Narrow Results

The important role of magnetic fields in the phenomena in and evolution of the Universe is well appreciated. A salient example of this is to make (often episodic) large magnetic fields in AGN accretion disks and their emanation of well-collimated and longitudinally extended astrophysical jets. Such typical cases or related astrophysical processes, we find, provide a fertile ground for exciting large-amplitude oscillations in the magnetic fields that constitute the spine of the jets. The energy sources of these oscillations can be traced originally to the gravitational energy of the central object. During their long propagation along the jet, because of the gradual changes of the density and magnetic fields, these large magnetic pulsations turn into relativistic amplitude electromagnetic (EM) pulses, which in turn induce intense wakefields that are capable of acceleration of electrons, positrons, and ions to high energies. In this review, we survey a variety of astrophysical objects ranging from as large as the cosmic AGN accretion disks and their jets to as small as microquasars, to find or predict that there exist common astrophysical processes of emission of high-energy particles and gamma (and other EM) emissions. A variety of these objects will be ideally observed and studied in the multimessenger astrophysical observations. One example that already stuck out was the case of the simultaneous observations of gravitational wave emission and gamma-ray pulse from the collision of the two neutron stars and their subsequent structure formation (such as a disk) around them.

We probe the role that the directional asymmetry, between relativistic outflows and kilo-parsec scale jets, play in the acceleration of cosmic rays. For this reason we use two powerful, nearby Active Galactic Nuclei (AGNs). These radio galaxies are atypical compared to the usual AGN as they contain ring-like features instead of hotspots. Our VLBI radio data have revealed a substantial misalignment between their small and large scale jets. Taking into account the overall information we have obtained about the AGNs themselves (VLA and VLBI radio data at 18 cm) and their clusters (X-ray observations) our study supports the present ideas of powerful radiogalaxies (radio quiet and radio loud) being sources of cosmic rays as well as their ability to accelarate the latter to ultra high energies.

Black hole surroundings and relativistic jets host magnetically dominated regions and fast magnetic reconnection are likely to play an important role concerning astrophysical phenomena associated with such regions. In this contribution, we highlight the works related to turbulence-driven reconnection processes. These processes have been studied by us using analytical as well as numerical methods which showed that fast reconnection processes are powerful ways to giving rise to relativistic particles and associated non-thermal emissions around stellar-mass and supermassive black holes. The power released from the reconnection can even compete with those of extraction from black hole spin.

Gamma-ray bursts (GRBs) are bright extragalactic flashes of gamma-ray radiation and briefly the most energetic explosions in the Universe. Their catastrophic origin —the merger of compact objects or the collapse of massive stars— drives the formation of a newborn compact remnant (black hole or magnetar) that powers two highly relativistic jets. As these jets continue to travel outwards, they collide with the external material surrounding the dying star, producing a long-lasting afterglow that can be seen across the entire electromagnetic spectrum, from the most energetic gamma-ray emission to radio wavelengths. But how can such material be accelerated and focused into narrow beams? The internal shock model proposes that repeated collisions between material blasted out during the explosion can produce the gamma-ray flash. The competing magnetic model credits primordial large-scale ordered magnetic fields that collimate and accelerate the relativistic outflows. To distinguish between these models and ultimately determine the power source for these energetic explosions, our team studies the polarization of the light during the first minutes after the explosion (using novel instruments on fully autonomous telescopes around the globe) to directly probe the magnetic field properties in these extragalactic jets. This technology allowed the detection of highly polarized optical light in GRB 120308A and confirmed the presence of mildly magnetized jets with large-scale primordial magnetic fields in a reduced sample of GRBs (e.g. GRB 090102, GRB 110205A, GRB 101112A, GRB 160625B). Here we discuss the observations of the most energetic and first GRB detected at very high TeV energies, GRB 190114C, which opens a new frontier in GRB magnetic field studies suggesting that some jets can be launched highly magnetized and that the collapse and destruction of these magnetic fields at very early times may have powered the explosion itself. Additionally, our most recent polarimetric observations of the jet of GRB 141220A indicate that, when the jetted ejected material is decelerated by the surrounding environment, the magnetic field amplification mechanisms at the front shock —needed to generate the observed synchrotron emission— produce small magnetic domains. These measurements validate theoretical expectations and contrast with previous observations that suggest large magnetic domains in collisionless shocks (i.e. GRB 091208B).

The discovery of hot and dense quantum chromodynamics (QCD) matter, known as Quark–Gluon Plasma (QGP), is an essential milestone in understanding the finite temperature QCD medium. Experimentalists around the world collect an unprecedented amount of data in heavy ion collisions, at Relativistic Heavy Ion Collider (RHIC), at Brookhaven National Laboratory (BNL) in New York, USA, and at the Large Hadron Collider (LHC), at CERN in Geneva, Switzerland. The experimentalists analyze these data to unravel the mystery of this new phase of matter that filled a few microseconds old universe just after the Big Bang. Recent advancements in theory, experimental techniques, and high computing facilities help us to better interpret experimental observations in heavy ion collisions. The exchange of ideas between experimentalists and theorists is crucial for the characterization of QGP. The motivation of this first conference, named Hot QCD Matter 2022 is to bring the community together to have a discourse on this topic. In this paper, there are 36 sections discussing various topics in the field of relativistic heavy ion collisions and related phenomena that cover a snapshot of the current experimental observations and theoretical progress. This paper begins with the theoretical overview of relativistic spin-hydrodynamics in the presence of the external magnetic field, followed by the Lattice QCD results on heavy quarks in QGP. Finally, it concludes with an overview of experimental results.

I report about the unification of relativistic jets from compact objects. The mass range is between 1.4 and 10 billion solar masses (i.e. from neutron stars to supermassive black holes in galaxies).

The location of the main emitting region responsible for the bulk of the Blazar emission is a puzzling issue in our understanding of jetted Active Galactic Nuclei. Fast flares and a high Compton dominance are more easily explained if the gamma-ray zone is well inside the Broad Line Region (BLR), while the absence of γ-γ absorption features in the Fermi-LAT spectra as well as the detection at Very High Energies (VHE) of some FSRQ put the blazar zone at much larger distances along the jet, beyond the BLR. The latter seems now to be the most typical behavior in FSRQ, questioning SED models based on the external Compton process on BLR photons.

The Event Horizon Telescope promises to construct an image of the supermassive black hole in the Galaxy center. Since the black hole event horizon is visible only when illuminated appropriately, its direct detectability depends on the structures and radiative properties of the plasma in the nearest vicinity of the black hole. General relativistic magnetohydrodynamics simulations and corresponding radiative transfer models allow us to predict the appearance of magnetized plasma near a supermassive, rotating black hole accurately. Here, we present the details of the three-dimensional models of accreting black hole scaled to the Galactic center object. We present expected appearance of these models at the Event Horizon Telescope observing wavelengths as a function of black hole’s angular velocity and thermodynamical properties of the plasma around it. In the near future, similar models will be used to interpret the Event Horizon Telescope observations. The observations will constrain the supermassive black hole spin value and orientation and will reveal the nature of the compact synchrotron emission produced nearby the black hole.

The Marcel Grossmann triennial meetings are focused on reviewing developments in gravitation and general relativity, aimed at understanding and testing Einstein’s theory of gravitation. The 15^th meeting (Rome, 2018) celebrated the 50^th anniversary of the first neutron star discovery (1967), and the birth of relativistic astrophysics. Another discovery of the same caliber is the detection of the binary neutron star GW170817 in 2017 — almost as if to celebrate the same jubilee — marking the beginning of multi-messenger gravitational wave astronomy. We present work in progress to craft open-sourced numerical tools that will enable the calculation of electromagnetic counterparts to gravita- tional waveforms: the GiRaFFE (General Relativistic Force-Free Electrodynamics) code. GiRaFFE numerically solves the general relativistic magnetohydrodynamics system of equations in the force-free limit, to model the magnetospheres surrounding compact binaries, in order (1) to characterize the nonlinear interaction between the source and its surrounding magnetosphere, and (2) to evaluate the electromagnetic counterparts of gravitational waves, including the production of collimated jets. We apply this code to various configurations of spinning black holes immersed in an external magnetic field, in order both to test our implementation and to explore the effects of (1) strong gravitational field, (2) high spins, and (3) tilt between the magnetic field lines and black hole spin, all on the amplification and collimation of Poynting jets. We will extend our work to collisions of black holes immersed in external magnetic field, which are prime candidates for coincident detection in both gravitational and electromagnetic spectra.

Higher collision energies at future colliders may eventually lead to the falsification of standard fixed-order perturbation theory and linear evolutions due to nonlinear structure of QCD at small-$x$. New physics research works that are strictly based on accurate jet measurements will undoubtedly have this observation known as Balitsky–Fadin–Kuraev–Lipatov (BFKL) effect via angular jet decorrelations taking into account the Mueller–Navelet jets. As one of the frontier colliders, FCC-ep, has a great observation potential on parton densities through asymmetrical collisions. We aim to test the observability of azimuthal angular jet decorrelations with the recent event generators (HERWIG, PYTHIA) at the generator and detector level for FCC-ep center of mass energies $\sqrt{s}=3.5$TeV in proton–electron collisions. Jets are reconstructed by the anti-$k_T$ algorithm ($R=0.5$), with $p_T>15$GeV and selected in the range of $|\eta |<6$. Relevant pseudo-rapidity regions have been analyzed with the azimuthal-angle difference between Mueller–Navelet Jets ($\mathrm{\Delta }\mathrm{\Phi }$) in the pseudo-rapidity separation ($\mathrm{\Delta }\eta$) and the distributions of $〈cos(n(\pi -\mathrm{\Delta }\varphi ))〉$ are presented in comparison as the result.

The Event Horizon Telescope promises to construct an image of the supermassive black hole in the Galaxy center. Since the black hole event horizon is visible only when illuminated appropriately, its direct detectability depends on the structures and radiative properties of the plasma in the nearest vicinity of the black hole. General relativistic magnetohydrodynamics simulations and corresponding radiative transfer models allow us to predict the appearance of magnetized plasma near a supermassive, rotating black hole accurately. Here, we present the details of the three-dimensional models of accreting black hole scaled to the Galactic center object. We present expected appearance of these models at the Event Horizon Telescope observing wavelengths as a function of black hole’s angular velocity and thermodynamical properties of the plasma around it. In the near future, similar models will be used to interpret the Event Horizon Telescope observations. The observations will constrain the supermassive black hole spin value and orientation and will reveal the nature of the compact synchrotron emission produced nearby the black hole.

As suggested by recent observations, we explore the influences of large-scale magnetic fields on the dynamics of slim disks. The magnetic fields are assumed to be self-similar and be a fixed fraction of the total pressure at the footpoint. The global solutions show that the radial velocity increases and the disk temperature decreases with enhancing magnetic fields. The ratio of the jet kinetic power to disk luminosity is less than 0.1, which indirectly support the argument that radio-loud (RL) narrow-line Seyfert 1 galaxies (NLS1s) are similar to blazars, with jets pointing to us.

We review recent theoretical developments in the study of the structure of jets that are produced in ultra relativistic heavy ion collisions. The core of the review focusses on the dynamics of the parton cascade that is induced by the interactions of a fast parton crossing a quark–gluon plasma. We recall the basic mechanisms responsible for medium induced radiation, underline the rapid disappearance of coherence effects, and the ensuing probabilistic nature of the medium induced cascade. We discuss how large radiative corrections modify the classical picture of the gluon cascade, and how these can be absorbed in a renormalization of the jet quenching parameter $\widehat{q}$. Then, we analyze the (wave)-turbulent transport of energy along the medium induced cascade, and point out the main characteristics of the angular structure of such a cascade. Finally, color decoherence of the in-cone jet structure is discussed. Modest contact with phenomenology is presented towards the end of the review.

Single spin asymmetry measurements ($A_{UT}$) of the azimuthal distribution of charged pions inside jets produced in transversely polarized proton collisions are sensitive to the transversity distribution and the Collins fragmentation function. The STAR Detector at the Relativistic Heavy Ion Collider is well suited for these types of measurements as it is capable of full jet reconstruction and charged pion identification in the mid-rapidity region ($|$$\eta$$|$$<1$). We report here the first observation of Collins $A_{UT}$ asymmetries in $\sqrt{s}=200$ GeV $p^{\uparrow }p$ collisions.

Various aspects of the high-energy emission from relativistic jets associated with compact astrophysical systems are reviewed. The main leptonic and hadronic processes responsible for the production of high-energy γ-rays, very-high-energy neutrinos and ultra-high energy cosmic rays are discussed. Relations between the γγ pair production and photomeson production opacities are derived, and their consequences for the relative emission of γ-rays and neutrinos are examined. The scaling of the size and location of the various emission zones and other quantities with black hole mass and dimensionless luminosity is elucidated. The results are applied to individual classes of objects, including blazars, microquasars and gamma-ray bursts. It is concluded that if baryons are present in the jet at sufficient quantities, then under optimal conditions most systems exhibiting relativistic jets may be detectable by upcoming neutrino telescopes. An exception is the class of TeV blazars, for which γ-ray observations imply neutrino yields well below detection limit.

Jet substructure in heavy-ion collisions is a rapidly evolving area with lots of intriguing new measurements. This contribution presents a selection of recent jet-substructure measurements from experiments at the LHC, in particular, soft-drop groomed radii of jets and reclustered large-radius jets from ATLAS, jet axis difference and generalized jet angularities from ALICE, as well as dijet shapes and b-jet shapes from the CMS experiment.

In the past decade the observation of cross section modification for leading hadrons, heavy flavor and two particle correlations in heavy ion collisions has provided important insights into the dynamics of parton propagation in dense strongly-interacting matter. The development of the theory of reconstructed jets and related experimental measurements have further shed light on the characteristics of in-medium parton showers. So far, experimental results from ultra-relativistic nuclear collisions at RHIC and LHC have been analyzed in the framework of parton energy loss, where the precision of the theoretical predictions cannot be systematically improved. Only recently have higher order calculations and applications of resummation and evolution to heavy ion collisions begun to emerge. Several examples of such advances are discussed in these proceedings.

Soft Collinear Effective Theory (SCET) is an effective field theory of Quantum Chromodynamics (QCD) for processes where there are energetic, nearly lightlike degrees of freedom interacting with one another via soft radiation. SCET has found many applications in high-energy and nuclear physics, especially in recent years the physics of hadronic jets in e⁺e^-, lepton-hadron, hadron-hadron, and heavy-ion collisions. SCET can be used to factorize multi-scale cross sections in these processes into single-scale hard, collinear, and soft functions, and to evolve these through the renormalization group to resum large logarithms of ratios of the scales that appear in the QCD perturbative expansion, as well as to study properties of nonperturbative effects. We overview the elementary concepts of SCET and describe how they can be applied in high-energy and nuclear physics.

In this paper a spatially resolved, fully self-consistent SSC model is presented. The observable spectral energy distribution (SED) evolves entirely from a low energetic delta distribution of injected electrons by means of the implemented microphysics of the jet. These are in particular the properties of the shock and the ambient plasma, which can be varied along the jet axis. Hence a large variety of scenarios can be computed, e.g. the acceleration of particles via multiple shocks. Two acceleration processes, shock acceleration and stochastic acceleration, are taken into account. From the resulting electron distribution the SED is calculated taking into account synchrotron radiation, inverse Compton scattering (full cross section) and synchrotron self absorption. The model can explain SEDs where cooling processes are crucial. It can verify high variability results from acausal simulations and produce variability not only via injection of particles, but due to the presence of multiple shocks. Furthermore a fit of the data, obtained in the 2010 multi-frequency campaign of Mrk501, is presented.

We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic (core) jet and an unmagnetized sheath plasma by the kinetic Kelvin-Helmholtz instability for different mass ratios (m_i/m_e = 1, 20, and 1836) and different jet Lorentz factors. We found that electron-positron cases have alternating magnetic fields instead of the DC magnetic fields found in electron-ion cases. We have also investigated particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma for electron-positron and electron-ion plasmas. Strong magnetic fields generated in the trailing shock lead to transverse deflection and acceleration of the electrons. We have self-consistently calculated the radiation from the electrons accelerated in the turbulent magnetic fields for different jet Lorentz factors. We find that the synthetic spectra depend on the bulk Lorentz factor of the jet, the jet temperature, and the strength of the magnetic fields generated in the shock.