Abstracts of Interest
Selected by:
Jose Bellido
Abstract: The Square Kilometre Array (SKA) is a planned large radio interferometer designed to operate over a wide range of frequencies, and with an order of magnitude greater sensitivity and survey speed than any current radio telescope. The SKA will address many important topics in astronomy, ranging from planet formation to distant galaxies. However, in this work, we consider the perspective of the SKA as a facility for studying physics. We review four areas in which the SKA is expected to make major contributions to our understanding of fundamental physics: cosmic dawn and reionisation; gravity and gravitational radiation; cosmology and dark energy; and dark matter and astroparticle physics. These discussions demonstrate that the SKA will be a spectacular physics machine, which will provide many new breakthroughs and novel insights on matter, energy and spacetime.
Abstract: We present a multi-wavelength study of IC 1531 (z=0.02564), an extragalactic radio source associated with the $\gamma$-ray object 3FGL J0009.9-3206 and classified as a blazar of uncertain type in the Third Fermi Large Area Telescope AGN Catalog (3LAC). A core-jet structure, visible in radio and X-rays, is enclosed within a $\sim$220 kpc wide radio structure. The morphology and spectral characteristics of the kiloparsec jet in radio and X-rays are typical of Fanaroff-Riley type I galaxies. The analysis of the radio data and optical spectrum and different diagnostic methods based on the optical, infrared and $\gamma$-ray luminosities also support a classification as a low-power radio galaxy seen at moderate angles ($\theta=$10$^\circ$-20$^\circ$). In the framework of leptonic models, the high-energy peak of the non-thermal nuclear spectral energy distribution can be explained in terms of synchrotron-self-Compton emission from a jet seen at $\theta\sim$15$^\circ$. Similarly to other misaligned AGNs detected by Fermi, the required bulk motion is lower ($\Gamma_{\rm bulk}=$4) than the values inferred in BL Lac objects, confirming that, because of the de-boosting of emission from the highly-relativistic blazar region, these nearby systems are valuable targets to probe the existence of multiple sites of production of the most energetic emission in the jets.
Abstract: Observed properties of the nonthermal afterglow emission of GW170817 from radio to X-ray are consistent with synchrotron radiation by electrons accelerated in the shock generated by outflow from the merger. However, previous studies modeling these data made a simplified assumption that all electrons in the shock are accelerated as a nonthermal population. Here we present a new modeling with a more natural electron energy distribution, in which the number fraction $f$ of electrons injected into particle acceleration is variable. Using two models (structured jet and radially-stratified spherical outflow) for the outflow geometry, model parameters are determined by fit to the observed data. Interestingly, new solutions are found with radio flux in the regime of low frequency synchrotron tail ($\nu < \nu_m$, where $\nu_m$ is the frequency corresponding to the lowest electron energy) in the early phase, in contrast to previous studies that found the radio frequency always above $\nu_m$. We encourage to take densely sampled low-frequency radio data in the early phase for future BNS merger events, which would detect $\nu_m$ passage and give a strong constraint on electron energy distribution and particle acceleration efficiency. In the context of the new solutions, the best-fit isotropic-equivalent jet energy and interstellar medium density are increased by 1--2 orders of magnitude from the conventional modeling, though these are still consistent with other constraints. Finally, we show that the cooling frequency becomes almost constant in the trans-relativistic regime, and implications are discussed about use of the cooling frequency to discriminate different models.
Abstract: The Simons Observatory (SO) will make precision temperature and polarization measurements of the cosmic microwave background (CMB) over angular scales between 1 arcminute and tens of degrees using over 60,000 detectors and sampling frequencies between 27 and 270 GHz. SO will consist of a six-meter-aperture telescope coupled to over 30,000 detectors and an array of half-meter aperture refractive cameras, coupled to an additional 30,000+ detectors. The unique combination of large and small apertures in a single CMB observatory will allow us to sample a wide range of angular scales over a common survey area while providing an important stepping stone towards the realization of CMB-Stage IV. CMB-Stage IV is a proposed project that will combine and expand on existing facilities in Chile and Antarctica to reach the ~500,000 detectors required for CMB-Stage IV's science objectives. SO and CMB-Stage IV will measure fundamental cosmological parameters of our universe, constrain primordial fluctuations, find high redshift clusters via the Sunyaev-Zeldovich effect, constrain properties of neutrinos, and trace the density and velocity of the matter in the universe over cosmic time. The complex set of technical and science requirements for SO has led to innovative instrumentation solutions which we will discuss. For instance, the SO large aperture telescope will couple to a cryogenic receiver that is 2.4 m in diameter and 2.4 m long. We will give an overview of the drivers for and designs of the SO telescopes and cameras as well as the current status of the project. We will also discuss the current status of CMB-Stage IV and important next steps in the project's development.
Abstract: We discuss the theory of pulsar-timing and astrometry probes of a stochastic gravitational-wave background with a recently developed "total-angular-momentum'" (TAM) formalism for cosmological perturbations. We review the formalism, emphasizing in particular the features relevant for this work. We describe the observables we consider (which we take to be the pulsar redshift and stellar angular displacement). We derive from the TAM approach angular power spectra for the observables and from them derive angular auto- and cross-correlation functions. We provide the full set of power spectra and correlation functions not only for the standard transverse-traceless propagating degrees of freedom in general relativity, but also for the four additional non-Einsteinian polarizations that may arise in alternative-gravity theories. We clarify the range of gravitational-wave frequencies that can be probed with pulsar timing and astrometry; speculate on the possibility to reconstruct the local three-dimensional metric perturbation with combined angular and time-sequence information; comment on the importance of testing the chirality of the gravitational-wave background; and describe briefly how to seek the type of power asymmetry that might arise if the signal is dominated by a handful of nearby sources. We also provide in an Appendix a simple re-derivation of the power spectra from the plane-wave formalism.
Abstract: IceCube detects more than 100,000 neutrinos per year in the GeV- to PeV-energy range. Among those, we have isolated a flux of high-energy cosmic neutrinos. I will discuss the instrument, the analysis of the data, the significance of the discovery of cosmic neutrinos, and the recent multi-messenger observation of a flaring TeV blazar in coincidence with the IceCube neutrino alert IC170922.
Abstract: Motivated by the occurrence of a moderately nearby supernova near the beginning of the Pleistocene, we investigate whether nitrate rainout resulting from the atmospheric ionization of enhanced cosmic ray flux could have, through its fertilizer effect, initiated carbon dioxide drawdown. Such a drawdown could possibly reduce the greenhouse effect and induce the climate change that led to the Pleistocene glaciations. We estimate that the nitrogen flux enhancement onto the surface from an event at 50 pc would be of order 10%, probably too small for dramatic changes. We estimate deposition of iron (another potential fertilizer) and find it also to be too small to be significant. There are also competing effects of opposite sign, including muon irradiation and reduction in photosynthetic yield caused by UV increase from stratospheric ozone layer depletion. We conclude that the effect may be of interest from much nearer supernovae in the geological past, but more work needs to be done to clarify the magnitude of various competing effects.
Abstract: We present 10 years of R-band monitoring data of 31 northern blazars which were either detected at very high energy (VHE) gamma rays or listed as potential VHE gamma-ray emitters. The data comprise 11820 photometric data points in the R-band obtained in 2002-2012. We analyze the light curves by determining their power spectral density (PSD) slopes assuming a power-law dependence with a single slope $\beta$ and a Gaussian probability density function (PDF). We use the multiple fragments variance function (MFVF) combined with a forward-casting approach and likelihood analysis to determine the slopes and perform extensive simulations to estimate the uncertainties of the derived slopes. We also look for periodic variations via Fourier analysis and quantify the false alarm probability through a large number of simulations. Comparing the obtained PSD slopes to values in the literature, we find the slopes in the radio band to be steeper than those in the optical and gamma rays. Our periodicity search yielded one target, Mrk 421, with a significant (p<5%) period. Finding one significant period among 31 targets is consistent with the expected false alarm rate, but the period found in Mrk~421 is very strong and deserves further consideration}.
Abstract: Cherenkov telescopes are also sensitive to the atmospheric fluorescence produced by the extensive air showers. However this contribution is neglected by the reconstruction algorithms of imaging air Cherenkov telescopes IACTs and wide-angle Cherenkov detectors WACDs. In this paper we evaluate the fluorescence contamination in the Cherenkov signals from MC simulations in both kinds of Cherenkov telescopes and for some typical observational situations. Results for an observation level of 2200 m a.s.l. are shown. In addition, the feasibility and capabilities of IACTs working as fluorescence telescopes are discussed with the assistance of some geometrical calculations.
Abstract: The ANtarctic Impulsive Transient Antenna (ANITA) long-duration balloon payload searches for Askaryan radio emission from ultra-high-energy ($>10^{18}$ eV) neutrinos interacting in Antarctic ice. ANITA is also sensitive to geomagnetic radio emission from extensive air showers (EAS). This talk summarizes recently released results from the third flight of ANITA, which flew during the 2014-2015 Austral summer. The most sensitive search from ANITA-III identified one neutrino candidate with an a priori background estimate of 0.7$^{+0.5}_{-0.3}$. When combined with previous flights, ANITA sets the best limits on diffuse neutrino flux at energies above $\sim10^{19.5}$ eV. Additionally, ANITA-III searches identified nearly 30 EAS candidates. One unusual event appears to correspond to an upward-going air shower, similar to an event from ANITA-I.
Abstract: The hybrid design of the Pierre Auger Observatory allows for the measurement of a number of properties of extensive air showers initiated by ultra-high energy cosmic rays. By comparing these measurements to predictions from air shower simulations, it is possible to both infer the cosmic ray's mass composition and test hadronic interactions beyond the energies reached by accelerators. In this paper, we will present a compilation of results of air shower measurements by Pierre Auger Observatory which are sensitive to the properties of hadronic interactions and can be used to constrain the hadronic interaction models. The inconsistencies found between the interpretation of different observables with regard to primary composition and between their measurements and simulations show that none of the currently used hadronic interaction models can provide a proper description of air showers and, in particular, of the muon production.
Abstract: These proceedings address a recent publication by the ANITA collaboration of four upward- pointing cosmic-ray-like events observed in the first flight of ANITA. Three of these events were consistent with stratospheric cosmic-ray air showers where the axis of propagation does not inter- sect the surface of the Earth. The fourth event was consistent with a primary particle that emerges from the surface of the ice suggesting a possible {\tau}-lepton decay as the origin of this event. These proceedings follow-up on the modeling and testing of the hypothesis that this event was of {\tau} neutrino origin.
Abstract: Telescope Array (TA) has completed analysis of nearly nine years of data measuring the atmospheric depth of air shower maximum ($X_{\textrm{max}}$) utilizing the TA surface detector array and the Black Rock Mesa and Long Ridge fluorescence detector stations. By using both the surface array and the fluorescence detector, the geometry and arrival time of air showers can be measured very precisely providing good resolution in determining $X_{\textrm{max}}$. $X_{\textrm{max}}$ is directly related to the air shower primary particle mass and is therefore important for understanding the composition of ultra high energy cosmic rays (UHECRs). UHECR composition will help answer questions such as the distance and location of their sources. We discuss the experimental apparatus, analysis method, and $X_{\textrm{max}}$ data collected. We compare the energy dependent distributions of the observed data to detailed Monte Carlo simulations of four chemical species, then test which individual species are not compatible with the data through an analysis of the shapes of the distributions. We also discuss the present state of composition analysis and interpretation between the Auger and TA experiments. These are the two largest UHECR observatories in the world with large exposures and should shed light on UHECR composition.