When nothing means something

You know how astronomy works, you look up at something with a telescope and “oh look”, Jupiter has moons or there’s a 7th planet. But you can also find nothing. One of the great things about science is that a null result is still a result. Hence by looking at your measurements carefully enough, you can actually say something interesting about what you haven’t seen.

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