Gravitational Waves & High Energy Neutrinos

Many of the violent phenomena observed in our Universe are potential emitters of gravitational waves and high energy neutrinos. Both these cosmic messengers can travel unimpeded over great distances, carrying information from unseen regions of our Universe. The challenge is to jointly analyze data of these different probes and check the hypothesis of a common origin : this is the GWHEN project - Gravitational Waves & High Energy Neutrinos coincidences, which gathers physicists from the Virgo/LIGO Scientific Collaboration, from the ANTARES Neutrino Telescope, and the IceCube Collaboration, from Europe and the USA. The feasibility of the project was studied between 2006 and 2008, and the actual collaboration between ANTARES, LIGO and Virgo started in 2009.

Since the discovery of high energy neutrinos of cosmic origin by IceCube in 2013, their exact origin is still unknown. It is one of the key questions in cosmic ray physics, as 98% of cosmic rays are of hadronic origin (protons, nuclei) and as neutrinos are expected to be produced in hadronic processes : identifying the sources of cosmic neutrinos would point directly to the production sites of hadronic cosmic rays.

After the announcement of the first detection of Gravitational Waves by LIGO in early 2016, another key question in high energy astrophysics is to understand the processes of jets formation and particle acceleration. The environment of compact objects, their formation through the merger of black holes or neutron stars or through the collapse of massive stars,  and their subsequent evolution, leads to gravitational wave emissions and potentially high energy neutrinos. The objective of GWHEN is to identify and study those common sources, which could be only visible through their gravitational and neutrino signals, because of the opacity of the sources to electromagnetic radiations.

The different detectors involved in the joint search : the 2 LIGO detectors in the US, Virgo in Italy (colored crosses), and the ANTARES Neutrino Telescope in the mediterranean sea (+ IceCube and the KM3NeT telescopes under construction, blue banners), together with the joint data taking periods and the status of the searches, as of september 2016.

Gravitational waves, predicted by Albert Einstein in 1916, are ripples in the space-time metric which are believed to propagate as a wave at the speed of light. These waves warp spacetime, changing the effective distance between nearby points in a characteristic pattern. Scientists attempt to detect gravitational waves using instruments called interferometers that bounce laser beams along two perpendicular arms. Measuring the interference between the beams allows to sense tiny variations in the arm lengths that may be caused by gravitational waves. LIGO is a network of three such instruments in the USA; one in Livingston, LA (4 km arm length) and two in Hanford, WA (4 km and 2 km arm lengths in 2007). Virgo is a 3 km detector located at the European Gravitational Observatory in Cascina, Italy.

The Virgo interferometer (left), LIGO Livingston interferometer (center), ANTARES neutrino telescope (right)

Neutrinos, on the other hand, are common yet enigmatic particles. They are stable, almost massless, and carry no electric charge, interacting with other particles through the weak force. The ANTARES collaboration has built an underwater neutrino telescope at a depth of 2475 m in the Mediterranean Sea to detect high-energy cosmic neutrinos using a three-dimensional array of roughly 900 light detectors (photomultipliers) distributed along 12 lines. Unlike conventional telescopes, ANTARES looks downward, using the Earth to act as a shield, or filter, against all particles except neutrinos (which can easily pass through the Earth). A small fraction of the neutrinos passing upwards through the Earth will interact with the rock in the seabed to produce charged particles called muons, moving at nearly the speed of light. As these muons move through the water, they produce a flash of light called Cherenkov radiation. The photomultipliers detect this light, and from its arrival times the flight direction of the original neutrino can be estimated.

Detection principle of high energy neutrinos.

Several known astrophysical sources are expected to produce both gravitational waves and high-energy neutrinos.  Soft Gamma Repeaters are X-ray pulsars in our galaxy that exhibit bursts of soft gamma rays ("flares"), which may be associated with star-quakes.  The deformation of the star during the outburst could produce gravitational waves, while neutrinos could emerge from the flares.  On the extragalactic scale, the most promising sources are gamma-ray bursts (GRBs), which are known to be very energetic. The most popular models for GRBs involve either the collapse of a rapidly rotating massive star or the merger of a binary system of compact objects (two neutron stars, or a neutron star and a black hole). In both scenarios, "jets" of particles moving close to the speed of light are produced that give rise to the observed gamma-ray burst.  The presence of protons or other hadrons in the jets would ensure the production of high-energy neutrinos, while gravitational waves would be produced by the binary merger or by any of several plausible mechanisms in the collapsing star scenario.

Artist's impression of a a magnetar (soft gamma-repeater, left), a GRB jet (center), a binary merger (right)

The first analysis combined data from ANTARES, LIGO, and Virgo from 2007 to search for gravitational waves coincident with neutrinos. ANTARES data were used to determine the arrival time and direction of candidate high-energy neutrino events. The LIGO-Virgo data were then scanned for a gravitational wave around the time of each putative neutrino. The ANTARES Collaboration has selected 216 potential neutrino events (this number being compatible with the expected background induced by cosmic ray interactions with the atmosphere). The neutrino track is reconstructed by using the time and charge of the hits on the photomultipliers. The subsequent LIGO-Virgo analysis exploits our knowledge of the time and possible directions of the neutrino event to improve the search sensitivity, allowing the detection of weaker gravitational-wave signals than would be possible without the neutrino information. We found no coincidences between a gravitational signal and a neutrino candidate. In addition, a statistical analysis of the gravitational-wave data for all neutrino candidates together showed no evidence for a weak collective signal. That means that if any any of the neutrino candidates came from the astrophysical sources being considered, they must have been too far away for the gravitational waves to be detectable. Simulations based on models of the maximum expected gravitational-wave emission from collapsing stars indicate that such sources would have to have been at least ~10 megaparsecs (33 million light years) away from the Earth.

More information about this first joint search here.

The LIGO detectors have been recently upgraded, allowing for the first detection of gravitational waves, announced in Feb. 2016. Two coalescence of binary black holes have been confirmed (GW150914 and GW151226), a third one being too weak to be confirmed (LVT151012). Within the GWHEN collaboration, high energy neutrinos from all 3 signals were searched for, using data of ANTARES and IceCube. The results of the search for the first signal, GW150914, have been published in Feb. 2016. The results related to the two other signals will be published by end 2016.

Parallely, a search for coincidence between low amplitude (below detection threshold) GW signals and high energy neutrinos is being finalized using all the data taken during LIGO's first observation run O1 between september 2015 and january 2016. The preparation of a similar analysis with data taken during the O2 run, due to start by end 2016, with Virgo joining in 2017, is under way.