We present ANNz2, a new implementation of the public software for photometric redshift (photo-z) estimation of Collister & Lahav, which now includes generation of full probability distribution ...functions (PDFs). ANNz2 utilizes multiple machine learning methods, such as artificial neural networks and boosted decision/regression trees. The objective of the algorithm is to optimize the performance of the photo-z estimation, to properly derive the associated uncertainties, and to produce both single-value solutions and PDFs. In addition, estimators are made available, which mitigate possible problems of non-representative or incomplete spectroscopic training samples. ANNz2 has already been used as part of the first weak lensing analysis of the Dark Energy Survey, and is included in the experiment's first public data release. Here we illustrate the functionality of the code using data from the tenth data release of the Sloan Digital Sky Survey and the Baryon Oscillation Spectroscopic Survey. The code is available for download at http://github.com/IftachSadeh/ANNZ.
ABSTRACT
We study the prompt phase of low-luminosity gamma-ray bursts (ll-GRBs) as potential source of very-high-energy (VHE) gamma-rays and ultra-high-energy cosmic rays (UHECRs). Within the ...internal shock model, we choose parameters for the relativistic outflow such that our representative events have observed properties similar to GRBs 980425, 100316D, and 120714B, and self-consistently calculate the full spectral and temporal properties in a leptonic synchrotron self-Compton scenario. To investigate the conditions under which inverse Compton radiation may lead to a peak in the GeV–TeV range, we vary the fraction of internal energy supplying the magnetic field. Further, we determine the maximal energies achievable for UHECR nuclei and derive constraints on the baryonic loading/typical duration by comparing to the extragalactic gamma-ray background. We find that ll-GRBs are potential targets for multiwavelength studies and in reach for Imaging Atmospheric Cherenkov Telescopes (IACTs) and optical/UV instruments. For comparable sub-MeV emission and similar dynamical evolution of the outflow, weak (strong) magnetic fields induce high (low) fluxes in the VHE regime and low (high) fluxes in the optical. VHE emission may be suppressed by γγ-absorption close to the engine or interactions with the extragalactic background light for redshifts z > 0.1. For UHECRs, the maximal energies of iron nuclei (protons) can be as high as ≃1011 GeV (1010 GeV) if the magnetic energy density is large (and the VHE component is correspondingly weak). These high energies are possible by decoupling the production regions of UHECR and gamma-rays in our multizone model. Finally, we find basic consistency with the energy budget needed to accommodate the UHECR origin from ll-GRBs.
Abstract The Ultraviolet Transient Astronomy Satellite (ULTRASAT) is scheduled to be launched to geostationary orbit in 2027. It will carry a telescope with an unprecedentedly large field of view ...(204 deg 2 ) and near-ultraviolet (NUV; 230–290 nm) sensitivity (22.5 mag, 5 σ , at 900 s). ULTRASAT will conduct the first wide-field survey of transient and variable NUV sources and will revolutionize our ability to study the hot transient Universe. It will explore a new parameter space in energy and timescale (months-long light curves with minutes cadence), with an extragalactic volume accessible for the discovery of transient sources that is >300 times larger than that of the Galaxy Evolution Explorer (GALEX) and comparable to that of the Vera Rubin Observatory’s Legacy Survey of Space and Time. ULTRASAT data will be transmitted to the ground in real time, and transient alerts will be distributed to the community in <15 minutes, enabling vigorous ground-based follow up of ULTRASAT sources. ULTRASAT will also provide an all-sky NUV image to >23.5 AB mag, over 10 times deeper than the GALEX map. Two key science goals of ULTRASAT are the study of mergers of binaries involving neutron stars, and supernovae. With a large fraction (>50%) of the sky instantaneously accessible, fast (minutes) slewing capability, and a field of view that covers the error ellipses expected from gravitational-wave (GW) detectors beyond 2026, ULTRASAT will rapidly detect the electromagnetic emission following binary neutron star/neutron star–black hole mergers identified by GW detectors, and will provide continuous NUV light curves of the events. ULTRASAT will provide early (hour) detection and continuous high-cadence (minutes) NUV light curves for hundreds of core-collapse supernovae, including for rarer supernova progenitor types.
We present a study of γ-ray emission from the core-collapse supernova remnant Cas A in the energy range from 0.1 GeV to 10 TeV. We used 65 hr of the Very Energetic Radiation Imaging Telescope Array ...System (VERITAS) data to cover 200 GeV-10 TeV, and 10.8 yr of Fermi-Large Area Telescope (LAT) data to cover 0.1-500 GeV. The spectral analysis of Fermi-LAT data shows a significant spectral curvature around 1.3 0.4stat GeV that is consistent with the expected spectrum from pion decay. Above this energy, the joint spectrum from Fermi-LAT and VERITAS deviates significantly from a simple power law, and it is best described by a power law with a spectral index of 2.17 0.02stat and a cutoff energy of 2.3 0.5stat TeV. These results, along with radio, X-ray, and γ-ray data, are interpreted in the context of leptonic and hadronic models. Assuming a one-zone model, we exclude a purely leptonic scenario and conclude that proton acceleration up to at least 6 TeV is required to explain the observed γ-ray spectrum. From modeling of the entire multiwavelength spectrum, a minimum magnetic field inside the remnant of Bmin 150 G is deduced.
The extragalactic background light (EBL), a diffuse photon field in the optical and infrared range, is a record of radiative processes over the universe's history. Spectral measurements of blazars at ...very high energies (>100 GeV) enable the reconstruction of the spectral energy distribution (SED) of the EBL, as the blazar spectra are modified by redshift- and energy-dependent interactions of the gamma-ray photons with the EBL. The spectra of 14 VERITAS-detected blazars are included in a new measurement of the EBL SED that is independent of EBL SED models. The resulting SED covers an EBL wavelength range of 0.56-56 m, and is in good agreement with lower limits obtained by assuming that the EBL is entirely due to radiation from cataloged galaxies.
Abstract We use the Very Energetic Radiation Imaging telescope Array System (VERITAS) imaging air Cherenkov telescope array to obtain the first measured angular diameter of β UMa at visual ...wavelengths using stellar intensity interferometry (SII) and independently constrain the limb-darkened angular diameter. The age of the Ursa Major moving group has been assessed from the ages of its members, including nuclear member Merak ( β UMa), an A1-type subgiant, by comparing effective temperature and luminosity constraints to model stellar evolution tracks. Previous interferometric limb-darkened angular-diameter measurements of β UMa in the near-infrared (Center for High Angular Resolution Astronomy (CHARA) Array, 1.149 ± 0.014 mas) and mid-infrared (Keck Nuller, 1.08 ± 0.07 mas), together with the measured parallax and bolometric flux, have constrained the effective temperature. This paper presents current VERITAS-SII observation and analysis procedures to derive squared visibilities from correlation functions. We fit the resulting squared visibilities to find a limb-darkened angular diameter of 1.07 ± 0.04 (stat) ± 0.05 (sys) mas, using synthetic visibilities from a stellar atmosphere model that provides a good match to the spectrum of β UMa in the optical wave band. The VERITAS-SII limb-darkened angular diameter yields an effective temperature of 9700 ± 200 ± 200 K, consistent with ultraviolet spectrophotometry, and an age of 390 ± 29 ± 32 Myr, using MESA Isochrones and Stellar Tracks. This age is consistent with 408 ± 6 Myr from the CHARA Array angular diameter.
We present results from a study of the photometric redshift performance of the Dark Energy Survey (DES), using the early data from a Science Verification period of observations in late 2012 and early ...2013 that provided science-quality images for almost 200 sq. deg. at the nominal depth of the survey. We assess the photometric redshift (photo-z) performance using about 15 000 galaxies with spectroscopic redshifts available from other surveys. These galaxies are used, in different configurations, as a calibration sample, and photo-z's are obtained and studied using most of the existing photo-z codes. A weighting method in a multidimensional colour-magnitude space is applied to the spectroscopic sample in order to evaluate the photo-z performance with sets that mimic the full DES photometric sample, which is on average significantly deeper than the calibration sample due to the limited depth of spectroscopic surveys. Empirical photo-z methods using, for instance, artificial neural networks or random forests, yield the best performance in the tests, achieving core photo-z resolutions ... ~ 0.08. Moreover, the results from most of the codes, including template-fitting methods, comfortably meet the DES requirements on photo-z performance, therefore, providing an excellent precedent for future DES data sets. (ProQuest: ... denotes formulae/symbols omitted.)
High-energy gamma-ray emission from supernova remnants (SNRs) has provided a unique perspective for studies of Galactic cosmic-ray acceleration. Tycho's SNR is a particularly good target because it ...is a young, type Ia SNR that has been well-studied over a wide range of energies and located in a relatively clean environment. Since the detection of gamma-ray emission from Tycho's SNR by VERITAS and Fermi-LAT, there have been several theoretical models proposed to explain its broadband emission and high-energy morphology. We report on an update to the gamma-ray measurements of Tycho's SNR with 147 hr of VERITAS and 84 months of Fermi-LAT observations, which represent about a factor of two increase in exposure over previously published data. About half of the VERITAS data benefited from a camera upgrade, which has made it possible to extend the TeV measurements toward lower energies. The TeV spectral index measured by VERITAS is consistent with previous results, but the expanded energy range softens a straight power-law fit. At energies higher than 400 GeV, the power-law index is 2.92 0.42stat 0.20sys. It is also softer than the spectral index in the GeV energy range, 2.14 0.09stat 0.02sys, measured in this study using Fermi-LAT data. The centroid position of the gamma-ray emission is coincident with the center of the remnant, as well as with the centroid measurement of Fermi-LAT above 1 GeV. The results are consistent with an SNR shell origin of the emission, as many models assume. The updated spectrum points to a lower maximum particle energy than has been suggested previously.
We present results from deep observations toward the Cygnus region using 300 hr of very high energy (VHE) γ-ray data taken with the VERITAS Cerenkov telescope array and over 7 yr of high-energy γ-ray ...data taken with the Fermi satellite at an energy above 1 GeV. As the brightest region of diffuse γ-ray emission in the northern sky, the Cygnus region provides a promising area to probe the origins of cosmic rays. We report the identification of a potential Fermi-LAT counterpart to VER J2031+415 (TeV J2032+4130) and resolve the extended VHE source VER J2019+368 into two source candidates (VER J2018+367* and VER J2020+368*) and characterize their energy spectra. The Fermi-LAT morphology of 3FGL J2021.0+4031e (the Gamma Cygni supernova remnant) was examined, and a region of enhanced emission coincident with VER J2019+407 was identified and jointly fit with the VERITAS data. By modeling 3FGL J2015.6+3709 as two sources, one located at the location of the pulsar wind nebula CTB 87 and one at the quasar QSO J2015+371, a continuous spectrum from 1 GeV to 10 TeV was extracted for VER J2016+371 (CTB 87). An additional 71 locations coincident with Fermi-LAT sources and other potential objects of interest were tested for VHE γ-ray emission, with no emission detected and upper limits on the differential flux placed at an average of 2.3% of the Crab Nebula flux. We interpret these observations in a multiwavelength context and present the most detailed γ-ray view of the region to date.
The Large Array Survey Telescope—Science Goals Ben-Ami, S.; Ofek, E. O.; Polishook, D. ...
Publications of the Astronomical Society of the Pacific,
08/2023, Letnik:
135, Številka:
1050
Journal Article
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