Ntutrino Emission urom Binary Ntutron btar Mtrgtrs

8.2.1 Prompt ans Exttnsts Emissions: Within Hunsrtss ou btronss

During the coalescence of two neutron stars or a black hole and a neutron star, part of the mater composing the neutron stars is ejected. Neutrino emissions can be expected from this mechanism as explained in the following, it will be qualiied asprompt emission. he neutrino emission can even occur from the inside of the object before any electromagnetic emission. In contrast, we will callextended emissionan emission on a longer time-scale of few hundreds of seconds. his extended emission is supposedly due to the luctuations of the outlow caused by the fall-back of the ejecta on the central engine, this results in a lower Lorentz factorα[114].

In more details, the high spin of the central engine induces very strong electromagnetic ields that collimate the ejected mater into a bipolar relativistic jet as can be seen in igureα8.4α[115].

his outlow of mater and energy is unsteady and causes internal shocks in the jet. Indeed, the mater ejected with a high Lorentz factor encounters layers of mater ejected earlier with a lower Lorentz factor, the relative motion of these two layers leads to a relativistic shock front.

As a consequence,α[114] propose that electrons and protons of the jet experience Fermi acceleration and that -rays are emited through synchrotron or inverse-Compton radiations generating a -ray burst. In the mean time, accelerated protons interact with photons or non-relativistic protons to produce charged pions and kaons which decay into muons and neutrinos.

he resulting muons decay also, into neutrinos and electrons:

� / → �⁰ � �^� � �⁻ � …

↓ ↓ ↓

� �^�� �_� �⁻� ̄�_�

↓ ↓

�� � � ̄�_� ⁻� ̄� � �_�

More details are given in sectionα1.4. As a consequence, two muon neutrinos are produced for one electron neutrino. However, we can assume equal luence at Earth in all lavour because of neutrino lavour oscillation as explained in sectionα4.3.2.

he high-energy neutrino lux resulting from these processes has been computed inα[114].

hey described the photon density in the jet by a broken power-law function. For cosmic rays, the canonical E l power-law spectrum has been used. hen, the spectrum of neutrinos pro-duced through the proton-proton or proton-gamma interactions described above is computed.

Previous short -ray bursts observations are used to estimate typical values of the physical quan-tities like the magnetic ield or the isotropic equivalent luminosity for each emission process.

Figureα8.5represents the resulting prompt and extended neutrino emission expected. he ex-tended emission is the largest because its lower Lorentz factor results in interactions closer to the central engine where the photon density is larger, therefore the meson production eiciency is higherα[116]. Two versions of the extended emissions are modelled with diferent assumptions on the parameter values, an optimistic and a moderate version.

A conservative time window in which the prompt neutrino emission from a typical -ray burst is expected has been estimated inα[117]. his time window is of [- 350 s, ⁻ 150 s] around the burst, considering possible a prompt emission since the central engine is active until the end of the -ray burst. his window has been extended to k 500 s in order to account for the extended emission and to be more conservative. It has been used in most of the previous searches for neutrino counterparts from gravitational wave events. A time window of k 1 h looking for even more extended emissions has been used as well.

8.2.2 Latt Emission: Attr Days

It has been predicted inα[112] that a high-energy neutrino emission, peaking days ater the merger can be expected if a long-lived millisecond magnetar results from the merger.

Depending on the signal to noise ratio, gravitational wave informations could probe the presence of such a magnetar for tenths of seconds following the merger. However gravitational wave data would not inform us on a potential collapse of the remnant into a black hole while

Figurt 8.4 – bnapshots ou tht rtst-mass stn-sity at stltrtts timts ou a binary ntutron star mtrgtr. he green arrows in the botom panel indicate plasma velocity and the white lin-es show the �-ield structure. he colorscale represents the density ⁸in log scale⁹ normalized to its initial maximum value of 5.9 · ��¹⁴ × (�.6�5�_Sun/�_NS)² g cm m. he time and distance for each snapshot is expressed in natural unit of

� = �.47·��⁻²×(�_NS/�.6�5�_Sun) ms= 4.4�×

(�_NS/�.6�5�_Sun) km. More details can be found inα[115].

Figurt 8.5–Ntutrino lutnrts urom txttnsts tmission (EE)for the moderate and optimistic models as well as prompt emission from a short -ray burst seen on-axis at a distance of 300 Mpc. Adapted fromα[114].

Figurt 8.6–Ntutrino lutnrt urom a stablt millistrons magnttaron time-scales from an hour to a year. he iducial magnetar model assumes an initial spin period of 1 ms, a surface dipole magnetic ield of 10s  G, an ejecta mass of 0.01 �_Sunand a source distance of 10 Mpc. Adapted fromα[112].

a neutrino detection with a characteristic light curve would give evidence of the presence of a long-lived neutron star remnantα[112]. X-rays could also point out the presence of a magnetar.

he mater ejected by the magnetar takes the form of a powerful magnetized wind thanks to the strong electromagnetic ields powered by the enormous rotational energy of the magnetar.

In the hours to days following the coalescence, these relativistic winds inlate a magnetized neb-ula in which particles can be accelerated to ultra-high energies. hree processes are considered:

thesurf-ridingof a particle on the magnetized wind, the magnetic reconnection process in the equatorial layer and later the Fermi acceleration at the termination shock.

At early times, synchrotron cooling of the protons is very important because of the strong magnetic ields, it suppresses neutrino production. It is ater roughly a day that high-energy neutrinos are produced eiciently as interactions with thermal photons comes to dominate in the nebula. he emission peaks ∼4αdays ater the merger with an energy of ∼10s  eV. Ater a week or so, cosmic rays escape the source without secondary production as the thermal photon density decreases suiciently. For this reason, we searched for late neutrino emissions in a time-window of 14αdays following the merger. he predicted neutrino luence can be seen in igureα8.6for diferent time windows ater the merger.

hese prompt, extended and late neutrino emissions have been searched for by A , I C and the Pierre Auger Observatory for the GW170817 event presented in chapterα9.