarf galaxy. That’s the thrill. And despite last night’s excitement being in vain, it was still thrilling to hope that we’d seen something amazing. And hell, finding out it was not an amazing event was still thrilling. It’s amazing to watch the corrective mechanisms of the scientific method in action, especially over the time span of a few hours.
Last night, science was asked a question: did something strange happen in M31? By this morning, we had the answer: no. That’s not a bad day for science. That’s a great one.
One final thought: one day, something amazing is going to happen in the Local Universe. Some star will explode, some neutron stars will collide or something we haven’t even imagined will happen. It is inevitable. The question is not whether it will happen. The question is: will we still be looking?