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.
Superluminous supernovae (SLSNe) are a rare class of stellar explosions with luminosities ~10-100 times greater than ordinary core-collapse supernovae. One popular model to explain the enhanced ...optical output of hydrogen-poor (Type I) SLSNe invokes energy injection from a rapidly spinning magnetar. A prediction in this case is that high-energy gamma rays, generated in the wind nebula of the magnetar, could escape through the expanding supernova ejecta at late times (months or more after optical peak). This paper presents a search for gamma-ray emission in the broad energy band from 100 MeV to 30 TeV from two Type I SLSNe, SN2015bn, and SN2017egm, using observations from Fermi-LAT and VERITAS. Although no gamma-ray emission was detected from either source, the derived upper limits approach the putative magnetar's spin-down luminosity. Prospects are explored for detecting very-high-energy (VHE; 100 GeV - 100 TeV) emission from SLSNe-I with existing and planned facilities such as VERITAS and CTA.
Blazars are variable emitters across all wavelengths over a wide range of timescales, from months down to minutes. It is therefore essential to observe blazars simultaneously at different ...wavelengths, especially in the X-ray and gamma-ray bands, where the broadband spectral energy distributions usually peak. In this work, we report on three " target-of-opportunity" observations of Mrk 421, one of the brightest TeV blazars, triggered by a strong flaring event at TeV energies in 2014. These observations feature long, continuous, and simultaneous exposures with XMM-Newton (covering the X-ray and optical/ultraviolet bands) and VERITAS (covering the TeV gamma-ray band), along with contemporaneous observations from other gamma-ray facilities (MAGIC and Fermi-Large Area Telescope) and a number of radio and optical facilities. Although neither rapid flares nor significant X-ray/TeV correlation are detected, these observations reveal subtle changes in the X-ray spectrum of the source over the course of a few days. We search the simultaneous X-ray and TeV data for spectral hysteresis patterns and time delays, which could provide insight into the emission mechanisms and the source properties (e. g., the radius of the emitting region, the strength of the magnetic field, and related timescales). The observed broadband spectra are consistent with a one-zone synchrotron self-Compton model. We find that the power spectral density distribution at greater than or similar to 4 x 10(-4) Hz from the X-ray data can be described by a power-law model with an index value between 1.2 and 1.8, and do not find evidence for a steepening of the power spectral index (often associated with a characteristic length scale) compared to the previously reported values at lower frequencies.
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.
M87 is one of the closest (z=0.00436) extragalactic sources emitting at very-high-energies (VHE, E > 100 GeV). The aim of this work is to locate the region of the VHE gamma-ray emission and to ...describe the observed broadband spectral energy distribution (SED) during the low VHE gamma-ray state. The data from M87 collected between 2012 and 2015 as part of a MAGIC monitoring programme are analysed and combined with multi-wavelength data from Fermi-LAT, Chandra, HST, EVN, VLBA and the Liverpool Telescope. The averaged VHE gamma-ray spectrum can be fitted from 100GeV to 10TeV with a simple power law with a photon index of (-2.41 \(\pm\) 0.07), while the integral flux above 300GeV is \((1.44 \pm 0.13) \times 10^{-12} cm^{-2} s^{-1}\). During the campaign between 2012 and 2015, M87 is generally found in a low emission state at all observed wavelengths. The VHE gamma-ray flux from the present 2012-2015 M87 campaign is consistent with a constant flux with some hint of variability (\(\sim3\sigma\)) on a daily timescale in 2013. The low-state gamma-ray emission likely originates from the same region as the flare-state emission. Given the broadband SED, both a leptonic synchrotron self Compton and a hybrid photo-hadronic model reproduce the available data well, even if the latter is preferred. We note, however, that the energy stored in the magnetic field in the leptonic scenario is very low suggesting a matter dominated emission region.
The aim of this study is to search for evidence of a common emission engine between radio giant pulses (GPs) and very-high-energy (VHE, E>100 GeV) gamma-rays from the Crab pulsar. 16 hours of ...simultaneous observations of the Crab pulsar at 1.4 GHz with the Effelsberg radio telescope and the Westerbork Synthesis Radio Telescope (WSRT), and at energies above 60 GeV with the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes were performed. We searched for a statistical correlation between the radio and VHE gamma-ray emission with search windows of different lengths and different time lags to the arrival times of a radio GP. A dedicated search for an enhancement in the number of VHE gamma-rays correlated with the occurrence of radio GPs was carried out separately for the P1 and P2 phase ranges respectively. 99444 radio GPs have been detected in the radio data sample. We find no significant correlation between the GPs and VHE photons in any of the search windows. Depending on phase cuts and the chosen search windows we find upper limits at 95% confidence level on an increase in VHE gamma-ray events correlated with radio GPs between 7% and 61% of the average Crab pulsar VHE flux for the P1 and P2 phase ranges respectively. This puts upper limits on the flux increase during a radio GP of 12% to 2900% (depending on search window duration and phase cuts) of the pulsed VHE flux. This is the most stringent upper limit on a correlation between gamma-ray emission and radio GPs reported so far.}
A&A 623, A175 (2019) The mechanisms producing fast variability of the $\gamma$-ray emission in
active galactic nuclei are under debate. The MAGIC telescopes detected a fast
very high energy (VHE, ...E$>100$ GeV) $\gamma$-ray flare from BL Lacertae on 2015
June 15. The flare had a maximum flux of $(1.5\pm 0.3)\times 10^{-10}$ photons
cm$^{-2}$ s$^{-1}$ and halving time of $26\pm8$ minutes. The MAGIC observations
were triggered by a high state in the optical and high energy (HE, E$>100$ MeV)
$\gamma$-ray bands. In this paper we present the MAGIC VHE $\gamma$-ray data
together with multiwavelength data from radio, optical, X-rays, and HE $\gamma$
rays from 2015 May 1 to July 31. Well-sampled multiwavelength data allow us to
study the variability in detail and compare it to the other epochs when fast
VHE $\gamma$-ray flares have been detected from this source. Interestingly, we
find that the behaviour in radio, optical, X-rays and HE $\gamma$-rays is very
similar to two other observed VHE $\gamma$-ray flares. In particular, also
during this flare there was an indication of rotation of the optical
polarization angle and of activity at the 43\,GHz core. These repeating
patterns indicate a connection between the three events. We also test modelling
of the spectral energy distribution, based on constraints from the light curves
and VLBA observations, with two different geometrical setups of two-zone
inverse Compton models. In addition we model the $\gamma$-ray data with the
star-jet interaction model. We find that all of the tested emission models are
compatible with the fast VHE $\gamma$-ray flare, but all have some tension with
the multiwavelength observations.
Monthly Notices of the Royal Astronomical Society, Volume 483,
Issue 4, March 2019, Pages 4578-4585 SNR G24.7+0.6 is a 9.5 kyrs radio and $\gamma$-ray supernova remnant evolving
in a dense medium. In ...the GeV regime, SNR G24.7+0.6
(3FHL\,J1834.1--0706e/FGES\,J1834.1--0706) shows a hard spectral index
($\Gamma$$\sim$2) up to $200$\,GeV, which makes it a good candidate to be
observed with Cherenkov telescopes such as MAGIC. We observed the field of view
of \snr\ with the MAGIC telescopes for a total of 31 hours. We detect very high
energy $\gamma$-ray emission from an extended source located 0.34\degr\ away
from the center of the radio SNR. The new source, named \mgc\ is detected up to
5\,TeV, and its spectrum is well-represented by a power-law function with
spectral index of $2.74 \pm 0.08$. The complexity of the region makes the
identification of the origin of the very-high energy emission difficult,
however the spectral agreement with the LAT source and overlapping position at
less than 1.5$\sigma$ point to a common origin. We analysed 8 years of
\fermi-LAT data to extend the spectrum of the source down to 60\,MeV.
\fermi-LAT and MAGIC spectra overlap within errors and the global broad band
spectrum is described by a power-law with exponential cutoff at
$1.9\pm0.5$\,TeV. The detected $\gamma$-ray emission can be interpreted as the
results of proton-proton interaction between the supernova and the CO-rich
surrounding.
