The BL Lacertae object 3C 66A was detected in a flaring state by the Fermi Large Area Telescope (LAT) and VERITAS in 2008 October. In addition to these gamma-ray observations, F-GAMMA, GASP-WEBT, ...PAIRITEL, MDM, ATOM, Swift, and Chandra provided radio to X-ray coverage. The available light curves show variability and, in particular, correlated flares are observed in the optical and Fermi-LAT gamma-ray band. The resulting spectral energy distribution can be well fitted using standard leptonic models with and without an external radiation field for inverse Compton scattering. It is found, however, that only the model with an external radiation field can accommodate the intra-night variability observed at optical wavelengths.
We present the results of 16 Swift-triggered Gamma-ray burst (GRB) follow-up observations taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) telescope array from 2007 ...January to 2009 June. The median energy threshold and response time of these observations were 260 GeV and 320 s, respectively. Observations had an average duration of 90 minutes. Each burst is analyzed independently in two modes: over the whole duration of the observations and again over a shorter timescale determined by the maximum VERITAS sensitivity to a burst with a t --1.5 time profile. This temporal model is characteristic of GRB afterglows with high-energy, long-lived emission that have been detected by the Large Area Telescope on board the Fermi satellite. No significant very high energy (VHE) gamma-ray emission was detected and upper limits above the VERITAS threshold energy are calculated. The VERITAS upper limits are corrected for gamma-ray extinction by the extragalactic background light and interpreted in the context of the keV emission detected by Swift. For some bursts the VHE emission must have less power than the keV emission, placing constraints on inverse Compton models of VHE emission.
We report results from an intensive multiwavelength campaign on the intermediate-frequency-peaked BL Lacertae object W Com (z = 0.102) during a strong outburst of very high energy gamma-ray emission ...in 2008 June. The very high energy gamma-ray signal was detected by VERITAS on 2008 June 7-8 with a flux F(>200 GeV) =(5.7 +/- 0.6) X 10-11 cm-2 s-1, about three times brighter than during the discovery of gamma-ray emission from W Com by VERITAS in 2008 March. The initial detection of this flare by VERITAS at energies above 200 GeV was followed by observations in high-energy gamma rays (AGILE; E Delta *g>= 100 MeV), X-rays (Swift and XMM-Newton), and at UV, and ground-based optical and radio monitoring through the GASP-WEBT consortium and other observatories. Here we describe the multiwavelength data and derive the spectral energy distribution of the source from contemporaneous data taken throughout the flare.
We present the results of observations of the TeV binary LS I +61? 303 with the VERITAS telescope array between 2008 and 2010, at energies above 300 GeV. In the past, both ground-based gamma-ray ...telescopes VERITAS and MAGIC have reported detections of TeV emission near the apastron phases of the binary orbit. The observations presented here show no strong evidence for TeV emission during these orbital phases; however, during observations taken in late 2010, significant emission was detected from the source close to the phase of superior conjunction (much closer to periastron passage) at a 5.6 standard deviation (5.6 Delta *s) post-trials significance. In total, between 2008 October and 2010 December a total exposure of 64.5 hr was accumulated with VERITAS on LS I +61? 303, resulting in an excess at the 3.3 Delta *s significance level for constant emission over the entire integrated data set. The flux upper limits derived for emission during the previously reliably active TeV phases (i.e., close to apastron) are less than 5% of the Crab Nebula flux in the same energy range. This result stands in apparent contrast to previous observations by both MAGIC and VERITAS which detected the source during these phases at 10% of the Crab Nebula flux. During the two year span of observations, a large amount of X-ray data were also accrued on LS I +61? 303 by the Swift X-ray Telescope and the Rossi X-ray Timing Explorer Proportional Counter Array. We find no evidence for a correlation between emission in the X-ray and TeV regimes during 20 directly overlapping observations. We also comment on data obtained contemporaneously by the Fermi Large Area Telescope.
Multiwavelength observations of the high-frequency-peaked blazar 1ES 2344+514 were performed from 2007 October to 2008 January. The campaign represents the first contemporaneous data on the object at ...very high energy (VHE, E >100 GeV) Delta *g-ray, X-ray, and UV energies. Observations with VERITAS in VHE Delta *g-rays yield a strong detection of 20 Delta *s with 633 excess events in a total exposure of 18.1 hr live time. A strong VHE Delta *g-ray flare on 2007 December 7 is measured at F(>300 GeV) = (6.76 ? 0.62) X 10--11 photons cm--2 s--1, corresponding to 48% of the Crab Nebula flux. Excluding this flaring episode, nightly variability at lower fluxes is observed with a time-averaged mean of F(>300 GeV) = (1.06 ? 0.09) X 10--11 photons cm--2 s--1 (7.6% of the Crab Nebula flux). The differential photon spectrum between 390 GeV and 8.3 TeV for the time-averaged observations excluding 2007 December 7 is well described by a power law with a photon index of Delta *G = 2.78 ? 0.09stat ? 0.15syst. On the flaring night of 2007 December 7 the measured VHE Delta *g-ray photon index was Delta *G = 2.43 ? 0.22stat ? 0.15syst. Over the full period of VERITAS observations contemporaneous X-ray and UV data were taken with Swift and RXTE. The measured 2-10 keV flux ranged by a factor of ~7 during the campaign. On 2007 December 8 the highest ever observed X-ray flux from 1ES 2344+514 was measured by Swift X-ray Telescope at a flux of F(2-10 keV) = (6.28 ? 0.31) X 10--11 erg cm--2 s--1. Evidence for a correlation between the X-ray flux and VHE Delta *g-ray flux on nightly timescales is indicated with a Pearson correlation coefficient of r = 0.60 ? 0.11. Contemporaneous spectral energy distributions (SEDs) of 1ES 2344+514 are presented for two distinct flux states. A one-zone synchrotron self-Compton (SSC) model describes both SEDs using parameters consistent with previous SSC modeling of 1ES 2344+514 from non-contemporaneous observations.
Abstract
In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent ...with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 10
9
M
⊙
. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous
γ
-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the
γ
-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
Giant X-ray outbursts, with luminosities of about 1037 erg s--1, are observed roughly every five years from the nearby Be/pulsar binary 1A 0535+262. In this article, we present observations of the ...source with VERITAS at very high energies (VHEs; E >100 GeV) triggered by the X-ray outburst in 2009 December. The observations started shortly after the onset of the outburst and provided comprehensive coverage of the episode, as well as the 111 day binary orbit. No VHE emission is evident at any time. We also examined data from the contemporaneous observations of 1A 0535+262 with the Fermi/Large Area Telescope at high-energy photons (E > 0.1 GeV) and failed to detect the source at GeV energies. The X-ray continua measured with the Swift/X-Ray Telescope and the RXTE/PCA can be well described by the combination of blackbody and Comptonized emission from thermal electrons. Therefore, the gamma-ray and X-ray observations suggest the absence of a significant population of non-thermal particles in the system. This distinguishes 1A 0535+262 from those Be X-ray binaries (such as PSR B1259-63 and LS I +61?303) that have been detected at GeV-TeV energies. We discuss the implications of the results on theoretical models.
The very high energy (VHE; E > 100 GeV) blazar Markarian 501 (Mrk 501) has a well-studied history of extreme spectral variability and is an excellent laboratory for studying the physical processes ...within the jets of active galactic nuclei. However, there are few detailed multiwavelength studies of Mrk 501 during its quiescent state, due to its low luminosity. A short-term multiwavelength study of Mrk 501 was coordinated in 2009 March, focusing around a multi-day observation with the Suzaku X-ray satellite and including Delta *g-ray data from VERITAS, MAGIC, and the Fermi Gamma-ray Space Telescope with the goal of providing a well-sampled multiwavelength baseline measurement of Mrk 501 in the quiescent state. The results of these quiescent-state observations are compared to the historically extreme outburst of 1997 April 16, with the goal of examining variability of the spectral energy distribution (SED) between the two states. The derived broadband SED shows the characteristic double-peaked profile. We find that the X-ray peak shifts by over two orders of magnitude in photon energy between the two flux states while the VHE peak varies little. The limited shift in the VHE peak can be explained by the transition to the Klein-Nishina (KN) regime. Synchrotron self-Compton models are matched to the data and the implied KN effects are explored.
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