Abstracts of Interest
Selected by:
Rami Alsulami
Abstract: 1906.10426
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Title:Searches for TeV gamma-ray counterparts to Gravitational Wave events with H.E.S.S
(Submitted on 25 Jun 2019 (v1), last revised 26 Jun 2019 (this version, v2))
Abstract: The search for electromagnetic counterparts for gravitational waves events is one of the main topics of multi-messenger Astrophysics. Among these searches is the one for high energy gamma-ray emission with the H.E.S.S. Imaging Atmospheric Cherenkov Telescopes in Namibia. During their second Observation Run O2, the Advanced Virgo detector in Italy and the two advanced LIGO detectors in Washington and Louisiana while conducting joint observations, detected for the first time, on August $14^{th}$, 2017 a transient GW signal due to the coalescence of two stellar masses black holes, an event labeled GW170814. The alert announcing the event was issued two hours later and H.E.S.S. observations could be scheduled for the nights of $16^{th}$, $17^{th}$ and $18^{th}$ August 2017. Three days after the binary BH merger, on August $17^{th}$, the coalescence of two neutron star was detected for the first time, followed by a GRB detection by Fermi's GBM starting a new era in multi-messenger Astronomy. Observations started 5.3 h after the merge and contained the counterpart SSS17a that was identified several hours later. It stands as the first data obtained by a ground-based pointing instrument on this object. In this contribution, we will present the results of the search of high-energy gamma ray emission as electromagnetic counterpart of these two GW events. No significant gamma ray emission was detected for either event. Nevertheless upper limit maps were derived constraining, for the first time, the non-thermal, high-energy emission on the remnant of a three detector binary black hole coalescence (GW170814), and a binary neutron star coalescence (GW170817).
Abstract: 1906.09420
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Title:Theoretical model of hydrodynamic jet formation from accretion disks with turbulent viscosity
(Submitted on 22 Jun 2019)
Abstract: We develop the theoretical model for the analytic description of hydrodynamic jets from protostellar disks employing the Beltrami-Bernoulli flow configuration of disk-jet structure. For this purpose we extend the standard turbulent viscosity prescription and derive several classes of analytic solutions using the flow parametrization in self-similar variables. Derived solutions describe the disk-jet structure, where for the first time jet properties are analytically linked with the properties of the accretion disk flow. The ratio of the jet ejection and disk accretion velocities is controlled by the turbulence parameter, while the ejection velocity increases with the decrease of local sound velocity and the jet launching radius. Derived solutions can be used to analyze the astrophysical jets from protostellar accretion disks and link the properties of outflows with the local observational properties of accretion disk flows.
Abstract: 1906.10954
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Full Text: [ PostScript, PDF]
Title:Constraints on gamma-ray and neutrino emission from NGC 1068 with the MAGIC telescopes
(Submitted on 26 Jun 2019)
Abstract: Starburst galaxies and star-forming active galactic nuclei (AGN) are among the candidate sources thought to contribute appreciably to the extragalactic gamma-ray and neutrino backgrounds. NGC 1068 is the brightest of the star-forming galaxies found to emit gamma rays from 0.1 to 50 GeV. Precise measurements of the high-energy spectrum are crucial to study the particle accelerators and probe the dominant emission mechanisms. We have carried out 125 hours of observations NGC 1068 with the MAGIC telescopes in order to search for gamma-ray emission in the very high energy band. We did not detect significant gamma-ray emission, and set upper limits at 95\% confidence level to the gamma-ray flux above 200 GeV f<5.1x10^{-13} cm^{-2} s ^{-1} . This limit improves previous constraints by about an order of magnitude and allows us to put tight constraints on the theoretical models for the gamma-ray emission. By combining the MAGIC observations with the Fermi-LAT spectrum we limit the parameter space (spectral slope, maximum energy) of the cosmic ray protons predicted by hadronuclear models for the gamma-ray emission, while we find that a model postulating leptonic emission from a semi-relativistic jet is fully consistent with the limits. We provide predictions for IceCube detection of the neutrino signal foreseen in the hadronic scenario. We predict a maximal IceCube neutrino event rate of 0.07 yr^{-1}.
Abstract: 1906.09553
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Full Text: [ PostScript, PDF]
Title:The impact of asymmetric neutrino emissions on nucleosynthesis in core-collapse supernovae
(Submitted on 23 Jun 2019)
Abstract: We investigate the impact of asymmetric neutrino emissions on the explosive nucleosynthesis in neutrino-driven core-collapse supernovae (CCSNe). We find that the asymmetric emissions tend to yield larger amounts of proton-rich ejecta (electron fraction, $Y_e > 0.51$) in the hemisphere of the higher $\nu_{\rm e}$ emissions, meanwhile neutron-rich matter ($Y_e < 0.49$) are ejected in the opposite hemisphere of the higher ${\bar \nu}_{\rm e}$ emissions. For larger asymmetric cases with $\ge 30\%$, the neutron-rich ejecta is abundantly produced, in which there are too much elements heavier than Zn compared to the solar abundances. This may place an upper limit of the asymmetric neutrino emissions in CCSNe. The characteristic features are also observed in elemental distribution; (1) abundances lighter than Ca are insensitive to the asymmetric neutrino emissions: (2) the production of Zn and Ge is larger in the neutron-rich ejecta even for smaller asymmetric cases with $\le 10\%$. We discuss these observational consequences, which may account for the (anti-)correlations among asymmetries of heavy elements and neutron star kicks in supernova remnants (SNRs). Future SNR observations of the direct measurement for the mass and spatial distributions of $\alpha$ elements, Fe, Zn and Ge will provide us the information on the asymmetric degree of neutrino emissions.
Abstract: 1906.11311
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Full Text: [ PostScript, PDF]
Title:Exact solutions for compact stars with CFL quark matter
(Submitted on 26 Jun 2019 (v1), last revised 28 Jun 2019 (this version, v2))
Abstract: The search for the true ground state of the dense matter remains open since Bodmer, Terazawa and other raised the possibility of stable quarks, boosted by Witten's $strange$ $matter$ hypothesis in 1984. Within this proposal, the strange matter is assumed to be composed of $strange$ quarks in addition to the usual $up$s and $down$s, having an energy per baryon lower than the strangeless counterpart, and even lower than that of nuclear matter. In this sense, neutron stars should actually be strange stars. Later work showed that a paired, symmetric in flavor, color-flavor locked (CFL) state would be preferred to the one without any pairing for a wide range of the parameters (gap $\Delta$, strange quark mass $m_s$, and bag constant B). We use an approximate, yet very accurate, CFL equation of state (EoS) that generalizes the MIT bag model to obtain two families of exact solutions for the static Einstein field equations constructing families anisotropic compact relativistic objects. In this fashion, we provide exact useful solutions directly connected with microphysics.
Abstract: 1906.11271
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Title:Cosmological simulations of massive black hole seeds: predictions for next generation electromagnetic and gravitational wave observations
(Submitted on 26 Jun 2019)
Abstract: We study how statistical properties of supermassive black holes depend on the frequency and conditions for massive seed formation in cosmological simulations of structure formation. We develop a novel method to re-calculate detailed growth histories and merger trees of black holes within the framework of the Illustris simulation for several seed formation models, including a physically motivated model where black hole seeds only form in progenitor galaxies that conform to the conditions for direct collapse black hole formation. While all seed models considered here are in a broad agreement with present observational constraints on black hole populations from optical, UV and X-ray studies, we find they lead to widely different black hole number densities and halo occupation fractions which are currently observationally unconstrained. In terms of future electromagnetic spectrum observations, the faint-end quasar luminosity function and the low mass-end black hole-host galaxy scaling relations are very sensitive to the specific massive seed prescription. Specifically, the direct collapse model exhibits a seeding efficiency which decreases rapidly with cosmic time and produces much fewer black holes in low mass galaxies, in contrast to the original Illustris simulation. We further find that the total black hole merger rate varies by more than one order of magnitude for different seed models, with the redshift evolution of the chirp mass changing as well. Supermassive black hole merger detections with LISA and International Pulsar Timing Array may hence provide the most direct means of constraining massive black hole seed formation in the early Universe.
Abstract: 1906.09454
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Title:Exploring particle escape in supernova remnants through gamma rays
(Submitted on 22 Jun 2019)
Abstract: The escape process of particles accelerated at supernova remnant (SNR) shocks is one of the poorly understood aspects of the shock acceleration theory. Here we adopt a phenomenological approach to study the particle escape and its impact on the gamma-ray spectrum resulting from hadronic collisions both inside and outside of a middle-aged SNR. Under the assumption that in the spatial region immediately outside of the remnant diffusion is suppressed with respect to the average Galactic one, we show that a significant fraction of particles are still located inside the SNR long time after their nominal release from the acceleration region. This fact results into a gamma-ray spectrum that resembles a broken power law, similar to those observed in several middle-aged SNRs. Above the break, the spectral steepening is determined by the diffusion coefficient outside of the SNR and by the time dependence of maximum energy. Consequently, the comparison between the model prediction and actual data will contribute to determining these two quantities, the former being particularly relevant within the predictions of the gamma-ray emission from the halo of escaping particles around SNRs which could be detected with future Cherenkov telescope facilities. We also calculate the spectrum of run-away particles injected into the Galaxy by an individual remnant. Assuming that the acceleration stops before the SNR enters the snowplow phase, we show that the released spectrum can be a featureless power law only if the accelerated spectrum has a slope alpha > 4.
Abstract: 1906.10302
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Full Text: [ PostScript, PDF]
Title:Rapid Particle Acceleration due to Re-collimation in Injected Jets with Helical Magnetic Fields
(Submitted on 25 Jun 2019)
Abstract: One of the key open questions in the study of relativistic jets is how magnetic reconnection occurs and whether it can effectively accelerate the jet's electrons. We investigate the evolution of an electron-proton relativistic jet containing helical magnetic fields, focusing on the interaction with the ambient plasma. We have performed 3D particle-in-cell (PIC) simulations of a jet containing a relatively large radius with embedded helical magnetic fields, in order to examine how the helical magnetic field excites kinetic instabilities such as the Weibel instability (WI), the kinetic Kelvin-Helmholtz instability (kKHI) and the mushroom instability (MI). In our simulations these kinetic instabilities are indeed excited and particles are accelerated. We observe a recollimation-like instability near the center of the jet at the linear stage. As the electron-proton jet evolves, the helical magnetic field becomes untangled due to a reconnection-like phenomena at the end of nonlinear stage, and electrons are further accelerated by multiple magnetic reconnection events/sites within the turbulent magnetic field.
Abstract: 1906.12257
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Full Text: [ PostScript, PDF]
Title:The origin of spin in binary black holes: Predicting the distributions of the main observables of Advanced LIGO
(Submitted on 28 Jun 2019)
Abstract: We study the formation of coalescing binary black holes via the evolution of isolated field binaries that go through the common envelope phase in order to obtain the combined distributions of the main observables of Advanced LIGO. We use a hybrid technique that combines the parametric binary population synthesis code COMPAS with detailed binary evolution simulations performed with the MESA code. We then convolve our binary evolution calculations with the redshift- and metallicity-dependent star-formation rate and the selection effects of gravitational-wave detectors to obtain predictions of observable properties. By assuming efficient angular momentum transport, we are able to present a model capable of predicting simultaneously the three main gravitational-wave observables: the effective inspiral spin parameter $\chi_{eff}$, the chirp mass $M_{chirp}$ and the cosmological redshift of merger $z_{merger}$. We find an excellent agreement between our model and the ten events from the first two advanced detector observing runs. We make predictions for the third observing run O3 and for Advanced LIGO design sensitivity. We expect 59% of events with $\chi_{eff} < 0.1$, while the remaining 41% of events with $\chi_{eff} \ge 0.1$ are split into 9% with $M_{chirp} < 15$ M$_\odot$ and 32% with $M_{chirp} \ge 15$ M$_\odot$. In conclusion, the favorable comparison of the existing LIGO/Virgo observations with our model predictions gives support to the idea that the majority, if not all of the observed mergers, originate from the evolution of isolated binaries. The first-born black hole has negligible spin because it lost its envelope after it expanded to become a giant star, while the spin of the second-born black hole is determined by the tidal spin up of its naked helium star progenitor by the first-born black hole companion after the binary finished the common-envelope phase.
Abstract: 1906.05184
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Full Text: [ PostScript, PDF]
Title:A model for microquasars of Population III
(Submitted on 12 Jun 2019)
Abstract: Current simulations indicate that the first stars were formed predominantly in binary systems. The study of the contribution of the first accreting binary systems to the reionization and heating of the intergalactic medium requires the formulation of a concrete model for microquasars of Population III. We aim at constructing a complete model for microquasars where the donor star is of Population III. We consider that the mas-loss of the star is caused exclusively by the spill of matter through the Roche lobe towards the black hole. We calculate the spectral energy distribution of the radiation produced by the accretion disk, the radiation-pressure driven wind, and the relativistic particles in the jets, within the framework of a lepto-hadronic model. In addition, we estimate the impact on the reionization by the first microquasars. We determine that Population III microquasars are powerful sources of ultraviolet radiation produced by the winds of their super-critical disks, and generate a broadband non-thermal emission in jets. Our results indicate that microquasars in the early Universe could have been important for the reionization and heating of the intergalactic medium.
Abstract: 1906.07399
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Full Text: [ PostScript, PDF]
Title:Constraining the Fraction of Core-Collapse Supernovae Harboring Choked Jets with High-energy Neutrinos
(Submitted on 18 Jun 2019 (v1), last revised 25 Jun 2019 (this version, v2))
Abstract: The joint observation of core-collapse supernovae with gamma-ray bursts shows that jets can be launched in the aftermath of stellar core collapse, likely by a newly formed black hole that accretes matter from the star. Such gamma-ray bursts have only been observed accompanying Type Ibc supernovae, indicating a stellar progenitor that lost its Hydrogen envelope before collapse. It is possible that jets are launched in core-collapse events even in the presence of a Hydrogen envelope, however, such jets are unlikely to be able to burrow through the star and will be stalled before escaping. High-energy neutrinos produced by such choked jets could escape the stellar envelope and could be observed. Here we examine how multi-messenger searches for high-energy neutrinos and core-collapse supernovae can detect or limit the fraction of stellar collapses that produce jets. We find that a high fraction of jet production is already limited by previous observational campaigns. We explore possibilities with future observations using LSST, IceCube and Km3NET.
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