On the 3rd of November 2005 a gamma-ray burst (GRB) was detected in the constellation of Ursa Major. Further examination found that there was a well-known galaxy in the vicinity, M81- Bode’s Galaxy. Could this violent event have come from one of amateur astronomy’s favourite objects?

M81, pretty aint it. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

During the latter half of the 20th century, astronomy moved away from being purely based on optical light to a wider range of wavelengths across the electromagnetic spectrum. From radio to submillimetre, infrared, UV, X-ray and gamma-ray, astronomers now have a vast array of tools for studying the visible universe. There are however other sources of information that come from astronomical sources.

Gravitational waves were first predicted by Einstein. While they haven’t been directly observed, their emission has been inferred from the orbit of a pair of neutron stars. Gravitational waves subtly stretch and compress spacetime. Hence to detect them you have to very accurately measure stretches and compressions. This is done at labs like LIGO where they measure this stretching over long distances (several miles). Such long distances are needed as the effect of gravitational waves is fractional. Hence the bigger the distance over which you measure the stretching, the bigger the stretch.

The gamma-ray burst in the vicinity of M81 was what is known as a short duration burst. While long duration bursts are the product of exploding massive stars, most short bursts are though to be formed when two compact objects (neutron stars or black holes) slam together after spiralling in due to energy lost by gravitational wave emission. However there is also another possible cause, a massive flare from a magnetar, a neutron star with an extremely high magnetic field.

One of the huge LIGO experiments. These measure the stretching thought to be caused by gravitational waves over several miles. Credit: IGO, Caltech, NSF

To investigate this, a team from LIGO searched through their data for a signal that could come from either a magentar or colliding compact objects. They found nothing.

But nothing can be interesting. After going back and looking at their measurement errors they were able to set upper limits on the flux of gravitational waves received from this gamma-ray burst. Consequently, by examining the flux they would expect to receive from merging compact objects they were able to set lower limits on the distance this burst was from Earth. Based on these limits they excluded a black hole – neutron star merger in M81 as the source of this GRB to at least 93% confidence. The constraint on a neutron star – neutron star merger was slightly weaker, but would require the event to have a very weakly beamed jet (and GRBs are known to almost always have tight, collimated jets). Based on a fairly generously unbeamed jet the LIGO results (seeing nothing) exclude an black hole – neutron star merger in M81 to greater than 99% confidence and a neutron star – neutron star merger to over 98% confidence. However the expected gravitational wave flux from an erupting magnetar is too low to be detected at the Earth – M81 distance so the results don’t rule that out.

So what was the cause of the bright flash of gamma-rays seen in Ursa Major seven years ago? Dunno, but seeing no gravitational wave signals tells us that it’s highly unlikely to be two massive compact objects slamming together in one of the sky’s prettiest galaxies.

The LIGO Scientific Collaboration, J. Abadie, B. P. Abbott, T. D. Abbott, R. et al. (2012). Implications For The Origin Of GRB 051103 From LIGO Observations Preprint arXiv: 1201.4413v1

The Solar System is a crowded place. Besides the eight planets and numerous dwarf planets there are millions of asteroids. Some of these have orbits that bring them close to the Earth. While most of these whizz by us, some are in orbits which mean that they can gravitationally interact with the Earth and the Moon and go in to orbit around it. These orbits are not stable and the objects will eventually be kicked out of the Earth-Moon system.

To date only one known object has been discovered to have undergone such a process. Known as 2006_RH120 it is a small body, only 3-5m across. In 2007-2008 it undertook four orbits of the Earth at a distance more than twice as far away as the Moon. But how often do objects like this perform their temporary dance with the Earth? Well a new paper of has been looking in to the rate of capture and when such events happen.

The authors use a simulation of the how asteroids will pass through the Earth-Moon System. They select a series of objects with orbital elements in the range where they could possibly be captured and then examine how they would be affected by coming close to the Earth and Moon. Previously it was thought that a close encounter with the Moon gave objects a gravitational tug allowing them to be captured by the Earth. However the new model finds that while the Moon does play a role in the capture, none of their simulated near-Earth objects came close enough to the Moon to get a sufficient enough tug for capture.

The model also found that capture most likely at aphelion and perihelion (when the Earth is furthest and closest to the Sun during its orbit). The same capture probability peaks were previously noted for temporary satellites of Jupiter. It’s also possible that the Moon itself could capture asteroids and get its own temporary satellites. However no objects in the simulation managed to complete an orbit of the Moon.

Objects in unstable orbits around the Earth will of course have the possibility entering the atmosphere and becoming meteors. About 1% of objects in the simulation impacted on the Earth, none on the moon. This means that a temporarily captured object is 3.5 times more likely to strike the Earth than an near-Earth object in a similar orbit. In total the authors estimate that a tenth of one percent of objects striking the Earth were in temporary orbit around us.

In all the authors estimate based on their model and the fact there aren’t a large population of observable temporary satellites that at any one time there is one object of approximately one metre in size temporarily orbiting the Earth along with potentially other smaller bodies. So the Earth only has one Moon, but it’s not the only natural object orbiting us.

Granvik, M., Vaubaillon, J., & Jedicke, R. (2011). The population of natural Earth satellites Icarus DOI: 10.1016/j.icarus.2011.12.003

Comet Lovejoy, seen here by the SOHO satellite, is on a final plunge towards the Sun. Later today it will get too close (perhaps as little as 140,000 km from the solar surface) and be destroyed by the extreme temperatures. The poor thing never had a chance of reaching its destination before it disintegrated; a ball of dirty ice couldn’t even cope with the heat of my kitchen! Oh, and before you complain that there’s no way I’d find a comet in my house, all I’d have to do is invite sometime-poster-on-this-blog Stuart round and provide him with some liquid nitrogen.

UPDATE: IT SURVIVED!! The Solar Dynamics Observatory saw it fly away:

This is a cross posting with the Astronomy Twitter Journal Club who are going to be discussing this topic on twitter (search for the #astrojc hashtag) this Thursday at 20:10 GMT. If you’re interested please come and join in.

Artist's impression (with festive enhancement) of a supernova explosion and resulting gamma-ray burst. Photo credit: NASA with apologies

The majority of gamma ray bursts are thought to be massive stellar explosions in distant galaxies, but the Christmas event didn’t fit this picture; the initial burst of gamma rays lasted for an unusually long time and, despite the efforts of powerful optical telescopes, no convincing candidate for the host galaxy could be found.

Nearly a year on from the initial observation two groups of astronomers have come up with two different, but equally plausible, explanations for the odd GRB 101225A. The first team suggest that it was caused by something small, like a comet, breaking apart and then falling into a neutron star within our own Milky Way. The alternative theory, put forward by the second team, also involves a neutron star, but in this case it’s merging with a young red giant star in another galaxy.

Unless the host of this gamma ray burst is found, and its distance measured, there’s no easy way to choose between these two options. Hopefully deeper optical data with, for example, a telescope like Hubble will provide the answers and settle this debate.

Thöne CC, de Ugarte Postigo A, Fryer CL, Page KL, Gorosabel J, Aloy MA, Perley DA, Kouveliotou C, Janka HT, Mimica P, Racusin JL, Krimm H, Cummings J, Oates SR, Holland ST, Siegel MH, De Pasquale M, Sonbas E, Im M, Park WK, Kann DA, Guziy S, García LH, Llorente A, Bundy K, Choi C, Jeong H, Korhonen H, Kubànek P, Lim J, Moskvitin A, Muñoz-Darias T, Pak S, & Parrish I (2011). The unusual γ-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33. Nature, 480 (7375), 72-4 PMID: 22129726 (alternative link for the paper here)

Campana S, Lodato G, D’Avanzo P, Panagia N, Rossi EM, Della Valle M, Tagliaferri G, Antonelli LA, Covino S, Ghirlanda G, Ghisellini G, Melandri A, Pian E, Salvaterra R, Cusumano G, D’Elia V, Fugazza D, Palazzi E, Sbarufatti B, & Vergani SD (2011). The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star. Nature, 480 (7375), 69-71 PMID: 22129725 (alternative link for the paper here)

Firstly, a new era of submillimetre astronomy began at the James Clerk Maxwell Telescope in Hawaii with the unveiling of the SCUBA-2 camera. This large instrument is the successor to SCUBA, which I’ve written about here before. Indeed it’s its faster, more sensitivity son, and should hopefully prove an invaluable tool in understanding the dusty Universe. To do this it has to be cooled to within 0.1 degree of absolute zero which the press release confidently states makes it “…colder than anything in the Universe that we know of”. Except, as was quickly pointed out on twitter, the Planck satellite which also has to operate at these chilly temperature.

The 4.5 tonne SCUBA-2 instrument mounted on the James Clerk Maxwell Telescope. Photo credit: Joint Astronomy Centre

The next story on my list is an intriguing e-petition calling for astronomy weather reports to be included in the normal weather forecast:

We the undersigned request that The Met Office produces regular Stargazing / Astronomy focused weather information, to be shown as part of the BBC Weather reports. Not only would this be a boon to amateur astronomers, it will also help keep the study of Astronomy prevalent in the public consciousness, which in turn helps encourage the study of science, which should be a boon to the economy.

Onto the Hubble Space Telescope and the publication of the 10,000th refereed scientific paper using its observations. Reaching this milestone really reflects how Hubble has managed to remain a cutting-edge instrument throughout its long life. The paper in question reports the finding of the faintest supernova ever associated with a long duration gamma ray burst – you can read it here. I wonder if the Space Telescope Science Institute will send the authors a prize for their achievement?

I was going to end with this story reporting the discovery of the most massive black holes ever seen. However, I really want to devote a separate post to this (coming soon, hopefully). Instead, here’s one of the most iconic images of our own planet – the blue marble hanging in space – which was taken nearly 40 years ago today. Thanks to Galileo’s Pendulum for the tip.

Our Earth as seen by Apollo 17. Photo credit: NASA

Curiosity will get to the martian surface in a new, untested, way. Instead of parachuting all the way down, it will complete its descent in a more controlled manner via a ‘sky crane’. I was looking for more details on this interesting maneuver and came across this great infographic from space.com explaining the whole process. Fingers crossed everything goes to plan!

At the height of the Cold War the US, concerned about the possibility of covert Soviet nuclear tests sent up a series of satellites to look for tell-tale flashes of radiation. These satellites began seeing something strange, they saw flashes. The Soviet military scientists hadn’t been working overtime, these were signals of astronomical origin. After decades of debate, it was shown that these were caused by the explosion of the most massive stars. Since then a succession of satellites have been flown to study light at the extreme end of the electromagnetic spectrum. One such instrument is the AGILE satellite, an Italian space observatory capable of surveying large chunks of the sky at once. As well as detecting distant Gamma Ray Bursts, it has also mapped events in our own Galaxy, such as the sudden brightening of the Crab Nebula in gamma rays last year. However in a strange completion of the historical circle it has also been detecting signals from the Earth.

Terrestrial Gamma-ray flashes are associated with spectacular lightning events in the upper atmosphere similar to this red Sprite. Credit:NASA

Terrestrial Gamma-ray Flashes (TGFs) are short bursts of gamma-ray radiation. These were first noticed by the Compton satellite and are associated with thunderstorms. Strong updrafts in clouds cause the formation of layers of positive and negative charge. There are typically eased by lightning strikes removing the net charge from one or more layers. However the strong electric fields in the clouds can accelerate electrons to velocities close to the speed of light. When a fast moving electron such as this (or from a source such as a cosmic ray) interacts with another electron it can accelerate it too leading to a run-away growth of fast-moving electrons. These are then diverted by interactions with atomic nuclei in the cloud releasing “braking radiation”. As the electrons are moving so fast, this radiation takes the form of extremely energetic gamma-rays.

As the AGILE satellite passes along its orbit it is capable of detecting TGFs from below. However an orbital inclination of 2.5 degrees limits the area where the satellite can detect these flashes to close to the equator. Happily this is where some of the Earth’s largest thunderstorms happen. From June 2008 to January 2010, AGILE scientists isolated over a hundred TGFs detections. The largest number of detected events came from Africa with others being found over Indonesia and a few over South America. However when it came to associating these events with lightning events they found something striking (excuse the pun). At first glance it appeared that the distributions of Terrestrial Gamma-Ray Flashes and lightning matched rather well. But when the distributions were studied in more detail, it was found that while in South America there was an 87% chance that that TGFs came from a random sub-sampling of lightning, this probability dropped to 3% in Africa. The authors don’t go on to explain this discrepancy, but it is clear there is still something unknown about the mechanisms driving some of the most violent events on the planet.

Source
AGILE Observations of Terrestrial Gamma-Ray Flashes , M. Marisaldi et al., 2011 Fermi Symposium proceedings

This is a cross posting with the Astronomy Twitter Journal Club who are going to be discussing this topic on twitter (search for the #astrojc hashtag) this Thursday at 20:10 GMT. If you’re interested please come and join in.

This ‘data tsunami’ is almost upon us, according to a new paper by G. Bruce Berriman and Steven Groom. The recent addition of large datasets from the Spitzer and WISE telescopes has massively increased queries to the online Infrared Science Archive (IRSA), and, unsurprisingly, slowed down the response time of the database. This is only going to get worse as the archive’s growth is expected to accelerate over the next few years.

The paper also points out that how astronomers use archives is going to change. At the moment, raw datasets are typically downloaded and then reduced on a user’s own computer. However, once data reach peta-byte scales it’s likely that they’ll have to be handled in situ, if only to avoid breaking the internet.

So what can be done? And, more importantly, can we do whatever we’re going to do in as cheap a way as possible? Firstly, we need better ways to search multiple online datasets efficiently – the excellent Virtual Observatory is already developing techniques to help here.

Next, we need to explore new technologies like cloud computing. The Square Kilometre Array (which will generate 10 gigabytes per second) will have theSkyNet, the (worryingly named) community based cloud which will harness the power of volunteers’ computers to process its data.

Finally we need to talk more, especially to IT experts in computer infrastructure, and then share what we’ve learned in the authors’ proposed new journal dedicated to information technology in astronomy. We then need to properly reward the effort people put into this area, as well as giving young astronomers a grounding in software engineering to better prepare them for this data-heavy future.

If we do all that then, the authors’ suggest, we’ll be able to survive the coming data flood. Fingers crossed.

G. Bruce Berriman, & Steven L. Groom (2011). How Will Astronomy Archives Survive The Data Tsunami? ACM Queue arXiv: 1111.0075v1