Steer clear of buses, Dr Higgs

A flurry of leaks and rumors about statistical fluctuations that may (or may not) have been observed by CERN's Atlas experiment has rather obscured the more significant particle physics news: the astonishing progress of the Large Hadron Colliders' luminosity ramp this year, which is well ahead of schedule. Recent physics runs have been amassing upward of 20 / pb of data (a measure of the number of particle collisions), with the total collected so far approaching 0.3 / fb. Unless they break the machine again, it seems likely that the official target for the year - 1 / fb - might be exceeded by a factor of 5 or more.

What does this mean? Most obviously, it means that by the end of the year (or soon after, when the data analysis is reported) we ought to know a lot more about the existence, or otherwise, of the Higgs Boson. More precisely, as the figure below from the Atlas collaboration shows, 5 / fb of data would serve to robustly exclude the existence of the Higgs across the entire range of masses where it is thought to be lurking. The green line shows the expected utility of the experiment to rule out the existence of hypothetical Higgs particles of different masses with this much data, with the exclusion being robust where the curve drops below 1 on the vertical axis.

So if the Standard Model Higgs does not exist - which paradoxically might be more interesting than its existence, though perhaps not to Peter Higgs - we'll know by the end of the year. If it does exist, then we'll also likely have firm hints, initially perhaps from ATLAS and CMS failing to do as well "as expected" at ruling out Higgs' of some given mass. An actual discovery might still take a little longer - there's some expectation that the Higgs might have a mass close to the lower limit of 115 GeV, which turns out to be the hardest to detect.

Scientifically, the existence of the Higgs is of only peripheral interest astrophysically (though sociologically I think some cosmologists already fear that their subject may become as frustratingly "successful" as particle physics, where a decade or more of experiments have failed to uncover much that is truly new). Of more direct relevance is the fact that the bounty of 2011 LHC data should also strongly constrain the existence of many types of hypothetical supersymmetric particles, which have long been leading candidates for making up the Universe's dark matter. Strictly speaking, failing to find dark matter at the LHC, or underground, would mean rather little - since while it would serve to exclude some popular models there is no shortage of other candidate dark matter particles that could escape direct detection for now. Still, it would be tremendously comforting to be able to finally root the mysterious dark matter in known physics, while failure to do so would leave a nagging worry that perhaps we have the dark matter / dark energy story more fundamentally wrong than we think.

Swift sees a gamma-ray burst that isn't a Gamma-Ray Burst

Time domain astrophysics - the systematic cataloging and study of transient and variable sources - is one of the most fashionable topics in modern astronomy. It's the motivation for a bunch of new telescopes that astronomers would like to build over the next decade, including the one most likely to actually happen, the Large Synoptic Survey Telescope. At first glance, this sudden enthusiasm for all things variable is a bit weird, as plenty of quintessentially transient objects - X-ray binaries, novae, supernovae... - have been studied for decades, if not millenia! When one looks more closely, though, one finds that our systematic knowledge of the transient sky is pretty woeful. To take just one example, there are millions of Active Galactic Nuclei out there, yet the number with well-sampled light curves is small enough that experts know each of them as close friends. And if we don't even understand the known variable sources all that well, who's to say that entirely new classes of rare transients might not be flashing away in the sky, unseen by anyone?

Such a discovery of a new type of variable source is exactly what's claimed in a recent preprint by Levan et al. ("An extremely luminous panchromatic outburst from the nucleus of a distant galaxy"). The outburst in question was detected in the gamma-rays by NASA's Swift satellite on 28th March, and in the month since the authors have marshalled follow-up observations in the optical, infra-red, X-ray and radio wavebands. They've been busy! The fruits of all this effort are summarized in the light curve plotted in the paper:

What we have here is already uncommon: an X-ray / gamma-ray source (the upper panel) that appears from nowhere and stays bright for (at least) a month, accompanied by infra-red emission (middle) and slowly rising radio flux (lower). But were this the only data in hand it wouldn't be unique - outbursts not too dissimilar to this are in fact quite characteristic of stellar mass black holes in close binary systems. The kicker is that both the Hubble image, and the VLBI radio position, pretty convincingly place the transient source at the very center of an otherwise unremarkable but moderately distant galaxy (z=0.35). It's not then an X-ray binary, and although the energetics on their own don't exclude it from being a stellar collapse (of the type thought to generate long Gamma-Ray Bursts) the duration of the high energy emission is much longer than any GRB previously observed. It does seem to be something new and different.

What then is it? In a companion paper, a bunch of the same authors offer an interpretation (I did say they'd been busy - remember this event was only discovered a month ago!). They suggest that what we're seeing is a relativistic jet produced following the tidal disruption of a star by a supermassive black hole in the nucleus of the galaxy in question. The idea is that a star, in the course of its random wanderings within the galaxy, is unlucky enough to stray too close to the black hole at its center. The tidal gravity of the black hole rips apart the star, forming a dense disk of gas that rapidly spirals in to be accreted by the black hole. As it does so, a small fraction of the mass is ejected at close to the speed of light as a jet, which just so happens to be aimed almost exactly in our direction. We see this as a bright transient not because the total energy is enormous (though it must still be large), but rather because it's been fired our way.

Is this interpretation correct? It's certainly plausible, though some of the plausibility comes from the fact that other obvious ideas seem even less likely! That the emission is beamed in our direction (the part that probably seems most outlandish) is almost inescapable: if it were not beamed the inferred luminosity in high-energy radiation would be ~10^{48} erg / s, and it's really hard to see how even a supermassive black hole in a relatively mundane galaxy could generate such an output. The theory of how a star can be shredded by a massive black hole is also well-established, though therein lies a slight problem, since the theory suggests that tidal disruptions should not normally look like this. The theoretical expectation, at least partially borne out by observations, is that the hot gas spiraling into the black hole ought to be bright in the optical and ultra-violet - exactly those wavelengths where no transient emission is seen in this case (see the grey points in the middle panel of the graph)! It's not unreasonable to assume that the intrinsic emission in the optical is simply hidden from us - a modest amount of dust along the line of sight would do the job perfectly well - but the lack of the classic tidal disruption flare signature is at least unfortunate.

What other clues are there? The volume of the Universe out to a redshift z=0.35 is about 10 Gpc^3, so if we assume a local number density of galaxies of 0.01 per cubic Mpc there are very roughly 100 million galaxies as close or closer than the suspected host of this event. Swift has been monitoring the gamma-ray sky for several years, and has seen one event so far, so by any accounting this type of event is rare: let's say of the order of 10^{-8} events per galaxy per year. Tidal disruptions are thought to be many orders of magnitude more common than that, so even if only a small fraction produce jets, and only a small fraction of those are pointed straight at us, the numbers could work out. We could also propose a new class of ultra-long GRBs (though then the association with the nucleus of the galaxy would be a coincidence), or that we're witnessing the sudden onset of ordinary AGN activity together with a blazar-like relativistic jet. The most exciting possibility of all - that this might be the first sighting of the merger of two supermassive black holes - doesn't pan out alas, as those events should be rarer still and we'd need to be very lucky to see one so close.

With only one example, there's an obvious limit to how confident one should be about any interpretation, so the key will be finding more. Now that we have a template, perhaps it may be possible to modify the Swift trigger routines to find more in gamma-rays? If not - or if Swift is turned off before another candidate emerges - then it ought to be possible to search for the signatures of relativistic jets among the more mundane tidal disruption events that should be discovered in large numbers by optical or infra-red surveys. In principle, that might tell us something new about the conditions needed for accreting black holes to produce powerful jets.