Abstracts of Interest
Selected by:
Peter Marinos
Abstract: 2205.06831
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Title:Aspects of Axions and ALPs Phenomenology
Download PDFAbstract: The physics of axions and axion-like particles (ALPs) is enjoying an incredibly productive period, with many new experimental proposals, theoretical idea, and original astrophysical and cosmological arguments which help the search effort. The large number of experimental proposals is likely to lead to fundamental advances (perhaps, a discovery?) in the coming years. The aim of this article is to provide a very brief overview of some of the recent developments in axions and ALP phenomenology, and to discuss some relevant aspects in this important field. A particular attention is given to the definition of motivated regions in the axion parameters space, which should be the targets of experimental searches.
Abstract: 2205.07038
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Title:Geminga contribution to the cosmic-ray positron excess according to the gamma-ray observations
Download PDFAbstract: We attempt to interpret the cosmic-ray positron excess by injection from the nearby pulsar Geminga, assuming a two-zone diffusion scenario and an injection spectrum with a low energy cutoff. Since the high energy positrons and electrons from Geminga can induce $\gamma$ rays via inverse Compton scattering, we take into account the extended $\gamma$-ray observations around Geminga from HAWC for $\sim 10$ TeV and from Fermi-LAT for $\mathcal{O}(10)$ GeV. According to the extended $\gamma$-ray observation claimed by an analysis of Fermi-LAT data, we find that Geminga could explain the positron excess for a $40\%$ energy conversion efficiency into positrons and electrons. However, based on the constraint on the extended $\gamma$ rays given by another Fermi-LAT analysis, positrons from Geminga would be insufficient to account for the positron excess. A further robust analysis of Fermi-LAT data for the extended $\gamma$ rays would be crucial to determine whether Geminga can explain the positron excess or not.
Abstract: 2205.07675
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Title:Further support and a candidate location for Planet 9
Download PDFAbstract: The existence of a hypothetical Planet 9 lurking in the outer solar system has been proposed as an explanation for the the anomalous clustering in the orbits of some trans-Neptunian objects. Here we propose to use meteors arriving at Earth as messengers with the potential of revealing the presence of a hitherto undiscovered massive object. The peculiar meteor CNEOS 2014-01-08, recenty proposed as the first interstellar meteor, might be one such messenger. Its procedence on the sky matches the predicted band of Planet 9's orbit and it is actually compatible with the highest probability region. The odds of this coincidence being due to chance are of ~0.5%. Furthermore, some statistical anomalies about CNEOS 2014-01-08 are resolved under the hypothesis that it was flung at Earth by Planet 9, as opposed to being the result of an unmediated interstellar encounter with our planet. Based on the available data, we propose the region at coordinates RA:50.0{\pm}4°, dec:11.8{\pm}1.8° in the constellation of Aries, as the first candidate location for Planet 9.
Abstract: 2205.07787
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Full Text: [ PostScript, PDF]
Title:Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$
Download PDFAbstract: Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime, allowing us to test whether the Kerr metric provides a good description of the space-time in the vicinity of the event horizons of supermassive BHs. We consider a wide range of well-motivated deviations from classical General Relativity solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A$^*$ (SgrA$^*$), connecting the size of the bright ring of emission to that of the underlying BH shadow and exploiting high-precision measurements of SgrA$^*$'s mass-to-distance ratio. The scenarios we consider, and whose fundamental parameters we constrain, include various regular BH models, string- and non-linear electrodynamics-inspired space-times, scalar field-driven violations of the no-hair theorem, alternative theories of gravity, new ingredients such as the generalized uncertainty principle and Barrow entropy, and BH mimickers including examples of wormholes and naked singularities. We demonstrate that SgrA$^*$'s image places particularly stringent constraints on models predicting a shadow size which is larger than that of a Schwarzschild BH of a given mass: for instance, in the case of Barrow entropy we derive constraints which are significantly tighter than the cosmological ones. Our results are among the first tests of fundamental physics from the shadow of SgrA$^*$ and, while the latter appears to be in excellent agreement with the predictions of General Relativity, we have shown that various well-motivated alternative scenarios (including BH mimickers) are far from being ruled out at present.
Abstract: 2205.07996
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Title:Horizons: Nuclear Astrophysics in the 2020s and Beyond
Download PDFAbstract: Nuclear Astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.
Abstract: 2205.08174
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Title:Turbulent diffusion of streaming cosmic rays in compressible, partially ionised plasma
Download PDFAbstract: Cosmic rays (CRs) are a dynamically important component of the interstellar medium (ISM) of galaxies. The $\sim$GeV CRs that carry most CR energy and pressure are likely confined by self-generated turbulence, leading them to stream along magnetic field lines at the ion Alfvén speed. However, the consequences of self-confinement for CR propagation on galaxy scales remain highly uncertain. In this paper, we use a large ensemble of magnetohydrodynamical turbulence simulations to quantify how the basic parameters describing ISM turbulence -- the sonic Mach number, $\mathcal{M}$ (plasma compressibility), Alfvén Mach number, $\mathcal{M}_{A0}$ (strength of the large-scale field with respect to the turbulence), and ionisation fraction by mass, $\chi$ -- affect the transport of streaming CRs. We show that the large-scale transport of CRs whose small-scale motion consists of streaming along field lines is well described as a combination of streaming along the mean field and superdiffusion both along (parallel to) and across (perpendicular to) it; $\mathcal{M}_{A0}$ drives the level of anisotropy between parallel and perpendicular diffusion and $\chi$ modulates the magnitude of the diffusion coefficients, while in our choice of units, $\mathcal{M}$ is unimportant except in the sub-Alfvénic ($\mathcal{M}_{A0} \lesssim 0.5$) regime. Our finding that superdiffusion is ubiquitous potentially explains the apparent discrepancy between CR diffusion coefficients inferred from measurements close to individual sources compared to those measured on larger, Galactic scales. Finally, we present empirical fits for the diffusion coefficients as a function of plasma parameters that may be used as sub-grid recipes for global interstellar medium, galaxy or cosmological simulations.
Abstract: 2205.08498
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Title:Curved jet motion. I. Orbiting and precessing jets
Download PDFAbstract: Astrophysical jets are often observed as bent or curved structures. We also know that the different jet sources may be binary in nature, which may lead to a regular, periodic motion of the jet nozzle, an orbital motion or precession. Here, we present the results of 2D (M)HD simulations in order to investigate how a precessing or orbiting jet nozzle affects the propagation of a high-speed jet. We have performed a parameter study of systems with different precession angles, orbital periods or separations, and different magnetic field strengths. We find that these kinds of nozzles lead to curved jet propagation which is determined by the main parameters that define the jet nozzle. We find C-shaped jets from orbiting nozzles and S-shaped jets from precessing nozzles. Over long time and long distances, the initially curved jet motion bores a broad channel into the ambient gas that is filled with high-speed jet material which lateral motion is damped, however. A strong (longitudinal) magnetic field can damp the jet curvature that is enforced by either precession of orbital motion of the jet sources. We have investigated the force balance across the jet and ambient medium and found that the lateral magnetic pressure and gas pressure gradients are almost balanced, but that a lack of gas pressure on the concave side of the curvature is leading to the lateral motion. Magnetic tension does not play a significant role. Our results are obtained in code units, but we provide scaling relations such that our results may be applied to young stars, micro-quasars, symbiotic stars or AGN.
Abstract: 2205.08544
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Title:Anisotropic diffusion cannot explain TeV halo observations
Download PDFAbstract: TeV halos are regions of enhanced photon emissivity surrounding pulsars. While multiple sources have been discovered, a self-consistent explanation of their radial profile and spherically-symmetric morphology remains elusive due to the difficulty in confining high-energy electrons and positrons within ~20 pc regions of the interstellar medium. One proposed solution utilizes anisotropic diffusion to confine the electron population within a "tube" that is auspiciously oriented along the line of sight. In this work, we show that while such models may explain a unique source such as Geminga, the phase space of such solutions is very small and they are unable to simultaneously explain the size and approximate radial symmetry of the TeV halo population.
Abstract: 2205.08566
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Title:GeV emission from a compact binary merger
Download PDFAbstract: An energetic $\rm \gamma$-ray burst (GRB), GRB 211211A, was observed on 2021 December 11 by the Neil Gehrels Swift Observatory. Despite its long duration, typically associated with bursts produced by the collapse of massive stars, the discovery of an optical-infrared kilonova and a quasi-periodic oscillation during a gamma-ray precursor points to a compact object binary merger origin. The complete understanding of this nearby ($\sim$ 1 billion light-years) burst will significantly impact our knowledge of GRB progenitors and the physical processes that lead to electromagnetic emission in compact binary mergers. Here, we report the discovery of a significant ($\rm >5 \sigma$) transient-like emission in the high-energy $\rm \gamma$-rays (HE; E$>0.1$ GeV) observed by Fermi/LAT starting at $10^3$ s after the burst. After an initial phase with a roughly constant flux ($\rm \sim 5\times 10^{-10}\ erg\ s^{-1}\ cm^{-2}$) lasting $\sim 2\times 10^4$ s, the flux started decreasing and soon went undetected. The multi-wavelength afterglow emission observed at such late times is usually in good agreement with synchrotron emission from a relativistic shock wave that arises as the GRB jet decelerates in the interstellar medium. However, our detailed modelling of a rich dataset comprising public and dedicated multi-wavelength observations demonstrates that GeV emission from GRB 211211A is in excess with respect to the expectation of this scenario. We explore the possibility that the GeV excess is inverse Compton emission due to the interaction of a long-lived, low-power jet with an external source of photons. We discover that the kilonova emission can provide the necessary seed photons for GeV emission in binary neutron star mergers.
Abstract: 2205.08898
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Title:Polarized x-rays from a magnetar
Download PDFAbstract: We report on the first detection of linearly polarized x-ray emission from an ultra-magnetized neutron star with the Imaging X-ray Polarimetry Explorer (IXPE). The IXPE 35 observations of the anomalous x-ray pulsar 4U 0142+61 reveal a linear polarization degree of $(12\pm 1)\%$ throughout the IXPE 2--8 keV band. We detect a substantial variation of the polarization with energy: the degree is $(14\pm 1)\%$ at 2--4 keV and $(41\pm 7)\%$ at 5.5--8 keV, while it drops below the instrumental sensitivity around 4--5 keV, where the polarization angle swings by $\sim 90^\circ$. The IXPE observations give us completely new information about the properties of the neutron star surface and magnetosphere and lend further support to the presence of the quantum mechanical effect of vacuum birefringence.
Abstract: 2205.09145
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Title:Multi-messenger constraints on the Hubble constant through combination of gravitational waves, gamma-ray bursts and kilonovae from neutron star mergers
Download PDFAbstract: The simultaneous detection of gravitational waves and light from the binary neutron star merger GW170817 led to independent measurements of distance and redshift, providing a direct estimate of the Hubble constant $H_0$ that does not rely on a cosmic distance ladder nor assumes a specific cosmological model. By using gravitational waves as ''standard sirens'', this approach holds promise to arbitrate the existing tension between the $H_0$ value inferred from the cosmic microwave background and those obtained from local measurements. However, the known degeneracy in the gravitational-wave analysis between distance and inclination of the source lead to a $H_0$ value from GW170817 that was not precise enough to resolve the existing tension. In this review, we summarize recent works exploiting the viewing-angle dependence of the electromagnetic signal, namely the associated short gamma-ray burst and kilonova, to constrain the system inclination and improve on $H_0$. We outline the key ingredients of the different methods, summarize the results obtained in the aftermath of GW170817 and discuss the possible systematics introduced by each of these methods.
Abstract: 2205.09225
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Title:Energetic Particle Perpendicular Diffusion: Simulations and Theory in Noisy Reduced Magnetohydrodynamic Turbulence
Download PDFAbstract: The transport of energetic charged particles (e.g., cosmic rays) in turbulent magnetic fields is usually characterized in terms of the diffusion parallel and perpendicular to a large-scale (or mean) magnetic field. The nonlinear guiding center theory (NLGC) has been a prominent perpendicular diffusion theory. A recent version of this theory, based on random ballistic spreading of magnetic field lines and a backtracking correction (RBD/BC), has shown good agreement with test particle simulations for a two-component magnetic turbulence model. The aim of the present study is to test the generality of the improved theory by applying it to the noisy reduced magnetohydrodynamic (NRMHD) turbulence model, determining perpendicular diffusion coefficients that are compared with those from the field line random walk (FLRW) and unified nonlinear (UNLT) theories and our test particle simulations. The synthetic NRMHD turbulence model creates special conditions for energetic particle transport, with no magnetic fluctuations at higher parallel wavenumbers so there is no resonant parallel scattering if the particle Larmor radius $R_{\rm L}$ is even slightly smaller than the minimum resonant scale. This leads to non-monotonic variation in the parallel mean free path $\lambda_\parallel$ with $R_{\rm L}$. Among the theories considered, only RBD/BC matches simulations within a factor of two over the range of parameters considered. This accuracy is obtained even though the theory depends on $\lambda_\parallel$ and has no explicit dependence on $R_{\rm L}$. In addition, the UNLT theory often provides accurate results and even the FLRW limit provides a very simple and reasonable approximation in many cases.
Abstract: 2205.09450
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Title:Search for Black Holes in the Galactic Halo by Gravitational Microlensing
Download PDFAbstract: Black hole-like objects with mass greater than $10 M_{\odot}$, as discovered by gravitational antennas, can produce long time-scale (several years) gravitational microlensing effects. Considered separately, previous microlensing surveys were insensitive to such events because of their limited duration of 6-7 years. We combined light curves from the EROS-2 and MACHO surveys to the Large Magellanic Cloud (LMC) to create a joint database for 14.1 million stars, covering a total duration of 10.6 years, with fluxes measured through 4 wide passbands. We searched for multi-year microlensing events in this catalog of extended light curves, complemented by 24.1 million light curves observed by only one of the surveys. Our analysis, combined with previous analysis from EROS, shows that compact objects with mass between $10^{-7}$ and $200 M_{\odot}$ can not constitute more than $\sim 20\%$ of the total mass of a standard halo (at $95\%$ CL). We also exclude that $\sim 50\%$ of the halo is made of Black Holes (BH) lighter than $1000 M_{\odot}$.
Abstract: 2205.09565
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Title:High-energy Gamma-rays from Magnetically Arrested Disks in Nearby Radio Galaxies
Download PDFAbstract: The origins of the GeV gamma-rays from nearby radio galaxies are unknown. Hadronic emission from magnetically arrested disks (MADs) around central black holes (BHs) is proposed as a possible scenario. Particles are accelerated in the MAD by magnetic reconnection and stochastic turbulence acceleration. We pick out the fifteen brightest radio galaxies in the GeV band from the Fermi 4LAC-DR2 catalog and apply the MAD model. We find that we can explain the data in the GeV bands by the MAD model if the accretion rate is lower than 0.1% of the Eddington rate. For a higher accretion rate, GeV gamma-rays are absorbed by two-photon interaction due to copious low-energy photons. If we assume another proposed prescription of the electron heating rate by magnetic reconnection, the MAD model fails to reproduce the GeV data for the majority of our sample. This indicates that the electron heating rate is crucial. We also apply the MAD model to Sgr A* and find that GeV gamma-rays observed at the Galactic center do not come from the MAD of Sgr A*. We estimate the cosmic ray intensity from Sgr A*, but it is too low to explain the high-energy cosmic ray intensity on Earth.
Abstract: 2205.09675
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Title:Fermi-LAT detection of a GeV afterglow from a compact stellar merger
Download PDFAbstract: It is usually thought that long-duration gamma-ray bursts (GRBs) are associated with massive star core collapse whereas short-duration GRBs are associated with mergers of compact stellar binaries. The discovery of a kilonova associated with a nearby (350 Mpc) long-duration GRB- GRB 211211A, however, indicates that the progenitor of this long-duration GRB is a compact object merger. Here we report the \emph{Fermi}-LAT detection of gamma-ray ($>100 {\rm \ MeV}$) afterglow emission from GRB 211211A, which lasts $\sim 20000$ s after the burst, the longest event for conventional short-duration GRBs ever detected. We suggest that this gamma-ray emission results mainly from afterglow synchrotron emission. The soft spectrum of GeV emission may arise from a limited maximum synchrotron energy of only a few hundreds of MeV at $\sim 20000$ s. The usually long duration of the GeV emission could be due to the proximity of this GRB and the long deceleration time of the GRB jet that is expanding in a low density cricumburst medium, consistent with the compact stellar merger scenario.
Abstract: 2205.09785
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Title:Neutrino pair annihilation above black-hole accretion disks in modified gravity
Download PDFAbstract: Using idealized models of the accretion disk, we investigate the effects induced by the modified theories of gravity on the annihilation of the neutrino pair annihilation into electron-positron pairs ($\nu{\bar \nu}\to e^-e^+$), occurring near the rotational axis. For the accretion disk, we have considered the models with temperature $T=constant$ and $T\propto r^{-1}$. In both cases, we find that the modified theories of gravity lead to an enhancement, up to more than one order of magnitude with respect to General Relativity, of the rate of the energy deposition rate of neutrino pair annihilation.
Abstract: 2205.10172
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Title:The Curvature of Spectral Energy Distribution and $γ$-ray Dominance of Fermi BL Lac Objects
Download PDFAbstract: The extragalactic $\gamma$-ray sky is dominated by blazars and their study plays an important role in understanding jet physics, cosmic evolution history and origin of ultra high energy cosmic rays. In this work, we study a large sample of BL Lac objects to investigate why some sources are detected in $\gamma$-rays, while others not. We selected 170 BL Lac objects, with measured synchrotron spectral curvature and Doppler factor, and divided them into Fermi-LAT detected (FBLs) and non-detected (NFBLs) sources. We show that FBLs have smaller curvature than NFBLs, even after getting rid of Doppler beaming effect. The BL Lac objects PKS 0048-09 and S5 0716+714 have similar synchrotron peak frequency and luminosity but different $\gamma$-ray dominance and their quasi-simultaneous broadband spectral energy distributions (SEDs) can be well fitted by a log-parabolic synchrotron self-Compton (SSC) model with same jet parameters except for the curvature and source size, assuming curvature being proportional to the size of emission region. Our results imply that for a given synchrotron luminosity, the different SED curvature and Compton dominance may account for the discrepancy between FBLs and NFBLs. We discuss these results in context of stochastic particle acceleration and radiation mechanisms.
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