The bright gamma-ray quasar 4C +55.17 is a distant source (z = 0.896) with a hard spectrum at GeV energies as observed by the Large Area Telescope (LAT) on board the Fermi satellite. This source is ...identified as a good source candidate for very high energy (VHE; >30 GeV) gamma-rays. In general, VHE gamma-rays from distant sources provide a unique opportunity to study the extragalactic background light (EBL) and underlying astrophysics. The flux intensity of this source in the VHE range is investigated. Then, constraints on the EBL are derived from the attenuation of gamma-ray photons coming from the distant blazar. We searched for a gamma-ray signal from this object using the 35 h observations taken by the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes between 2010 November and 2011 January. No significant VHE gamma-ray signal was detected. We computed the upper limits of the integrated gamma-ray flux at the 95 per cent confidence level of 9.4 × 10−12 and 2.5 × 10−12 cm−2 s−1 above 100 and 200 GeV, respectively. The differential upper limits in four energy bins in the range from 80 to 500 GeV are also derived. The upper limits are consistent with the attenuation predicted by low-flux EBL models on the assumption of a simple power-law spectrum extrapolated from LAT data.
We use the VERITAS imaging air Cherenkov Telescope (IACT) array to obtain the
first measured angular diameter of $\beta$ UMa at visual wavelengths using
stellar intensity interferometry (SII) and ...independently constrain the
limb-darkened angular diameter. The age of the Ursa Major moving group has been
assessed from the ages of its members, including nuclear member Merak ($\beta$
UMa), an A1-type subgiant, by comparing effective temperature and luminosity
constraints to model stellar evolution tracks. Previous interferometric
limb-darkened angular-diameter measurements of $\beta$ UMa in the near-infrared
(CHARA Array, $1.149 \pm 0.014$ mas) and mid-infrared (Keck Nuller, $1.08 \pm
0.07$ mas), together with the measured parallax and bolometric flux, have
constrained the effective temperature. This paper presents current VERITAS-SII
observation and analysis procedures to derive squared visibilities from
correlation functions. We fit the resulting squared visibilities to find a
limb-darkened angular diameter of $1.07 \pm 0.04 {\rm (stat)} \pm 0.05$ (sys)
mas, using synthetic visibilities from a stellar atmosphere model that provides
a good match to the spectrum of $\beta$ UMa in the optical wave band. The
VERITAS-SII limb-darkened angular diameter yields an effective temperature of
$9700\pm200\pm 200$ K, consistent with ultraviolet spectrophotometry, and an
age of $390\pm 29 \pm 32 $ Myr, using MESA Isochrones and Stellar Tracks
(MIST). This age is consistent with $408 \pm 6$ Myr from the CHARA Array
angular diameter.
We have searched for radio pulsations towards 49 Fermi Large Area Telescope (LAT) 1FGL Catalog \(\gamma\)-ray sources using the Green Bank Telescope at 350 MHz. We detected 18 millisecond pulsars ...(MSPs) in blind searches of the data; 10 of these were discoveries unique to our survey. Sixteen are binaries, with eight having short orbital periods \(P_B < 1\) day. No radio pulsations from young pulsars were detected, although three targets are coincident with apparently radio-quiet \(\gamma\)-ray pulsars discovered in LAT data. Here, we give an overview of the survey and present radio and \(\gamma\)-ray timing results for the 10 MSPs discovered. These include the only isolated MSP discovered in our survey and six short-\(P_B\) binary MSPs. Of these, three have very low-mass companions (\(M_c\) \(\ll\) 0.1M\(_{\odot}\)) and hence belong to the class of black widow pulsars. Two have more massive, non-degenerate companions with extensive radio eclipses and orbitally modulated X-ray emission consistent with the redback class. Significant \(\gamma\)-ray pulsations have been detected from nine of the discoveries. This survey and similar efforts suggest that the majority of Galactic \(\gamma\)-ray sources at high Galactic latitudes are either MSPs or relatively nearby non-recycled pulsars, with the latter having on average a much smaller radio/\(\gamma\)-ray beaming ratio as compared to MSPs. It also confirms that past surveys suffered from an observational bias against finding short-\(P_B\) MSP systems.
Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but ...still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole. We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet.
G106.3$+$2.7, commonly considered a composite supernova remnant (SNR), is
characterized by a boomerang-shaped pulsar wind nebula (PWN) and two distinct
("head" & "tail") regions in the radio band. A ...discovery of very-high-energy
(VHE) gamma-ray emission ($E_\gamma > 100$ GeV) followed by the recent
detection of ultra-high-energy (UHE) gamma-ray emission ($E_\gamma > 100$ TeV)
from the tail region suggests that G106.3$+$2.7 is a PeVatron candidate. We
present a comprehensive multi-wavelength study of the Boomerang PWN (100"
around PSR J2229+6114) using archival radio and Chandra data obtained from two
decades ago, a new NuSTAR X-ray observation from 2020, and upper limits on
gamma-ray fluxes obtained by Fermi and VERITAS observatories. The NuSTAR
observation allowed us to detect a 51.67 ms spin period from the pulsar PSR
J2229+6114 and the PWN emission characterized by a power-law model with $\Gamma
= 1.52\pm0.06$ up to 20 keV. Contrary to the previous radio study by Kothes et
al. 2006, we prefer a much lower PWN B-field ($B\sim3$ $\mu$G) and larger
distance ($d \sim 8$ kpc) based on (1) the non-varying X-ray flux over the last
two decades, (2) the energy-dependent X-ray PWN size resulting from synchrotron
burn-off and (3) the multi-wavelength spectral energy distribution (SED) data.
Our SED model suggests that the PWN is currently re-expanding after being
compressed by the SNR reverse shock $\sim 1000$ years ago. In this case, the
head region should be formed by GeV--TeV electrons injected earlier by the
pulsar propagating into the low density environment.
Compilation of papers presented by the VERITAS Collaboration at the 38th International Cosmic Ray Conference (ICRC), held July 26 through August 3, 2023 in Nagoya, Japan.
We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2.2\(^\circ\) away from the best-fit position of the IceCube neutrino event IceCube-211208A ...detected on December 8, 2021. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV gamma-ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray data from Swift-XRT and NuSTAR characterize the transition between the low-energy and high-energy components of the broadband spectral energy distribution (SED), and the gamma-ray data from Fermi -LAT, VERITAS, and H.E.S.S. require a spectral cut-off near 100 GeV. Both X-ray and gamma-ray measurements provide strong constraints on the leptonic and hadronic models. We analytically explore a synchrotron self-Compton model, an external Compton model, and a lepto-hadronic model. Models that are entirely based on internal photon fields face serious difficulties in matching the observed SED. The existence of an external photon field in the source would instead explain the observed gamma-ray spectral cut-off in both leptonic and lepto-hadronic models and allow a proton jet power that marginally agrees with the Eddington limit in the lepto-hadronic model. We show a numerical lepto-hadronic model with external target photons that reproduces the observed SED and is reasonably consistent with the neutrino event despite requiring a high jet power.
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 \(\lesssim M_{\chi} \lesssim\) 100 TeV), a mass range accessible with current Imaging Atmospheric Cherenkov Telescopes. As ultraheavy dark matter (UHDM; \(M_{\chi} \gtrsim\) 100 TeV) has been suggested as an under-explored 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 gamma-ray observatory. With 216 hours of observations of four dwarf spheroidal galaxies, we perform an unbinned likelihood analysis. We find no evidence of a \(\gamma\)-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.
The ground-based gamma-ray observatory VERITAS (Very Energetic Radiation Imaging Telescope Array System) is sensitive to photons of astrophysical origin with energies in the range between \(\approx ...85\) GeV to \(\approx 30\) TeV. The instrument consists of four 12-m diameter imaging Cherenkov telescopes operating at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. VERITAS started four-telescope operations in 2007 and collects about 1100 hours of good-weather data per year. The VERITAS collaboration has published over 100 journal articles since 2008 reporting on gamma-ray observations of a large variety of objects: Galactic sources like supernova remnants, pulsar wind nebulae, and binary systems; extragalactic sources like star forming galaxies, dwarf-spheroidal galaxies, and highly-variable active galactic nuclei. This note presents VTSCat: the catalog of high-level data products from all VERITAS publications.
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