Abstracts of Interest
Selected by:
Imogen Barnsley
Abstract: 2404.01239
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Title:Characterization of Atmosphere-Skimming Cosmic-Ray Showers in High-Altitude Experiments
View PDF HTML (experimental)Abstract:Atmosphere-skimming showers are initiated by cosmic rays with incoming directions such that the full development of the cascade occurs inside the atmosphere without ever reaching the ground. This new class of showers has been observed in balloon-borne experiments such as ANITA, but a characterisation of their properties is lacking. The interplay between the Earth's magnetic field, the long distances over which atmosphere-skimming showers develop, and the low density of the atmosphere they traverse gives rise to several effects that are not seen in downward-going cascades, and require detailed modeling. In this article, we used the latest version of the ZHAireS-RASPASS shower simulation program to tackle this problem, and dwell on the particular phenomena that arises from the peculiar environment on which these showers develop. We focus in particular on the properties of the longitudinal profile of the shower and its fluctuations as a function of cosmic-ray energy, direction and primary mass. We have also studied the phase-space of cosmic-ray arrival directions where detection in high-altitude experiments is more likely, and have found that only in a small range of directions the showers are sufficiently developed before reaching the altitude of the detector. Our results are relevant for the design of high-altitude and in particular balloon-borne experiments, and for the interpretation of the data they collect.
Abstract: 2404.01456
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Title:Monster shocks, gamma-ray bursts and black hole quasi-normal modes from neutron-star collapse
View PDF HTML (experimental)Abstract:We perform the first magnetohydrodynamic simulation tracking the magnetosphere of a collapsing magnetar. The collapse is expected for massive rotating magnetars formed in merger events, and it may occur many hours after the merger. The results suggest a novel gamma-ray burst (GRB) scenario, which creates a delayed high-energy counterpart of the merger gravitational waves. The collapse launches an outgoing magnetospheric shock, and a hot magnetized outflow forms behind the shock. The outflow is modulated by the ring-down of the nascent black hole, imprinting its kilohertz quasi-normal modes on the GRB tail.
Abstract: 2404.01457
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Title:A Semi-blind Reconstruction of the History of Effective Number of Neutrinos Using CMB Data
View PDF HTML (experimental)Abstract:We explore the possibility of redshift-dependent deviations in the contribution of relativistic degrees of freedom to the radiation budget of the cosmos, conventionally parameterized by the effective number of neutrinos $N_{\rm eff}$, from the predictions of the standard model. We expand the deviations $\Delta N_{\rm eff}(z)$ in terms of top-hat functions and treat their amplitudes as the free parameters of the theory to be measured alongside the standard cosmological parameters by the Planck measurements of the cosmic microwave background (CMB) anisotropies and Baryonic Acoustic Oscillations, as well as performing forecasts for futuristic CMB surveys such as PICO and CMB-S4. We reconstruct the history of $\Delta N_{\rm eff}$ and find that with the current data the history is consistent with the standard scenario. Inclusion of the new degrees of freedom in the analysis increases $H_0$ to $68.71\pm 0.44$, slightly reducing the Hubble tension. With the smaller forecasted errors on the $\Delta N_{\rm eff}(z)$ parametrization modes from future CMB surveys, very accurate bounds are expected within the possible range of dark radiation models.
Abstract: 2404.01502
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Title:Turning Spherical Cows into Spherical Cheeses: a bold and flavourful re-Interpretation of the Moon's spectrum
View PDF HTML (experimental)Abstract:"Everyone knows the moon is made of cheese..." This line, famously uttered by Wallace to his canine sidekick Gromit in the 80s classic, may be one of the most cruelly underappreciated movie quotations of our time. Indeed, while most scientists today would simply reject Wallace's claim as preposterous, we aim to revisit his theory on the composition of our natural satellite, revealing that it may not be as implausible as the scientific consensus would have it. Through a revelatory novel analysis of existing data, we will show that very simple cheese-based models can provide a convincing explanation of the Lunar surface's spectral characteristics in the near-infrared. Using the tried and tested PLS (Partial Least Squares) method, we efficiently and reliably retrieve the concentrations of various cheese types in different locations of the Lunar surface. Our results bring to light a bold and flavourful prediction about the Moon's composition, which lays the groundwork for an important paradigm shift in planetary sciences. We urge the scientific community to take a serious notice of this piquant novel interpretation, and strongly consider it in their future models of planetary composition and formation in our solar system and beyond.
Abstract: 2404.01649
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Title:Observations of the Crab Nebula with MACE (Major Atmospheric Cherenkov Experiment)
View PDF HTML (experimental)Abstract:The Major Atmospheric Cherenkov Experiment (MACE) is a large size (21m) Imaging Atmospheric Cherenkov Telescope (IACT) installed at an altitude of 4270m above sea level at Hanle, Ladakh in northern India. Here we report the detection of Very High Energy (VHE) gamma-ray emission from Crab Nebula above 80 GeV. We analysed ~15 hours of data collected at low zenith angle between November 2022 and February 2023. The energy spectrum is well described by a log-parabola function with a flux of ~(3.46 +/- 0.26stat) x 10-10 TeV-1 cm-2 s-1, at 400 GeV with spectral index of 2.09 +/- 0.06stat and a curvature parameter of 0.08 +/- 0.07stat. The gamma-rays are detected in an energy range spanning from 80 GeV to ~5 TeV. The energy resolution improves from ~34% at an analysis energy threshold of 80 GeV to ~21% above 1 TeV. The daily light curve and the spectral energy distribution obtained for the Crab Nebula is in agreement with previous measurements, considering statistical and systematic uncertainties.
Abstract: 2404.01888
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Title:Estimation of the atmospheric absorption profile with isotropic background events observed by Imaging Atmospheric Cherenkov Telescopes
View PDF HTML (experimental)Abstract:Atmospheric Cherenkov telescopes rely on the Earth's atmosphere as part of the detector. The presence of clouds affects observations and can introduce biases if not corrected for. Correction methods typically require an atmospheric profile, that can be measured with external atmospheric monitoring devices. We present a novel method for measuring the atmospheric profile using the data from Imaging Atmospheric Cherenkov telescopes directly. The method exploits the comparison of average longitudinal distributions of the registered Cherenkov light between clear atmosphere and cloud presence cases. Using Monte Carlo simulations of a subarray of four Large-Sized Telescopes of the upcoming Cherenkov Telescope Array Observatory and a simple cloud model we evaluate the accuracy of the method in determining the basic cloud parameters. We find that the method can reconstruct the transmission of typical clouds with an absolute accuracy of a few per cent. For low-zenith observations, the height of the cloud centre can be reconstructed with a typical accuracy of a few hundred metres, while the geometrical thickness can be accurately reconstructed only if it is >= 3 km. We also evaluate the robustness of the method against the typical systematic uncertainties affecting atmospheric Cherenkov telescopes.
Abstract: 2404.01926
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Title:Synchro-curvature description of $γ$-ray light curves and spectra of pulsars: global properties
View PDF HTML (experimental)Abstract:This work presents a methodological approach to generate realistic $\gamma$-ray light curves of pulsars, resembling reasonably well the observational ones observed by the Fermi-Large Area Telescope instrument, fitting at the same time their high-energy spectra. The theoretical light curves are obtained from a spectral and geometrical model of the synchro-curvature emission. Despite our model relies on a few effective physical parameters, the synthetic light curves present the same main features observed in the observational $\gamma$-ray light curve zoo, such as the different shapes, variety in the number of peaks, and a diversity of peak widths. The morphological features of the light curves allows us to statistically compare the observed properties. In particular, we find that the proportion on the number of peaks found in our synthetic light curves is in agreement with the observational one provided by the third Fermi-LAT pulsar catalog. We also found that the detection probability due to beaming is much higher for orthogonal rotators (approaching 100%) than for small inclination angles (less than 20%).The small variation on the synthetic skymaps generated for different pulsars indicates that the geometry dominates over timing and spectral properties in shaping the gamma-ray light curves. This means that geometrical parameters like the inclination angle can be in principle constrained by gamma-ray data alone independently on the specific properties of a pulsar. At the same time, we find that $\gamma$-ray spectra seen by different observers can slightly differ, opening the door to constraining the viewing angle of a particular pulsar.
Abstract: 2404.02202
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Title:Constraints on the Cosmic Neutrino Background from NGC 1068
View PDF HTML (experimental)Abstract:We use recent evidence of TeV neutrino events from NGC 1068, detected by the IceCube experiment, to constrain the overdensity of relic neutrinos locally and globally. Since these high-energy neutrinos have travelled long distances through a sea of relic neutrinos, they could have undergone scattering, altering their observed flux on Earth. Considering only Standard Model interactions, we constrain the relic overdensity to be $\eta\leq 3.85 \times 10^8 (5.39 \times 10^{11})$ at the 95$\%$ confidence level for overdensities with a radius of 14 Mpc (10 kpc), assuming the sum of neutrino masses saturates the cosmological bound, $\sum_i m_i = 0.13$ eV. We demonstrate that this limit improves with larger neutrino masses and how it depends on the scale of the overdensity region.
Abstract: 2404.02627
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Title:Possible X-ray Cocoon Emission from GRB 050709
View PDF HTML (experimental)Abstract:The detection of the short gamma-ray burst (SGRB) 050709 by the HETE-2 satellite opened a new window into understanding the nature of SGRBs, offering clues about their emission mechanism and progenitors, with the crucial aid of optical follow-up observations. Here, we revisit the prompt emission of GRB 050709. Our analysis reveals an initial hard spike ~200 ms long, followed by a subsequent soft tail emission lasting ~300 ms. These components could be common among other SGRBs originating from binary neutron merger events, such as GW/GRB 170817A. Detailed temporal and spectral analyses indicate that the soft tail emission might be attributed to the cocoon formed by the relativistic jet depositing energy into the surrounding material. We find the necessary cocoon parameters at the breakout, as consistent with numerical simulation results. We compared the physical parameters of this cocoon with those of other SGRBs. The relatively higher cocoon pressure and temperature in GRB 050709 may indicate a more on-axis jet compared to GRB 170817A and GRB 150101B.
Abstract: 2404.03229
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Title:Relation between the keV-MeV and TeV emission of GRB 221009A and its implications
View PDF HTML (experimental)Abstract:Gamma-ray bursts (GRBs) are believed to launch relativistic jets, which generate prompt emission by their internal processes and drive external shocks into surrounding medium, accounting for the long-lasting afterglow emission. However, how the jet powers the external shock is an open question. The unprecedented observations of the keV-MeV emission with GECAM and the TeV emission with LHAASO of so far the brightest burst, GRB 221009A, offer a great opportunity to study the prompt-to-afterglow transition and the early dynamical evolution of the external shock. In this letter, we find that the cumulative light curve of keV-MeV emission could well fit the rising stage of the TeV light curve of GRB 221009A, with a time delay of $4.45^{+0.26}_{-0.26}$\,s for TeV emission. Moreover, both the rapid increase in the initial stage and the excess from about \T+260\,s to 270\,s in the TeV light curve could be interpreted by inverse Compton (IC) scatterings of the inner-coming photons by the energetic electrons in external shock. Our results not only reveal a close relation between the keV-MeV and TeV emission, but also indicate a continuous, rather than impulsive, energy injection to the external shock. Assuming an energy injection rate proportional to the keV-MeV flux, we build a continuous energy injection model which well fits the TeV light curve of GRB 221009A, and provides an estimate of the Lorentz factor of the jet.
Abstract: 2404.03533
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Title:Impact of the Magnetic Horizon on the Interpretation of the Pierre Auger Observatory Spectrum and Composition Data
View PDF HTML (experimental)Abstract:The flux of ultra-high energy cosmic rays reaching Earth above the ankle energy (5 EeV) can be described as a mixture of nuclei injected by extragalactic sources with very hard spectra and a low rigidity cutoff. Extragalactic magnetic fields existing between the Earth and the closest sources can affect the observed CR spectrum by reducing the flux of low-rigidity particles reaching Earth. We perform a combined fit of the spectrum and distributions of depth of shower maximum measured with the Pierre Auger Observatory including the effect of this magnetic horizon in the propagation of UHECRs in the intergalactic space. We find that, within a specific range of the various experimental and phenomenological systematics, the magnetic horizon effect can be relevant for turbulent magnetic field strengths in the local neighbourhood of order $B_{\rm rms}\simeq (50-100)\,{\rm nG}\,(20\rm{Mpc}/{d_{\rm s})( 100\,\rm{kpc}/L_{\rm coh}})^{1/2}$, with $d_{\rm s}$ the typical intersource separation and $L_{\rm coh}$ the magnetic field coherence length. When this is the case, the inferred slope of the source spectrum becomes softer and can be closer to the expectations of diffusive shock acceleration, i.e., $\propto E^{-2}$. An additional cosmic-ray population with higher source density and softer spectra, presumably also extragalactic and dominating the cosmic-ray flux at EeV energies, is also required to reproduce the overall spectrum and composition results for all energies down to 0.6~EeV.
This page created: Tue Apr 9 18:22:43 ACST 2024 by Imogen Barnsley
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