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.
Abstract
The Galactic Center (GC) region hosts a variety of powerful astronomical sources and rare astrophysical processes that emit a large flux of nonthermal radiation. The inner 375 pc × 600 pc ...region, called the Central Molecular Zone, is home to the supermassive black hole Sagittarius A*, massive cloud complexes, and particle accelerators such as supernova remnants (SNRs). We present the results of our improved analysis of the very-high-energy gamma-ray emission above 2 TeV from the GC using 125 hr of data taken with the Very Energetic Radiation Imaging Telescope Array System imaging-atmospheric Cerenkov telescope between 2010 and 2018. The central source VER J1745–290, consistent with the position of Sagittarius A*, is detected at a significance of 38 standard deviations above the background level (38
σ
), and we report its spectrum and light curve. Its differential spectrum is consistent with a power law with exponential cutoff, with a spectral index of
, a flux normalization at 5.3 TeV of
TeV
−1
cm
−2
s
−1
, and cutoff energy of
TeV. We also present results on the diffuse emission near the GC, obtained by combining data from multiple regions along the GC ridge, which yield a cumulative significance of 9.5
σ
. The diffuse GC ridge spectrum is best fit by a power law with a hard index of 2.19 ± 0.20, showing no evidence of a cutoff up to 40 TeV. This strengthens the evidence for a potential accelerator of PeV cosmic rays being present in the GC. We also provide spectra of the other sources in our field of view with significant detections, composite SNR G0.9+0.1, and HESS J1746–285.
Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive photomultiplier tube (PMT) cameras. Exposure to high levels of background illumination degrades the efficiency of and ...potentially destroys these photo-detectors over time, so IACTs cannot be operated in the same configuration in the presence of bright moonlight as under dark skies. Since September 2012, observations have been carried out with the VERITAS IACTs under bright moonlight (defined as about three times the night-sky-background (NSB) of a dark extragalactic field, typically occurring when Moon illumination > 35%) in two observing modes, firstly by reducing the voltage applied to the PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to the cameras. This has allowed observations at up to about 30 times previous NSB levels (around 80% Moon illumination), resulting in 30% more observing time between the two modes over the course of a year. These additional observations have already allowed for the detection of a flare from the 1ES 1727 + 502 and for an observing program targeting a measurement of the cosmic-ray positron fraction. We provide details of these new observing modes and their performance relative to the standard VERITAS observations.
HESS J0632+057 is a gamma-ray binary composed of a compact object orbiting a Be star with a period of about 315 days. Extensive X-ray and TeV gamma-ray observations have revealed a peculiar light ...curve containing two peaks, separated by a dip. We present the results of simultaneous observations in hard X-rays with NuSTAR and in TeV gamma-rays with VERITAS, performed in 2017 November and December. These observations correspond to the orbital phases φ 0.22 and 0.3, where the fluxes are rising toward the first light-curve peak. A significant variation of the spectral index from 1.77 0.05 to 1.56 0.05 is observed in the X-ray data. The multiwavelength spectral energy distributions (SED) derived from the observations are interpreted in terms of a leptonic model, in which the compact object is assumed to be a pulsar and nonthermal radiation is emitted by high-energy electrons accelerated at the shock formed by the collision between the stellar and pulsar wind. The results of the SED fitting show that our data can be consistently described within this scenario, and allow us to estimate the magnetization of the pulsar wind at the location of the shock formation. The constraints on the pulsar wind magnetization provided by our results are shown to be consistent with those obtained from other systems.
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.
Combined with measurements made by very-long-baseline interferometry, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful ...of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL Lacertae observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). with a rise time of ∼2.3 hr and a decay time of ∼36 min. The peak flux above 200 GeV is (4.2 0.6) × 10−6 photon m−2 s−1 measured with a 4-minute-binned light curve, corresponding to ∼180% of the flux that is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in Very Long Baseline Array observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models that invoke relativistic plasma passing stationary shocks.
The very high energy (VHE; E> 100 GeV) blazar Markarian 501 was observed between April 17 and May 5 (MJD 54 938–54 956), 2009, as part of an extensive multi-wavelength campaign from radio to VHE. ...Strong VHE γ-ray activity was detected on May 1st with Whipple and VERITAS, when the flux (E> 400 GeV) increased to 10 times the pre-flare baseline flux (3.9 × 10-11 ph cm-2 s-1), reaching five times the flux of the Crab Nebula. This coincided with a decrease in the optical polarization and a rotation of the polarization angle by 15°. This VHE flare showed a fast flux variation with an increase of a factor ~4 in 25 min, and a falling time of ~50 min. We present the observations of the quiescent state previous to the flare and of the high state after the flare, focusing on the flux and spectral variability from Whipple, VERITAS, Fermi-LAT, RXTE, and Swift combined with optical and radio data.
HESS J1943+213 is a very high energy (VHE; >100 GeV) γ-ray source in the direction of the Galactic plane. Studies exploring the classification of the source are converging toward its identification ...as an extreme synchrotron BL Lac object. Here we present 38 hr of VERITAS observations of HESS J1943+213 taken over 2 yr. The source is detected with a significance of ∼20 standard deviations, showing a remarkably stable flux and spectrum in VHE γ-rays. Multifrequency Very Long Baseline Array (VLBA) observations of the source confirm the extended, jet-like structure previously found in the 1.6 GHz band with the European VLBI Network and detect this component in the 4.6 and 7.3 GHz bands. The radio spectral indices of the core and the jet and the level of polarization derived from the VLBA observations are in a range typical for blazars. Data from VERITAS, Fermi-LAT, Swift-XRT, the FLWO 48″ telescope, and archival infrared and hard X-ray observations are used to construct and model the spectral energy distribution (SED) of the source with a synchrotron self-Compton model. The well-measured γ-ray peak of the SED with VERITAS and Fermi-LAT provides constraining upper limits on the source redshift. Possible contribution of secondary γ-rays from ultra-high-energy cosmic-ray-initiated electromagnetic cascades to the γ-ray emission is explored, finding that only a segment of the VHE spectrum can be accommodated with this process. A variability search is performed across X-ray and γ-ray bands. No statistically significant flux or spectral variability is detected.
We report the detection of very-high-energy (VHE) gamma-ray emission from supernova remnant (SNR) G106.3+2.7. Observations performed in 2008 with the VERITAS atmospheric Cherenkov gamma-ray telescope ...resolve extended emission overlapping the elongated radio SNR. The 7.3sigma (pre-trials) detection has a full angular extent of roughly 0.{sup 0}6 by 0.{sup 0}4. Most notably, the centroid of the VHE emission is centered near the peak of the coincident {sup 12}CO (J = 1-0) emission, 0.{sup 0}4 away from the pulsar PSR J2229+6114, situated at the northern end of the SNR. Evidently the current-epoch particles from the pulsar wind nebula are not participating in the gamma-ray production. The VHE energy spectrum measured with VERITAS is well characterized by a power law dN/dE = N {sub 0}(E/3 TeV){sup -G}AMMA with a differential index of GAMMA = 2.29 +- 0.33{sub stat} +- 0.30{sub sys} and a flux of N{sub 0} = (1.15 +- 0.27{sub stat} +- 0.35{sub sys}) x 10{sup -13} cm{sup -2} s{sup -1} TeV{sup -1}. The integral flux above 1 TeV corresponds to {approx}5 percent of the steady Crab Nebula emission above the same energy. We describe the observations and analysis of the object and briefly discuss the implications of the detection in a multiwavelength context.
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