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VERITAS Observations of day-scale flaring of M87 in 2010 April  [PDF]
E. Aliu,T. Arlen,T. Aune,M. Beilicke,W. Benbow,A. Bouvier,S. M. Bradbury,J. H. Buckley,V. Bugaev,K. Byrum,A. Cannon,A. Cesarini,L. Ciupik,E. Collins-Hughes,M. P. Connolly,W. Cui,R. Dickherber,C. Duke,M. Errando,A. Falcone,J. P. Finley,G. Finnegan,L. Fortson,A. Furniss,N. Galante,D. Gall,S. Godambe,S. Griffin,J. Grube,R. Guenette,G. Gyuk,D. Hanna,J. Holder,H. Huan,G. Hughes,C. M. Hui,T. B. Humensky,A. Imran,P. Kaaret,N. Karlsson,M. Kertzman,D. Kieda,H. Krawczynski,F. Krennrich,M. J. Lang,S. LeBohec,A. S Madhavan,G. Maier,P. Majumdar,S. McArthur,A. McCann,P. Moriarty,R. Mukherjee,P. D Nunez,R. A. Ong,M. Orr,A. N. Otte,N. Park,J. S. Perkins,A. Pichel,M. Pohl,H. Prokoph,J. Quinn,K. Ragan,L. C. Reyes,P. T. Reynolds,E. Roache,H. J. Rose,J. Ruppel,D. B. Saxon,M. Schroedter,G. H. Sembroski,G. D. ?entürk,C. Skole,D. Staszak,G. Te?i?,M. Theiling,S. Thibadeau,K. Tsurusaki,J. Tyler,A. Varlotta,V. V. Vassiliev,S. Vincent,M. Vivier,S. P. Wakely,J. E. Ward,T. C. Weekes,A. Weinstein,T. Weisgarber,D. A. Williams,B. Zitzer
Physics , 2011, DOI: 10.1088/0004-637X/746/2/141
Abstract: VERITAS has been monitoring the very-high-energy (VHE; >100GeV) gamma-ray activity of the radio galaxy M87 since 2007. During 2008, flaring activity on a timescale of a few days was observed with a peak flux of (0.70 +- 0.16) X 10^{-11} cm^{-2} s^{-1} at energies above 350GeV. In 2010 April, VERITAS detected a flare from M87 with peak flux of (2.71 +- 0.68) X 10^{-11} cm^{-2} s^{-1} for E>350GeV. The source was observed for six consecutive nights during the flare, resulting in a total of 21 hr of good quality data. The most rapid flux variation occurred on the trailing edge of the flare with an exponential flux decay time of 0.90^{+0.22}_{-0.15} days. The shortest detected exponential rise time is three times as long, at 2.87^{+1.65}_{-0.99} days. The quality of the data sample is such that spectral analysis can be performed for three periods: rising flux, peak flux, and falling flux. The spectra obtained are consistent with power-law forms. The spectral index at the peak of the flare is equal to 2.19 +- 0.07. There is some indication that the spectrum is softer in the falling phase of the flare than the peak phase, with a confidence level corresponding to 3.6 standard deviations. We discuss the implications of these results for the acceleration and cooling rates of VHE electrons in M87 and the constraints they provide on the physical size of the emitting region.
VERITAS Observations of TeV Binaries  [PDF]
Andrew W. Smith for the VERITAS Collaboration
Physics , 2013,
Abstract: Since the commissioning of the array in Spring 2007, the VERITAS array (sensitive in the 0.1-50 TeV energy range) has acquired over 300 hours of observations investigating the TeV emission from X-ray binary star systems, in particular focusing on the known TeV binary targets LS I +61 303 and HESS J0632+057. Both TeV binaries have been monitored by VERITAS for several years and the resulting dataset is continuing to yield important results in the characterization of these poorly understood systems. We present these results, as well as the contemporaneous observations of these sources taken with Fermi-LAT and Swift-XRT. In the case of LS I +61 303, simultaneous observations taken with VERITAS and Fermi-LAT reveal a break in emission in the 10-200 GeV range. For HESS J0632 057, the extended VERITAS observations have allowed for the first identification of a binary system through TeV gamma-ray observations.
VERITAS Observations of Gamma-Ray Bursts Detected by Swift  [PDF]
VERITAS Collaboration,V. A. Acciari,E. Aliu,T. Arlen,T. Aune,M. Beilicke,W. Benbow,S. M. Bradbury,J. H. Buckley,V. Bugaev,K. Byrum,A. Cannon,A. Cesarini,J. L. Christiansen,L. Ciupik,E. Collins-Hughes,M. P. Connolly,W. Cui,C. Duke,M. Errando,A. Falcone,J. P. Finley,G. Finnegan,L. Fortson,A. Furniss,N. Galante,D. Gall,S. Godambe,S. Griffin,J. Grube,R. Guenette,G. Gyuk,D. Hanna,J. Holder,G. Hughes,C. M. Hui,T. B. Humensky,D. J. Jackson,P. Kaaret,N. Karlsson,M. Kertzman,D. Kieda,H. Krawczynski,F. Krennrich,M. J. Lang,A. S Madhavan,G. Maier,S. McArthur,A. McCann,P. Moriarty,M. D. Newbold,R. A. Ong,M. Orr,A. N. Otte,N. Park,J. S. Perkins,M. Pohl,H. Prokoph,J. Quinn,K. Ragan,L. C. Reyes,P. T. Reynolds,E. Roache,H. J. Rose,J. Ruppel,D. B. Saxon,M. Schroedter,G. H. Sembroski,G. D. ?entürk,A. W. Smith,D. Staszak,S. P. Swordy,G. Te?i?,M. Theiling,S. Thibadeau,K. Tsurusaki,A. Varlotta,V. V. Vassiliev,S. Vincent,M. Vivier,S. P. Wakely,J. E. Ward,T. C. Weekes,A. Weinstein,T. Weisgarber,D. A. Williams,M. Wood
Physics , 2011, DOI: 10.1088/0004-637X/743/1/62
Abstract: We present the results of sixteen Swift-triggered GRB follow-up observations taken with the VERITAS telescope array from January, 2007 to June, 2009. The median energy threshold and response time of these observations was 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 time scale 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 (LAT) on-board the Fermi satellite. No significant 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 (EBL) 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.
MAGIC observations of the giant radio galaxy M87 in a low emission state between 2005 and 2007  [PDF]
Karsten Berger,Dijana Dominis Prester,Fabrizio Tavecchio,Tomislav Terzi?,for the MAGIC Collaboration
Physics , 2011, DOI: 10.7529/ICRC2011/V08/1093
Abstract: We present the results of a long M87 monitoring campaign in very high energy $\gamma$-rays with the MAGIC-I Cherenkov telescope. A total of 150 hours of data was gathered between 2005 and 2007. No flaring activity was found during that time. Nevertheless, we have found an apparently steady and weak signal at the level of $7\sigma$. We present the spectrum between 100 GeV and 2 TeV, which is consistent with a simple power law with a spectral index $-2.21\pm0.21$ and a flux normalization (at 300 GeV) of $5.4\pm1.1 \times 10^{-8} \frac{1}{\mathrm{TeV s m}^{2}}$. It complements well with the previously published Fermi spectrum, covering an energy range of four orders of magnitude without apparent change in the spectral index.
VERITAS Observations of Blazars  [PDF]
Wystan Benbow
Physics , 2010,
Abstract: 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, in most cases with the deepest-ever VHE exposure. These observations have resulted in the detection of VHE gamma-rays from 16 AGN (15 blazars), including 8 for the first time at these energies. The VERITAS blazar KSP is summarized in this proceeding and selected results are presented.
Highlights of Recent Multiwavelength Observations of VHE Blazars with VERITAS  [PDF]
J. Grube,for the VERITAS collaboration
Physics , 2009,
Abstract: 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.
VERITAS Observations of Magnetars  [PDF]
R. Guenette,for the VERITAS Collaboration
Physics , 2009,
Abstract: Magnetars are rotating neutron stars with extremely strong magnetic fields (~ 10^14-10^15 G). X-ray and soft gamma-ray observations have revealed the existence of non-thermal particle populations which may suggest emission of very high energy photons. VERITAS, an array of four 12m imaging atmospheric Cherenkov telescopes, is designed to observe gamma-ray emission between 100 GeV and 30 TeV. Here we present the results of VERITAS observations of two magnetars, 4U 0142+61 and 1E 2259+586.
Search for Pulsed Emission in Archival VERITAS Data  [PDF]
Avery Archer,for the VERITAS Collaboration
Physics , 2015,
Abstract: Since the 2011 VERITAS discovery of very high energy (VHE; E>100 GeV) gamma rays from the Crab pulsar, there has been concerted effort by the gamma-ray astrophysics community to detect other pulsars in the VHE band in order to place better constraints on emission models. Pulsar modelling demonstrates that much of the magnetosphere is opaque to VHE photons, limiting emission regions to the outer magnetosphere or beyond the light cylinder. The locations of 19 known pulsars have been observed by VERITAS since full observations began in 2007 with 11 locations having more than 20 hours of observations. Observations of VHE emission from more sources could provide key data to help constrain current models of emission location and mechanisms. We present the status of the ongoing VERITAS program searching for pulsed emission in archival data.
VERITAS 2008 - 2009 monitoring of the variable gamma-ray source M87  [PDF]
The VERITAS Collaboration,V. A. Acciari,E. Aliu,T. Arlen,T. Aune,M. Beilicke,W. Benbow,D. Boltuch,S. M. Bradbury,J. H. Buckley,V. Bugaev,K. Byrum,A. Cannon,A. Cesarini,Y. C. Chow,L. Ciupik,P. Cogan,W. Cui,R. Dickherber,C. Duke,J. P. Finley,G. Finnegan,P. Fortin,L. Fortson,A. Furniss,N. Galante,D. Gall,G. H. Gillanders,S. Godambe,J. Grube,R. Guenette,G. Gyuk,D. Hanna,J. Holder,C. M. Hui,T. B. Humensky,A. Imran,P. Kaaret,N. Karlsson,M. Kertzman,D. Kieda,A. Konopelko,H. Krawczynski,F. Krennrich,M. J. Lang,S. LeBohec,G. Maier,S. McArthur,A. McCann,M. McCutcheon,J. Millis,P. Moriarty,R. A. Ong,A. N. Otte,D. Pandel,J. S. Perkins,A. M. J. Quinn,K. Ragan,L. C. Reyes,P. T. Reynolds,E. Roache,H. J. Rose,A. C. Rovero,M. Schroedter,G. H. Sembroski,G. Demet Senturk,A. W. Smith,D. Steele,S. P. Swordy,M. Theiling,S. Thibadeau,A. Varlotta,S. Vincent,R. G. Wagner,S. P. Wakely,J. E. Ward,T. C. Weekes,A. Weinstein,T. Weisgarber,D. A. Williams,S. Wissel,M. Wood,B. Zitzer Chandra M87 Monitoring,:,D. E. Harris,F. Massaro
Physics , 2010, DOI: 10.1088/0004-637X/716/1/819
Abstract: M87 is a nearby radio galaxy that is detected at energies ranging from radio to VHE gamma-rays. Its proximity and its jet, misaligned from our line-of-sight, enable detailed morphological studies and extensive modeling at radio, optical, and X-ray energies. Flaring activity was observed at all energies, and multi-wavelength correlations would help clarify the origin of the VHE emission. In this paper, we describe a detailed temporal and spectral analysis of the VERITAS VHE gamma-ray observations of M87 in 2008 and 2009. In the 2008 observing season, VERITAS detected an excess with a statistical significance of 7.2 sigma from M87 during a joint multi-wavelength monitoring campaign conducted by three major VHE experiments along with the Chandra X-ray Observatory. In February 2008, VERITAS observed a VHE flare from M87 occurring over a 4-day timespan. The peak nightly flux above 250GeV was 7.7% of the Crab Nebula flux. M87 was marginally detected before this 4-day flare period, and was not detected afterwards. Spectral analysis of the VERITAS observations showed no significant change in the photon index between the flare and pre-flare states. Shortly after the VHE flare seen by VERITAS, the Chandra X-ray Observatory detected the flux from the core of M87 at a historical maximum, while the flux from the nearby knot HST-1 remained quiescent. Acciari et al. (2009) presented the 2008 contemporaneous VHE gamma-ray, Chandra X-ray, and VLBA radio observations which suggest the core as the most likely source of VHE emission, in contrast to the 2005 VHE flare that was simultaneous with an X-ray flare in the HST-1 knot. In 2009, VERITAS continued its monitoring of M87 and marginally detected a 4.2 sigma excess corresponding to a flux of ~1% of the Crab Nebula. No VHE flaring activity was observed in 2009.
Multiwavelength Observations of LS I +61 303 with VERITAS, Swift and RXTE  [PDF]
VERITAS Collaboration,V. A. Acciari,E. Aliu,T. Arlen,M. Bautista,M. Beilicke,W. Benbow,M. Bottcher,S. M. Bradbury,V. Bugaev,Y. Butt,Y. Butt,K. Byrum0,A. Cannon,A. Cesarini,Y. C. Chow,L. Ciupik,P. Cogan,P. Colin,W. Cui,M. Daniel,R. Dickherber,T. Ergin,A. Falcone,S. J. Fegan,J. P. Finley,P. Fortin,L. Fortson,A. Furniss,D. Gall,G. H. Gillanders,J. Grube,R. Guenette,G. Gyuk,D. Hanna,E. Hays,J. Holder,D. Horan,C. M. Hui,T. B. Humensky,P. Kaaret,N. Karlsson,D. Kieda,J. Kildea,A. Konopelko,H. Krawczynski,F. Krennrich,M. J. Lang,S. LeBohec,G. Maier,A. McCann,M. McCutcheon,J. Millis,P. Moriarty,R. Mukherjee,T. Nagai,R. A. Ong,A. N. Otte,D. Pandel,J. S. Perkins,J. S. Perkins,M. Pohl,J. Quinn,K. Ragan,L. C. Reyes,P. T. Reynolds,E. Roache,H. J. Rose,M. Schroedter,G. H. Sembroski,A. W. Smith,D. Steele,M. Stroh,S. Swordy,M. Theiling,J. A. Toner,A. Varlotta,V. V. Vassiliev,R. G. Wagner,S. P. Wakely,J. E. Ward,T. C. Weekes,A. Weinsteiny,R. J. White,D. A. Williams,S. Wissely,M. Woody,B. Zitzer
Physics , 2009, DOI: 10.1088/0004-637X/700/2/1034
Abstract: We present results from a long-term monitoring campaign on the TeV binary LSI +61 303 with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between September 2006 and February 2008. The binary was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8 sigma (2006/2007) and 7.3 sigma (2007/2008) significance level for emission above 500 GeV. The source was detected during active periods with flux values ranging from 5 to 20% of the Crab Nebula, varying over the course of a single orbital cycle. Additionally, the observations conducted in the 2007-2008 observing season show marginal evidence (at the 3.6 sigma significance level) for TeV emission outside of the apastron passage of the compact object around the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show large variability with flux values typically varying between 0.5 and 3.0*10^-11 ergs cm^-2 s^-1 over a single orbital cycle. The contemporaneous X-ray and TeV data are examined and it is shown that the TeV sampling is not dense enough to detect a correlation between the two bands.
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