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Abstract The radio galaxy NGC 1275 is the brightest cluster galaxy in the Perseus Cluster. It is well studied across all wave bands, including very high-energy (VHE;E ≥ 100 GeV)γ-rays, and with radio observations over the last 20 yr tracking an unusual radio component, “C3.” NGC 1275 was observed in an exceptional VHE-flaring state between 2016 December 31 and 2017 January 3. The flare peak reached ∼1.5 Crab units as measured by the MAGIC observatory. We report on the observations of NGC 1275 conducted by VERITAS and multiwavelength data collected during this flaring state and, for context, data taken between 2009 and 2017 inclusively. VERITAS detected the declining state of the flare on 2017 January 2 (MJD 57755) and 3 (MJD 57756) at an average flux state of 0.5 Crab units. VERITAS spectra show an overall long-term trend of harder when brighter. During the flare, theγ-ray spectrum obtained from the combined Fermi-LAT, MAGIC, and VERITAS observations changes from a power law with an exponential cutoff on January 1 to a log-parabola on January 2. To study the evolution of the flare in more detail, multiband spectral energy distributions (SEDs) were constructed for the nights of 2017 January 1 and 2 corresponding to the shift from the peak to the decline of the flare. A blob-in-jet modeling of the SEDs results in support for a two-component model with aθ = 10^∘jet angle to the line of sight and theγ-ray emission zone located in the vicinity of the C3 radio component. 

Abstract Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the Large High Altitude Air Shower Observatory (LHAASO) collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Swift-XRT observations have been used to explore the double-peaked broadband emission. Models of the spectral energy distribution (SED) are currently unconstrained due to the lack of contemporaneous multiwavelength data at either peak. Here, we report serendipitous observations of NGC 4278 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS), made possible by the contemporaneous observations of the nearby blazars 1ES 1218+304, 1ES 1215+303, and W Comae, each of which are located within 2° of NGC 4278. VERITAS did not detect any gamma-ray emission, and a flux upper limit was calculated. The flux upper limits constrain the photon spectrum of the quasi-quiet period, and together with Fermi-LAT, indicate a peak in the SED between 100 GeV and 2 TeV. We present an interpretation of the broadband SED that is based on acceleration of protons in the corona of the active galactic nuclei, followed byp–γinteractions and optically thinγ-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube. 

Abstract Axion-like particles (ALPs) are hypothetical particles and compelling candidates for cold dark matter. Their existence could be probed through their conversions into photons in the presence of magnetic fields. In this work, we explore the effect of these photon-ALP conversions by searching for an attenuation in the observed gamma ray spectra of galactic sources that emit at energies of hundreds of TeV. We analyze data from the High-Altitude Water Cherenkov (HAWC) Observatory for the source 3HWC J1908+063. No evidence of photon-ALP conversions was found, and we set constraints on the ALP parameter space. Specifically, we derive exclusion limits for ALPs with masses in the range 10^-8 eV ≤m_a≤ 10^-6 eV and photon-ALP couplings in the range 10^-12 GeV^-1≤g_aγ≤ 10^-10 GeV^-1, based on HAWC observations. 

Abstract Observations of GeV gamma-ray emission from the well-studied mixed-morphology supernova remnant (SNR) W44 by Fermi-Large Area Telescope and AGILE imply that it is a site of significant cosmic-ray acceleration. The spectral energy distribution (SED) derived from the GeV data suggests that the gamma-ray emission likely originates from the decay of neutral pions generated by cosmic-ray interactions. It is essential to measure the SED of W44 in the X-ray and very-high-energy (VHE) gamma-ray bands to verify the hadronic origin of the emission and to gauge the potential contributions from leptonic emission. We report an upper limit of the nonthermal X-ray flux from W44 of 5  × 10⁻¹³erg cm⁻²s⁻¹in the 0.5–8.0 keV band based on  ∼300 ks of XMM-Newton observations. The X-ray upper limit is consistent with previously estimated hadronic models, but in tension with the leptonic models. We estimate the VHE flux upper limit of  ∼1.2  × 10⁻¹²erg s⁻¹cm⁻²in the 0.5–5.0 TeV range from W44 using data from the Very Energetic Radiation Imaging Telescope Array System. Our nondetection of W44 at VHE wavelengths is in agreement with observations from other imaging atmospheric Cherenkov telescopes and is perhaps consistent with the evolutionary stage of the SNR. 

Abstract In 2017 September, a high-energy neutrino event detected by the IceCube Neutrino Observatory (IceCube-170922A) was associated, at the 3σlevel, with a gamma-ray flare from the blazar TXS 0506+056. Cosmic rays that are accelerated in astrophysical sources can escape from their jets and interact with background radiation fields. Interactions with the extragalactic background light can produce pions and hence neutrinos, while interactions with the cosmic microwave background predominantly drive inverse Compton scattering, contributing to electromagnetic cascades in intergalactic space. The resulting secondary gamma-ray emission can be detected with high-energy gamma-ray telescopes. Here, we report on a new search for such cosmogenic cascade emission from the blazar TXS 0506+056, using a combined data set from the Fermi Large Area Telescope and VERITAS. We compare the gamma-ray spectrum and neutrino observations with the predictions of cosmic-ray-induced cascades in intergalactic space. The observed gamma-ray spectrum is modeled as a combination of the primary spectrum and the cascade spectrum. We apply a Monte Carlo simulation with a Δχ²-based likelihood analysis to jointly determine the best-fit parameters of a proton emission spectrum describing the data and derive constraints on the proton escape luminosity. Assuming an LP primary photon spectrum, we find consistency with a proton injection spectral index ofα_p ≃ 2.0 and a cutoff energy ofE_p,max ≃ 1.3 × 10¹⁶eV and constrain the isotropic proton escape luminosity to 1 × 10⁴⁴erg s⁻¹≲L_p,esc ≲ 3 × 10⁴⁵erg s⁻¹at the 90% confidence level. 

Abstract We use the stellar intensity interferometry system implemented with the Very Energetic Radiation Imaging Telescope Array System at Fred Lawrence Whipple Observatory as a light collector to obtain measurements of the rapid rotator starγCassiopeiae, at a wavelength of 416 nm. Using data from baselines sampling different position angles, we extract the size, oblateness, and projected orientation of the photosphere. Fitting the data with a uniform ellipse model yields a minor-axis angular diameter of 0.43 ± 0.02 mas, a major-to-minor-radius ratio of 1.28 ± 0.04, and a position angle of 116° ± 5° for the axis of rotation. A rapidly rotating stellar atmosphere model that includes limb and gravity darkening describes the data well with a fitted angular diameter of $0.604_{-0.034}^{+0.041}$ mas corresponding to an equatorial radius of ${10.9}_{-0.6}^{+0.8}{R}_{\odot }$ , a rotational velocity with a 1σlower limit at 97.7% of breakup velocity, and a position angle of $114\stackrel{\mathrm{°}}{.}{7}_{-5.7}^{+6.4}$ . These parameters are consistent with Hαline spectroscopy and infrared-wavelength Michelson interferometric measurements of the star’s decretion disk. This is the first measurement of an oblate photosphere using intensity interferometry. 

Abstract We present a new study on the MeV–TeVγ-ray origin of HESS J1857+026 using data collected from the Fermi–LAT, VERITAS, and HAWC observatories. A spatial and spectral study of HESS J1857+026 including radiative modeling of the MeV–TeV spectrum determines the likely dominantγ-ray origin as a pulsar wind nebula (PWN) powered by the energetic pulsar PSR J1856+0245. The MeV–TeV spectrum is further characterized through basic evolutionary radiative modeling assuming a PWN origin to constrain the physical properties of the system such as the magnetic field strength and PWN age. The results of the PWN evolutionary model are consistent with the observational constraints of the system, finding an age of the system betweenτ = [16, 21] kyr and a magnetic field strength betweenB = [0.4, 1.6]μG. These estimates support an evolved PWN scenario where the observedγ-ray emission is generated by the relativistic electrons' inverse Compton (IC) scattering off local photon fields; however, the low-energy (E < 10 GeV) spectral component could be dominated by hadronic emission originating from a supernova remnant (SNR). For a PWN component above 10 GeV, we measure the conditions for particle diffusion, finding that the local diffusion (D(50 TeV) ∼ 10²⁸cm⁻²s⁻¹) is suppressed compared to the interstellar medium (ISM) value, in agreement with similar TeV PWNe. By measuring the radial surface brightness profiles of theγ-ray source across multiple instruments, we demonstrate that the combined MeV–TeV spatial information is a powerful tool to constrain particle diffusion properties.