Abstracts of Interest
Selected by:
Abstract: 1904.05882
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Full Text: [ PostScript, PDF]
Title:Planning the scientific applications of the Five-hundred-meter Aperture Spherical radio Telescope
(Submitted on 12 Apr 2019)
Abstract: The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is by far the largest telescope of any kind ever built. FAST produced its first light in September 2016 and it is now under commissioning, with normal operation to commence in late 2019. During testing and early science operation, FAST has started making astronomical discoveries, particularly pulsars of various kinds, including millisecond pulsars, binaries, gamma-ray pulsars, etc. The papers in this mini-volume propose ambitious observational projects to advance our knowledge of astronomy, astrophysics and fundamental physics in many ways. Although it may take FAST many years to achieve all the goals explained in these papers, taken together they define a powerful strategic vision for the next decade.
Abstract: 1904.05898
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:FSRQ/BL Lac dichotomy as the magnetized advective accretion process around black holes: a unified classification of blazars
(Submitted on 11 Apr 2019)
Abstract: The $Fermi$ blazar observations show a strong correlation between $\gamma$-ray luminosities and spectral indices. BL Lac objects are less luminous with harder spectra than flat-spectrum radio quasars (FSRQs). Interestingly FSRQs are evident to exhibit a Keplerian disc component along with a powerful jet. We compute the jet intrinsic luminosities by beaming corrections determined by different cooling mechanisms. Observed $\gamma$-ray luminosities and spectroscopic measurements of broad emission lines suggest a correlation of the accretion disc luminosity with jet intrinsic luminosity. Also, theoretical and observational inferences for these jetted sources indicate a signature of hot advective accretion flow and a dynamically dominant magnetic field at jet-footprint. Indeed it is difficult to imagine the powerful jet launching from a geometrically thin Keplerian disc. We propose a magnetized, advective disc-outflow symbiosis with explicit cooling to address a unified classification of blazars by controlling both the mass accretion rate and magnetic field strength. The large scale strong magnetic fields influence the accretion dynamics, remove angular momentum from the infalling matter, help in the formation of strong outflows/jets, and lead to synchrotron emissions simultaneously. We suggest that the BL Lacs are more optically thin and magnetically dominated than FSRQs at the jet-footprint to explain their intrinsic $\gamma$-ray luminosities.
Abstract: 1904.06324
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Full Text: [ PostScript, PDF]
Title:Direct measurement of stellar angular diameters by the VERITAS Cherenkov Telescopes
(Submitted on 12 Apr 2019)
Abstract: The angular size of a star is a critical factor in determining its basic properties. Direct measurement of stellar angular diameters is difficult: at interstellar distances stars are generally too small to resolve by any individual imaging telescope. This fundamental limitation can be overcome by studying the diffraction pattern in the shadow cast when an asteroid occults a star, but only when the photometric uncertainty is smaller than the noise added by atmospheric scintillation. Atmospheric Cherenkov telescopes used for particle astrophysics observations have not generally been exploited for optical astronomy due to the modest optical quality of the mirror surface. However, their large mirror area makes them well suited for such high-time-resolution precision photometry measurements. Here we report two occultations of stars observed by the VERITAS Cherenkov telescopes with millisecond sampling, from which we are able to provide a direct measurement of the occulted stars' angular diameter at the $\leq0.1$ milliarcsecond scale. This is a resolution never achieved before with optical measurements and represents an order of magnitude improvement over the equivalent lunar occultation method. We compare the resulting stellar radius with empirically derived estimates from temperature and brightness measurements, confirming the latter can be biased for stars with ambiguous stellar classifications.
Abstract: 1904.06261
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Search for gamma-ray emission from $p$-wave dark matter annihilation in the Galactic Center
(Submitted on 12 Apr 2019)
Abstract: Indirect searches for dark matter through Standard Model products of its annihilation generally assume a cross-section which is dominated by a term independent of velocity ($s$-wave annihilation). However, in many DM models an $s$-wave annihilation cross-section is absent or helicity suppressed. To reproduce the correct DM relic density in these models, the leading term in the cross section is proportional to the DM velocity squared ($p$-wave annihilation). Indirect detection of such $p$-wave DM is difficult because the average velocities of DM in galaxies today are orders of magnitude slower than the DM velocity at the time of decoupling from the primordial thermal plasma, suppressing the annihilation cross-section today by some five orders of magnitude relative to its value at freeze out. Thus $p$-wave DM is out of reach of traditional searches for DM annihilations in the Galactic halo. Near the region of influence of a central supermassive black hole, such as Sgr A$^*$, however, DM can form a localized over-density known as a `spike'. In such spikes the DM is predicted to be both concentrated in space and accelerated to higher velocities, allowing the $\gamma$-ray signature from its annihilation to potentially be detectable above the background. We use the $Fermi$ Large Area Telescope to search for the $\gamma$-ray signature of $p$-wave annihilating DM from a spike around Sgr A$^*$ in the energy range 10 GeV-600 GeV. Such a signal would appear as a point source and would have a sharp line or box-like spectral features difficult to mimic with standard astrophysical processes, indicating a DM origin. We find no significant excess of $\gamma$ rays in this range, and we place upper limits on the flux in $\gamma$-ray boxes originating from the Galactic Center. This result, the first of its kind, is interpreted in the context of different models of the DM density near Sgr A$^*$.
Abstract: 1904.06218
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Full Text: [ PostScript, PDF]
Title:Ground-based Gamma-Ray Astronomy: an Introduction
(Submitted on 11 Apr 2019)
Abstract: During the last two decades Gamma-Ray Astronomy has emerged as a powerful tool to study cosmic ray physics. In fact, photons are not deviated by galactic or extragalactic magnetic fields so their directions bring the information of the production sites and are easier to detect than neutrinos. Thus the search for $\gamma$ primarily address in the framework of the search of cosmic ray sources and to the investigation of the phenomena in the acceleration sites. This note is not a place for a review of ground-based gamma-ray astronomy. We will introduce the experimental techniques used to detect photons from ground in the overwhelming background of CRs and briefly describe the experiments currently in data taking or under installation.
Abstract: 1904.06155
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Full Text: [ PostScript, PDF]
Title:Pushing the Limits of Exoplanet Discovery via Direct Imaging with Deep Learning
(Submitted on 12 Apr 2019)
Abstract: Further advances in exoplanet detection and characterisation require sampling a diverse population of extrasolar planets. One technique to detect these distant worlds is through the direct detection of their thermal emission. The so-called direct imaging technique, is suitable for observing young planets far from their star. These are very low signal-to-noise-ratio (SNR) measurements and limited ground truth hinders the use of supervised learning approaches. In this paper, we combine deep generative and discriminative models to bypass the issues arising when directly training on real data. We use a Generative Adversarial Network to obtain a suitable dataset for training Convolutional Neural Network classifiers to detect and locate planets across a wide range of SNRs. Tested on artificial data, our detectors exhibit good predictive performance and robustness across SNRs. To demonstrate the limits of the detectors, we provide maps of the precision and recall of the model per pixel of the input image. On real data, the models can re-confirm bright source detections.
Abstract: 1904.06135
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Cosmological constraints from cosmic homogeneity
(Submitted on 12 Apr 2019)
Abstract: In this paper, we study the normalised characteristic scale of transition to cosmic homogeneity, $\mathcal{R}_H/d_V$, as a cosmological probe. We use a compilation of the SDSS galaxy samples, comprising more than $10^6$ galaxies in the redshift range $0.17 \leq z \leq 2.2$ within the largest comoving volume to date, $\sim 8 h^{-3}Gpc^3$. We show that these samples can be described by a single bias model as a function of redshift. By combining our measurements with prior Cosmic Microwave Background and Lensing information from the Planck satellite, we constrain the total matter density ratio of the universe $\Omega_m = 0.340 \pm 0.029$ and the Dark Energy density ratio $\Omega_{\Lambda} = 0.668 \pm 0.023$. Our results are compatible with a flat $\Lambda$CDM model. These results show the complementarity of the normalised homogeneity scale with other cosmological probes and open new roads to cosmometry.
Abstract: 1904.05882
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Planning the scientific applications of the Five-hundred-meter Aperture Spherical radio Telescope
(Submitted on 12 Apr 2019)
Abstract: The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is by far the largest telescope of any kind ever built. FAST produced its first light in September 2016 and it is now under commissioning, with normal operation to commence in late 2019. During testing and early science operation, FAST has started making astronomical discoveries, particularly pulsars of various kinds, including millisecond pulsars, binaries, gamma-ray pulsars, etc. The papers in this mini-volume propose ambitious observational projects to advance our knowledge of astronomy, astrophysics and fundamental physics in many ways. Although it may take FAST many years to achieve all the goals explained in these papers, taken together they define a powerful strategic vision for the next decade.
Abstract: 1904.05721
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
(Submitted on 11 Apr 2019)
Abstract: We present a 0.16% precise and 0.27% accurate determination of R0, the distance to the Galactic Center. Our measurement uses the star S2 on its 16-year orbit around the massive black hole Sgr A* that we followed astrometrically and spectroscopically for 27 years. Since 2017, we added near-infrared interferometry with the VLTI beam combiner GRAVITY, yielding a direct measurement of the separation vector between S2 and Sgr A* with an accuracy as good as 20 micro-arcsec in the best cases. S2 passed the pericenter of its highly eccentric orbit in May 2018, and we followed the passage with dense sampling throughout the year. Together with our spectroscopy, in the best cases with an error of 7 km/s, this yields a geometric distance estimate: R0 = 8178 +- 13(stat.) +- 22(sys.) pc. This work updates our previous publication in which we reported the first detection of the gravitational redshift in the S2 data. The redshift term is now detected with a significance level of 20 sigma with f_redshift = 1.04 +- 0.05.
Abstract: 1904.05589
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Cherenkov in the Sky: Measuring the Sound Speed of Primordial Scalar Fluctuations
(Submitted on 11 Apr 2019)
Abstract: We consider production of additional relativistic particles coupled to the inflaton. We show that the imprints of these particles on the spectrum of primordial perturbations can be used for the direct measurement of the speed of sound of scalar perturbations, regardless of the mechanism of the production of this species. We study a model where these relativistic localized sources are decay products of heavier particles generated via a resonance mechanism. These particles emit in-phase inflaton particles which interfere constructively on the the so-called sound boom, leading to an "inflationary Cherenkov effect". The resulting shock waves lead to distinctive patterns on the temperature anisotropies of the cosmic microwave background. Moreover, we show that the model predicts unique features on the power spectrum of curvature perturbation and sizeable flattened non-gaussianity for a suitable range of parameters.
Abstract: 1904.05821
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Holography for the very early Universe and the classic puzzles of Hot Big Bang cosmology
(Submitted on 11 Apr 2019)
Abstract: We show that standard puzzles of hot Big Bang cosmology that motivated the introduction of cosmological inflation, such as the smoothness and horizon problem, the flatness problem, the relic problem, the way to generate enough entropy and perturbations, are also solved by holographic models for very early universe based on perturbative three dimensional QFT. In the holographic setup, cosmic evolution is mapped to inverse renormalization group (RG) flow of the dual QFT, and the resolution of the puzzles relies on properties of the RG flow.
Abstract: 1904.05594
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Several Problems in Particle Physics and Cosmology Solved in One SMASH
(Submitted on 11 Apr 2019)
Abstract: The Standard Model (SM) of particle physics is a big success. However, it lacks explanations for cosmic inflation, the matter-anti-matter asymmetry of the Universe, dark matter, neutrino oscillations, and the feebleness of CP violation in the strong interactions. The latter may be explained by a complex scalar field charged under a spontaneously broken global U(1) Peccei-Quinn (PQ) symmetry. Moreover, the pseudo Nambu-Goldstone boson of this breaking -- the axion -- may play the role of the dark matter. Furthermore, the modulus of the PQ field is a candidate for driving inflation. If additionally three extra SM singlet neutrinos (whose mass is induced by the PQ field) are included, the five aforementioned problems can be addressed at once. We review the SM extension dubbed SMASH -for SM-Axion-Seesaw-Higgs portal inflation-, discuss its predictions and tests in astrophysics, cosmology, and laboratory experiments. Variants of SMASH are also considered and commented on.
Abstract: 1904.05471
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Signatures for quark matter from multi-messenger observations
(Submitted on 10 Apr 2019 (v1), last revised 12 Apr 2019 (this version, v2))
Abstract: We review the prospects for detecting quark matter in neutron star cores. We survey the proposed signatures and emphasize the importance of data from neutron star mergers, which provide access to dynamical properties that operate on short timescales that are not probed by other neutron star observables.
Abstract: 1904.05287
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Searching for quantum black hole structure with the Event Horizon Telescope
(Submitted on 10 Apr 2019 (v1), last revised 12 Apr 2019 (this version, v2))
Abstract: The impressive images from the Event Horizon Telescope sharpen the conflict between our observations of gravitational phenomena and the principles of quantum mechanics. Two related scenarios for reconciling quantum mechanics with the existence of black hole-like objects, with "minimal" departure from general relativity and local quantum field theory, have been explored; one of these could produce signatures visible to EHT observations. A specific target is temporal variability of images, with a characteristic time scale determined by the classical black hole radius. The absence of evidence for such variability in the initial observational span of seven days is not expected to strongly constrain such variability. Theoretical and observational next steps towards investigating such scenarios are outlined.
Abstract: 1904.05139
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:H.E.S.S. observations of the flaring gravitationally lensed galaxy PKS 1830-211
(Submitted on 10 Apr 2019)
Abstract: PKS 1830-211 is a known macrolensed quasar located at a redshift of z=2.5. Its high-energy gamma-ray emission has been detected with the Fermi-LAT instrument and evidence for lensing was obtained by several authors from its high-energy data.
Observations of PKS 1830-211 were taken with the H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes in August 2014, following a flare alert by the Fermi- LAT collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20-27 days from the alert flare, as expected from observations at other wavelengths.
More than twelve hours of good quality data were taken with an analysis threshold of $\sim67$ GeV. The significance of a potential signal is computed as a function of the date as well as the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT.
No photon excess or significant signal is detected. An upper limit on PKS 1830-211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.
Abstract: 1904.04934
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Simulations of imaging the event horizon of Sagittarius A* from space
(Submitted on 9 Apr 2019)
Abstract: It has been proposed that Very Long Baseline Interferometry (VLBI) at sub-millimeter waves will allow us to image the shadow of the black hole in the center of our Milky Way, Sagittarius A* (Sgr A*), and thereby test basic predictions of general relativity. This paper presents imaging simulations of a new Space VLBI mission concept. An initial design study of the concept has been presented as the Event Horizon Imager (EHI). The EHI may be suitable for imaging Sgr A* at high frequencies (up to ~690 GHz), which has significant advantages over performing ground-based VLBI at 230 GHz. The concept EHI design consists of two or three satellites in polar or equatorial circular Medium-Earth Orbits with slightly different radii. Due to the relative drift of the satellites along the individual orbits, this setup will result in a dense spiral-shaped uv-coverage with long baselines (up to ~60 Glambda), allowing for extremely high-resolution and high-fidelity imaging of radio sources. We simulate EHI observations of general relativistic magnetohydrodynamics models of Sgr A* and calculate the expected noise based on preliminary system parameters. On long baselines, where the signal-to-noise ratio may be low, fringes could be detected if the system is sufficiently phase stable and the satellite orbits can be reconstructed with sufficient accuracy. Averaging visibilities accumulated over multiple epochs of observations could then help improving the image quality. With three satellites, closure phases could be used for imaging. Our simulations show that the EHI could be capable of imaging the black hole shadow of Sgr A* with a resolution of 4 uas (about 8% of the shadow diameter) within several months of observing time. The EHI concept could thus be used to measure black hole shadows much more precisely than with ground-based VLBI, allowing for stronger tests of general relativity and accretion models.
Abstract: 1904.04923
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project
(Submitted on 9 Apr 2019)
Abstract: Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope (EHT) and high precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD and KORAL. Agreement between the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.
Abstract: 1904.04654
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Electric charge of black holes: Is it really always negligible?
(Submitted on 9 Apr 2019)
Abstract: We discuss the problem of the third black hole parameter, an electric charge. While the mass and the spin of black holes are frequently considered in the majority of publications, the charge is often neglected and implicitly set identically to zero. However, both classical and relativistic processes can lead to a small non-zero charge of black holes. When dealing with neutral particles and photons, zero charge is a good approximation. On the other hand, even a small charge can significantly influence the motion of charged particles, in particular cosmic rays, in the vicinity of black holes. Therefore, we stress that more attention should be paid to the problem of a black-hole charge and hence, it should not be neglected a priori, as it is done in most astrophysical studies nowadays. The paper looks at the problem of the black-hole charge mainly from the astrophysical point of view, which is complemented by a few historical as well as philosophical notes when relevant. In particular, we show that a cosmic ray or in general elementary charged particles passing a non-neutral black hole can experience an electromagnetic force as much as sixteen times the gravitational force for the mass of the Galactic centre black hole and its charge being seventeen orders of magnitude less than the extremal value. Furthermore, a Kerr-Newman rotating black hole with the maximum likely charge of 1 Coulomb per solar mass can have the position of its innermost stable circular orbit (ISCO) moved by both rotation and charge in ways that can enhance or partly cancel each other, putting the ISCO not far from the gravitational radius or out at more than 6 gravitational radii. An interpretation of X-ray radiation from near the ISCO of a black hole in X-ray binaries is then no longer unique.
Abstract: 1904.04355
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Extragalactic neutrinos as tracers of Dark Matter?
(Submitted on 8 Apr 2019)
Abstract: Neutrinos produced in extragalactic sources may experience flavor-oscillations and decoherence on their way to Earth due to their interaction with dark matter (DM). As a result, they may be detected in pointer-states other than the flavor states at the source. The oscillation pattern and the structure of the pointer-states can give us information on the characteristics of the DM and the kind of interaction that has taken place. From this perspective, neutrinos can be viewed as DM-tracers. We study the local evolution of neutrino flavor-eigenstates due to local effects produced by the presence of DM. To explore the sensitivity of the model, we consider different DM density profiles, masses and interactions. Starting from the eigenstates of the neutrino-mass Hamiltonian, we construct the flavor-states with the neutrino mixing-matrix in vacuum. We then include local interactions with DM, acting along the neutrino path towards the Earth, and analyse the resulting probabilities. In doing so, we adopt different DM density profiles, e.g. a constant, a local isotropic and a Navarro-Frenk-White density distribution. Finally, by following the time evolution of the flavor-states, we identify pointer-states and interpret the results in terms of the adopted DM model. Due to the interaction with DM, neutrinos experience the MSW effect, the extent of which depends on the DM density profile. The interaction with DM produces the enhancement or suppression of oscillations. Decoherence effects may take place. We model the time evolution of extragalactic neutrino flavor-states by letting them interact with DM. The features of the calculated response seem to support the notion that these neutrinos can be taken as DM tracers. From a theoretical point of view, the coexistence and/or competition of decoherence and MSW effects is sustained by the results.
Abstract: 1904.04074
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Alternative approach to gravity and MOND
(Submitted on 8 Apr 2019)
Abstract: The classical gravitational two-body problem is generalized in order to be applicable also to weak gravitational fields. The equation of motion holds both for terrestrial and large cosmic scales, the Newtonian gravitational law represents a mathematical limit of the generalized form. Motivation comes from observational results on rotation curves of galaxies. Existence of a dark matter is not assumed. The crucial laws of physics hold and also the potential energy of the system is symmetric with respect to masses of the two bodies. Shortcomings of the results published for decades, including MOND theories and false-yet-familiar approaches, are overcome. The impact on searching for a fundamental physical theory is stressed. Some of the conventional ideas of the past centuries do not hold for the zone of small accelerations, e.g., the principle of least action using the Lagrangian density of potentials and fields does not work. We may look forward to great changes in our understanding of the evolution of the Universe.
Abstract: 1904.03944
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Ensuring Uninterrupted Power Supply to Lunar Installations Through an Organic Rankine Cycle
(Submitted on 8 Apr 2019 (v1), last revised 12 Apr 2019 (this version, v2))
Abstract: We propose using the temperature gradients between the Moon's surface and the soil at a certain depth to power an Organic Rankine Cycle that could supply a permanent installation, particularly at night, when solar power is not available. Our theoretical and engineering considerations show that, with existing working fluids and quite feasible technical requirements, it is possible to continuously yield $25\,{\rm kW}$ to sustain a 3 member crew.
Abstract: 1904.03894
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Probing galactic cosmic ray distribution with TeV gamma-ray sky
(Submitted on 8 Apr 2019)
Abstract: The distribution of cosmic rays in the Galaxy at energies above few TeVs is still uncertain and this affects the expectations for the diffuse gamma flux produced by hadronic interactions of cosmic rays with the interstellar gas. We show that the TeV gamma-ray sky can provide interesting constraints. Namely, we compare the flux from the galactic plane measured by Argo-YBJ, HESS, HAWC and Milagro with the expected flux due to diffuse emission and point-like and extended sources observed by HESS showing that experimental data can already discriminate among different hyphoteses for cosmic ray distribution. The constraints can be strengthened if the contribution of sources not resolved by HESS is taken into account.
Abstract: 1904.03243
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Accelerating AGN jets to parsec scales using general relativistic MHD simulations
(Submitted on 5 Apr 2019)
Abstract: Accreting black holes produce collimated outflows, or jets, that traverse many orders of magnitude in distance, accelerate to relativistic velocities, and collimate into tight opening angles. Of these, perhaps the least understood is jet collimation due to the interaction with the ambient medium. In order to investigate this interaction, we carried out axisymmetric general relativistic magnetohydrodynamic simulations of jets produced by a large accretion disc, spanning over 5 orders of magnitude in time and distance, at an unprecedented resolution. Supported by such a disc, the jet attains a parabolic shape, similar to the M87 galaxy jet, and the product of the Lorentz factor and the jet half-opening angle, $\gamma\theta\ll 1$, similar to values found from very long baseline interferometry (VLBI) observations of active galactic nuclei (AGN) jets; this suggests extended discs in AGN. We find that the interaction between the jet and the ambient medium leads to the development of pinch instabilities, which produce significant radial and lateral variability across the jet by converting magnetic and kinetic energy into heat. Thus pinched regions in the jet can be detectable as radiating hotspots and may provide an ideal site for particle acceleration. Pinching also causes gas from the ambient medium to become squeezed between magnetic field lines in the jet, leading to enhanced mass-loading of the jet and potentially contributing to the spine-sheath structure observed in AGN outflows.
Abstract: 1904.04142
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:The masses and shadows of the black holes Sagittarius A* and M87 in modified gravity (MOG)
(Submitted on 8 Apr 2019)
Abstract: We present calculations for the anticipated shadow sizes of Sgr A* and the supermassive black hole in the galaxy M87 in the context of our modified theory of gravity (MOG, also known as Scalar-Tensor-Vector-Gravity, or STVG). We demonstrate that mass estimates derived from stellar dynamics in the vicinity of these black holes are the Newtonian masses of the black holes even in the MOG theory. Consequently, shadow sizes increase as a function of the key MOG parameter $\alpha$, and may offer an observational means to distinguish the MOG theory from standard general relativity.
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