Background:
Since 2010, the IceCube Observatory, utilizing a cubic-km volume of ice in Antartica, has been detecting neutrinos with energies exceeding 100 TeV (~10^13 times the energy of a visible photon), which likely originate from astrophysical sources outside of the Milky Way Galaxy. However, their sources remained unknown, mainly due to the limited accuracy of their localizations in the sky. From 2016, IceCube initiated a new, automated alert program that rapidly identifies significant neutrino candidates and widely disseminates their sky positions, so that telescopes around the world can immediately search for potential counterparts at various wavelengths.

Result:
On September 22, 2017, a neutrino with energy ~300 TeV (dubbed IceCube-170922A) was detected with relatively good sky localization, and was rapidly followed up by numerous telescopes operating across the electromagnetic spectrum. The Fermi-LAT satellite and the MAGIC telescopes identified an object (named TXS 0506+056) shining brightly in gamma-rays (Fig. 1). The object is classified as a “blazar”, a type of supermassive black hole that is actively ejecting “jets" of plasma at relativistic velocities nearly toward us. This is the first time that a likely source of high-energy neutrinos has been identified with reasonable confidence [1]. These results were published in the July 13 issue of Science magazine, in a paper authored by more than 1100 scientits in 16 collaborations, including Susumu Inoue of iTHEMS as a member of the MAGIC Collaboration [2].

Implications:
The production of such high-energy neutrinos requires the acceleration of hadrons (proton or nuclei) to extremely high energies. This implies special physical conditions in the jets from supermassive black holes, and offer valuable clues on the formation mechanism of the jets, which is not well understood. This may also be the first step in solving the long-standing mystery of the origin of ultra-high-energy cosmic rays, the highest energy particles known to exist in the Universe [3]. Finally, it may shed new light on the properties of neutrinos at energies far beyond the capability of terrestrial accelerator facilities.

Prospects:
Following on the heels of GW170817, the binary neutron star merger event discovered in August 2017 in gravitational waves and then identified in electromagnetic waves, this signals the dawn of the “electroweak" sector of multi-messenger astronomy involving neutrinos and photons. Vigorous efforts will continue in the next years, with bright prospects for elucidating the physics of supermassive black holes and their jets, the origin of high-energy neutrinos and cosmic rays, etc.

Figure 1:
Image of the sky at optical wavelengths of the region of interest. Overlayed are the positional uncertainties of the neutrino IceCube-170922A, and those of the blazar TXS 0506+056 at optical wavelengths and in gamma rays observed by Fermi and MAGIC.

Series of lectures "Frontiers of Mathematical Sciences: Universe, Matter, Life and Information" were at Komaba Campus, Univ. of Tokyo, on every Wed. April-July, 2018, for the 1st and 2nd year undergraduate students in Univ. Tokyo. Six researchers from RIKEN iTHEMS (Y. Inoue, Y. Yokokura, M. Tachikawa, J. Fawcett, T. Doi and M. Taki) gave 14 lectures altogether.
The photo is the very last slide of M. Taki and himself (left) who gave the last lecture on July 11, as well as T. Tsuboi who organized this class. Undergraduate students seem to enjoy these lectures which cover the wide range of topics selected from the point of view of mathematical sciences.
We plan to publish these lectures as a book in the near future.

The episodic dynamics of the magnetic eruption of spinning black hole (BH) accretion discs and the associated intense shape-up of their jets are studied via three-dimensional general-relativistic magnetohydrodynamics (GRMHD). The embedded magnetic fields in the disc are amplified by a magnetorotational instability (MRI) so large as to cause an eruption of the magnetic field (reconnection) and large chunks of matter accrete episodically toward the roots of the jets upon such an event. We also find that the eruption events produce intensive Alfvén pulses, which propagate through the jets. After the eruption, the disc returns to the weakly magnetic state. Such disc activities cause short-time variabilities in mass accretion rate at the event horizon, as well as electromagnetic luminosity inside the jet. Since the dimensionless strength parameter a0 = eE/m_eωc of these Alfvén wave pulses is extremely high for a substantial fraction of Eddington accretion rate accretion flows on to supermassive black holes, the Alfvén shocks turn into ultrarelativistic (a0 >> 1) bow wake acceleration, manifesting as ultra-high-energy cosmic rays and electrons, which finally emit gamma-rays. Since our GRMHD model has universality in its spatial and temporal scales, it is applicable to a wide range of astrophysical objects, ranging from active galactic nuclei (AGNs, the primary target of this research) to micro-quasars. Properties such as the time variabilities of blazar gamma-ray flares and the spectrum observed by the Fermi Gamma-ray Observatory are explained well by linear acceleration of electrons by a bow wake.

Figure caption:
A snap shot of accreting gas onto a spinning black hole and Poynting flux dominated jet formation by 3-dimensional general relativistic magneto-hydrodynamic simulation. Log-scaled inverse of plasma beta (magnetic pressure / thermal pressure) (x-y plane), mass density (y-z plane), and magnetic pressure (x-z plane) are shown.