Follow-up of Binary Neutron Star Merger GW170817 in neutrinos by members of IGFAE

Follow-up of Binary Neutron Star Merger GW170817 in neutrinos by members of IGFAE

On October 16, 2017, the LIGO and Virgo collaborations announced publicly the detection of gravitational waves (GW) from the merger of two neutron stars in a binary system (BNS). The merger had occurred on August 17, 2017 at 14h41m04 s local time in Santiago de Compostela. The same collision was also seen 1.7 s later in gamma-rays by the Fermi and INTEGRAL satellites as a result of a short duration (< 2 s) gamma-ray burst (GRB) and, subsequently, across the electromagnetic spectrum, with radio, optical, and X-ray detections by a plethora of observatories across the globe. These observations made it possible for the first time to pinpoint the source location of a gravitational wave event in the Southern terrestrial hemisphere at equatorial declination -23o22’53” and right ascension 13h09m48s in the outskirts of the galaxy NGC 4993 at approximately 40 Mpc distance from the Earth. The event, dubbed GW170817, marks the beginning of a new era in Astronomy and Astrophysics since it is the first source of GW seen also in electromagnetic radiation. The remarkable observation led to a series of almost 80 papers, many of them published in Science, Nature, Physical Review Letters and The Astrophysical Journal (see http://kilonovae.org), including a summary of all observations performed (http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9/meta)

In a joint and unprecedented effort by the neutrino observatories ANTARES and IceCube, the Pierre Auger cosmic-ray and neutrino observatory along with LIGO and Virgo GW detectors, scientists have searched for neutrino emission from this merger. The detection of a GRB (the closest one with known redshift ever observed) indicates particle acceleration and high-energy emission which is currently not well understood from compact objects. The observation of high-energy neutrinos in spatial and temporal coincidence with the observed BNS would reveal acceleration of cosmic rays in the source as well as the dissipation mechanism in the relativistic outflows (jets) known to be produced in GRBs.

Remarkably, at the moment of the merger the source was in an almost ideal position in the sky for neutrino detection with the surface detector array of the Pierre Auger Observatory, while it was in a less optimal position for IceCube and ANTARES (see Figure 1 in https://arxiv.org/abs/1710.05839). The search looked for high-energy neutrinos in the energy band from 1011 eV up to 1020 eV. The three detectors complement each other in the energy bands in which they are most sensitive. No candidate neutrino events were found in directional coincidence with the position of the source within +/- 500 s of the onset of the BNS merger, nor in the subsequent 14 days. The non detection allowed us to put limits on neutrino production in BNS for the first time. 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, which is most likely the case for the detected GRB. These results have been submitted to The Astrophysical Journal and are available in the arXiv preprint server (https://arxiv.org/abs/1710.05839).

The Astroparticle Physics group at IGFAE led by Prof. Enrique Zas proposed in the 90’s to use the Surface Detector of the Pierre Auger Observatory to search for ultra-high energy neutrinos, typically above 1017 eV. The search for UHE neutrinos from GW170817 was a collaborative effort of the Auger groups at IGFAE and at Univ. of Wuppertal, Germany, with the invaluable support of the Pierre Auger Collaboration. “We were fortunate that this event was in such an optimal position…” says Jaime Alvarez-Muñiz who coordinates the work in Auger devoted to the search for ultra-high neutrinos in Auger, “…and we were prepared to respond to the alert issued by LIGO/Virgo on August 17 and search for a neutrino counterpart”, points out Francisco Pedreira who actually performed the search in Auger data.

The Astroparticle Physics group at IGFAE is composed of five senior members (Jaime Alvarez-Muñiz, Gonzalo Parente, Inés Valiño, Ricardo Vázquez and Enrique Zas) as well as several PhD students (Aida López Casado, Francisco Pedreira and Guillermo Torralba). The group participates in the Pierre Auger Observatory since 2002 when Spain became a full member of the Pierre Auger Collaboration consisting of more than 400 scientists from 20 countries.

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