## Neutrino Astrophysics

### Peter Mészáros

For a more detailed recent review see here. Below is an abbreviated introduction and an outline of some specific research projects.

The same shocks which accelerate the electrons responsible for the radiation of non-thermal γ-rays in GRBs, supernovae, AGNs nd clusters of galaxies should also accelerate protons present in the shocks. Both the GRB internal and external reverse shocks as well as the supenovae and cluster shocks are mildly relativistic and can lead to highly relativistic proton energy spectra of the form E^{-2}, i.e. to cosmic rays (CRs). The typical values of the shock radius, magnetic field and the bulk Lorentz factor which explain the electron acceleration and gamma-ray emission in these high energy sources indicate that the protons reach Lorentz factors that may reach up to 10^{11}. This makes them potential cosmic ray sources. These CRs can collide with intra-sourrce target photons (pγ) or other thermal protons (pp), leading to charged and neutral pions, the former decaying to charged muons and eventually neutrinos, while the second decay into a pair of γ-rays (figure on left). The resulting veru high energy (VHE) neutrino fluxes are expected to be detectable above the atmospheric neutrino background with the Antarctic Cubic Kilometer ICECUBE Cherenkov detector (figure on right), Penn State being involved in the science and data analysis aspects. IceCube construction was finished (2011), with sensitivity up to about 100 PeV, and its Deep Core" sub-array can measure down to 10-15 GeV energies.

A diffuse flux of VHE neutrinos up to a few PeV of undoubted astrophysical origin was discovered by IceCube in 2013, end its extenson down to TeV energies in 2015. The spectrum is well above the atmospheric neutrino background by at least 7σ (figure on left). The flavor ratio is compatible with 1:1:1 as one would expect from pion decay and vacuum oscillations over cosmological distances, and the arrivale directions are compatible with an isotropic distribution, with no significant correlation with any class of extragalactic objects, strongly suggesting that it is of extragalactic origin.

GRBs have been considered promising candidate sources for these VHE neutrinos, e,g, from internal shocks (Waxman & Bahcall, 1997), and general properties of GRBs as well as AGN jets as CR and VHE neutrino sources were discussed by, e.g. Rachen & Mészáros, 1998. These VHE neutrino spectra extended to the PeV range, through pγ interactions with the γ rays in the sources themselves. Interaction with the optical/UV phtotons in the GRBs would leade to 10^{17}-10^{19} eV neutrios (Waxman & Bahcall, 1999). Another mechanism for neutrino production in GRB is inelastic nuclear collisions, if the the outflow has a substantial neutron/proton ratio, which occur when the nuclear scattering time scale becomes comparable to the expansion time scale and the relative velocities of the nuclei become large enough to collide inelastically, resulting in charged pions and ~few GeV neutrinos (Bahcall & Meszaros 2000); this is expected also in outflows with Lorentz factor transverse inhomogeneities (Meszaros & Rees 2000)

Considering a simple GRB internal shock models with a given Fermi acceleration spectral index, say $s=2-2.2$, and assuming a relativistic proton to electron ratio f_{pe}, one can predict a VHE neutrino flux for a given observed γ-ray spectrum for GRBs measured and localized in direction and time, say by Swift. and one can compare this against the energy, direction and arrival time of masured VHE neutrinos. IceCube has carried out such an analysis (IceCUbe, 2015; IceCube, 2017), using the predictions the simplest internal shock models, as well as a modified version including magnetic dissipation and a simple jet photospheric model. For values of f_{pe}~1-10 (taken to be typical, e.g., if the Auger-detected GZK cosmic rays also are due to GRBs), they conclude that less than ~1% of the EM-detected classical GRBs can be associated with the observed VHE neutrinos. Of course there are a number of uncertainties and approximations in the models of the acceleration and the photon and neutrino production, but taken at face value, this lack of agreement could imply that f_{pe} << 1, or else that the shocks or the disspation occur at larger radii, where the pγ efficiency is much reduced, and thus it is desirable to investigate also other source candidates.

A possible alternative VHE neutrino source may be provided by CR acceleration in the shocks of supernovae and hypernovae in starburst galaxies. The supernovae can accelrate CRs up to PeV energies, and the hypernovae up to at least 100 PeV, and such CRs undergoing pp collisons in their host galaxy and, after escape, in their host galaxy cluster intergalactic medium, produce VHE neutrinos extending up to about 5 PeV, with a neutrino spectrum and flux which is roughly comparable to that of IceCube ( Senno et al, 20150; also, the secondary VHE γ-rays, after undergoing γγ cascades against the EBL in intergalactic space, marginally satisfy the difuse γ-ray background observed by Fermi (red data points in figure at right). However, since this background is already up to 85% already explained by blazars, one needs an explanation which produces significantly less γ-rays. This is possible if the bulk of the neutrinos (and the corresponding γ-rays) are produced at redshifts z > 3-4 (Xiao et al, 2016), since from those redshifts the EBL γγ attenuation is so strong that the 15% residual (magenta zone in figure at right) Fermi background can be satisfied, as well as much of the neutrino background (blue data points in figure at right).

A more attractive type of alternative source are the low-luminosity GRBs (LLGRBs). Basically, they form a sequence going from choked GRBs, to shock-breakout GRBs, to low-luminosity emergent GRBs (Senno et al, 2016, from left to right in the figure at left). The choked jets are collapsars where the jet never emerged, but they can undergo neutrino production while stil inside the star; in the shock break-outs the jet impelled a shock to break out, producing neutrinos and low luminosity γ-rays; and the third type are emergent,but low luminosity jets, all three being electromagnetically dark or faint, but together providing a neutrino flux which is approxumately that etected by IceCube (spectrum at right).

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. Large imaging air Cherenkov telescopes and large water Cherenkov detectors such as HAWC, as well as space-based large area solid state detectors such as on FERMI measure photons in this energy range, which might 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).

The section below is a discussion of earlier work

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, may be detectable by 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 in 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 IceCube for a high enough baryon load fireball. 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 also calculated in detail the neutrino detection prospects for a standard internal shock in a nearby GRB such as GRB030329 by a km scale detector such as ICECUBE .

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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MeV-GeV emission from neutron-loaded short Gamma-ray Bursts" , Razzaque, S and Meszaros, P, 2006, ApJ, in press (astro-ph/0601652)

Enhancement of the electron anti-neutrino flux from astrophysical sources by two photon annihilation interactions" , Razzaque, S, Meszaros, P and Waxman, E, 2006, PRD, 73, 103005 (astro-ph/0509186)

High Energy Neutrinos from a Slow Jet Model of Core Collapse Supernovae", Razzaque, S, Meszaros, P and Waxman, E, 2005, Mod. Phys. Letts. A, 20, No. 31, 2351-2367

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Neutrino Tomography of Gamma Ray Bursts and Massive Stellar Collapses", Razzaque, S, Meszaros, P & Waxman, E, 2003, Phys.Rev.D, 68:3001

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