Search for: All records

We report the first observations of a long-duration very-high-energy (VHE; E>100 GeV) flare from BL Lacertae (VER J2202+422), taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). On October 15, 2022, the Fermi-Large Area Telescope (LAT) detected elevated GeV activity originating from this blazar. This triggered a multiwavelength campaign, which includes observations from VERITAS, Swift, NuSTAR, and select optical and radio observatories. VERITAS observed the source for a total of ~9.8 hours between September 1, 2022 and December 1, 2022. An analysis of these data yields a ~28 sigma detection of the source. While previously observed VHE flares from BL Lacertae have lasted on time-scales of minutes to days, VERITAS continued to detect flaring activity from the source for over a month (~40 days) after the original flaring activity was detected with Fermi-LAT. Broadband spectral modeling shows that a synchrotron self-Compton (SSC) model with an external inverse-Compton (EC) component is preferred over a one-zone SSC model. 

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 Microquasars such as SS 433 are considered potential contributors to cosmic rays up to the knee of the cosmic-ray energy spectrum (∼4 PeV), where a transition in the dominant acceleration processes is expected. The SS 433 system, located within the W50 supernova remnant, is a Galactic microquasar with relativistic jets interacting with the surrounding medium over parsec scales, providing an example for studying jet-driven particle acceleration. A deep morphological and spectral study of SS 433 is performed using more than 150 hr of observations with VERITAS, sensitive toγ-ray energies >100 GeV. With an angular resolution better than 0 $\stackrel{\text{°}}{.}$ 1, extended TeVγ-ray emission is resolved from both the eastern and western jet lobes, located tens of parsecs from the central binary. The emission appears elongated along the jet axis and coincides with regions where the jets interact with the surrounding supernova remnant. No TeV emission is detected from the central binary, nor is significant emission observed between the central binary and the jet lobes. Phase-resolved analyses show no evidence for variability in the orbital or precessional phase, supporting a steady emission scenario. The observed morphology and spectra are consistent with scenarios where particles are accelerated in the lobes of the jets, possibly through shocks or alternative processes such as magnetic reconnection. The extended TeV emission from the jet lobes of SS 433 favors a leptonic origin in the VERITAS energy range, suggesting that any hadronic acceleration is subdominant. The results offer valuable constraints on how microquasar jets may contribute to the Galactic cosmic-ray population toward the knee. 

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 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. 

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 Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum – a feature associated with flow imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGP formation in smaller systems. For the first time, we demonstrate the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton–lead and proton–proton collisions at the Large Hadron Collider (LHC). These results are described by a model including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions.