We report on the discovery of two gamma-ray sources at E > 200 GeV with VERITAS: RX J0648.7+1516 and VER J0521+211. Both sources are located at low galactic latitudes (|b| < 10 deg), and were previously classified as active galactic nuclei of unknown type. The discovery and characterization of gamma-ray emission together with follow-up optical spectroscopy permitted the identification of RX J0648.7+1516 and VER J0521+211 as BL Lac-type blazars. In the case of RX J0648.7+1516, a spectroscopic redshift of z = 0.179 was derived.
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona is currently the world's most-sensitive detector of astrophysical very high energy (VHE; E>100 GeV) gamma rays. Approximately 25 blazars are known to emit VHE photons, and observations of blazars are one of the VERITAS collaboration's key science projects (KSP). More than 50 of these objects have been observed with the array, in most cases with the deepest-ever VHE exposure. This contribution summarizes the VERITAS blazar KSP, with a focus on the upper limits measured in the blazar discovery program. VERITAS blazar …
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona is one of the world's most-sensitive detectors of very-high-energy (VHE, E>100 GeV) gamma rays. More than 50 extragalactic sources are known to emit VHE photons; these include blazars, radio galaxies, and starburst galaxies. Blazar observations are one of the VERITAS Collaboration's Key Science Projects. More than 400 hours per year are devoted to this program and ~100 blazars have already been observed with the array, in most cases with the deepest ever VHE exposure. These observations have resulted in 21 detection…
VERITAS is an array of four imaging atmospheric Cherenkov telescopes that observes gamma-ray sources in the very high energy range (VHE, E>100 GeV). A large fraction of the known gamma-ray sources are of extragalactic origin and belong to the blazar class: active galactic nuclei with relativistic jets pointing along the line of sight. Gamma-ray emission from blazars is typically variable. Part of the VERITAS observation program is devoted to follow-up triggers on flaring blazars, initiated by observations at other wavelengths. We will present those target of opportunity observations based on o…
VERITAS is an array of four 12-m imaging Cherenkov telescopes, sensitive to gamma rays in the energy range from 85 GeV to 30 TeV. VERITAS dedicates roughly 40% of its total observing time to blazars. We present recent highlights from the VERITAS blazar discovery and long-term monitoring programs. These observations have led to 25 blazar detections, including 10 VHE discoveries. The VERITAS cameras were recently upgraded, lowering the energy threshold and enhancing the sensitivity. This upgrade helped to establish four new blazar detections with data taken after the completion of the upgrade in…
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona began full-scale operations in 2007, and it is one of the world's most sensitive detectors of astrophysical VHE (E > 100 GeV) gamma rays. Forty-one blazars are known to emit VHE photons, and observations of blazars are one of the VERITAS Collaboration's Key Science Projects (KSPs). More than 400 hours per year are devoted to this program, and ~100 blazars have already been observed with the array, in most cases with the deepest-ever VHE exposure. These observations have resulted in 20 detections, including 10 new VH…
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona began full-scale operations in 2007, and is one of the world's most-sensitive detectors of astrophysical VHE (E>100 GeV) gamma rays. Approximately 50 blazars are known to emit VHE photons, and observations of blazars are a major focus of the VERITAS Collaboration. Nearly 2000 hours have been devoted to this program and ~130 blazars have already been observed with the array, in most cases with the deepest-ever VHE exposure. These observations have resulted in 21 detections, including 10 VHE discoveries. Recent highli…
The VERITAS array of four 12-m diameter imaging atmospheric-Cherenkov telescopes in southern Arizona is used to study very high energy (VHE; E>100 GeV) gamma-ray emission from astrophysical objects. VERITAS is currently the most sensitive VHE gamma-ray observatory in the world and one of the VERITAS collaboration's Key Science Projects (KSP) is the study of blazars. These active galactic nuclei (AGN) are the most numerous class of identified VHE sources, with ~30 known to emit VHE photons. More than 70 AGN, almost all of which are blazars, have been observed with the VERITAS array since 2007, …
The VERITAS array of Cherenkov telescopes was used to observe ~130 blazars from 2007 to 2012. Of these, 25 were detected as very-high-energy (VHE; E > 100 GeV) sources. We present here the results of the analysis of 65 VERITAS non-detected blazars, including upper limits on their VHE flux. Results from a stacked analysis of the entire data set and of smaller sub-sets (defined as a function of the redshift and the blazar class) are presented and discussed.
The VERITAS array of 12-m atmospheric-Cherenkov telescopes is used in an intensive observation program focused on the discovery of VHE (E > 100 GeV) gamma-ray-emitting blazars. Since VERITAS began full-scale operation in 2007, more than 1000 hours of observation, on ~90 targets, have been devoted to the VHE blazar discovery effort and to timely follow-up in multi-wavelength (MWL) observation campaigns. These data have resulted in the discovery of VHE emission from 10 blazars (6 HBLs and 4 IBLs). A summary of these VHE discoveries is presented.
VERITAS, an array of imaging atmospheric-Cherenkov telescopes, studies blazars in the energy range between ~100 GeV and ~30 TeV. With its excellent sensitivity at these energies, and ever-deepening source exposures, VERITAS is in a position to probe distant blazars for clear absorption signatures in their very-high-energy gamma-ray spectra due to interactions with the extragalactic background light (EBL). We discuss results from recent VERITAS observations of PG 1553+113 (z > 0.4) which have resulted in the most significant very-high-energy detection ever obtained for this source. The most rec…
A non-zero intergalactic magnetic field (IGMF) would potentially produce detectable effects on cascade emission from blazars. Depending on the strength of the IGMF, the cascade emission may be time delayed or angularly broadened compared to the blazar's primary, unscattered emission. Ground-based imaging atmospheric-Cherenkov telescopes, such as VERITAS, have the precise angular resolution needed to search for magnetically-broadened emission. We present the latest VERITAS results on the search for extended gamma-ray emission, based on observations of a number of strongly-detected TeV blazars a…
The VERITAS collaboration has approved long-term observations on several distant, hard-spectrum blazars. We present first results from VERITAS long-term observations of 1ES1218+304, 1ES0229+200, and 1ES0414+009. Gamma-ray observations of distant, hard-spectrum blazars has emerged as an effective, indirect probe of the universe's diffuse extragalactic background light (EBL), due to the extinction of gamma-rays via the pair production interaction $γ_{TeV}γ_{EBL}\rightarrow e^{+}e^{-}$, which is expected to produce absorption features in distant gamma-ray sources. The sources presented here emit …
The intermediate-frequency-peaked BL Lac object 3C 66A was detected in a flaring state by the Fermi-LAT and VERITAS observatories in October 2008. These data and follow-up observations at other wavelengths create a rich sample of light curves and a constraining spectral energy distribution (SED). This is the first time that simultaneous observations at GeV and TeV energies were obtained for a flaring blazar. Results from these joint Fermi-LAT and VERITAS observations are presented in this conference proceeding.
1ES 1218+304 is among a group of several TeV blazars that exhibit surprisingly hard energy spectra given their redshifts. The VERITAS collaboration has carried out an intensive observing campaign of 1ES 1218+304 in early 2009 to characterize the emission properties of one of these hard-spectra distant blazars in more detail. While hard-spectra TeV blazars are most promising for unveiling putative signatures from EBL absorption, they may also have implications on shock acceleration scenarios in relativistic jets. Our observations of 1ES 1218+304 demonstrate significant flux variability. Further…
VERITAS is an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes located in southern Arizona. Its aim is to study the very high energy (VHE: E > 100 GeV) gamma-ray emission from astrophysical objects. In addition to the study of blazars, the VERITAS extragalactic science program develops a comprehensive observational program of extragalactic non-blazar sources. The study of Active Galactic Nuclei (AGN) is intensively pursued through the large MWL observational campaign on non-blazar radio galaxies. The success of these MWL campaigns has for the first time provided insights to…
The gamma-ray blazar TXS 0506+056 was found with an enhanced gamma-ray emission state in spatial and temporal coincidence with the IceCube high energy neutrino event IC170922A. This is the most significant association by far between a high-energy neutrino event and a blazar in a flaring state. Studying the time evolution and spectral behavior of the blazar emission may help in identifying the sources of the diffuse neutrino flux observed by IceCube and the origin of energetic cosmic rays. TXS 0506+056 was detected by the VERITAS gamma-ray observatory with a significance of 5.8 standard deviati…
We present long-term observations of several VHE (E > 100 GeV) blazars with VERITAS, together with contemporaneous Swift and RXTE X-ray data. The observed targets include Mrk 421, Mrk 501, 1ES 2344+514. Strong flux and spectral variability is seen in Mrk 421 on nightly time-scales between January and June 2008, revealing a highly correlated X-ray to GeV/TeV connection. Modest X-ray variability is evident in Mrk 501. Observations of 1ES 2344+514 in December 2007 show VHE gamma-ray and X-ray flux doubling on nightly time-scales.
The VERITAS array of imaging atmospheric Cherenkov telescopes has collected nearly 6000 hours of observations of active galactic nuclei (AGNs). It has detected 39 very-high-energy (VHE, >100 GeV) AGNs at redshifts up to z = 0.9, of which 24 are classified as high-frequency-peaked BL Lacertae objects (HBLs). VERITAS has obtained an extensive dataset of HBL, including extreme HBL (xHBL), observations, with lightcurves spanning up to 11 years, allowing the characterization of their long-term and short-term variability. A study of various xHBLs/HBLs in the VERITAS dataset is presented, and the cor…
VERITAS is an array of four 12-m atmospheric Cherenkov telescopes, designed to observe the very-high-energy (VHE; E $\geq$100 GeV) sky. Since 2007, it has detected more than 20 extra-galactic sources, the majority of which are active-galactic-nuclei of the blazar class. In this paper we present a selection of the most recent results from the VERITAS blazar observing program, in particular in the context of broad-band multi-wavelength campaigns. Four results are highlighted: the detection of the flat-spectrum-radio-quasar PKS 1222+216 (4C +21.35) during March 2014; the $γ$-ray flare from the BL…
VERITAS is an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes located in southern Arizona. Its aim is to study the very high energy (VHE: E > 100 GeV) gamma-ray emission from astrophysical objects. The study of Active Galactic Nuclei (AGN) is intensively pursued through the VERITAS blazar key science project, but also through the large MWL observational campaigns on radio galaxies. The successful VERITAS AGN research program has provided insights to the jet inner structures and started a more detailed classification of blazars. Moreover, the synergy between Fermi and VERIT…
The high-synchrotron-peaked blazar 1ES 1959+650 was among the first extragalactic sources detected in the very high energy gamma ray band (VHE, E > 100 GeV). In October 2015, the source entered an extended period of activity that continued through July 2016, during which several strong VHE flares were observed. This flaring activity in the TeV band was accompanied by a strong increase in the optical, X-ray, and GeV gamma-ray flux of the source, surpassing its brightest recorded flux states. The VERITAS telescope array performed observations of 1ES 1959+650 between October 2015 and June 2016, a…
Gamma-ray blazars are among the most extreme astrophysical sources, harboring phenomena far more energetic than those attainable by terrestrial accelerators. These galaxies are understood to be active galactic nuclei that are powered by accretion onto supermassive black holes and have relativistic jets pointed along the Earth line of sight. The emission displayed is variable at all wavelengths and timescales probed thus far, necessitating contemporaneous broadband observations to disentangle the details of the emission processes within the relativistic jets. The very high energy (VHE; $E\ge$10…
We present here the results of the analysis of the $γ$-ray blazar, 1ES 1215+303, over a 10-year period, from 2008 to 2017, measured at high energies (HE; 200 MeV $< E <$ 100 GeV) by the Fermi Large Area Telescope (LAT) and at very high energies (VHE; $E >$ 100 GeV) by Fermi-LAT and VERITAS. This is the longest temporal study of this high-frequency-peaked BL Lac object (HBL) at $γ$-ray energies to date. The spectrum follows a log parabola over this time period, and its HE and VHE spectra are well-connected. Its flux is sufficiently strong at HE to allow us to bin the Fermi-LAT data in 3-day int…
We present results from an intensive VERITAS monitoring campaign of the high-frequency peaked BL Lac object 1ES 1218+304 in 2008/2009. Although 1ES 1218+304 was detected previously by MAGIC and VERITAS at a persistent level of ~6% of the Crab Nebula flux, the new VERITAS data reveal a prominent flare reaching ~20% of the Crab. While very high energy (VHE) flares are quite common in many nearby blazars, the case of 1ES 1218+304 (redshift z = 0.182) is particularly interesting since it belongs to a group of blazars that exhibit unusually hard VHE spectra considering their redshifts. When correct…
1ES 1218+304 is a moderate-redshift (z = 0.182) high-frequency-peaked BL Lac object (HBL). Detected by both the MAGIC and VERITAS gamma-ray observatories, 1ES 1218+304 is frequently monitored by VERITAS as part of its long-term blazar monitoring program. On the 3rd of January 2019, during a regularly scheduled blazar snapshot, VERITAS observed 1ES 1218+304 to have an elevated VHE flux state which continued through January 5 (ATEL #12360). In addition to VERITAS observations, MAGIC also detected elevated VHE emission (ATEL #12354) and Swift-BAT/XRT and Tuorla observations, in X-ray and optical …
Between the beginning of its full-scale scientific operations in 2007 and 2012, the VERITAS Cherenkov telescope array observed more than 130 blazars; of these, 26 were detected as very-high-energy (VHE; E>100 GeV) γ-ray sources. In this work, we present the analysis results of a sample of 114 undetected objects. The observations constitute a total live-time of ~570 hours. The sample includes several unidentified Fermi-Large Area Telescope (LAT) sources (located at high Galactic latitude) as well as all the sources from the second Fermi-LAT catalog which are contained within the field of view o…
The intermediate-frequency peaked BL Lacertae (IBL) object 3C 66A is detected during 2007 - 2008 in VHE (very high energy: E > 100 GeV) gamma-rays with the VERITAS stereoscopic array of imaging atmospheric Cherenkov telescopes. An excess of 1791 events is detected, corresponding to a significance of 21.2 standard deviations (sigma), in these observations (32.8 hours live time). The observed integral flux above 200 GeV is 6% of the Crab Nebula's flux and shows evidence for variability on the time-scale of days. The measured energy spectrum is characterized by a soft power law with photon index …
We report on the discovery of high-energy (HE; E > 0.1 GeV) and very high-energy (VHE; E > 100 GeV) gamma-ray emission from the blazar RBS 0413 with Fermi Large Area Telescope (LAT) and VERITAS. The spectral energy distribution (SED), including contemporaneous X-ray and optical observations, is presented. Synchrotron self-Compton, external-Compton and lepto-hadronic models are applied to the SED and the results are discussed.
Intermediate-frequency-peaked BL Lacertae objects (IBLs) are a class of blazars characterized by a spectral energy distribution (SED) with a lower-energy synchrotron peak than the majority of extragalactic sources detected by ground-based imaging atmospheric Cherenkov telescopes (IACTs). Consequently, the peak gamma-ray flux falls outside the very-high-energy regime (VHE, >100 GeV) covered by IACTs such as VERITAS, making IBLs difficult to detect except during infrequent episodes of elevated flux. However, the study of these sources in a low-flux state is essential for developing a complete un…
We report on VERITAS very-high-energy (VHE; E>100 GeV) observations of six blazars selected from the Fermi Large Area Telescope First Source Catalog (1FGL). The gamma-ray emission from 1FGL sources was extrapolated up to the VHE band, taking gamma-ray absorption by the extragalactic background light into account. This allowed the selection of six bright, hard-spectrum blazars that were good candidate TeV emitters. Spectroscopic redshift measurements were attempted with the Keck Telescope for the targets without Sloan Digital Sky Survey (SDSS) spectroscopic data. No VHE emission is detected dur…
The extragalactic background light (EBL) contains all the radiation emitted by nuclear and accretion processes since the epoch of recombination. Direct measurements of the EBL in the near-IR to mid-IR waveband are extremely difficult due mainly to the zodiacal foreground light. Instead, gamma-ray astronomy offers the possibility to indirectly set limits on the EBL by studying the effects of gamma-ray absorption in the spectra of detected sources in the very high energy range (VHE: $>$100 GeV). These effects can be generally seen in the spectra of VHE blazars as a softening (steepening) of the …
Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy $γ$-rays, through the investigation of the blazar 3C 66A. We model the high energy spectrum of 3C 66A, which has been observed recently with the Fermi-LAT and VERITAS telescope. The spectrum has a hard change from the energy range of 0.2-100 GeV to 200-500 GeV in recent almost contemporaneous observations of two telescopes. The de-absorbed VERITAS spectrum greatly depends on the redshift, which is highly uncertain. If z=0.444 is adopted, we are able to use the SSC model to produce the Fermi-LAT…
High-frequency-peaked BL Lacs (HBLs) dominate the extragalactic TeV sky, with more than 50 objects detected by the current generation of TeV observatories. Still, the properties of TeV-emitting HBLs as a population are poorly understood due to biases introduced by the observing strategies of Cherenkov Telescopes, limiting our ability to estimate the potential contribution of TeV blazars to the diffuse neutrino, gamma-ray, and cosmic-ray background as well as their role in the late-stage evolution of active galactic nuclei. The VERITAS telescope array has designed a program to quantify and mini…
HESS J1943+213 is a very high energy (VHE; 100 GeV) γ-ray source in the direction of the Galactic plane. 38 hours of deep VERITAS observations taken over two seasons detect the source with ~20 σ significance. Monitoring observations of HESS J1943+213 show a remarkably stable flux and spectrum in VHE γ-rays. Studies exploring the classification of HESS J1943+213 are converging towards accepting the source as an extreme synchrotron BL Lac object. Specifically, overall SED characteristics of the source, the detection of a potential host galaxy in near-IR imaging, and VLBI observations of the HESS…
We present a search for magnetically broadened gamma-ray emission around active galactic nuclei (AGN), using VERITAS observations of seven hard-spectrum blazars. A cascade process occurs when multi-TeV gamma rays from AGN interact with extragalactic background light (EBL) photons to produce electron-positron pairs, which then interact with cosmic microwave background (CMB) photons via inverse-Compton scattering to produce gamma rays. Due to the deflection of the electron-positron pairs, a non-zero intergalactic magnetic field (IGMF) would potentially produce detectable effects on the angular d…
Ground-based $γ$-ray observatories, such as the VERITAS array of imaging atmospheric Cherenkov telescopes, provide insight into very-high-energy (VHE, $\mathrm{E}>100\,\mathrm{GeV}$) astrophysical transient events. Examples include the evaporation of primordial black holes, gamma-ray bursts and flaring blazars. Identifying such events with a serendipitous location and time of occurrence is difficult. Thus, employing a robust search method becomes crucial. An implementation of a transient detection method based on deep-learning techniques for VERITAS will be presented. This data-driven approach…
On 2017 September 22, the IceCube Neutrino Observatory reported the detection of the high-energy neutrino event \icnu, of potential astrophysical origin. It was soon determined that the neutrino direction was consistent with the location of the gamma-ray blazar \txs~(3FGL J0509.4+0541), which was in an elevated gamma-ray emission state as measured by the \emph{Fermi} satellite. VERITAS observations of the neutrino/blazar region started on 2017 September 23 in response to the neutrino alert and continued through 2018 February 6. While no significant very-high-energy (VHE; E $>$ 100 GeV) emissio…
During May 2013, a gamma-ray flare from the BL Lac object 1ES 1727+502 (z=0.055) has been detected with the VERITAS Cherenkov telescopes. This detection represents the first evidence of very-high-energy (E>100 GeV) variability from this blazar and has been achieved using a reduced-high-voltage configuration which allows observations under bright moonlight. The integral flux is about five times higher than the archival VHE flux measured by MAGIC. The detection triggered additional VERITAS observations during standard dark-time and multiwavelength observations from infrared to X-rays with the FL…
Markarian 421 is one of the brightest BL-Lac objects in gamma-rays in the northern hemisphere. Because of its brightness, the source has been the focus of several coordinated multi-wavelength (MWL) campaigns designed to study the physical processes responsible for the non-thermal continuum emission. The blazar monitoring program of VERITAS recently received a ToO during a strong flaring event by Markarian 421 in February 2010. The source was seen at flux level of approximately 8 Crab units and exhibited spectral evolution and variability features. A multi-wavelength campaign with other MWL par…
The extragalactic background light (EBL), a diffuse photon field in the optical and infrared range, is a record of radiative processes over the Universe's history. Spectral measurements of blazars at very high energies ($>$100 GeV) enable the reconstruction of the spectral energy distribution (SED) of the EBL, as the blazar spectra are modified by redshift- and energy-dependent interactions of the gamma-ray photons with the EBL. The spectra of 14 VERITAS-detected blazars are included in a new measurement of the EBL SED that is independent of EBL SED models. The resulting SED covers an EBL wave…
The giant radio galaxy M 87 is located at a distance of ~16 Mpc and harbors a supermassive black hole in its center. The structure of its relativistic plasma jet is resolved at radio, optical and X-ray wavelengths. M 87 belongs to the class of active galactic nuclei (AGN) and is one of the few extragalactic TeV gamma-ray source not belonging to the class of blazars. M 87 is also detected by Fermi in the GeV energy range. This makes it a unique laboratory for the study of the jet substructures and the morphology of the non-thermal emission processes. In spring 2010 a major flare was observed at…
In this review, recent progress in theoretical models for the broadband (radio through gamma-ray) emission from blazars are summarized. The salient features of both leptonic and hadronic models are reviewed. I present sample modeling results of spectral energy distributions (SEDs) of different types of Fermi-detected blazars along the traditional blazar sequence, using both types of models. In many cases, the SEDs of high-frequency peaked blazars (HBLs) have been found to be well represented by simple synchrotron + synchrotron self-Compton (SSC) models. However, a few HBLs recently discovered …
Located in southern Arizona, VERITAS is amongst the most sensitive detectors for astrophysical very high energy (VHE; E>100 GeV) gamma rays and has been operational since April 2007. We highlight some recent results from VERITAS observations. These include the long-term observations of the gamma-ray binaries HESS J0632+057 and LS I +61° 303, the observations of the Galactic Center region, and of the supernova remnant Cas~A. We discuss the results from a decade of multi-wavelength observations of the blazar 1ES 1215+303, the EHT 2017 campaign on the M87 galaxy, the discovery of 3C 264 in VHE, a…
It is notoriously difficult to organize simultaneous observations from space and from the ground. This is particularly so at GeV-TeV energies where the space observations from all sky telescopes and the ground-based observations (mostly using the atmospheric Cherenkov technique). Because of observing constraints, the latter observations are limited to "snap-shots" of sources with short exposure times. For the past several years the observing program with the Whipple Observatory 10m Gamma-ray Telescope has been devoted to the observation of several bright TeV gamma-ray emitting AGN (including M…
HESS J1943+213 is a very-high-energy (VHE; $>$100 GeV) $γ$-ray source in the direction of the Galactic Plane. Studies exploring the classification of the source are converging towards its identification as an extreme synchrotron BL Lac object. Here we present 38 hours of VERITAS observations of HESS J1943+213 taken over two years. The source is detected with $\sim$20 standard deviations significance, showing a remarkably stable flux and spectrum in VHE $γ$-rays. Multi-frequency very-long-baseline array (VLBA) observations of the source confirm the extended, jet-like structure previously found …
Over seventy blazars identified as sources of TeV gamma ray emission have been detected, including approximately sixty BL Lac objects and seven flat spectrum radio quasars. The distribution in space of these objects can be described by a luminosity function, which gives their comoving space density as a function of their luminosity. We investigate a source selection method to be used for determining the luminosity function of TeV gamma-ray blazars using observations from VERITAS, a ground-based gamma-ray observatory consisting of an array of four atmospheric Cherenkov telescopes located in sou…
Since its launch in April 2007, the AGILE satellite detected with its Gamma-Ray Imaging Detector (GRID) several blazars at high significance: 3C 279, 3C 454.3, PKS 1510-089, S5 0716+714, 3C 273, W Comae, Mrk 421 and PKS 0537-441. Moreover, AGILE was able both to rapidly respond to sudden changes in blazar activity state at other wavelengths and to alert other telescopes quickly in response to changes in the gamma-ray fluxes. Thus, we were able to obtain multiwavelength data from other observatories such as Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, as well as radio-to-optical cove…
The Cygnus region is a very active region of our Galaxy with many sources of GeV and TeV gamma-ray emission, such as supernova remnants, pulsar wind nebulae, and massive star clusters. A detailed study of the Cygnus region at these energies can give insight into the processes of particle acceleration in astrophysical sources. VERITAS (Very Energetic Radiation Imaging Telescope Array System) is an array of four 12 meter diameter imaging atmospheric Cherenkov telescopes located at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. From 2007 through 2012 VERITAS observed the Cygnus…
B2013+370 and B2023+336 are two blazars at low-galactic latitude that were previously proposed to be the counterparts for the EGRET unidentified sources, 3EG J2016+3657 and 3EG J2027+3429. Gamma-ray emission associated with the EGRET sources has been detected by the Fermi Gamma-ray Space Telescope, and the two sources, 1FGL J2015.7+3708 and 1FGL J2027.6+3335, have been classified as unidentified in the 1-year catalog. This analysis of the Fermi-LAT data collected during 31 months reveals that the 1FGL sources are spatially compatible with the blazars, and are significantly variable, supporting…
The very high energy (VHE; E > 100 GeV) gamma-ray blazar Markarian 501 has a history of extreme spectral variability and is an excellent laboratory for studying the physical processes within the jets of active galactic nuclei. A short term multi-wavelength study of Markarian 501 was coordinated in March 2009 using the Suzaku X-ray satellite as well as the VERITAS and MAGIC experiments to cover the critical X-ray and VHE gamma-ray bands. The results of the quiescent-state observations with VERITAS and Suzaku are combined with public data from the Fermi Gamma-ray Space Telescope and compared to …
We present the redshift lower limit of z>0.6035 for the very-high-energy (VHE; E>100 GeV) emitting blazar PKS 1424+240 (PG 1424+240). This limit is inferred from Lyman beta and gamma absorption observed in the far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph. No VHE-detected blazar has shown solid spectroscopic evidence of being more distant. At this distance, VHE observations by VERITAS are shown to sample historically large gamma-ray opacity values at 500 GeV, extending beyond tau=4 for low-level models of the extragalactic background light (EBL) and beyond…
Very high energy (VHE: >100 GeV) gamma rays coming from AGN can pair-produce on the intergalactic background light generating an electromagnetic cascade. If the Intergalactic Magnetic Field (IGMF) is sufficiently strong, this cascade may result in an extended isotropic emission of photons around the source, or halo. Using VERITAS observations of the blazar Markarian 421, we search for extended emission by comparing the source angular distribution ($θ^2$) from a quiescent period with one coming from a flare period, which can be considered as halo-free. $χ^2$ test showed no significant statistic…
Thanks to the wide field of view of its gamma-ray imager, the AGILE satellite obtained a long term monitoring of the brightest blazars in the sky and during the first 3 years of operation detected several blazars in a high gamma-ray state: 3C 279, 3C 454.3, PKS 1510-089, S5 0716+714, 3C 273, W Comae, and Mrk 421. Through the rapid dissemination of our alerts we were able to obtain also multi-wavelength data from many observatories such as Spitzer, Swift, RXTE, Suzaku, XMM-Newton, INTEGRAL, MAGIC, VERITAS, and ARGO as well as radio-to-optical coverage by means of the MOJAVE project, the GASP pr…
Most of the ~50 blazars detected to date at TeV energies (E>0.1 TeV) are high-frequency-peaked BL Lacs (HBLs). Only a handful episodic TeV detections of low- and intermediate-frequency-peaked BL Lacs (LBL/IBLs, with synchrotron peak frequencies in the infrared and optical regime) have been reported, typically during high-flux states. The VERITAS array, a ground-based TeV observatory located in southern Arizona has observed five known TeV LBL/IBLs since 2009: 3C 66A, W Comae, PKS 1424+240, S5 0716+714 and BL Lacertae, with at least 5-10 hours/year, which so far resulted in the detection of a br…
BL Lacertae is the prototype of the blazar subclass known as BL Lac type objects. BL Lacertae object itself is a low-frequency-peaked BL Lac(LBL). Very high energy (VHE) gamma ray emission from this source was discovered in 2005 by MAGIC observatory while the source was at a flaring state. Since then, VHE gamma rays from this source has been detected several times. However, all of those times the source was in a high activity state. Former studies suggest several non-thermal zones emitting in gamma-rays, then gamma-ray flare should be composed of a convolution. Observing the BL Lacertae object…
The Cygnus region of the Galactic plane is a promising target for high-energy and very high energy (VHE) gamma-ray telescopes as it is home to many potential sources, such as supernova remnants, pulsar wind nebulae, X-ray binaries and massive star clusters. The VHE gamma-ray observatory VERITAS (Very Energetic Radiation Imaging Telescope Array System) is an array of four 12 m diameter imaging atmospheric Cherenkov telescopes located at Mt. Hopkins, AZ, USA. Over the period of 2007 to 2012, VERITAS has carried out extensive observations in the direction of Cygnus. These observations were initia…
The Cygnus region of the Galactic plane contains many known supernova remnants, pulsars, X-ray and GeV gamma-ray emitters which make it a prime candidate for a Very High Energy (VHE) gamma-ray survey in the Northern Hemisphere. The VERITAS observatory, an array of four atmospheric Cherenkov telescopes located at the base of Mt. Hopkins in southern Arizona, USA, has carried out an extensive survey of the Cygnus region between 67 and 82 degrees in galactic longitude and between -1 and 4 degrees in galactic latitude. The survey, comprising more than 140 hours of observations, reaches an average V…
