The Galactic Center region imaged by VERITAS Beilicke, M.
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
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The Galactic Center has long been a region of interest for high-energy and very-high-energy observations. Many potential sources of GeV/TeV γ-ray emission have been suggested, e.g., the accretion of ...matter onto the black hole, cosmic rays from a nearby supernova remnant, or the annihilation of dark matter particles. The Galactic Center has been detected at MeV/GeV energies by EGRET and recently by Fermi/LAT. At GeV/TeV energies, the Galactic Center was detected by different ground-based Cherenkov telescopes such as CANGAROO, Whipple 10m, HESS, and MAGIC. We present the results from 15h of VERITAS observations conducted at large zenith angles, resulting in a >10 standard deviation detection. The combined Fermi/VERITAS results are compared to astrophysical models.
Aims. We aim to constrain the structural variations within the HST-1 region downstream of the radio jet of M 87, in general as well as in connection to the episodes of activity at very high energy ...(VHE). Methods. We analyzed and compared 26 VLBI observations of the M 87 jet, obtained between 2006 and 2011 with the Very Long Baseline Array (VLBA) at 1.7 GHz and the European VLBI Network (EVN) at 5 GHz. Results. HST-1 is detected at all epochs; we model-fitted its complex structure with two or more components, the two outermost of which display a significant proper motion with a superluminal velocity around ~4 c. The motion of a third feature that is detected upstream is more difficult to characterize. The overall position angle of HST-1 has changed during the time of our observations from −65° to −90°, while the structure has moved by over 80 mas downstream. Our results on the component evolution suggest that structural changes at the upstream edge of HST-1 can be related to the VHE events.
The accretion-powered X-ray pulsar GX 301−2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late 2018 December, with contiguous observations by the Neutron star Interior ...Composition Explorer Mission (NICER) X-ray telescope, the Swift X-ray Telescope and Burst Alert Telescope, and the Fermi Gamma-ray Burst Monitor spanning several months. The observations detected the pulsar in a rare apastron flaring state coinciding with a significant spin up of the pulsar discovered with the Fermi Gamma-ray Burst Monitor. The X-Calibur, NICER, and Swift observations reveal a pulse profile strongly dominated by one main peak, and the NICER and Swift data show strong variation of the profile from pulse to pulse. The X-Calibur observations constrain for the first time the linear polarization of the 15-35 keV emission from a highly magnetized accreting neutron star, indicating a polarization degree of % (90% confidence limit) averaged over all pulse phases. We discuss the spin up and the X-ray spectral and polarimetric results in the context of theoretical predictions. We conclude with a discussion of the scientific potential of future observations of highly magnetized neutron stars with the more sensitive follow-up mission XL-Calibur.
Although Galactic cosmic rays (protons and nuclei) are widely believed to be mainly accelerated by the winds and supernovae of massive stars, definitive evidence of this origin remains elusive nearly ...a century after their discovery. The active regions of starburst galaxies have exceptionally high rates of star formation, and their large size—more than 50 times the diameter of similar Galactic regions—uniquely enables reliable calorimetric measurements of their potentially high cosmic-ray density. The cosmic rays produced in the formation, life and death of massive stars in these regions are expected to produce diffuse -ray emission through interactions with interstellar gas and radiation. M82, the prototype small starburst galaxy, is predicted to be the brightest starburst galaxy in terms of -ray emission. Here we report the detection of >700-GeV -rays from M82. From these data we determine a cosmic-ray density of 250 eV cm-3 in the starburst core, which is about 500 times the average Galactic density. This links cosmic-ray acceleration to star formation activity, and suggests that supernovae and massive-star winds are the dominant accelerators.
We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was ...observed during a 34.6 minute exposure, when the integral flux above 200 GeV reached (3.4 + or - 0.6) x 10 super(-6) photons m super(-2) s super(-1), roughly 125% of the Crab Nebula flux measured by VERITAS. The light curve indicates that the observations missed the rising phase of the flare but covered a significant portion of the decaying phase. The exponential decay time was determined to be 13 + or - 4 minutes, making it one of the most rapid gamma-ray flares seen from a TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft, with a photon index of 3.6 + or - 0.4, which is in agreement with the measurement made previously by MAGIC in a lower flaring state. Contemporaneous radio observations of the source with the Very Long Baseline Array revealed the emergence of a new, superluminal component from the core around the time of the TeV gamma-ray flare, accompanied by changes in the optical polarization angle. Changes in flux also appear to have occurred at optical, UV, and GeV gamma-ray wavelengths at the time of the flare, although they are difficult to quantify precisely due to sparse coverage. A strong flare was seen at radio wavelengths roughly four months later, which might be related to the gamma-ray flaring activities. We discuss the implications of these multiwavelength results.
The high-frequency-peaked BL Lacertae object 1ES 0229+200 is a relatively distant (z = 0.1396), hard-spectrum (Gamma ~ 2.5), very-high-energy (VHE; E > 100 GeV) emitting gamma -ray blazar. VHE ...measurements of this active galactic nucleus have been used to place constraints on the intensity of the extragalactic background light and the intergalactic magnetic field (IGMF). A multi-wavelength study of this object centered around VHE observations by Very Energetic Radiation Imaging Telescope Array System (VERITAS) is presented. This study obtained, over a period of three years, an 11.7 standard deviation detection and an average integral flux F(E > 300 GeV) = (23.3 + or - 2.8 sub(stat) + or - 5.8 sub(sys)) x 10 super(-9) photons m super(-2) s super(-1), or 1.7% of the Crab Nebula's flux (assuming the Crab Nebula spectrum measured by H.E.S.S). Supporting observations from Swift and RXTE are analyzed. The Swift observations are combined with previously published Fermi observations and the VHE measurements to produce an overall spectral energy distribution which is then modeled assuming one-zone synchrotron-self-Compton emission. The chi super(2) probability of the TeV flux being constant is 1.6%. This, when considered in combination with measured variability in the X-ray band, and the demonstrated variability of many TeV blazars, suggests that the use of blazars such as 1ES 0229+200 for IGMF studies may not be straightforward and challenges models that attribute hard TeV spectra to secondary gamma -ray production along the line of sight.
We report our intensive, high angular resolution radio monitoring observations of the jet in M 87 with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from 2011 ...February to 2012 October, together with contemporaneous high-energy (100 MeV < E < 100 GeV) gamma -ray light curves obtained by the Fermi Large Area Telescope. We detected a remarkable (up to ~70%) increase of the radio flux density from the unresolved jetbase (radio core) with VERA at 22 and 43 GHz coincident with the VHE activity. These results in the radio bands strongly suggest that the VHE gamma -ray activity in 2012 originates in the jet base within 0.03 pc or 56 Schwarzschild radii (the VERA spatial resolution of 0.4 mas at 43 GHz) from the central supermassive black hole. This indicates that a new radio-emitting component was created near the black hole in the period of the VHE event, and then propagated outward with progressively decreasing synchrotron opacity.
We report the discovery of TeV gamma-ray emission from the Type Ia supernova remnant (SNR) G120.1+1.4, known as Tycho's SNR. Observations performed in the period 2008-2010 with the VERITAS ...ground-based gamma-ray observatory reveal weak emission coming from the direction of the remnant, compatible with a point source located at 00h25m27.s0, + 64?10'50'' (J2000). The TeV photon spectrum measured by VERITAS can be described with a power law dN/dE = C(E/3.42 TeV)-- Delta *G with Delta *G = 1.95 ? 0.51stat ? 0.30sys and C = (1.55 ? 0.43stat ? 0.47sys) X 10--14 cm--2 s--1 TeV--1. The integral flux above 1 TeV corresponds to ~0.9% of the steady Crab Nebula emission above the same energy, making it one of the weakest sources yet detected in TeV gamma rays. We present both leptonic and hadronic models that can describe the data. The lowest magnetic field allowed in these models is ~80 Delta *mG, which may be interpreted as evidence for magnetic field amplification.
Observations of radio halos and relics in galaxy clusters indicate efficient electron acceleration. We report here on VHE gamma-ray observations of the Coma galaxy cluster with the VERITAS array of ...imaging Cerenkov telescopes, with complementing Fermi Large Area Telescope observations at GeV energies. We use the gamma-ray upper limits to constrain cosmic rays (CRs) and magnetic fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio is constrained to be <16% from VERITAS data and <1.7% from Fermi data (averaged within the virial radius). These upper limits are starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Finally, since galaxy clusters are dark matter (DM) dominated, the VERITAS upper limits have been used to place constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the DM particles, left angle bracket sigmavright angle bracket.
TeV J2032+4130 was the first unidentified source discovered at very high energies (VHEs; E > 100 GeV), with no obvious counterpart in any other wavelength. It is also the first extended source to be ...observed in VHE gamma rays. Following its discovery, intensive observational campaigns have been carried out in all wavelengths in order to understand the nature of the object, which have met with limited success. We report here on a deep observation of TeV J2032+4130 based on 48.2 hr of data taken from 2009 to 2012 by the Very Energetic Radiation Imaging Telescope Array System experiment. The source is detected at 8.7 standard deviations (sigma) and is found to be extended and asymmetric with a width of 9'.5 + or - 1'.2 along the major axis and 4'.0 + or - 0'.5 along the minor axis. The spectrum is well described by a differential power law with an index of 2.10 + or - 0.14 sub(stat) + or - 0.21 sub(sys) and a normalization of (9.5 + or - 1.6 sub(stat) + or - 2.2 sub(sys)) x 10 super(-13) TeV super(-1) cm super(-2) s super(-1) at 1 TeV. We interpret these results in the context of multiwavelength scenarios which particularly favor the pulsar wind nebula interpretation.
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