Abstracts of Interest
Selected by:
Tiffany Collins
Abstract: 2206.04699
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Title:Pulsars Do Not Produce Sharp Features in the Cosmic-Ray Electron and Positron Spectra
Download PDFAbstract: Pulsars are considered to be the leading explanation for the excess in cosmic-ray positrons detected by PAMELA and AMS-02. A notable feature of standard pulsar models is the sharp spectral cutoff produced by the increasingly efficient cooling of very-high-energy electrons by synchrotron and inverse-Compton processes. This spectral break has been employed to: (1) constrain the age of pulsars that contribute to the excess, (2) argue that a large number of pulsars must significantly contribute to the positron flux, and (3) argue that spectral cutoffs cannot distinguish between dark matter and pulsar models. We prove that this spectral feature does not exist -- it appears due to approximations that treat inverse-Compton scattering as a continuous, instead of as a discrete and catastrophic, energy-loss process. Astrophysical sources do not produce sharp spectral features via cooling, reopening the possibility that such a feature would provide incontrovertible evidence for dark matter.
Abstract: 2206.05019
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Title:Physical origin of the dark spot at the image of supermassive black hole SgrA* revealed by the EHT collaboration
Download PDFAbstract: In this comment we elucidate the physical origin of the dark spot at the image of supermassive black hole SgrA* presented very recently by the EHT collaboration. It is argued that this dark spot, which is noticeably smaller of the classical black hole shadow, is the northern hemisphere of the event horizon globe. At the same time, the outer boundary of this dark spot is an equator on the event horizon globe.
Abstract: 2206.05151
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Title:EOS-ESTM: A flexible climate model for habitable exoplanets
Download PDFAbstract: Rocky planets with temperate conditions provide the best chance for discovering habitable worlds and life outside the Solar System. In the last decades, new instrumental facilities and large observational campaigns have been driven by the quest for habitable worlds. Climate models aimed at studying the habitability of rocky planets are essential tools to pay off these technological and observational endeavours. In this context, we present EOS-ESTM, a fast and flexible model aimed at exploring the impact on habitability of multiple climate factors, including those unconstrained by observations. EOS-ESTM is built on ESTM, a seasonal-latitudinal energy balance model featuring an advanced treatment of the meridional and vertical transport. The novel features of EOS-ESTM include: (1) parameterizations for simulating the climate impact of oceans, land, ice, and clouds as a function of temperature and stellar zenith distance; (2) a procedure (EOS) for calculating the radiative transfer in atmospheres with terrestrial and non-terrestrial compositions illuminated by solar- and non-solar-type stars. By feeding EOS-ESTM with Earth's stellar, orbital and planetary parameters we derive a reference model that satisfies a large number of observational constraints of the Earth's climate system. Validation tests of non-terrestrial conditions yield predictions that are in line with comparable results obtained with a hierarchy of climate models. The application of EOS-ESTM to planetary atmospheres in maximum greenhouse conditions demonstrates the possibility of tracking the snowball transition at the outer edge of the HZ for a variety of planetary parameters, paving the road for multi-parametric studies of the HZ.
Abstract: 2206.05296
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Title:Deep learning techniques for Imaging Air Cherenkov Telescopes
Download PDFAbstract: Very High Energy (VHE) gamma rays and charged cosmic rays (CCRs) provide an observational window into the acceleration mechanisms of extreme astrophysical environments. One of the major challenges at Imaging Air Cherenkov Telescopes (IACTs) designed to look for VHE gamma rays, is the separation of air showers initiated by CCRs which form a background to gamma ray searches. Two other less well studied problems at IACTs are a) the classification of different primary nuclei among the CCR events and b) identification of anomalous events initiated by Beyond Standard Model particles that could give rise to shower signatures which differ from the standard images of either gamma rays or CCR showers. The problems of categorizing the primary particle that initiates a shower image, or the problem of tagging anomalous shower events in a model independent way, are problems that are well suited to a machine learning (ML) approach. Traditional studies that have explored gamma ray/CCR separation have used a multivariate analysis based on derived shower properties, which contains significantly reduced information about the shower. In our work, we address the problems outlined above by using ML architectures trained on full simulated shower images, as opposed to training on just a few derived shower properties. We illustrate the techniques of binary and multi-category classification using convolutional neural networks, and we also pioneer the use of autoencoders for anomaly detection at VHE gamma ray experiments. As a case study, we apply our techniques to the H.E.S.S. experiment. However, the real strength of the techniques that we broach here in the context of VHE gamma ray observatories, is that these methods can be applied broadly to any other IACT, such as the upcoming Cherenkov Telescope Array (CTA), or can even be suitably adapted to CCR experiments.
Abstract: 2206.05299
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Title:Impact of late-time neutrino emission on the Diffuse Supernova Neutrino Background
Download PDFAbstract: In the absence of high-statistics supernova neutrino measurements, estimates of the diffuse supernova neutrino background (DSNB) hinge on the precision of simulations of core-collapse supernovae (CCSNe). Understanding the cooling phase of protoneutron star (PNS) evolution ($\gtrsim1\,{\rm s}$ after core bounce) is crucial, since approximately 50% of the energy liberated by neutrinos is emitted during the cooling phase. We model the cooling phase with a hybrid method, by combining the neutrino emission predicted by 3D hydrodynamic simulations with several cooling phase estimates, including a novel two-parameter correlation depending on the final baryonic PNS mass and the time of shock revival. We find that the predicted DSNB event rate at Super-Kamiokande can vary by a factor of $\sim2-3$ depending on the cooling phase treatment. We also find that except for one cooling estimate, the range in predicted DSNB events is largely driven by the uncertainty in the neutrino mean energy. With a good understanding of the late time neutrino emission, more precise DSNB estimates can be made for the next generation of DSNB searches.
Abstract: 2206.06390
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Title:The Cosmic History of Long Gamma Ray Bursts
Download PDFAbstract: The cosmic formation rate of long Gamma Ray Bursts (LGRBs) encodes the evolution, across cosmic times, of their progenitors' properties and of their environment. The LGRB formation rate and the luminosity function, with its redshift evolution, are derived by reproducing the largest set of observations collected in the last four decades, namely the observer-frame prompt emission properties of GRB samples detected by the Fermi and Compton Gamma Ray Observatory (CGRO) satellites and the redshift, luminosity and energy distributions of flux-limited, redshift complete, samples of GRBs detected by Swift. The model that best reproduces all these constraints consists of a GRB formation rate increasing with redshift $\propto (1+z)^{3.2}$, i.e. steeper than the star formation rate, up to $z\sim3$ followed by a decrease $\propto(1+z)^{-3}$. On top of this, our model predicts also a moderate evolution of the characteristic luminosity function break $\propto(1+z)^{0.6}$. Models with only luminosity or rate evolution are excluded at $>5\sigma$ significance. The cosmic rate evolution of LGRBs is interpreted as their preference to occur in environments with metallicity $12+\log(\rm O/H)<8.6$, consistently with theoretical models and host galaxy observations. The LGRB rate at $z=0$, accounting for their collimation, is $\rho_0=79^{+57}_{-33}$ Gpc$^{-3}$ yr$^{-1}$ (68% confidence interval). This corresponds to $\sim$1\% of broad-line Ibc supernovae producing a successful jet in the local Universe. This fraction increases up to $\sim$7% at $z\ge3$. Finally, we estimate that at least $\approx0.2-0.7$ yr$^{-1}$ of Swift and Fermi detected bursts at $z<0.5$ are jets observed slightly off-axis.
Abstract: 2206.07969
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Title:Properties of pasta phases in catalyzed neutron stars
Download PDFAbstract: Exotic non-spherical configurations of nuclei, known as ``pasta" phases, are expected to be present at the bottom of the inner crust of a neutron star. We study the properties of these configurations in catalyzed neutron stars within a compressible liquid-drop model approach, with surface parameters optimized to reproduce experimental nuclear masses. Our results show that the properties of the pasta phases exhibit strong model dependence. To estimate the model uncertainties, a Bayesian analysis is performed, combining information from nuclear physics experiments and chiral perturbation theoretical calculations with astrophysical observations. The inferred posterior distributions are discussed, with particular focus on the effect of the low-density energy functional on the predictions.
Abstract: 2206.05423
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Title:Role of magnetic fields in fueling Seyfert nuclei
Download PDFAbstract: Molecular gas is believed to be the fuel for star formation and nuclear activity in Seyfert galaxies. To identify the role of magnetic fields in funneling molecular gas into the nuclear region, measurements of the magnetic fields embedded in molecular gas are needed. By applying the new velocity gradient technique (VGT) to ALMA and PAWS's CO isotopolog data, we obtain for the first time the detection of CO-associated magnetic fields in several nearby Seyfert galaxies and their unprecedented high-resolution magnetic field maps. The VGT-measured magnetic fields globally agree with the one inferred from existing HAWC+ dust polarization and VLA synchrotron polarization. An overall good alignment between the magnetic fields traced by the VGT-CO measurement and synchrotron polarization supports the correlation between star formation and cosmic ray generation. We find that CO-traced magnetic fields have a more significant radial component in the central regions of most Seyferts in our sample, where efficient molecular gas inflows are expected. In particular, we find the misalignment between the magnetic fields traced by CO and dust polarization within the nuclear ring of NGC 1097, and the former follows the secondary central bar. It reveals different magnetic field configurations in different gas phases and provides an observational diagnostic for the ongoing multi-phase fueling of Seyfert activity.
Abstract: 2206.05453
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Title:Satellite galaxies' drag on field stars in the Milky Way
Download PDFAbstract: With Gaia EDR3 data, velocity dispersion of Milky Way field stars around satellite galaxies have been investigated. We have fitted velocity dispersion against distance to satellite galaxy and found the gradient of velocity dispersion is related to the mass of satellite galaxy. With order-of-magnitude approximations, a linear correlation has been fitted between the mass of satellite galaxy and gradient of velocity dispersion caused by its gravitational drag. Though our result is an observational qualitative result, it shows better relation could be obtained with more observations in the future.
Abstract: 2206.05803
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Title:Jets in radio galaxies and quasars: an observational perspective
Download PDFAbstract: This article gives a brief historical introduction and reviews our current understanding of jets in radio galaxies and quasars from an observational perspective, with an emphasis on observations at radio wavelengths. Recent results on the Fanaroff-Riley classification scheme, and the nature of radio structures and jets in the FR classes as well as in high-excitation and low-excitation radio galaxies are summarized. The collimation and propagation of jets from nuclear sub-pc to hundreds of kpc scales from both observatinoal and theoretical work have been discussed. The jets exhibit evidence of interaction with a clumpy interstellar medium, especially in young radio sources, and could trigger both star formation as well as suppress star formation depending on the physical conditions. Observational evidence for such interactions and jet feedback which have profound implications in our understanding of galaxy evolution have been presented. Recurrent jet activity which has been seen over a wide range of projected linear size and time scales has been discussed. This review article concludes with a brief discussion of unresolved questions on jets which new telescopes should help address.
Abstract: 2206.05847
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Title:Single Pulse Variability in Gamma-ray Pulsars
Download PDFAbstract: The Fermi Large Area Telescope receives $\ll$1 photon per rotation from any $\gamma$-ray pulsar. However, out of the billions of monitored rotations of the bright pulsars Vela (PSR~J0835$-$4510) and Geminga (PSR~J0633$+$1746), a few thousand have $\geq$2 pulsed photons. These rare pairs encode information about the variability of pulse amplitude and shape. We have cataloged such pairs and find the observed number to be in good agreement with simple Poisson statistics, limiting any amplitude variations to $<$19% (Vela) and $<$22% (Geminga) at 2$\sigma$ confidence. Using an array of basis functions to model pulse shape variability, the observed pulse phase distribution of the pairs limits the scale of pulse shape variations of Vela to $<$13% while for Geminga we find a hint of $\sim$20% single-pulse shape variability most associated with the pulse peaks. If variations last longer than a single rotation, more pairs can be collected, and we have calculated upper limits on amplitude and shape variations for assumed coherence times up to 100 rotations, finding limits of $\sim$1% (amplitude) and $\sim$3% (shape) for both pulsars. Because a large volume of the pulsar magnetosphere contributes to $\gamma$-ray pulse production, we conclude that the magnetospheres of these two energetic pulsars are stable over one rotation and very stable on longer time scales. All other $\gamma$-ray pulsars are too faint for similar analyses. These results provide useful constraints on rapidly-improving simulations of pulsar magnetospheres, which have revealed a variety of large-scale instabilities in the thin equatorial current sheets where the bulk of GeV $\gamma$-ray emission is thought to originate.
Abstract: 2206.06967
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Title:The Fermi paradox: Impact of astrophysical processes and dynamical evolution
Download PDFAbstract: The Fermi paradox has given rise to various attempts to explain why no evidence of extraterrestrial civilisations was found so far on Earth and in our Solar System. Here, we present a dynamical model for the development of such civilisations, which accounts for self-destruction, colonisation and astrophysical destruction mechanisms of civilisations including gamma-ray bursts, type Ia and type II supernovae as well as radiation from the supermassive black hole. We adopt conservative estimates regarding the efficiency of such processes and find that astrophysical effects can influence the development of intelligent civilisations and change the number of systems with such civilisations by roughly a factor of 2; potentially more if the feedback is enhanced. Our results show that non-equilibrium evolution allows for solutions in-between extreme cases such as "rare Earth" or extreme colonisation, including scenarios with civilisation fractions between 10^{-2} and 10^{-7}. These would imply still potentially large distances to the next such civilisations, particularly when persistence phenomena are being considered. As previous studies, we confirm that the main uncertainties are due to the lifetime of civilisations as well as the assumed rate of colonisation. For SETI-like studies, we believe that unbiased searches are needed considering both the possibilities that the next civilisations are nearby or potentially very far away.
Abstract: 2206.07324
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Title:Advection of Accelerated Electrons in Radio/X-ray Knots of AGN Jets
Download PDFAbstract: The X-ray emission from the knots of the kilo-parsec scale jet of active galactic nuclei (AGN) suggests the high energy emission process is different from the radio/optical counterpart. Interpretation based on the Inverse Compton scattering of cosmic microwave photons has been ruled out through Fermi gamma-ray observations for low redshift sources. As an alternate explanation, synchrotron emission from a different electron population is suggested. We propose a model considering the advected electron distribution from the sites of particle acceleration in AGN knots. This advected electron distribution is significantly different from the accelerated electron distribution and satisfies the requirement of the second electron population. The synchrotron emission from the accelerated and the advected electron distribution can successfully reproduce the observed radio to X-ray fluxes of the knots of 3C 273. For the chosen combination of the model parameters, the spectrum due to inverse Compton scattering of cosmic microwave photons falls within the Fermi gamma-ray upper limits.
Abstract: 2206.08035
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Title:X-ray timing and spectral analysis of reverberating active galactic nuclei
Download PDFAbstract: We use the publicly available XMM-Newton archive to select a sample of 20 active galactic nuclei (AGN) known to exhibit reverberation signatures caused by the reflection of X-rays from the corona off the accretion disc that feeds the central black hole engine. Inverse Compton scattering by energetic electrons, coupled with accretion disc fluctuations give rise to the highly variable observed X-ray spectrum, the behaviour of which is still not fully understood. We use 121 observations in 3 - 4 distinct spectral states for each source and calculate the time lags as a function of frequency. We fit the relativistic reflection model RELXILL and explore parameter correlations. The known scaling relationship between the black hole mass and time lag is well recovered and the continuum flux is coupled strongly to the disc reflection flux. We also find that 1H 0707-495 and IRAS 13224-3809 are well described using reflection and absorption modelling in a variety of flux states. The reflection fraction is strongly coupled to the power law photon index and may be linked to dynamics of the emitting region. The data reveals hints of the power law evolutionary turnover when the 2 - 10 keV Eddington fraction is $\sim0.02$, the origin of which is not fully understood. Finally, we report the covering fraction is inversely correlated with the flux and power law photon index in IRAS 13224-3809. These findings support recent studies of 1H 0707-495 where the covering fraction may contribute to the observed variability via flux modulations from non-uniform orbiting clouds.
Abstract: 2206.05624
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Title:A Challenge to the Standard Cosmological Model
Download PDFAbstract: We present the first joint analysis of catalogs of radio galaxies and quasars to determine if their sky distribution is consistent with the standard $\Lambda$CDM model of cosmology. This model is based on the cosmological principle, which asserts that the universe is isotropic and homogeneous on large scales, so the observed dipole anisotropy in the cosmic microwave background (CMB) must be attributed to our local peculiar motion. We test the null hypothesis that there is a dipole anisotropy in the sky distribution of radio galaxies and quasars consistent with the motion inferred from the CMB, as is expected for cosmologically distant sources. Our two samples, constructed respectively from the NRAO VLA Sky Survey and the Wide-field Infrared Survey Explorer, are systematically independent and have no shared objects. Using a completely general, two dimensional definition of the $p$-value that accounts for correlation between the found dipole amplitude and its directional offset from the CMB dipole, the null hypothesis is independently rejected with $p=7.9\times10^{-3}$ and $p=9.9\times10^{-6}$ for the radio galaxy and quasar samples, respectively, corresponding to $2.7\sigma$ and $4.4\sigma$ significance. The joint significance, using sample size-weighted $Z$-scores, is $5.2\sigma$. We show that the radio galaxy and quasar dipoles are consistent with each other and find no evidence for any frequency dependence of the amplitude.
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