Abstracts of Interest
Selected by:
Peter Marinos
Abstract: 2208.14459
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Title:The velocity distribution of outflows driven by choked jets in stellar envelopes
Download PDFAbstract: Many stripped envelope supernovae (SNe) present a signature of high-velocity material responsible for broad absorption lines in the observed spectrum. These include SNe that are associated with long gamma-ray bursts (LGRBs) and low-luminosity GRBs (llGRBs), and SNe that are not associated with GRBs. Recently it was suggested that this high velocity material originates from a cocoon that is driven by a relativistic jet. In LGRBs this jet breaks out successfully from the stellar envelope, while in llGRBs and SNe that are not associated with GRBs the jet is choked. Here we use numerical simulations to explore the velocity distribution of an outflow that is driven by a choked jet and its dependence on the jet and progenitor properties. We find that in all cases where the jet is not choked too deep within the star, the outflow carries a roughly constant amount of energy per logarithmic scale of proper velocity over a wide range of velocities, which depends mostly on the cocoon volume at the time of its breakout. This is a universal property of jets driven outflows, which does not exist in outflows of spherically symmetric explosions or when the jets are choked very deep within the star. We therefore conclude that jets that are choked (not too deep) provide a natural explanation to the fast material seen in the early spectra of stripped envelope SNe that are not associated with LGRBs and that properties of this material could reveal information on the otherwise hidden jets.
Abstract: 2208.14460
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Full Text: [ PostScript, PDF]
Title:Astrometric Microlensing of Primordial Black Holes with Gaia
Download PDFAbstract: The Gaia space telescope allows for unprecedented accuracy for astrometric measurements of stars in the Galaxy. In this work, we explore the sensitivity of Gaia to detect primordial black hole (PBH) dark matter through the distortions that PBHs would create in the apparent trajectory of background stars, an effect known as astrometric microlensing (AML). We present a novel calculation of the lensing probability, and we combine this with the existing publicly released Gaia eDR3 stellar catalog to predict the expected rate of AML events that Gaia will see. We also compute the expected distribution of a few event observables, which will be useful for reducing backgrounds. We argue that the astrophysical background rate of AML like events due to other sources is negligible (except possibly for very long duration events), and we use this to compute the potential exclusion that could be set on the parameter space of PBHs with a monochromatic mass function. We find that Gaia is sensitive to PBHs in the range of $0.4~M_\odot$ - $5\times10^7~M_\odot$, and has peak sensitivity to PBHs of $\sim 10~M_\odot$ for which it can rule out as little as a fraction $3\times10^{-4}$ of dark matter composed of PBHs. With this exquisite sensitivity, Gaia has the potential to rule out a PBH origin for the gravitational wave signals seen at LIGO. Our novel calculation of the lensing probability includes for the first time, the effect of intermediate duration lensing events, where the lensing event lasts for a few years, but for a period which is still shorter than the Gaia mission lifetime. The lower end of our predicted mass exclusion is especially sensitive to these type of lensing events. As and when time-series data for Gaia is released, our prediction of the lensing rate and event observable distributions will be useful to estimate the true exclusion/discovery of the PBH parameter space utilizing this data.
Abstract: 2208.14469
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Title:Exploiting stellar explosion induced by the QCD phase transition in large-scale neutrino detectors
Download PDFAbstract: The centers of the core-collapse supernovae are one of the densest environments in the universe. Under such conditions, it is conceivable that a first-order phase transition from ordinary nuclear matter to the quark-gluon plasma occurs. This transition releases a large amount of latent heat that can drive a supernova explosion and may imprint a sharp signature in the neutrino signal. We show how this snap feature, if observed at large-scale neutrino detectors, can set competitive limits on the neutrino masses and assist the localization of the supernova via triangulation. The 95%C.L. limit on the neutrino mass can reach 0.16 eV in Ice-Cube, 0.22 eV in Hyper-Kamiokande, and 0.58 eV in DUNE, for a supernova at a distance of 10 kpc. For the same distance and in the most optimistic neutrino conversion case, the triangulation method can constrain the $1\sigma$ angular uncertainty of the supernova localization within $\sim 0.3-9.0$ deg in the considered pairs of the detectors, leading to an improvement up to an order of magnitude with respect to the often considered in the literature rise time of the neutronization burst.
Abstract: 2208.14491
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Title:The supernova remnant SN 1006 as a Galactic particle accelerator
Download PDFAbstract: The origin of cosmic rays is a pivotal open issue of high-energy astrophysics. Supernova remnants are strong candidates to be the Galactic factory of cosmic rays, their blast waves being powerful particle accelerators. However, supernova remnants can power the observed flux of cosmic rays only if they transfer a significant fraction of their kinetic energy to the accelerated particles, but conclusive evidence for such efficient acceleration is still lacking. In this scenario, the shock energy channeled to cosmic rays should induce a higher post-shock density than that predicted by standard shock conditions. Here we show this effect, and probe its dependence on the orientation of the ambient magnetic field, by analyzing deep X-ray observations of the Galactic remnant of SN 1006. By comparing our results with state-of-the-art models, we conclude that SN 1006 is an efficient source of cosmic rays and obtain an observational support for the quasi-parallel acceleration mechanism.
Abstract: 2208.14659
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Title:The ALPACA experiment: The project of the first sub-PeV gamma-ray observation in the southern sky
Download PDFAbstract: The ALPACA experiment is a project aiming to observe sub-PeV gamma rays for the first time in the southern hemisphere. The main goal of ALPACA is to identify PeVatrons, the accelerators of Galactic PeV cosmic rays, by observing sub-PeV pion-decay gamma rays generated in interactions between PeV cosmic rays and the interstellar medium. This new air shower experiment is located at an altitude of 4,740 m above sea level in the middle of Mt. Chakartaya in Bolivia. The air shower array consists of 401 scintillation counters covering an 83,000 m$^2$ surface area. In addition, a water-Cherenkov-type muon detector array with an area of 3,700 m$^2$ is installed to discriminate gamma rays from background cosmic rays. The prototype array ALPAQUITA will start data taking in 2022 and will extend to ALPACA in 2024. We report on a general introduction to ALPACA, the progress of the project, and the sensitivity to sub-PeV gamma rays.
Abstract: 2208.14781
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Title:A Machine Learning Approach to Predict Missing Flux Densities in Multi-band Galaxy Surveys
Download PDFAbstract: We present a new method based on information theory to find the optimal number of bands required to measure the physical properties of galaxies with a desired accuracy. As a proof of concept, using the recently updated COSMOS catalog (COSMOS2020), we identify the most relevant wavebands for measuring the physical properties of galaxies in a Hawaii Two-0 (H30)- and UVISTA-like survey for a sample of $i<25$ AB mag galaxies. We find that with available $i$-band fluxes, $r$, $u$, IRAC/$ch2$ and $z$ bands provide most of the information regarding the redshift with importance decreasing from $r$-band to $z$-band. We also find that for the same sample, IRAC/$ch2$, $Y$, $r$ and $u$ bands are the most relevant bands in stellar mass measurements with decreasing order of importance. Investigating the inter-correlation between the bands, we train a model to predict UVISTA observations in near-IR from H30-like observations. We find that magnitudes in $YJH$ bands can be simulated/predicted with an accuracy of $1\sigma$ mag scatter $\lesssim 0.2$ for galaxies brighter than 24 AB mag in near-IR bands. One should note that these conclusions depend on the selection criteria of the sample. For any new sample of galaxies with a different selection, these results should be remeasured. Our results suggest that in the presence of a limited number of bands, a machine learning model trained over the population of observed galaxies with extensive spectral coverage outperforms template-fitting. Such a machine learning model maximally comprises the information acquired over available extensive surveys and breaks degeneracies in the parameter space of template-fitting inevitable in the presence of a few bands.
Abstract: 2208.15003
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Title:Insights on the origin of ORCs from cosmological simulations
Download PDFAbstract: We investigate shock structures driven by merger events in high-resolution simulations that result in a galaxy with a virial mass M ~ 1e12 Msol. We find that the sizes and morphologies of the internal shocks resemble remarkably well those of the newly-detected class of odd radio circles (ORCs). This would highlight a so-far overlooked mechanism to form radio rings, shells and even more complex structures around elliptical galaxies. Mach numbers of M = 2-3 for such internal shocks are in agreement with the spectral indices of the observed ORCs. We estimate that ~5 percent of galaxies could undergo merger events which occasionally lead to such prominent structures within the galactic halo during their lifetime, explaining the low number of observed ORCs. At the time when the shock structures are matching the physical sizes of the observed ORCs, the central galaxies are typically classified as early-type galaxies, with no ongoing star formation, in agreement with observational findings. Although the energy released by such mergers could potentially power the observed radio luminosity already in Milky-Way-like halos, our predicted luminosity from a simple, direct shock acceleration model is much smaller than the observed one. Considering the estimated number of candidates from our cosmological simulations and the higher observed energies, we suggest that the proposed scenario is more likely for halo masses around 1e13 Msol in agreement with the observed stellar masses of the galaxies at the center of ORCs.
Abstract: 2209.00006
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Title:A Phantom Menace: On the Morphology of the Galactic Center Excess
Download PDFAbstract: The characteristics of the Galactic Center Excess (GCE) emission observed in gamma-ray energies -- especially the morphology of the GCE -- remain a hotly debated subject. The manner in which the dominant diffuse gamma-ray background is modeled has been claimed to have a determining effect on the preferred morphology. In this work, we compare two distinct approaches to the galactic diffuse gamma-ray emission background: the first approach models this emission through templates calculated from a sequence of well-defined astrophysical assumptions, while the second approach divides surrogates for the background gamma-ray emission into cylindrical galactocentric rings with free independent normalizations. At the latitudes that we focus on, we find that the former approach works better, and that the overall best fit is obtained for an astrophysically motivated fit when the GCE follows the morphology expected of dark matter annihilation. Quantitatively, the improvement compared to the best ring-based fits is roughly 6500 in the chi^2 and roughly 4000 in the log of the Bayesian evidence.
Abstract: 2209.00305
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Title:Effects of galactic magnetic field on the UHECR correlation studies with starburst galaxies
Download PDFAbstract: We estimate the biases caused by the coherent deflection due to the galactic magnetic field (GMF) in the previous maximum-likelihood analysis for searching the UHECR sources. We simulate the mock event datasets with a set of assumptions for the starburst galaxy (SBG) source model, coherent deflection by a GMF model, and the mixed-mass composition, then conducted a maximum-likelihood analysis with ignorance of the GMF in the same manner as previous studies. We find that the anisotropic fraction $f_{\rm ani}$ is estimated systematically lower than the true value. We estimate the true parameters which are compatible with the best-fit parameters with the observation. We find that except for a narrow region with a large anisotropic fraction and small separation angular scale wide parameter space is still compatible with the experimental results. We also develop the maximum-likelihood method with consideration of the GMF model and confirm that the estimated parameters would be improved.
Abstract: 2209.00744
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Title:Probing hadronic interaction models with the hybrid data of the Pierre Auger Observatory
Download PDFAbstract: Presently large systematic uncertainties remain in the description of hadronic interactions at ultra-high energies and a fully consistent description of air-shower experimental data is yet to be reached. The amount of data collected by the Pierre Auger Observatory using simultaneously the fluorescence and surface detectors in the energy range $10^{18.5}-10^{19.0}$ eV has provided opportunity to perform a multi-parameter test of model predictions. We apply a global method to simultaneously fit the mass composition of cosmic rays and adjustments to the simulated depth of shower maximum ($X_\text{max}$), and hadronic signals at ground level ($R_\text{Had}$). The best description of hybrid data is obtained for a deeper scale of simulated $X_\text{max}$ than predicted by hadronic interaction models tuned to the LHC data. Consequently, the deficit of the simulated hadronic signal at ground level, dominated by muons, is alleviated with respect to the unmodified hadronic interaction models. Because of the size of the adjustments $\Delta X_\text{max}$ and $R_\text{Had}$ and the large number of events in the sample, the statistical significance of these assumed adjustments is large, greater than 5$\sigma_\text{stat}$, even for the combination of the systematic experimental shifts within 1$\sigma_\text{sys}$ that are the most favorable for the models.
Abstract: 2209.00882
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Title:Dark Matter: DAMA/LIBRA and its perspectives
Download PDFAbstract: The long-standing model-independent annual modulation effect measured by DAMA deep underground at Gran Sasso Laboratory with different experimental configurations is summarized and perspectives will be highlighted. DAMA/LIBRA-phase2 set-up, $\simeq$ 250 kg highly radio-pure NaI(Tl) confirms the evidence of a signal that meets all the requirements of the model independent Dark Matter annual modulation signature at high C.L.; the full exposure is 2.86 ton $\times$ yr over 22 annual cycles. The experiment is currently collecting data in the DAMA/LIBRA-phase2 empowered configuration with an even lower software energy threshold. Other recent claims are shortly commented.
Abstract: 2209.01302
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Full Text: [ PostScript, PDF]
Title:Observation of Spectral Structures in the Flux of Cosmic-Ray Protons from 50 GeV to 60 TeV with the Calorimetric Electron Telescope on the International Space Station
Download PDFAbstract: A precise measurement of the cosmic-ray proton spectrum with the Calorimetric Electron Telescope (CALET) is presented in the energy interval from 50 GeV to 60 TeV, and the observation of a softening of the spectrum above 10 TeV is reported. The analysis is based on the data collected during $\sim$6.2 years of smooth operations aboard the International Space Station and covers a broader energy range with respect to the previous proton flux measurement by CALET, with an increase of the available statistics by a factor of $\sim$2.2. Above a few hundred GeV we confirm our previous observation of a progressive spectral hardening with a higher significance (more than 20 sigma). In the multi-TeV region we observe a second spectral feature with a softening around 10 TeV and a spectral index change from =2.6 to -2.9 consistently, within the errors, with the shape of the spectrum reported by DAMPE. We apply a simultaneous fit of the proton differential spectrum which well reproduces the gradual change of the spectral index encompassing the lower energy power-law regime and the two spectral features observed at higher energies.
Abstract: 2209.02121
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Title:Magnetic energy dissipation and gamma-ray emission in energetic pulsars
Download PDFAbstract: Some of the most energetic pulsars exhibit rotation-modulated gamma-ray emission in the 0.1 to 100 GeV band. The luminosity of this emission is typically 0.1-10% of the pulsar spin-down power (gamma-ray efficiency), implying that a significant fraction of the available electromagnetic energy is dissipated in the magnetosphere and reradiated as high-energy photons. To investigate this phenomenon we model a pulsar magnetosphere using 3D particle-in-cell simulations with strong synchrotron cooling. We particularly focus on the dynamics of the equatorial current sheet where magnetic reconnection and energy dissipation take place. Our simulations demonstrate that a fraction of the spin-down power dissipated in the magnetospheric current sheet is controlled by the rate of magnetic reconnection at microphysical plasma scales and only depends on the pulsar inclination angle. We demonstrate that the maximum energy and the distribution function of accelerated pairs is controlled by the available magnetic energy per particle near the current sheet, the magnetization parameter. The shape and the extent of the plasma distribution is imprinted in the observed synchrotron emission, in particular, in the peak and the cutoff of the observed spectrum. We study how the strength of synchrotron cooling affects the observed variety of spectral shapes. Our conclusions naturally explain why pulsars with higher spin-down power have wider spectral shapes and, as a result, lower gamma-ray efficiency.
Abstract: 2209.02295
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Title:Cosmic-ray propagation under consideration of a spatially resolved source distribution
Download PDFAbstract: Cosmic rays (CRs) are an integral component of the interstellar medium, producing broadband emission while interacting with other Galactic matter components like the interstellar gas or magnetic fields. In addition to observations, numerical simulations of CR propagation through the Galaxy help to increase the level of understanding of Galactic CR transport and diffuse $\gamma$-ray emission as seen by different experiments. Up to now, the standard approach at modelling source distributions used as input for such transport simulations often rely on radial symmetry and analytical functions rather than individual, observation-based sources. We aim at a redefinition of existing CR source distributions by combining sources observed with the H.E.S.S. experiment and simulated random sources, which follow the matter density in the Milky Way. As a result, H.E.S.S.-inspired Galactic CR source distributions are inferred. We use the PICARD code to perform 3D-simulations of nuclei and electrons in CR propagation using our hybrid source distribution models. Furthermore, also gamma-ray maps and spectra, simulated with the redefined source models, are evaluated in different regions in the Galaxy and compared with each other to determine the statistical scatter of the underlying distributions. We find global consistency between our models and in comparison to previous simulations, with only some localised fluctuations, e.g. in the spiral arms. This implementation of a three-dimensional source model based on observations and simulations enables a new quality of propagation modelling. It offers possibilities for more realistic CR transport scenarios beyond radial symmetry and delivers meaningful results in both the arm and interarm regions of the Galaxy. This gives a more realistic picture of the Galactic $\gamma$-ray sky by including structures from the source model and not just the gas distributions.
Abstract: 2209.02352
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Full Text: [ PostScript, PDF]
Title:A pilot ASKAP survey for radio transients towards the Galactic Centre
Download PDFAbstract: We present the results of a radio transient and polarisation survey towards the Galactic Centre, conducted as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients pilot survey. The survey region consisted of five fields covering $\sim265\,{\rm deg}^2$ ($350^\circ\lesssim l\lesssim10^\circ$, $\vert b\vert \lesssim 10^\circ$). Each field was observed for 12\,minutes, with between 7 and 9 repeats on cadences of between one day and four months. We detected eight highly variable sources and seven highly circularly-polarised sources (14 unique sources in total). Seven of these sources are known pulsars including the rotating radio transient PSR~J1739--2521 and the eclipsing pulsar PSR~J1723--2837. One of them is a low mass X-ray binary, 4U 1758--25. Three of them are coincident with optical or infrared sources and are likely to be stars. The remaining three may be related to the class of Galactic Centre Radio Transients (including a highly likely one, VAST~J173608.2--321634, that has been reported previously), although this class is not yet understood. In the coming years, we expect to detect $\sim$40 bursts from this kind of source with the proposed four-year VAST survey if the distribution of the source is isotropic over the Galactic fields.
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