Abstracts of Interest
Selected by:
Jason Ahumada
Abstract: 2306.06750
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Title:Aerosol parameters for night sky brightness modelling estimated from daytime sky images
Download PDFAbstract: Atmospheric turbidity is one of the key factors influencing the propagation of artificial light into the environment during cloudless nights. High aerosol loading can reduce the visibility of astronomical objects, and thus information on atmospheric pollution is critical for the prediction of the night sky brightness (NSB) distribution. In particular, the aerosol optical depth (AOD) and asymmetry parameter (g) are among the most important aerosol properties influencing the NSB amplitudes. However, these two parameters are rarely available at astronomical sites. Here, we develop a method for AOD and g retrievals from clear-sky radiometry carried out around sunset or sunrise, shortly before or after night-time observation is intended. The method allows for reducing the number of unknowns needed in the processing and interpretation of night sky radiances, and thus provides an efficient tool for gathering input data to present skyglow simulators. The practice of collecting information about aerosols in this way could become a routine part of astronomical observations, much like observing standard stars to obtain extinction coefficients. If the procedure were conducted around sunset and the data were quickly reduced, it could offer an on-the-spot estimate of the NSB for the night ahead. The error analysis is performed using the theoretical model, while taking into account experimental errors of radiance readings. The capability of the method is demonstrated in a field experiment conducted under cloudless conditions.
Abstract: 2306.06415
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Full Text: [ PostScript, PDF]
Title:Detecting gamma rays with high resolution and moderate field of view: the air Cherenkov technique
Download PDFAbstract: The Imaging Atmospheric Cherenkov technique allows to detect very high energy gamma rays from few tens of GeV to hundreds of TeV using ground-based instrumentation. At these energies a gamma ray generates a shower of secondary particles when it enters the Earth's atmosphere. These particles emit Cherenkov light in the visible and near UV ranges. The Cherenkov light produced by the shower reaches the ground as a short pulse of a few nanosecond duration over a large circle of around 100 m radius (a light pool). This pulse of light can be imaged with telescopes provided with fast photodetectors and electronics. Combining the images of several telescopes distributed over this light pool allows to estimate the gamma-ray energy and incident direction, and to reject gamma rays from the strong background of charged cosmic rays. The collection area of an array of a few telescopes is of the order of the area of the light pool, i.e. $>$10$^5$m$^2$. Such an array reaches a sensitivity of a few millicrabs at 100 GeV energies in 50 hours of observations, an angular resolution of $\sim$5 arcmin and a spectral resolution of $\sim$10%. This chapter describes the technical implementation of Imaging Atmospheric Cherenkov telescopes and describes how the data are analyzed to reconstruct the physical parameters of the primary gamma rays.
Abstract: 2306.05900
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Title:Design and Expected Performance of the IceCube-Gen2 Surface Array and its Radio Component (ARENA2022)
Download PDFAbstract: IceCube-Gen2, the next generation of the IceCube Neutrino Observatory at the South Pole, will consist of three co-located arrays: a deep Optical Array and a more shallow and larger Radio Array for neutrino detection in the ice, and a Surface Array above the footprint of the Optical Array. The Surface Array will be comprised of hybrid stations featuring elevated radio antennas and scintillation detectors, following the design of a prototype station successfully operating at the South Pole since 2020. Besides providing a veto for neutrino detection, the Surface Array will make IceCube-Gen2 a unique laboratory for cosmic-ray air showers. Compared to the current IceCube detector with its IceTop surface array, the aperture for coincident air-shower measurements detected by both, the deep optical and surface arrays, will increase by about a factor of 30. In addition to particle physics questions, such as the production of PeV muons and neutrinos in prompt decays, these surface-deep coincidences will be used to target astrophysical questions of the most energetic Galactic cosmic rays. The combination of particle and radio measurements at the surface and high-energy muons measured in the ice promises unprecedented accuracy for the mass composition in the energy range of the presumed Galactic-to-extragalactic transition - complementing the multimessenger science case of IceCube-Gen2. This proceeding provides an overview of the IceCube-Gen2 Surface Array and, in particular, its radio component.
Abstract: 2306.06372
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Title:A tera-electronvolt afterglow from a narrow jet in an extremely bright gamma-ray burst 221009A
Download PDFAbstract: Some gamma-ray bursts (GRBs) have an afterglow in the tera-electronvolt (TeV) band, but the early onset of this afterglow has not been observed. We report observations with the Large High Altitude Air Shower Observatory of the bright GRB 221009A, which serendipitously occurred within the instrument field of view. More than 64,000 photons (above 0.2~TeV) were detected within the first 3000 seconds. The TeV photon flux began several minutes after the GRB trigger, then rose to a flux peak about 10 seconds later. This was followed by a decay phase, which became more rapid at $\sim 650\,{\rm s}$ after the peak. The emission can be explained with a relativistic jet model with half-opening angle $\sim 0.8^\circ$, consistent with the core of a structured jet. This interpretation could explain the high isotropic energy of this GRB.
Abstract: 2306.05080
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Title:FAST reveals new evidence for M94 as a merger
Download PDFAbstract: We report the first high-sensitivity HI observation toward the spiral galaxy M94 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). From these observations, we discovered that M94 has a very extended HI disk, twice larger than that observed by THINGS, which is accompanied by an HI filament and seven HVCs (high velocity clouds) at different distances. The projected distances of these clouds and filament are less than 50 kpc from the galactic center. We measured a total integrated flux (including all clouds/filament) of 127.3 ($\pm$1) Jy km s$^{-1}$, corresponding to a H I mass of (6.51$\pm$0.06)$\times$10$^{8}$M$_{\odot}$, which is 63.0% more than that observed by THINGS. By comparing numerical simulations with the HI maps and the optical morphology of M94, we suggest that M94 is likely a remnant of a major merger of two galaxies, and the HVCs and HI filament could be the tidal features originated from the first collision of the merger happened about 5 Gyr ago. Furthermore, we found a seemingly isolated HI cloud at a projection distance of 109 kpc without any optical counterpart detected. We discussed the possibilities of the origin of this cloud, such as dark dwarf galaxy and RELHIC (REionization-Limited HI Cloud). Our results demonstrate that high-sensitivity and wide-field HI imaging is important in revealing the diffuse cold gas structures and tidal debris which is crucial to understanding the dynamical evolution of galaxies.
Abstract: 2306.07324
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Title:Stochastic dynamics of multi-waterfall hybrid inflation and formation of primordial black holes
Download PDFAbstract: We show that a hybrid inflation model with multiple waterfall fields can result in the formation of primordial black hole (PBH) with an astrophysical size, by using an advanced algorithm to follow the stochastic dynamics of the waterfall fields. This is in contrast to the case with a single waterfall field, where the wavelength of density perturbations is usually too short to form PBHs of the astrophysical scale (or otherwise PBH are overproduced and the model is ruled out) unless the inflaton potential is tuned. In particular, we demonstrate that PBHs with masses of order $10^{20}\, {\rm g}$ can form after hybrid inflation consistently with other cosmological observations if the number of waterfall fields is about 5 for the case of instantaneous reheating. Observable gravitational waves are produced from the second-order effect of large curvature perturbations as well as from the dynamics of texture or global defects that form after the waterfall phase transition.
Abstract: 2306.06177
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Title:Prospects for Direct Detection of Black Hole Formation in Neutron Star Mergers with Next-Generation Gravitational-Wave Detectors
Download PDFAbstract: A direct detection of black hole formation in neutron star mergers would provide invaluable information about matter in neutron star cores and finite temperature effects on the nuclear equation of state. We study black hole formation in neutron star mergers using a set of 196 numerical relativity simulations consisting of long-lived and black hole-forming remnants. The postmerger gravitational-wave spectrum of a long-lived remnant has greatly reduced power at a frequency $f$ greater than $f_{\rm peak}$, for $f \gtrsim 4\,\rm kHz$, with $f_{\rm peak} \in [2.5, 4]\,\rm kHz$. On the other hand, black-hole-forming remnants exhibit excess power in the same large $f$ region and manifest exponential damping in the time domain characteristic of a quasi-normal mode. We demonstrate that the gravitational-wave signal from a collapsed remnant is indeed a quasi-normal ringing. We report on the opportunity for direct detections of black hole formation with next-generation gravitational-wave detectors such as Cosmic Explorer and Einstein Telescope and set forth the tantalizing prospect of such observations up to a distance of 100 Mpc.
Abstract: 2306.04577
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Full Text: [ PostScript, PDF]
Title:Cosmological significance of the early bright galaxies observed with JWST
Download PDFAbstract: The recent discovery of objects with redshift $z>10$ with the help of James Webb Space Telescope (JWST) poses serious challenges to the $\Lambda$CDM cosmological model, which has been in vogue for some time now. The new data indicate that galaxy formation must have taken place much earlier than expected in this model. Another viable class of cosmological models is that of the so-called coasting models, in which the scale factor of the universe varies proportionately with time. In these models, the universe at redshift $z=12$ has ample time ($\sim 1070$ Myrs) for galaxy formation. The earliest such model is the one proposed by E.A. Milne, based on his `kinematic relativity', but it is considered unrealistic for not treating gravity as relevant at cosmological scales. A closed version of an eternal coasting FLRW model was proposed by the present authors even before SNe Ia data began to pour in. Subsequently we developed a more general model of the same class, which is valid for all the three possible geometries, with open, closed or flat spatial sections. In the nonrelativistic era, this model makes the falsifiable prediction that the ratio of matter density to dark energy density is 2. This avoids the cosmic coincidence problem. Moreover, this eternal coasting model allows room for creation of matter from dark energy, that may speed up galaxy and structure formation at the early epochs, as implied by the JWST data. The paper also attempts to review some similar coasting models, but emphasizes the eternal coasting cosmology as the most probable candidate model capable of explaining the presence of high redshift galaxies discovered by JWST.
Abstract: 2306.04053
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Full Text: [ PostScript, PDF]
Title:Detection of the Cosmological Time Dilation of High Redshift Quasars
Download PDFAbstract: A fundamental prediction of relativistic cosmologies is that, due to the expansion of space, observations of the distant cosmos should be time dilated and appear to run slower than events in the local universe. Whilst observations of cosmological supernovae unambiguously display the expected redshift-dependent time dilation, this has not been the case for other distant sources. Here we present the identification of cosmic time dilation in a sample of 190 quasars monitored for over two decades in multiple wavebands by assessing various hypotheses through Bayesian analysis. This detection counters previous claims that observed quasar variability lacked the expected redshift-dependent time dilation. Hence, as well as demonstrating the claim that the lack of the redshift dependence of quasar variability represents a significant challenge to the standard cosmological model, this analysis further indicates that the properties of quasars are consistent with them being truly cosmologically distant sources.
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