Direct measurements of the extragalactic background light (EBL) in the near-IR to mid-IR waveband are extremely difficult due to an overwhelming foreground from the zodiacal light that outshines the ...faint cosmological diffuse radiation field by more than an order of magnitude. Indirect constraints on the EBL are provided by Delta *g-ray observations of active galactic nuclei. Using the combination of the Fermi Gamma-Ray Space Telescope together with the current generation of ground-based air Cherenkov telescopes (H.E.S.S., MAGIC, and VERITAS) provides unprecedented sensitivity and spectral coverage for constraining the EBL in the near- to mid-IR. In this paper, we present new limits on the EBL based on the analysis of the broadband spectra of a select set of Delta *g-ray blazars covering 200 MeV to several TeV. The EBL intensity at 15 Delta *mm is constrained to be 1.36 ? 0.58 nW m--2 sr--1. We find that the fast evolution and baseline EBL models of Stecker et al., as well as the model of Kneiske et al., predict significantly higher EBL intensities in the mid-IR (15 Delta *mm) than is allowed by the constraints derived here. In addition, the model of Franceschini et al. and the fiducial model of Dominguez et al. predict near- to mid-IR ratios smaller than that derived from our analysis. Namely, their intensities in the near-IR are too low while their intensities in the mid-IR are marginally too high. All of the aforementioned models are inconsistent with our analysis at the >3 Delta *s level.
We present a detailed analysis of week-long simultaneous observations of the blazar Mrk 421 at 2-60 keV X-rays (RXTE) and TeV -rays (Whipple and HEGRA) in 2001. Accompanying optical monitoring was ...performed with the Mt. Hopkins 48 inch telescope. The unprecedented quality of this data set enables us to establish the existence of the correlation between the TeV and X-ray luminosities, and also to start unveiling some of its characteristics, in particular its energy dependence and time variability. The source shows strong variations in both X-ray and -ray bands, which are highly correlated. No evidence of an X-ray/-ray interband lag tau is found on the full week data set, with tau image 3 ks. A detailed analysis of the March 19 flare, however, reveals that data are not consistent with the peak of the outburst in the 2-4 keV X-ray and TeV band being simultaneous. We estimate a image ks TeV lag. The amplitudes of the X-ray and -ray variations are also highly correlated, and the TeV luminosity increases more than linearly with respect to the X-ray one. The high degree of correlation lends further support to the standard model in which a unique electron population produces the X-rays by synchrotron radiation and the -ray component by inverse Compton scattering. However, the finding that for the individual best observed flares the -ray flux scales approximately quadratically with respect to the X-ray flux poses a serious challenge to emission models for TeV blazars, as it requires rather special conditions and/or fine tuning of the temporal evolution of the physical parameters of the emission region. We briefly discuss the astrophysical consequences of these new findings in the context of the competing models for the jet emission in blazars.
Abstract The Accelerator Neutrino Neutron Interaction Experiment (ANNIE) is a 26-ton water Cherenkov neutrino detector installed on the Booster Neutrino Beam (BNB) at Fermilab. Its main physics goals ...are to perform a measurement of the neutron yield from neutrino-nucleus interactions, as well as a measurement of the charged-current cross section of muon neutrinos. An equally important focus is the research and development of new detector technologies and target media. Specifically, water-based liquid scintillator (WbLS) is of interest as a novel detector medium, as it allows for the simultaneous detection of Cherenkov light and scintillation. This paper presents the deployment of a 366 L WbLS vessel in ANNIE in March 2023 and the subsequent detection of both Cherenkov light and scintillation from the WbLS. This proof-of-concept allows for the future development of reconstruction and particle identification algorithms in ANNIE, as well as dedicated analyses within the WbLS volume, such as the search for neutral-current events and the hadronic scintillation component.
We combine the recent estimate of the contribution of galaxies to the 3.6 mum intensity of the extragalactic background light (EBL) with optical and near-infrared (IR) galaxy counts to set new limits ...on intrinsic spectra of some of the most distant TeV blazars, 1ES 0229+200, 1ES 1218+30.4, and 1ES 1101-232, located at redshifts 0.1396, 0.182, and 0.186, respectively. The new lower limit on the 3.6 mum EBL intensity is significantly higher than the previous one set by the cumulative emission from resolved Spitzer galaxies. Correcting for attenuation by the revised EBL, we show that the differential spectral index of the intrinsic spectrum of the three blazars is image or harder. These results present blazar emission models with the challenge of producing extremely hard intrinsic spectra in the sub-TeV to multi-TeV regime. These results also question the reliability of recently derived upper limits on the near-IR EBL intensity that are solely based on the assumption that intrinsic blazar spectra should not be harder than 1.50.
The Accelerator Neutrino Neutron Interaction Experiment (ANNIE) aims to make a unique measurement of neutron yield from neutrino-nucleus interactions and to perform R&D for the next generation of ...water-based neutrino detectors. In this paper, we characterize beam-induced neutron backgrounds in the experimental hall at Fermi National Accelerator Laboratory. It is shown that the background levels are sufficiently low to allow the next stage of the experiment to proceed. These measurements are relevant to other Booster Neutrino Beam (BNB) 1 experiments located adjacent to ANNIE Hall, where dirt neutrons and sky-shine could present similar backgrounds.
The Very Energetic Radiation Imaging Telescope Array System (VERITAS) represents an important step forward in the study of extreme astrophysical processes in the universe. It combines the power of ...the atmospheric Cherenkov imaging technique using a large optical reflector with the power of stereoscopic observatories using arrays of separated telescopes looking at the same shower. The seven identical telescopes in VERITAS, each of aperture 10 m, will be deployed in a filled hexagonal pattern of side 80 m; each telescope will have a camera consisting of 499 pixels with a field of view of 3.5°. VERITAS will substantially increase the catalog of very high energy (
E>100 GeV) γ-ray sources and greatly improve measurements of established sources.
Very high energy ( similar to TeV) gamma -rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity ...for determining the EBL intensity if their intrinsic spectrum is known. Conversely, knowledge of the EBL intensity can be used to determine the intrinsic blazar spectrum. Unfortunately, neither the EBL intensity nor the intrinsic blazar spectrum is known with high enough precision to accurately derive one quantity from the other. In this paper we use the most recent data on the EBL to construct 12 different realizations representing all possible permutations between EBL limits and the detections in the different wavelength regions. These realizations explore a significantly larger range of allowable EBL spectra than any previous studies. We show that these realizations can be used to explore the effects of the EBL on the inferred spectra of blazars. Concentrating on the two relatively nearby (z approximately 0.03) blazars Mrk 421 and 501, we derive their intrinsic spectra and peak gamma -ray energies for the different EBL realizations. Some EBL spectra give rise to "unphysical" intrinsic blazar spectra, characterized by an exponential rise at high TeV energies. We use the F-test to show that some of these exponential rises are statistically insignificant. Consequently, statements regarding the existence of an "IR background-TeV gamma -ray crisis" are unfounded on the basis of our present knowledge of the EBL. EBL spectra that do give rise to unphysical blazar spectra are regarded as invalid realizations of the EBL. Those that do not thus define new constraints on the EBL spectrum and are used to derive new limits on the intensity and the peak gamma -ray energy of these two blazars. In particular, we derive an upper limit of similar to 15 nW m super(-2) sr super(-1) on the 60 mu m EBL intensity and find the peak energies of the Mrk 421 and Mrk 501 sources to be very similar, between 0.5-1.2 and 0.8-2.5 TeV, respectively. We also show that the intrinsic spectrum of Mrk 421 during a period of intense flaring activity has a peak energy that seems to shift to higher energies at higher flux states. For the most distant blazar H1426+428 (z = 0.129), most EBL realizations give rise to spectra that peak between similar to 1 and 5 TeV, with some suggesting peaks below 400 GeV and above similar to 10 TeV. These results provide important constraints on the different particle acceleration mechanisms and the generation of the gamma -ray emission. Finally, we also show that uncertainties in the absolute calibration of the gamma -ray energies with atmospheric Cerenkov telescopes have an important impact on the intrinsic blazar spectra. These systematic uncertainties will be improved with the next generations of telescopes that will also cover a wider range of gamma -ray energies, providing further insights into the intrinsic spectrum of TeV blazars.
The observation of two dramatic outbursts of TeV photons from the active galaxy Markarian 421 is reported. The data suggest that the emission region is extremely small, perhaps even smaller than the ...Solar System.
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