ANTARES/AUGER/IceCube search for neutrino emission from the binary neutron star merger GW170817

[Published Oct. 16th, 2017]

Today, the LIGO and Virgo collaborations have announced the detection of a new gravitational wave event, GW170817, which constitutes the first time that a binary neutron star merger has been detected with the LIGO-Virgo observatories. This unique observation is even more compelling since the same collision was seen by the Fermi and INTEGRAL satellites as a result of a short gamma-ray burst (GRB) and, subsequently, across the electromagnetic spectrum, with radio, optical, and X-ray detections. These observations made it possible for the first time to pinpoint the source location of a gravitational wave event. The source was found to be in a galaxy 130 million light years away, known as NGC 4993. The joint observations are published in a « Multi-Messenger » paper, signed by 50 different collaborations, among which ANTARES.

Figure 1 : Observatories around the world, from space or ground, that have searched for emissions from GW170817. ANTARES is one of the blue point in the Mediterranean Sea [Credit : LIGO-Virgo].

In a joint effort by the ANTARES, IceCube, Pierre Auger, LIGO, and Virgo collaborations, scientists have searched for neutrino emission from this merger. The search looked for neutrinos in the GeV to EeV energy range and did not find any neutrino in directional coincidence with the host galaxy (see figure 2). The nondetection agrees well with our expectation from short GRB models of observations at a large off-axis angle, which is most likely the case for the GRB detected in conjunction with GW170817. These results are being submitted to The Astrophysical Journal and is available here.

Figure 2 : Localizations and sensitive sky areas at the time of the GW event in equatorial coordinates: GW 90% credible- level localization (red contour), direction of NGC4993 (black plus symbol), directions of IceCube’s and Antares’s neutrino candidates within 500 s of the merger (green crosses and blue diamonds, respectively), Antares’s horizon separating down-going (north of horizon) and up-going (south of horizon) neutrino directions (dashed-blue line), and Auger’s fields of view for Earth-skimming (darker blue) and down-going (lighter blue) directions. IceCube’s up-going and down-going directions are on the northern and southern hemispheres, respectively. The zenith angle of the source at the time of the merger was 73.8° for Antares, 66.6° for IceCube, and 91.9° for Auger. [Taken from Albert et al., 2017]

The detection of a gravitational wave event is always exciting news for astrophysicists, but having one also confirmed for the first time by other instruments that detect photons is even more exciting. It marks the beginning of a new era of multimessenger astrophysics. For ANTARES, Auger and IceCube scientists, it is another step toward the time when cosmic events will be detected and studied with neutrinos, light, and gravitational waves together.

The combined gravitational wave and gamma-ray observation of this event tells us that there was particle acceleration by the source, which provides insight into the merger process and also means that very high energy neutrinos might have been produced. The detection of neutrinos would reveal even more information about the merger, including the density and energy of the hadrons involved and the energy dissipation mechanisms.

The search for neutrino emission from this collision of two neutron stars has been conducted in collaboration with both the IceCube and Pierre Auger observatories, which bring improved sensitivities at different energies. At the exact time of the merger, the source could fortuitously be seen from an ideal viewing angle by the Pierre Auger observatory. For IceCube and ANTARES, the direction to the source was in a part of the sky where the detectors are less sensitive, but still capable of making significant observations.

The first study concentrated on a time window within 500 seconds around the detection of GW170817, and yielded no associated neutrino events. Later, scientists expanded the search to include possible emission of high-energy neutrinos up to 14 days after the merger, suggested by theoretical predictions, and also searched for lower energy neutrinos associated with the merger remnant and ejected material. These follow-up studies did not find any significant emission either. The lack of neutrino emission from GW170817 is in good agreement with the observation of a GRB from a high viewing angle, i.e., with the outflow jet not pointing toward Earth.

ANTARES has already searched for neutrino emission from gravitational wave sources, e.g. the Binary Black Holes GW150914, jointly with IceCube, or more recently GW170104. With a growing networks of GW detectors, it is expected that many more neutron star mergers will be discovered in the near future, expanding the capability to study these sources through high energy neutrinos with ANTARES (and the forthcoming KM3NeT telescopes), IceCube and Auger.

Writing  credit : S. Bravo @ IceCube + ANTARES Collaboration