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A search for enhanced very high energy GAMMA-RAY emission from the 2013 march crab nebula flare
(2014)
In 2013 March, a flaring episode from the Crab Nebula lasting similar to 2 weeks was detected by Fermi-LAT (Large Area Telescope on board the Fermi Gamma-ray Space Telescope). The Very Energetic Radiation Imaging Telescope Array System (VERITAS) provides simultaneous observations throughout this period. During the flare, Fermi-LAT detected a 20 fold increase in flux above the average synchrotron flux >100 MeV seen from the Crab Nebula. Simultaneous measurements with VERITAS are consistent with the non-variable long-term average Crab Nebula flux at TeV energies. Assuming a linear correlation between the very high energy flux change >1 TeV and the flux change seen in the Fermi-LAT band >100 MeV during the period of simultaneous observations, the linear correlation factor can be constrained to be at most 8.6 x 10(-3) with 95% confidence.
The variability of the blazar Markarian 421 in TeV gamma rays over a 14-year time period has been explored with the Whipple 10 m telescope. It is shown that the dynamic range of its flux variations is large and similar to that in X-rays. A correlation between the X-ray and TeV energy bands is observed during some bright flares and when the complete data sets are binned on long timescales. The main database consists of 878.4 h of observation with the Whipple telescope, spread over 783 nights. The peak energy response of the telescope was 400 GeV with 20% uncertainty. This is the largest database of any TeV-emitting active galactic nucleus (AGN) and hence was used to explore the variability profile of Markarian 421. The tithe-averaged flux from Markarian 421 over this period was 0.446 +/- 0.008 Crab flux units. The flux exceeded 10 Crab flux units on three separate occasions. For the 2000-2001 season the average flux reached 1.86 Crab units, while in the 1996-1997 season the average flux was only 0.23 Crab units.
The Galactic center is an interesting region for high-energy (0.1-100 GeV) and very-high-energy (E > 100 GeV) gamma-ray observations. Potential sources of GeV/TeV gamma-ray emission have been suggested, e.g., the accretion of matter onto the supermassive black hole, cosmic rays from a nearby supernova remnant (e.g., Sgr A East), particle acceleration in a plerion, or the annihilation of dark matter particles. The Galactic center has been detected by EGRET and by Fermi/LAT in the MeV/GeV energy band. At TeV energies, the Galactic center was detected with moderate significance by the CANGAROO and Whipple 10 m telescopes and with high significance by H.E.S.S., MAGIC, and VERITAS. We present the results from three years of VERITAS observations conducted at large zenith angles resulting in a detection of the Galactic center on the level of 18 standard deviations at energies above similar to 2.5 TeV. The energy spectrum is derived and is found to be compatible with hadronic, leptonic, and hybrid emission models discussed in the literature. Future, more detailed measurements of the high-energy cutoff and better constraints on the high-energy flux variability will help to refine and/or disentangle the individual models.
A SEARCH FOR VERY HIGH ENERGY GAMMA RAYS FROM THE MISSING LINK BINARY PULSAR J1023+0038 WITH VERITAS
(2016)
The binary millisecond radio pulsar PSR J1023+0038 exhibits many characteristics similar to the gamma-ray binary system PSR B1259-63/LS 2883, making it an ideal candidate for the study of high-energy nonthermal emission. It has been the subject of multiwavelength campaigns following the disappearance of the pulsed radio emission in 2013 June, which revealed the appearance of an accretion disk around the neutron star. We present the results of very high energy (VHE) gamma-ray observations carried out by the Very Energetic Radiation Imaging Telescope Array System before and after this change of state. Searches for steady and pulsed emission of both data sets yield no significant gamma-ray signal above 100 GeV, and upper limits are given for both a steady and pulsed gamma-ray flux. These upper limits are used to constrain the magnetic field strength in the shock region of the PSR J1023+0038 system. Assuming that VHE gamma rays are produced via an inverse Compton mechanism in the shock region, we constrain the shock magnetic field to be greater than similar to 2 G before the disappearance of the radio pulsar and greater than similar to 10 G afterward.
We present the results of 71.6 hr of observations of the Geminga pulsar (PSR J0633+1746) with the VERITAS very-high-energy gamma-ray telescope array. Data taken with VERITAS between 2007 November and 2013 February were phase-folded using a Geminga pulsar timing solution derived from data recorded by the XMM-Newton and Fermi-LAT space telescopes. No significant pulsed emission above 100 GeV is observed, and we report upper limits at the 95% confidence level on the integral flux above 135 GeV (spectral analysis threshold) of 4.0x10(-13) s(-1) cm(-2) and 1.7 x 10(-13) s(-1) cm(-2) for the two principal peaks in the emission profile. These upper limits, placed in context with phase-resolved spectral energy distributions determined from 5 yr of data from the Fermi-Large Area Telescope (LAT), constrain possible hardening of the Geminga pulsar emission spectra above similar to 50 GeV.
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) gamma-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 similar to 570 hr. 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 that are contained within the field of view of the VERITAS observations. We have also performed optical spectroscopy measurements in order to estimate the redshift of some of these blazars that do not have spectroscopic distance estimates. We present new optical spectra from the Kast instrument on the Shane telescope at the Lick observatory for 18 blazars included in this work, which allowed for the successful measurement or constraint on the redshift of four of them. For each of the blazars included in our sample, we provide the flux upper limit in the VERITAS energy band. We also study the properties of the significance distributions and we present the result of a stacked analysis of the data set, which shows a 4s excess.
We present the results of a multi-wavelength campaign targeting the blazar 1ES 1218+30.4 with observations with the 1.3 m McGraw-Hill optical telescope, the Rossi X-ray Timing Explorer (RXTE), the Fermi Gamma-Ray Space Telescope, and the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The RXTE and VERITAS observations were spread over a 13 day period and revealed clear evidence for flux variability, and a strong X-ray and gamma-ray flare on 2009 February 26 (MJD 54888). The campaign delivered a well-sampled broadband energy spectrum with simultaneous RXTE and VERITAS very high energy (VHE, > 100 GeV) observations, as well as contemporaneous optical and Fermi observations. The 1ES 1218+30.4 broadband energy spectrum-the first with simultaneous X-ray and VHE gamma-ray energy spectra-is of particular interest as the source is located at a high cosmological redshift for a VHE source (z = 0.182), leading to strong absorption of VHE gamma rays by photons from the optical/infrared extragalactic background light (EBL) via gamma VHE +gamma EBL -> e(+) e(-)pair-creation processes. We model the data with a one-zone synchrotron self-Compton (SSC) emission model and with the extragalactic absorption predicted by several recent EBL models. We find that the observations are consistent with the SSC scenario and all the EBL models considered in this work. We discuss observational and theoretical avenues to improve on the EBL constraints.
The TeV binary system LS I +61 degrees 303 is known for its regular, non-thermal emission pattern that traces the orbital period of the compact object in its 26.5 day orbit around its B0 Ve star companion. The system typically presents elevated TeV emission around apastron passage with flux levels between 5% and 15% of the steady flux from the Crab Nebula (> 300 GeV). In this article, VERITAS observations of LS I + 61 degrees. 303 taken in late 2014 are presented, during which bright TeV flares around apastron at flux levels peaking above 30% of the Crab Nebula flux were detected. This is the brightest such activity from this source ever seen in the TeV regime. The strong outbursts have rise and fall times of less than a day. The short timescale of the flares, in conjunction with the observation of 10 TeV photons from LS I + 61 degrees 303 during the flares, provides constraints on the properties of the accelerator in the source.
We present results from VERITAS observations of the BL Lac object PG 1553+113 spanning the years 2010, 2011, and 2012. The time-averaged spectrum, measured between 160 and 560 GeV, is well described by a power law with a spectral index of 4.33 +/- 0.09. The time-averaged integral flux above 200 GeV measured for this period was (1.69 +/- 0.06) x 10(-11) photons cm(-2) s(-1), corresponding to 6.9% of the Crab Nebula flux. We also present the combined gamma-ray spectrum from the Fermi Large Area Telescope and VERITAS covering an energy range from 100 MeV to 560 GeV. The data are well fit by a power law with an exponential cutoff at 101.9 +/- 3.2 GeV. The origin of the cutoff could be intrinsic to PG 1553+113 or be due to the gamma-ray opacity of our universe through pair production off the extragalactic background light (EBL). Given lower limits to the redshift of z > 0.395 based on optical/UV observations of PG 1553+113, the cutoff would be dominated by EBL absorption. Conversely, the small statistical uncertainties of the VERITAS energy spectrum have allowed us to provide a robust upper limit on the redshift of PG 1553+113 of z <= 0.62. A strongly elevated mean flux of (2.50 +/- 0.14) x10(-11) photons cm(-2) s(-1) (10.3% of the Crab Nebula flux) was observed during 2012, with the daily flux reaching as high as (4.44 +/- 0.71) x10(-11) photons cm(-2) s(-1) (18.3% of the Crab Nebula flux) on MJD 56048. The light curve measured during the 2012 observing season is marginally inconsistent with a steady flux, giving a chi(2) probability for a steady flux of 0.03%.
We report the detection of a new TeV gamma-ray source, VER J0521+211, based on observations made with the VERITAS imaging atmospheric Cherenkov Telescope Array. These observations were motivated by the discovery of a cluster of >30 GeV photons in the first year of Fermi Large Area Telescope observations. VER J0521+211 is relatively bright at TeV energies, with a mean photon flux of (1.93 +/- 0.13(stat) +/- 0.78(sys)) x 10(-11) cm(-2) s(-1) above 0.2 TeV during the period of the VERITAS observations. The source is strongly variable on a daily timescale across all wavebands, from optical to TeV, with a peak flux corresponding to similar to 0.3 times the steady Crab Nebula flux at TeV energies. Follow-up observations in the optical and X-ray bands classify the newly discovered TeV source as a BL Lac-type blazar with uncertain redshift, although recent measurements suggest z = 0.108. VER J0521+211 exhibits all the defining properties of blazars in radio, optical, X-ray, and gamma-ray wavelengths.
We present the results of a multiwavelength observational campaign on the TeV binary system LS I +61 degrees 303 with the VERITAS telescope array (>200 GeV), Fermi-LAT (0.3-300 GeV), and Swift/XRT (2-10 keV). The data were taken from 2011 December through 2012 January and show a strong detection in all three wavebands. During this period VERITAS obtained 24.9 hr of quality selected livetime data in which LS I +61 degrees 303 was detected at a statistical significance of 11.9 sigma. These TeV observations show evidence for nightly variability in the TeV regime at a post-trial significance of 3.6 sigma. The combination of the simultaneously obtained TeV and X-ray fluxes do not demonstrate any evidence for a correlation between emission in the two bands. For the first time since the launch of the Fermi satellite in 2008, this TeV detection allows the construction of a detailed MeV-TeV spectral energy distribution from LS I +61 degrees 303. This spectrum shows a distinct cutoff in emission near 4 GeV, with emission seen by the VERITAS observations following a simple power-law above 200 GeV. This feature in the spectrum of LS I +61 degrees 303, obtained from overlapping observations with Fermi-LAT and VERITAS, may indicate that there are two distinct populations of accelerated particles producing the GeV and TeV emission.
We present very high energy (VHE) imaging of MGRO J2019+37 obtained with the VERITAS observatory. The bright extended (similar to 2 degrees) unidentified Milagro source is located toward the rich star formation region Cygnus-X. MGRO J2019+37 is resolved into two VERITAS sources. The faint, point-like source VER J2016+371 overlaps CTB 87, a filled-center remnant (SNR) with no evidence of a supernova remnant shell at the present time. Its spectrum is well fit in the 0.65-10 TeV energy range by a power-law model with photon index 2.3 +/- 0.4. VER J2019+378 is a bright extended (similar to 1 degrees) source that likely accounts for the bulk of the Milagro emission and is notably coincident with PSR J2021+3651 and the star formation region Sh 2-104. Its spectrum in the range 1-30 TeV is well fit with a power-law model of photon index 1.75 +/- 0.3, among the hardest values measured in the VHE band, comparable to that observed near Vela-X. We explore the unusual spectrum and morphology in the radio and X-ray bands to constrain possible emission mechanisms for this source.
Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive photomultiplier tube (PMT) cameras. Exposure to high levels of background illumination degrades the efficiency of and potentially destroys these photo-detectors over time, so IACTs cannot be operated in the same configuration in the presence of bright moonlight as under dark skies. Since September 2012, observations have been carried out with the VERITAS IACTs under bright moonlight (defined as about three times the night-sky-background (NSB) of a dark extragalactic field, typically occurring when Moon illumination > 35%) in two observing modes, firstly by reducing the voltage applied to the PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to the cameras. This has allowed observations at up to about 30 times previous NSB levels (around 80% Moon illumination), resulting in 30% more observing time between the two modes over the course of a year. These additional observations have already allowed for the detection of a flare from the 1ES 1727 + 502 and for an observing program targeting a measurement of the cosmic-ray positron fraction. We provide details of these new observing modes and their performance relative to the standard VERITAS observations. (C) 2017 Elsevier B.V. All rights reserved.
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 high-frequency-peaked BL Lac object RBS 0413. VERITAS, a ground-based gamma-ray observatory, detected VHE. rays from RBS 0413 with a statistical significance of 5.5 standard deviations (sigma) and a gamma-ray flux of (1.5 +/- 0.6(stat) +/- 0.7(syst)) x 10(-8) photons m(-2) s(-1) (similar to 1% of the Crab Nebula flux) above 250 GeV. The observed spectrum can be described by a power law with a photon index of 3.18 +/- 0.68(stat) +/- 0.30(syst). Contemporaneous observations with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope detected HE gamma rays from RBS 0413 with a (stat)istical significance of more than 9 sigma, a power-law photon index of 1.57 +/- 0.12(stat-0.12sys')(+0.11) and a gamma-ray flux between 300 MeV and 300 GeV of (1.64 +/- 0.43(stat-0.22sys)(+ 0.31)) x 10(-5) photons m(-2) s(-1). We present the results from Fermi-LAT and VERITAS, including a spectral energy distribution modeling of the gamma-ray, quasi-simultaneous X-ray (Swift-XRT), ultraviolet (Swift-UVOT), and R-band optical (MDM) data. We find that, if conditions close to equipartition are required, both the combined synchrotron self-Compton/external-Compton and the lepto-hadronic models are preferred over a pure synchrotron self-Compton model.
Cosmic-ray electrons and positrons (CREs) at GeV-TeV energies are a unique probe of our local Galactic neighborhood. CREs lose energy rapidly via synchrotron radiation and inverse-Compton scattering processes while propagating within the Galaxy, and these losses limit their propagation distance. For electrons with TeV energies, the limit is on the order of a kiloparsec. Within that distance, there are only a few known astrophysical objects capable of accelerating electrons to such high energies. It is also possible that the CREs are the products of the annihilation or decay of heavy dark matter (DM) particles. VERITAS, an array of imaging air Cherenkov telescopes in southern Arizona, is primarily utilized for gamma-ray astronomy but also simultaneously collects CREs during all observations. We describe our methods of identifying CREs in VERITAS data and present an energy spectrum, extending from 300 GeV to 5 TeV, obtained from approximately 300 hours of observations. A single power-law fit is ruled out in VERITAS data. We find that the spectrum of CREs is consistent with a broken power law, with a break energy at 710 +/- 40(stat) +/- 140(syst) GeV.
Discovery of Very-high-energy Emission from RGB J2243+203 and Derivation of Its Redshift Upper Limit
(2017)
Very-high-energy (VHE; > 100 GeV) gamma-ray emission from the blazar RGB J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the period between 2014 December 21 and 24. The VERITAS energy spectrum from this source can be fitted by a power law with a photon index of 4.6 +/- 0.5, and a flux normalization at 0.15 TeV of (6.3 +/- 1.1) x 10(-10) cm(-2) s(-1) TeV-1. The integrated Fermi-LAT flux from 1 to 100 GeV during the VERITAS detection is (4.1 +/- 0.8) x 10(-8) cm(-2) s(-1), which is an order of magnitude larger than the four-year-averaged flux in the same energy range reported in the 3FGL catalog, (4.0 +/- 0.1 x 10(-9) cm(-2) s(-1)). The detection with VERITAS triggered observations in the X-ray band with the Swift-XRT. However, due to scheduling constraints Swift-XRT observations were performed 67 hr after the VERITAS detection, rather than simultaneously with the VERITAS observations. The observed X-ray energy spectrum between 2 and 10 keV can be fitted with a power law with a spectral index of 2.7 +/- 0.2, and the integrated photon flux in the same energy band is (3.6 +/- 0.6) x 10(-13) cm(-2) s(-1). EBL-model-dependent upper limits of the blazar redshift have been derived. Depending on the EBL model used, the upper limit varies in the range from z < 0.9 to z < 1.1.
We present a new measurement of the energy spectrum of iron nuclei in cosmic rays from 20 TeV to 500 TeV; The measurement makes use of a template-based analysis method, which, for the first time, is applied to the energy reconstruction of iron-induced air showers recorded by the VERITAS array of imaging atmospheric Cherenkov telescopes. The event selection makes use of the direct Cherenkov light which is emitted by charged particles before the first interaction, as well as other parameters related to the shape of the recorded air shower images. The measured spectrum is well described by a power law dF/dE = f(0) center dot (E/E-0)(-gamma) over the full energy range, with gamma = 2.82 +/- 0.30(stat)(-0.27)(+0.24)(syst) and f(0) = (4.82 +/- 0.98(stat)(-2.70)(+2.12)(syst)) x 10(-7) m(-2) s(-1) TeV-1 sr(-1) at E-0 = 50 TeV, with no indication of a cutoff or spectral break. The measured differential flux is compatible with previous results, with improved statistical uncertainty at the highest energies.
Combined with measurements made by very-long-baseline interferometry, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL. Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL. Lacertae observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). with a rise time of similar to 2.3 hr and a decay time of similar to 36 min. The peak flux above 200 GeV is (4.2 +/- 0.6) x 10(-6) photon m(-2) s(-1) measured with a 4-minute-binned light curve, corresponding to similar to 180% of the flux that is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in Very Long Baseline Array observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models that invoke relativistic plasma passing stationary shocks.
Multiwavelength observations of the AGN 1ES 0414+009 with veritas, FERMI-LAT, SWIFT-XRT, AND MDM
(2012)
We present observations of the BL Lac object 1ES 0414+009 in the >200 GeV gamma-ray band by the VERITAS array of Cherenkov telescopes. 1ES 0414+009 was observed by VERITAS between 2008 January and 2011 February, resulting in 56.2 hr of good quality pointed observations. These observations resulted in a detection of 822 events from the source corresponding to a statistical significance of 6.4 standard deviations (6.4 sigma) above the background. The source flux, showing no evidence for variability, is measured as (5.2 +/- 1.1(stat) +/- 2.6(sys)) x 10(-12) photons cm(-2) s(-1) above 200 GeV, equivalent to approximately 2% of the Crab Nebula flux above this energy. The differential photon spectrum from 230 GeV to 850 GeV is well fit by a power law with a photon index of Gamma = 3.4 +/- 0.5(stat) +/- 0.3(sys) and a flux normalization of (1.6 +/- 0.3(stat) +/- 0.8(sys)) x 10(-11) photons cm(-2) s(-1) at 300 GeV. We also present multiwavelength results taken in the optical (MDM), x-ray (Swift-XRT), and GeV (Fermi-LAT) bands and use these results to construct a broadband spectral energy distribution (SED). Modeling of this SED indicates that homogenous one-zone leptonic scenarios are not adequate to describe emission from the system, with a lepto-hadronic model providing a better fit to the data.
On 2015 March 23, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) responded to a Swift-Burst Alert Telescope (BAT) detection of a gamma-ray burst, with observations beginning 270 s after the onset of BAT emission, and only 135 s after the main BAT emission peak. No statistically significant signal is detected above 140 GeV. The VERITAS upper limit on the fluence in a 40-minute integration corresponds to about 1% of the prompt fluence. Our limit is particularly significant because the very-high-energy (VHE) observation started only similar to 2 minutes after the prompt emission peaked, and Fermi-Large Area Telescope observations of numerous other bursts have revealed that the high-energy emission is typically delayed relative to the prompt radiation and lasts significantly longer. Also, the proximity of GRB 150323A (z = 0.593) limits the attenuation by the extragalactic background light to similar to 50% at 100-200 GeV. We conclude that GRB 150323A had an intrinsically very weak high-energy afterglow, or that the GeV spectrum had a turnover below similar to 100 GeV. If the GRB exploded into the stellar wind of a massive progenitor, the VHE non-detection constrains the wind density parameter to be A greater than or similar to 3 x 10(11) g . cm(-1), consistent with a standard Wolf-Rayet progenitor. Alternatively, the VHE emission from the blast wave would be weak in a very tenuous medium such as the interstellar medium, which therefore cannot be ruled out as the environment of GRB 150323A.
We report on deep observations of the extended TeV gamma-ray source MGRO J1908+06 made with the VERITAS very high energy gamma-ray observatory. Previously, the TeV emission has been attributed to the pulsar wind nebula (PWN) of the Fermi-LAT pulsar PSR J1907+0602. We detect MGRO J1908+06 at a significance level of 14 standard deviations (14 sigma) and measure a photon index of 2.20 +/- 0.10(stat) +/- 0.20(sys). The TeV emission is extended, covering the region near PSR J1907+0602 and also extending toward SNR G40.5-0.5. When fitted with a two-dimensional Gaussian, the intrinsic extension has a standard deviation of sigma(src) = 0 degrees.44 +/- 0 degrees.02. In contrast to other TeV PWNe of similar age in which the TeV spectrum softens with distance from the pulsar, the TeV spectrum measured near the pulsar location is consistent with that measured at a position near the rim of G40.5-0.5, 0 degrees.33 away.
The Galactic Center ridge has been observed extensively in the past by both GeV and TeV gamma-ray instruments revealing a wealth of structure, including a diffuse component and the point sources G0.9+0.1 (a composite supernova remnant) and Sgr A* (believed to be associated with the supermassive black hole located at the center of our Galaxy). Previous very high energy (VHE) gamma-ray observations with the H.E.S.S.. experiment have also detected an extended TeV gamma-ray component along the Galactic plane in the >300 GeV gamma-ray regime. Here we report on observations of the Galactic Center ridge from 2010 to 2014 by the VERITAS telescope array in the >2 TeV energy range. From these observations we (1) provide improved measurements of the differential energy spectrum for Sgr A* in the >2 TeV gamma-ray regime, (2) provide a detection in the >2 TeV gamma-ray emission from the composite SNR G0.9+0.1 and an improved determination of its multi-TeV gamma-ray energy spectrum, and. (3) report on the detection of VER J1746-289, a localized enhancement of >2 TeV gamma-ray emission along the Galactic plane.