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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 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. 

Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing would be expected to leave an imprint on the spectra of TeV $\gamma$ -ray sources. In 2017, the VERITAS $\gamma$ -ray observatory recorded the second day of a dramatic flare of the radio galaxy NGC 1275, embedded at the center of the Perseus galaxy cluster. This serendipitous locale provides a spatially-extended magnetic field of strength $\mathcal{O}\left(10\mathrm{\mu }\mathrm{G}\right)$ through which escaping photons traverse, making it an excellent target to study ALPs. We analyze the VERITAS data of NGC 1275’s 2017 flare with the gammapy analysis package. Extensive fitting and modeling are performed to ultimately conduct a likelihood analysis used to search for any evidence of a preference for ALPs and to explore the confidence with which constraints can be set. We adopt the ${\mathrm{CL}}_{\mathrm{s}}$ method for this study for its conservative approach to setting limits in regimes where the search has limited sensitivity. No evidence for the existence of ALPs is found, and no combination of mass and coupling strength can be excluded at or above 95% confidence level. We provide a map showing the strength of our exclusions in the mass and coupling parameter space. The strongest exclusions are found in the mass range $2\times {10}^{-7}\mathrm{eV}\lesssim m_a\lesssim 4\times {10}^{-7}\mathrm{eV}$ and at the coupling strength of $g_{a\gamma }\gtrsim 3\times {10}^{-11}{\mathrm{GeV}}^{-1}$ up to 80% confidence level, which are consistent with previous studies. We find the ${\mathrm{CL}}_{\mathrm{s}}$ to be a trustworthy approach, and advocate for its continued usage in future studies. We note that many of the limitations contributing to the limited sensitivity seen by VERITAS in this study will be improved with next-generation $\gamma$ -ray instruments, such as the Cherenkov Telescope Array Observatory. 

Abstract Assuming Galactic cosmic rays originate in supernovae and the winds of massive stars, starburst galaxies should produce very-high-energy (VHE;E > 100 GeV) gamma-ray emission via the interaction of their copious quantities of cosmic rays with the large reservoirs of dense gas within the galaxies. Such VHE emission was detected by VERITAS from the starburst galaxy M82 in 2008–09. An extensive, multiyear campaign followed these initial observations, yielding a total of 254 hr of good-quality VERITAS data on M82. Leveraging modern analysis techniques and the larger exposure, these VERITAS data show a more statistically significant VHE signal (∼6.5 standard deviations,σ). The corresponding photon spectrum is well fit by a power law (Γ = 2.3 ± 0.3_stat ± 0.2_sys), and the observed integral flux isF(>450 GeV) = (3.2 ± 0.6_stat ± 0.6_sys) × 10⁻¹³cm⁻²s⁻¹, or ∼0.4% of the Crab Nebula flux above the same energy threshold. The improved VERITAS measurements, when combined with various multiwavelength data, enable modeling of the underlying emission and transport processes. A purely leptonic scenario is found to be a poor representation of the gamma-ray spectral energy distribution (SED). A lepto-hadronic scenario with cosmic rays following a power-law spectrum in momentum (indexs ≃ 2.25) and with significant bremsstrahlung below 1 GeV provides a good match to the observed SED. The synchrotron emission from the secondary electrons indicates that efficient nonradiative losses of cosmic-ray electrons may be related to advective escape from the starburst core. 

Abstract We present an analysis of new multiwavelength observations of the TeV gamma-ray binary HESS J0632+057, conducted using SALT, Swift, NuSTAR, and VERITAS in 2023–2024. By combining these new data with archival observations, we confirm previous suggestions of orbital variability in the source’s X-ray spectrum, including increased X-ray absorption at the orbital phase interval ofϕ ≈ 0.3–0.4. The source’s X-ray flux within this phase interval seems to have exhibited a significant change on an orbital timescale. Additionally, occasional short-term variations in the X-ray band on a timescale of less than 3 days have been observed. The measured duration of the increased absorbing column density and the flux variability timescales can provide clues about the interaction between the putative pulsar and the Be companion’s disk if, as previously suggested, the pulsar crosses the disk at this phase interval. Moreover, the new contemporaneous X-ray and TeV observations around the pulsar-crossing phases revealed independent variability in the X-ray and TeV fluxes, contrary to a previous observation of concurrent flux increases. While these observations alone cannot provide definitive conclusions, we discuss our results in the context of pulsar–disk interaction and intrabinary shock emission scenarios. 

Understanding the nature and identity of dark matter is a key goal in the physics community. In the case that TeV-scale dark matter particles decay or annihilate into standard model particles, very-high-energy (VHE) gamma rays (greater than 100 GeV) will be present in the final state. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an imaging atmospheric Cherenkov telescope array that can indirectly detect VHE gamma rays in an energy range of 100 GeV to > 30 TeV. Dwarf spheroidal galaxies (dSphs) are ideal candidates in the search for dark matter due to their high dark matter content, high mass-to-light ratios, and their low gamma-ray fluxes from astrophysical processes. This study uses a legacy data set of 638 hours collected on 17 dSphs, built over 11 years with an observing strategy optimized according to the dark matter content of the targets. The study addresses a broad dark matter particle mass range, extending from 200 GeV to 30 PeV. In the absence of a detection, we set the upper limits on the dark matter velocity-weighted annihilation cross section 

Abstract Dark matter is a key piece of the current cosmological scenario, with weakly interacting massive particles (WIMPs) a leading dark matter candidate. WIMPs have not been detected in their conventional parameter space (100 GeV ≲ M χ ≲ 100 TeV), a mass range accessible with current Imaging Atmospheric Cherenkov Telescopes. As ultraheavy dark matter (UHDM; M χ ≳ 100 TeV) has been suggested as an underexplored alternative to the WIMP paradigm, we search for an indirect dark matter annihilation signal in a higher mass range (up to 30 PeV) with the VERITAS γ -ray observatory. With 216 hr of observations of four dwarf spheroidal galaxies, we perform an unbinned likelihood analysis. We find no evidence of a γ -ray signal from UHDM annihilation above the background fluctuation for any individual dwarf galaxy nor for a joint-fit analysis, and consequently constrain the velocity-weighted annihilation cross section of UHDM for dark matter particle masses between 1 TeV and 30 PeV. We additionally set constraints on the allowed radius of a composite UHDM particle. 

Abstract This paper investigates the origin of theγ-ray emission from MGRO J1908+06 in the GeV–TeV energy band. By analyzing the data collected by the Fermi Large Area Telescope, the Very Energetic Radiation Imaging Telescope Array System, and High Altitude Water Cherenkov, with the addition of spectral data previously reported by LHAASO, a multiwavelength study of the morphological and spectral features of MGRO J1908+06 provides insight into the origin of theγ-ray emission. The mechanism behind the bright TeV emission is studied by constraining the magnetic field strength, the source age, and the distance through detailed broadband modeling. Both spectral shape and energy-dependent morphology support the scenario that inverse Compton emission of an evolved pulsar wind nebula associated with PSR J1907+0602 is responsible for the MGRO J1908+06γ-ray emission with a best-fit true age ofT= 22 ± 9 kyr and a magnetic field ofB= 5.4 ± 0.8μG, assuming the distance to the pulsard_PSR= 3.2 kpc.