## Neutrino Astrophysics

### Peter Mészáros

The same shocks which the electrons responsible for the non-thermal gamma-rays in GRB should also accelerate protons present in the shockis. Both the internal and the external reverse shocks are mildly relativistic, and are expected to lead to relativistic proton energy spectra of the form E^{-2}. For this, the acceleration time must be shorter than both the radiation or adiabatic loss time and the escape time from the acceleration region. The same constraints on the magnetic field and the bulk Lorentz factor required to obtain efficient electron acceleration and gamma-ray emission at ~1 MeV indicate that the protons reach Lorentz factors ranging up to 10^{11}. This makes them potential cosmic ray sources. The accelerated protons in the GRB jet can interact with the fireball photons, leading to charged pions, muons and neutrinos. This reaction peaks at the energy threshold for the photo-meson Delta resonance. For internal shocks producing observed 1 MeV photons this implies ~ 10^{16} eV protons, and neutrinos with ~ 5 % of that energy, E_nu ~ 10^{14} eV. Above this threshold, the fraction of the proton energy lost to pions is ~ 20% for typical fireball parameters, and the typical spectrum of neutrino energy per decade is flat, E_nu^2 Phi_nu ~ constant (Waxman & Bahcall, 1997). Synchrotron and adiabatic losses limit the muon lifetimes (Rachen & Meszaros 1998), suppressing the neutrino flux above E_nu ~ 10^{16} eV. Another copious source of target photons in the UV is the afterglow reverse shock, for which the resonance condition requires higher energy protons leading to neutrinos of 10^{17}-10^{19} eV (Waxman & Bahcall, 1999). These neutrino fluxes are expected to be detectable above the atmospheric neutrino background with the planned cubic kilometer ICECUBE Cherenkov detector, Penn State being involved in the science and data analysis aspects. IceCube construction is finished (2011), including its Deep Core" array for measuring down to 10-15 GeV energies.

Another mechanism for neutrino production in GRB is inelastic nuclear collisions. Provided the fireball has a substantial neutron/proton ratio, as expected in most GRB progenitors, the inelastic process is most intense when the nuclear scattering time scale becomes comparable to the expansion time scale, at which point the relative velocities of the nuclei become large enough to collide inelastically, resulting in charged pions and ~few GeV neutrinos (Bahcall & Meszaros 2000). Inelastic collisions can also occur in fireball outflows with transverse inhomogeneities in the bulk Lorentz factor (Meszaros & Rees 2000). The typical neutrino energies are in the 10 GeV range , which could be detectable with the IceCube Deep Core array in coincidence with nearby observed GRBs (see also Meszaros & Rees (2011).

The photo-pion and inelastic collisions responsible for the ultra-high energy neutrinos will also lead to neutral pions and electron-positron pair cascades, resulting in GeV to TeV energy photons. A tentative ~ 0.1 TeV detection of a GRB has been reported (Atkins et al 2001) with the water Cherenkov detector Milagrito, protoptype of larger MILAGRO. Other large atmospheric Cherenkov detectors, such as HAWC, as well as space-based large area solid state detectors such as on FERMI are able to measure photons in this energy range, which would be coincident with the neutrino pulses and the usual MeV gamma-ray event. Their detection would provide important constraints on the emission mechanism of GRBs. GeV emission is also a feature of many AGNs, in particular blazars. For jets with high jet Lorentz factors and small inclination to the observer, photons out to ~10 TeV have been measured with ground air Cherenkov telescopes. In such cases, a secondary reprocessed GeV photon halo may be detectable, from inverse Compton scattering on CMB photons by pairs produced in TeV-IR photon-photon interactions (Dai, Zhang, Gou, Meszaros & Waxman, 2002).

A potentially important source of high energy neutrinos in GRB is expected in collapsar models. The core collapse of massive stars resulting in a relativistic jet which breaks through the stellar envelope is a widely discussed scenario for gamma-ray burst production. For very extended or slow rotating stars, the jet may be unable to break through the envelope. Both penetrating and choked jets (Meszaros & Waxman 2001) will produce, by photo-meson interactions of accelerated protons, a burst of ~3-5 TeV neutrinos while propagating in the stellar envelope. The predicted flux, from both penetrating and choked jets, should be easily detectable by planned cubic kilometer neutrino telescopes. The contribution of pp collisions between accelerated jet protons and stellar envelope nucleons gives a more prominent TeV component (Razzaque, Meszaros & Waxman 2003b).

High energy neutrinos may also be produced in magnetars, which are ultra-high magnetic field neutron stars that can accelerate cosmic rays to high energies through the unipolar effect, as well as being copious soft X-ray emitters. Zhang, Dai, Meszaros & Waxman (2002) show that young, fast-rotating magnetars should emit TeV neutrinos through photomeson interactions. An exciting possibility is that the recent giant flare of the Soft Gamma Repeater SGR 1806-20 recently detected (December 28 2005) in gamma-rays by Swift may also produce cosmic rays and neutrinos. Ioka, Razzaque, Kobayashi and Meszaros calculated the TeV neutrino flux expected from this SGR. Proton acceleration and p,gamma interactions would produce signals detectable with AMANDA for a high enough baryon load fireball, and analysis of dat taken by Amanda II are underway. This emission would be associated also with detectable TeV gamma-ray emission.

Long Gamma-Ray Bursts (lasting longer than ~10 s) are also sometimes found associated with a supernova, which is of interest for the X-ray and optical afterglow, as well as for constraining progenitor scenarios. A test of the presence of such a SNR shell preceding the GRB explosion, as in the supranova scenario, would lead to distinctive neutrino spectra in the TeV range (Razzaque, Meszaros and Waxman 2003a). Razzaque, Meszaros and Waxman calculated in detail the neutrino detection prospects from a nearby GRB such as GRB030329 by a km scale detector such which ICECUBE should be able to detect.

Alvarez-Muniz and Meszaros (2004) developed a quantitative model of radio-quiet AGNs as sources of cosmic rays and high energy neutrinos, related to X-ray emission models. These sources, which do not have significant jets, may be ten times more numerous than blazars, and hence may be important ICECUBE candidates.

Razzaque, Meszaros and Waxman (2005) investigated the possibility of semi-relativistic jets being present in core collapse supernovae which are not related to GRB, as suggested by anisotropic, polarized remnant observations. Proton acceleration in shocks in these incipient jets could lead to neutrino emission detectable with ICECUBE out to 20 Mpc.

Ioka, Kobayashi and Meszaros (2005) interpreted the anisotropic supernova remnant W49B as the result of a collapsar gamma-ray burst, where proton acceleration leads to neutrons decaying far from the remnant. The decay electrons lead to inverse Compton which results in GeV-TeV photons, with a flux which could in principle be detectable with FERMI and ground-based air Cherenkov telescopes.

As part of the ICECUBE collaboration, Meszaros and Razzaque calculated models and participated in the evaluation of the sensitivity of this detector to high energy muon neutrinos from specific sources, as well as on an evauation of the first year performance of ICECUBE .

Neutron-rich material in both short and long GRB is expected to be ejected by the central engine. The free neutrons beta decay to a proton, an electron and an anti-neutrino in about fifteen minutes in its rest frame. The sudden creation of a relativistic electron is accompanied by radiation with unique temporal and spectral signature. Razzaque and Meszaros calculated this radiation signature collectively emitted by all beta decay electrons from neutron-rich outflow. Detection of this signature, e.g. by FERMI, may thus provide strong evidence for not only neutron but also for proton content in the relativistic gamma-ray burst jets.

The ratio of anti-electron to total neutrino flux, expected from p,gamma interactions in astrophysical sources is generally 1:15. However this ratio is enhanced by the decay of muon-antimuon pairs, created by the annihilation of secondary high energy photons from the decay of the neutral pions produced in p,gamma interactions. Razzaque, Meszaros and Waxman (2006) showed that the anti-electron to total neutrino ratio may be significantly enhanced in gamma-ray burst (GRB) fireballs, and that detection at the Glashow resonance of $\bar{\nu}_e$ in kilometer scale neutrino detectors may constrain GRB fireball model parameters, such as the magnetic field and energy dissipation radius.

The afterglow emission from short gamma-ray bursts suggests that binary neutron star or NS-BH mergers may be the progenitors. Razzaque and Meszaros (2006) considered a neutron-rich relativistic jet model of short bursts, which predicts a high energy neutrino and photon emission as neutrons and protons decouple. Upcoming neutrino telescopes are unlikley to detect the 50 GeV neutrinos expected in this model, but for bursts at z~0.1, FERMI and ground-based Cherenkov telescopes should be able to detect prompt 100 MeV and 100 GeV photon signatures, which may help test the progenitor identification.

Young magnetars may be born with milisecond rotation periods, and the ultrastrong magnetic field will result in a Poynting dominated outgoing wavefield, which can accelerate cosmic rays to GZK energies through wake-field acceleration. In this case, one could probe the birth of fast rotating magnetars through high-energy neutrinos, (Murase, Meszaros and Zhang, 2009), which are produced when the hultra-high energy protons interact with the ejected outer stellar envelope (the supernova remnant).

An interesting direct generation mechanism of production of High Energy Neutrinos and Photons from Curvature Pions in Magnetars was investigated by Herpay, Razzaque, Patkós and Mészáros. This is expected through the curvature radiation of pions in strongly magnetized pulsars or magnetars. This mechanism operate only in magetars, since it requires the very high fields measured in these objects. The production of TeV energy neutrinos associated with this is expected to be detectable by cubic kilometer scale detectors, while the high energy photons are in the range of space detectors.

With graduate student Shan Gao and postdoc Kenji Toma (2011) we studied the properties of very high redshift (10 Pop. III GRBs have a very hard neutrinos spectrum, PeV to EeV, and they could be detected by IceCube in 5 years. The figure on the right is for a 300 solar mass object.

This research is partly sponsored by NSF

## References:

"High energy neutrino emission from the earliest gamma-ray bursts", Gao, S., Toma, K. and Mészáros, P., 2011, Phys.Rev. D, 83:103004.

"GeV Emission from collisional magnetized gamma-ray bursts", Mészáros, P. and Rees, M.J., 2011, ApJ(Lett.), 733:L40.

Probing the Birth of Fast Rotating Magnetars through High-Energy Neutrinos", Murase, K., Meszaros, P. and Zhang, B., 2009, PRD, 79:103001 (arXiv:0904.2509)

"High Energy Neutrinos and Photons from Curvature Pions in Magnetars", Herpay, T., Razzaque, R., Patkós, A. and Mészáros, P., 2008, JCAP, JCAP08.025 (arXiv:0807.4914)

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