Jet Radiation Research Articles

Blazar Optical Polarimetry: Current Progress in Observations and Theories

Polarimetry has been a standard tool to probe the active galactic nucleus (AGN) jet magnetic field. In recent years, several optical polarization monitoring programs have been carried out, bringing in many exciting new results and insights into jet dynamics and emission. This article discusses current progress in blazar optical polarimetry. The main focus is the variability of polarization signatures, which has spurred a lot of theoretical studies. These novel developments have provided unique constraints on the blazar flares and emphasized the role of the magnetic field in jet evolution. Optical polarimetry will continue to act as an essential component in the multi-messenger study of AGN jets, in particular with the upcoming high-energy polarimetry. Comparing to first-principle numerical simulations, future multi-wavelength polarimetry can shed light on jet dynamics, particle acceleration, and radiation processes.

Particle Accelerators in Space: Recent News from VERITAS

Our universe carries a small but important population of highly energetic denizens: supernova remnants with fast shocks, pulsars with powerful winds, intensely-interacting binary systems built from a compact object and a massive star, relativistic jets launched by supermassive black holes. All of these environments conspire to generate populations of nonthermal particles, and observations of the very high energy (VHE; E > 100 GeV) gamma rays produced by these particles are gradually revealing the methods by which Nature accelerates cosmic rays, as well as the ways in which those cosmic rays escape and diffuse into the interstellar medium. These observations include studies of cosmic-ray acceleration in the supernova remnants Cassiopeia A and IC 443, follow-up of unidentified HAWC sources, and the remarkable Fall 2017 periastron passage of VER J2032+4127, the 50-year-period binary system containing PSR J2032+4127 and a Be star. Fast TeV gamma-ray flares coincidental with the emergence of superluminal radio knots from the blazar BL Lac can be interpreted in terms of a coherent scenario of jet particle flow and radiation. The recent TeV gamma-ray discovery of the radio galaxy 3C 264 adds a new member to the small population of off-axis jets available for study. Meanwhile, the direct detection for the first time of gravitational wave (GW) transients by Advanced LIGO has motivated searches for their electromagnetic counterparts at all wavelengths. Neutrino astronomy is an emerging area of study in high-energy astrophysics, and astrophysical neutrinos are natural cousins of VHE gamma rays. The VERITAS gamma-ray observatory has an active program of follow-up observations in the directions of potentially astrophysical high-energy neutrinos detected by IceCube, as well as in the direction of GW transients. In this talk, we discuss recent results from the VERITAS galactic, extragalactic, and multi-messenger Follow-up programs.

The black hole mass, jet power and accretion in blazars and flat-spectrum radio-loud narrow-line Seyfert 1 galaxies

We use a large sample of Fermi flat-spectrum radio quasars (FSRQs), BL Lacertae objects (BL Lacs) and flat-spectrum radio-loud narrow-line Seyfert 1 galaxies (F-RLNLS1s) to study the relationship between them. Our main results are as follows: (i) Compared with FSRQs, the F-RLNLS1s have lower redshift, black hole mass and jet kinetic power. However, the F-RLNLS1s have higher redshift, broad-line region luminosity, accretion rate and jet kinetic power than BL Lacs on the average. When we use the median black hole mass to normalize the jet kinetic and radiation power, we find a small difference between FSRQs and F-RLNLS1s in normalized jet kinetic power. (ii) According to the BZ theory, we find that both of FSRQs and most of F-RLNLS1s may have a strong accretion disk which dominated by the radiation-pressure. (iii) Most of these F-RLNLS1s and FSRQs have accretion disk luminosity ( $L_{\text{Disk}}$ ) larger than jet radiation power ( $P_{\text{rad}}$ ), $\log L_{\text{Disk}} > \log P_{\text{rad}}$ , which suggests that FSRQs and F-RLNLS1s may have a strong accretion disk. We use a linear regression to analyze the relationship between jet power and accretion disk luminosity. It is interesting to note that the slopes of FSRQs are similar to that of F-RLNLS1s in jet power vs. accretion disk luminosity panel. These results above may suggest that the formation mechanism of jets in F-RLNLS1s is similar to that of FSRQs. (iv) By using the multiple linear regression analysis, we find that the accretion and black hole mass may have a different contribution to jet kinetic power for F-RLNLS1s. What’s more, the contribution of black hole mass is larger than that of accretion rate. These results imply that the jet kinetic power of F-RLNLS1s less than FSRQs may due to the low black hole mass of F-RLNLS1s.

Falling water films over vertical surfaces due to orthogonal water jet impingement

In AHWR rod bundle assembly, LOCA could cause failure of the system due to overheating. To overcome this problem, it is proposed to introduce holes in the axial and circumferential direction of central water tube so that water discharges through these holes in the form of a jet and strikes the high-temperature fuel rods resulting in their cooling by gravity-driven flow mechanism. This cooling involves a different mechanism of heat transfer such as two-phase flow, single phase jet impingement, droplet impingement, evaporation, natural convection, and radiation due to the high temperature of the fuel rods. Among all these heat transfer mechanisms, the present study focuses on the horizontal water jet impingement on a vertically oriented surface. The scarcity of research in such cases is the motivation to study the hydrodynamics of the water jet impingement on a vertically oriented surface. A proper understanding of hydrodynamics of the water film on a vertical surface is important in the understanding of heat removal processes. Present study discusses the hydrodynamics of falling water films on a flat surface and a curved surface exposed to horizontal water jet impingement. Effect of the jet exit to surface distance (Z/d), Reynolds number, nozzle diameter (d), and the ratio of curvature of the surface to the diameter of the pipe (D/d) were examined on lateral and vertical spread of the water film at vertically oriented flat and curved surface.

Acoustic radiation of subsonic jets in the vicinity of an inclined flat plate.

Acoustic measurements of turbulent jets in the vicinity of a flat plate, mimicking a neighbouring wing, were compared to results from two wavepacket-based source models previously studied in the literature: the Tailored Green's Function method, which considers the radiation of the turbulent structure in the vicinity of a semi-infinite flat plate, and the Boundary Element Method, which can represent the full geometry of the plate used in the experiments. Particular interest is given to analysing how the angle of attack of the plate (α) affects the sound radiated by an installed jet with trailing edge 6 diameters away from the nozzle and 1 diameter away from the centerline for 0° ≤ α ≤ 45°. The results herein confirm the behaviour identified by the models: the scattered acoustic field follows the rotation of the plate, shifting a silence region with negligible scattered sound, and creating regions with lower noise levels in positions that correspond to the ground for an aircraft with engines under its wings. This is further explored by means of a Mach number analysis for M = 0.5, 0.7, and 0.9, showing that this trend is present whenever trailing-edge scattering of jet disturbances is dominant in the acoustic field.

RATAN-600 and RadioAstron reveal the neutrino-associated blazar TXS 0506+056 as a typical variable AGN

The possible association with the high-energy neutrino event IceCube-170922A has sparked interest in the blazar TXS 0506+056. We present 72 instantaneous 1–22 GHz spectra measured over the past 20 years with the RATAN-600 telescope and compare them with the results of observations of 700 variable Active Galactic Nuclei (AGN) studied within the same program. The recent radio flare of TXS 0506+056 started from a minimum in 2013 and reached its first peak in December 2017 and a second peak in May-June 2018. This was the third strong radio flare in this source since 1997. The spectrum remains nearly flat during the flares. The spectral shape and variability pattern observed in TXS 0506+056 are typical for variable AGN. RadioAstron Space VLBI observations in 2013–2015 did not detect TXS 0506+056 on space-ground baselines of more than 9 Earth diameters. However, an observation on 23 September 2015 resulted in the detection of interferometric signal on 6 Earth diameter baselines at 18 cm close to the detection limit. We consider the possibility that TXS 0506+056 and other AGN may accelerate relativistic protons more efficiently than electrons. Relativistic protons are necessary to produce both the high-energy neutrinos observed in the IceCube Observatory and the high AGN brightness temperatures implied by the RadioAstron detection. They may also provide the main contribution to the observed synchrotron radiation of parsec-scale AGN jets. This supports the suggestion that relativistic protons may play a much more important part in extragalactic astrophysics than earlier anticipated.

Probing the Emission Mechanism and Magnetic Field of Neutrino Blazars with Multiwavelength Polarization Signatures

Abstract The characteristic two-component blazar spectral energy distribution (SED) can be of either leptonic and/or hadronic origins. The potential association of the high-energy neutrino event IceCube-170922A with the flaring blazar TXS 0506+056 indicates that hadronic processes may operate in a blazar jet. Despite multiwavelength follow-ups of the event and extensive theoretical modelings, the radiation mechanisms and the underlying magnetic field strength and configuration remain poorly understood. In this paper, we consider generic leptonic and hadronic blazar spectral models with distinct magnetic field strengths and radiation mechanisms. We analytically reproduce the SEDs and neutrino flux of hadronic models, and predict their X-ray to γ-ray polarization degrees. Furthermore, by performing relativistic magnetohydrodynamic simulations taking into account the polarization-dependent radiation transfer, we study the time-dependent multiwavelength polarization variability of the proton synchrotron (PS) model under a shock scenario. Our results suggest that the high-energy polarization degree and the neutrino flux can be jointly used to pinpoint the leptonic and/or hadronic blazar radiation mechanisms in the X-ray and γ-ray bands, and to infer the magnetic field strength in the emission region. In addition, the temporal multiwavelength polarization signatures in the PS shed light on the jet energy composition and the dynamical importance of magnetic fields in the blazar emission region. Future multiwavelength polarimetry facilities such as the Imaging X-ray Polarimetry Explorer and All-sky Medium Energy Gamma-ray Observatory together with neutrino telescopes such as IceCube can provide unprecedented observational constraints to probe the blazar radiation mechanisms and jet dynamics.

Limits on Electromagnetic Counterparts of Gravitational-wave-detected Binary Black Hole Mergers

Abstract Unlike mergers of two compact objects containing a neutron star (NS), binary black hole (BBH) mergers are not accompanied by the production of tidally disrupted material and hence lack the most direct source of accretion to power a jet and generate electromagnetic (EM) radiation. However, following a tentative detection by the Fermi GBM of a γ-ray counterpart to GW150914, several ideas were proposed for driving a jet and producing EM radiation. If such jets were in fact produced, they would, however, lack the cocoon emission that makes jets from binary NSs also bright at large viewing angles. Here, via Monte Carlo simulations of a population of BBH mergers with properties consistent with those inferred from the existing LIGO/Virgo observations and the angular emission characteristic of jets propagating into the interstellar medium, we derive limits on the allowed energetics and Lorentz factors of such jets from EM follow-ups to GW-detected BBH merger events to date, and we make predictions that will help tighten these limits with broadband EM follow-ups to events in future LIGO/Virgo runs. The condition that ≲1 event out of 10 GW-detected BBH mergers be above the Fermi/GBM threshold imposes that any currently allowed emission model has to satisfy the condition (E iso/1049 erg)(θ jet/20°) ≲ 1.

Mathematical simulation of aircraft engine jet exhausts radiation

The paper deals with the problems of mathematical simulation of aircraft engine jet exhausts radiation, the simulation being carried out by means of shader subroutines for the concurrent computation of the radiative transfer equation on the video card resources. The combination of an analytical model of an isobaric jet and ray tracing of computation of the radiative transfer equation allows us to develop a flexible model of aircraft jet radiation, the model taking into account the main parameters of streams in the jet and in the co-current flow, the spectral lines of the radiating components, and provides real-time computation. For the graphic implementation of the model, the OpenGL standard is used

Factorisation and subtraction beyond NLO

We provide a general method to construct local infrared subtraction counterterms for unresolved radiative contributions to differential cross sections, to any order in perturbation theory. We start from the factorised structure of virtual corrections to scattering amplitudes, where soft and collinear divergences are organised in gauge-invariant matrix elements of fields and Wilson lines, and we define radiative eikonal form factors and jet functions which are fully differential in the radiation phase space, and can be shown to cancel virtual poles upon integration by using completeness relations and general theorems on the cancellation of infrared singularities. Our method reproduces known results at NLO and NNLO, and yields substantial simplifications in the organisation of the subtraction procedure, which will help in the construction of efficient subtraction algorithms at higher orders.

LHC luminosity and energy upgrades confront natural supersymmetry models

The electroweak fine-tuning measure ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{EW}}$ allows for correlated supersymmetry (SUSY) soft terms as are expected in any ultraviolet complete theory. Requiring no less than 3% electroweak fine-tuning implies upper bounds of about 360 GeV on all Higgsinos, while top squarks are lighter than $\ensuremath{\sim}3\text{ }\text{ }\mathrm{TeV}$ and gluinos are bounded by $\ensuremath{\sim}6--9\text{ }\text{ }\mathrm{TeV}$. We examine the reach for SUSY of the planned high luminosity (HL: $3\text{ }\text{ }{\mathrm{ab}}^{\ensuremath{-}1}$ at 14 TeV) and the proposed high energy (HE: $15\text{ }\text{ }{\mathrm{ab}}^{\ensuremath{-}1}$ at 27 TeV) upgrades of the LHC via four LHC collider search channels relevant for natural SUSY: 1. gluino pair production followed by gluino decay to third generation (s)quarks, 2. top squark pair production followed by decay to third generation quarks and light Higgsinos, 3. neutral Higgsino pair production with QCD jet radiation (resulting in monojet events with soft dileptons), and 4. wino pair production followed by decay to light Higgsinos leading to same-sign diboson production. We confront our reach results with upper limits on superpartner masses in four natural SUSY models: natural gravity mediation via the 1. two- and 2. three-extra-parameter nonuniversal Higgs models, 3. natural minilandscape models with generalized mirage mediation and 4. natural anomaly mediation We find that while the HL-LHC can probe considerable portions of natural SUSY parameter space in all these models, the HE-LHC will decisively cover the entire natural SUSY parameter space with better than 3% fine-tuning.

Drivers of the Low-Cloud Response to Poleward Jet Shifts in the North Pacific in Observations and Models

The long-standing expectation that poleward shifts of the midlatitude jet under global warming will lead to poleward shifts of clouds and a positive radiative feedback on the climate system has been shown to be misguided by several recent studies. On interannual time scales, free-tropospheric clouds are observed to shift along with the jet, but low clouds increase across a broad expanse of the North Pacific Ocean basin, resulting in negligible changes in total cloud fraction and top-of-atmosphere radiation. Here it is shown that this low-cloud response is consistent across eight independent satellite-derived cloud products. Using multiple linear regression, it is demonstrated that the spatial pattern and magnitude of the low-cloud-coverage response is primarily driven by anomalous surface temperature advection. In the eastern North Pacific, anomalous cold advection by anomalous northerly surface winds enhances sensible and latent heat fluxes from the ocean into the boundary layer, resulting in large increases in low-cloud coverage. Local increases in low-level stability make a smaller contribution to this low-cloud increase. Despite closely capturing the observed response of large-scale meteorology to jet shifts, global climate models largely fail to capture the observed response of clouds and radiation to interannual jet shifts because they systematically underestimate how sensitive low clouds are to surface temperature advection, and to a lesser extent, low-level stability. More realistic model simulations of cloud–radiation–jet interactions require that parameterizations more accurately capture the sensitivity of low clouds to surface temperature advection.

Jet Radiation Properties of 4C +49.22: from the Core to Large-scale Knots

Abstract 4C +49.22 is a γ-ray flat-spectrum radio quasar with a bright and knotty jet. We investigate the properties of the core and large-scale knots by using their spectral energy distributions (SEDs). Analysis of the Fermi/LAT data from 4C +49.22 over the past eight years reveals a long-term steady γ-ray emission component besides bright outbursts. For the core region, the γ-ray emission together with the simultaneous emission in the low-energy bands at different epochs is explained with the single-zone leptonic model. The derived magnetization parameters and radiation efficiencies of the radio-core jet decrease as the γ-ray flux decays, likely indicating that a large part of the magnetic energy is converted to the kinetic energy of particles on a parsec scale. For the large-scale knots, their radio–optical–X-ray SEDs can be reproduced with the leptonic model by considering the inverse Compton scattering of cosmic microwave background photons. The sum of the predicted γ-ray fluxes of these knots is comparable to that observed with LAT at ∼1024 Hz of the steady γ-ray component, indicating that the steady γ-ray emission may be partially contributed by these large-scale knots. This may conceal the flux variations of the low-level γ-ray emission from the radio core. The derived bulk Lorentz factors of the knots decrease with increasing distance from the core, illustrating the deceleration of the jet on a large scale. The powers of the core and knots are roughly of the same order, but the jet changes from highly magnetized in the core region to particle-dominated in the large-scale knots.

Study of human response in conditions of surface heating, asymmetric radiation and variable air jet direction

Low temperature radiant heating systems are widely used in buildings being advantageous both from energy use and thermal comfort point of views. However, attention should be paid to asymmetric radiation especially in the case of wall heating. The aim of our research was to analyse the perception of thermal asymmetry in combination with different air velocities. A series of measurements were carried out in controlled environment involving 20 subjects (10 female, 10 male). It was shown that the thermal asymmetry is perceived at low air velocities, but increasing the air velocity to 0.1 m/s, 0.15 m/s or 0.2 m/s the perception of air movement overwrite the perception of thermal asymmetry. The heat removed by convection by the same air flow and air jet velocity was different for women and men group. The higher decrease of the skin temperature in case of women (around 2 K) leads to thermal sensation votes under −0.5 (even though the air and mean radiant temperature were 24 °C). The responses of women and men group regarding draught perception were different as well.

Spectral analysis of jet turbulence and radiated sound

Informed by LES data and resolvent analysis of the mean flow, we examine the structure of turbulence in jets in the subsonic, transonic, and supersonic regimes. Spectral (frequency-space) proper orthogonal decomposition is used to extract energy spectra and decompose the flow into energy-ranked coherent structures. We demonstrate that two distinct mechanisms, which can be distinguished by their characteristic frequency scaling and spatial support, lead to the formation of wavepackets—coherent structures that are known for their acoustic importance in the aft-angle radiation of high subsonic and supersonic jets. We compare these characteristics to acoustic source features extracted from hologram sound pressure measurements in a recent publication. The evidence strongly suggests that both mechanisms are active in full-scale jets and comprise the experimentally educed sources of sound.

Next-to-leading order electroweak corrections to off-shell WWW production at the LHC

Triboson processes allow for a measurement of the triple and quartic couplings of the Standard Model gauge bosons, which can be used to constrain anomalous gauge couplings. In this paper we calculate the next-to-leading order electroweak corrections to fully off-shell W−W+W+ production, namely the production of a {ell}_1^{-}{ell}_2^{+}{ell}_3^{+}{overline{nu}}_{ell_1}{nu}_{ell_2}{nu}_{ell_3} final state with (ℓi = e, μ), including all triple, double, single and non resonant topologies and interferences of diagrams with all different vector boson (W, Z, γ) intermediate states. We find large cancellations of the electroweak correction to the qoverline{q} -induced channel, which includes the exchange of virtual electroweak gauge bosons, and photon-induced jet radiation processes. This accidental compensation is found to be strongly phase space and observable dependent. The resulting corrections in a suitably defined fiducial region thus amount to −2.0%, but rise rapidly for other observables.

Calculation of Gas Dynamics and Radiation of High-Altitude Jets

Calculation of Gas Dynamics and Radiation of High-Altitude Jets

A precessing and nutating jet in OJ287

AbstractWe re-analyzed OJ287 in 120 Very Long Baseline Array (VLBA, MOJAVE) observations (at 15 GHz) covering the time span between Apr. 1995 and Apr. 2017. We find that the radio jet motion over the sky is consistent with a precessing and nutating jet source. The variability of the radio flux-density can be explained by Doppler beaming due to a change in the viewing angle. We suggest that part of the optical emission is due to synchrotron emission related to the jet radiation. We find a strikingly similar scaling for the timescales for precession and nutation as indicated for SS433 with a factor of roughly 50 times longer in OJ287.

Flux and spectral variation characteristics of 3C 454.3 at the GeV band

We analyze the long-term lightcurve of 3C 454.3 observed with Fermi/LAT and investigate its relation to flux in the radio, optical and X-ray bands. By fitting the 1-day binned GeV lightcurve with multiple Gaussian functions (MGF), we propose that the typical variability timescale in the GeV band is 1–10 d. The GeV flux variation is accompanied by the spectral variation characterized as flux-tracking, i.e., “harder when brighter.” The GeV flux is correlated with the optical and X-ray fluxes, and a weak correlation between γ-ray flux and radio flux is also observed. The γ-ray flux is not correlated with the optical linear polarization degree for the global lightcurves, but they show a correlation for the lightcurves before MJD 56000. The power density spectrum of the global lightcurve shows an obvious turnover at ∼ 7.7 d, which may indicate a typical variability timescale of 3C 454.3 in the γ-ray band. This is also consistent with the derived timescales by fitting the global lightcurve with MGF. The spectral evolution and an increase in the optical linear polarization degree along with the increase in γ-ray flux may indicate that the radiation particles are accelerated and the magnetic field is ordered by the shock processes during the outbursts. In addition, the nature of 3C 454.3 may be consistent with a self-organized criticality system, similar to Sagittarius A*, and thus the outbursts could be from plasmoid ejections driven by magnetic reconnection. This may further support the idea that the jet radiation regions are magnetized.

Constraining the compressed spectrum of the top squark and chargino along the W corridor

Studying superpartner production together with a hard initial state radiation (ISR) jet has been a useful strategy for searches of supersymmetry with a compressed spectrum at the Large Hadron Collider (LHC). In the case of the top squark (stop), the ratio of the missing transverse momentum from the lightest neutralinos and the ISR momentum, defined as $\bar{R}_M$, turns out to be an effective variable to distinguish the signal from the backgrounds. It has helped to exclude the stop mass below 590 GeV along the top corridor where $m_{\tilde{t}} - m_{\tilde{\chi}_1^0} \approx m_t$. On the other hand, the current experimental limit is still rather weak in the $W$ corridor where $m_{\tilde{t}} - m_{\tilde{\chi}_1^0} \approx m_W +m_b$. In this work we extend this strategy to the parameter region around the $W$ corridor by considering the one lepton final state. In this case the kinematic constraints are insufficient to completely determine the neutrino momentum which is required to calculate $\bar{R}_M$. However, the minimum value of $\bar{R}_M$ consistent with the kinematic constraints still provides a useful discriminating variable, allowing the exclusion reach of the stop mass to be extended to $\sim 550$ GeV based on the current 36 fb$^{-1}$ LHC data. The same method can also be applied to the chargino search with $m_{\tilde{\chi}_1^\pm} -m_{\tilde{\chi}_1^0} \approx m_W$ because the analysis does not rely on $b$ jets. If no excess is present in the current data, a chargino mass of 300 GeV along the $W$ corridor can be excluded, beyond the limit obtained from the multilepton search.