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Title: VERITAS 2008-2009 MONITORING OF THE VARIABLE GAMMA-RAY SOURCE M 87

Abstract

M 87 is a nearby radio galaxy that is detected at energies ranging from radio to very high energy (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 M 87 in 2008 and 2009. In the 2008 observing season, VERITAS detected an excess with a statistical significance of 7.2 standard deviations ({sigma}) from M 87 during a joint multi-wavelength monitoring campaign conducted by three major VHE experiments along with the Chandra X-ray Observatory. In 2008 February, VERITAS observed a VHE flare from M 87 occurring over a 4 day timespan. The peak nightly flux above 250 GeV was (1.14 {+-} 0.26) x 10{sup -11} cm{sup -2} s{sup -1}, which corresponded to 7.7% of the Crab Nebula flux. M 87 was marginally detected before this 4 day flare period, and was not detected afterward. Spectral analysis of the VERITAS observations showed no significant change in themore » 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 M 87 at a historical maximum, while the flux from the nearby knot HST-1 remained quiescent. Acciari et al. presented the 2008 contemporaneous VHE gamma-ray, Chandra X-ray, and Very Long Baseline Array 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 M 87 and marginally detected a 4.2{sigma} excess corresponding to a flux of {approx}1% of the Crab Nebula. No VHE flaring activity was observed in 2009.« less

Authors:
;  [1];  [2]; ;  [3];  [4]; ; ; ;  [5];  [6];  [7];  [8];  [9];  [10];  [11];  [12]; ;  [13];  [14]
  1. Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645 (United States)
  2. Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027 (United States)
  3. Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States)
  4. Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064 (United States)
  5. Department of Physics, Washington University, St. Louis, MO 63130 (United States)
  6. Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716 (United States)
  7. School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT (United Kingdom)
  8. Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 (United States)
  9. School of Physics, University College Dublin, Belfield, Dublin 4 (Ireland)
  10. School of Physics, National University of Ireland Galway, University Road, Galway (Ireland)
  11. Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605 (United States)
  12. Physics Department, McGill University, Montreal, QC H3A 2T8 (Canada)
  13. Department of Physics, Purdue University, West Lafayette, IN 47907 (United States)
  14. Department of Physics, Grinnell College, Grinnell, IA 50112-1690 (United States)
Publication Date:
OSTI Identifier:
21451112
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 716; Journal Issue: 1; Other Information: DOI: 10.1088/0004-637X/716/1/819
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COSMIC GAMMA SOURCES; CRAB NEBULA; FLARING; GAMMA RADIATION; PHOTON EMISSION; PHOTONS; RADIO GALAXIES; WAVELENGTHS; BOSONS; COSMIC RADIO SOURCES; COSMIC RAY SOURCES; ELECTROMAGNETIC RADIATION; ELEMENTARY PARTICLES; EMISSION; GALAXIES; IONIZING RADIATIONS; MASSLESS PARTICLES; NEBULAE; RADIATIONS; SUPERNOVA REMNANTS

Citation Formats

Acciari, V. A., Benbow, W., Aliu, E., Arlen, T., Chow, Y. C., Aune, T., Beilicke, M., Buckley, J. H., Bugaev, V., Dickherber, R., Boltuch, D., Bradbury, S. M., Byrum, K., Cannon, A., Cesarini, A., Ciupik, L., Cogan, P., Cui, W., Finley, J. P., and Duke, C., E-mail: cmhui@physics.utah.ed. VERITAS 2008-2009 MONITORING OF THE VARIABLE GAMMA-RAY SOURCE M 87. United States: N. p., 2010. Web. doi:10.1088/0004-637X/716/1/819.
Acciari, V. A., Benbow, W., Aliu, E., Arlen, T., Chow, Y. C., Aune, T., Beilicke, M., Buckley, J. H., Bugaev, V., Dickherber, R., Boltuch, D., Bradbury, S. M., Byrum, K., Cannon, A., Cesarini, A., Ciupik, L., Cogan, P., Cui, W., Finley, J. P., & Duke, C., E-mail: cmhui@physics.utah.ed. VERITAS 2008-2009 MONITORING OF THE VARIABLE GAMMA-RAY SOURCE M 87. United States. doi:10.1088/0004-637X/716/1/819.
Acciari, V. A., Benbow, W., Aliu, E., Arlen, T., Chow, Y. C., Aune, T., Beilicke, M., Buckley, J. H., Bugaev, V., Dickherber, R., Boltuch, D., Bradbury, S. M., Byrum, K., Cannon, A., Cesarini, A., Ciupik, L., Cogan, P., Cui, W., Finley, J. P., and Duke, C., E-mail: cmhui@physics.utah.ed. Thu . "VERITAS 2008-2009 MONITORING OF THE VARIABLE GAMMA-RAY SOURCE M 87". United States. doi:10.1088/0004-637X/716/1/819.
@article{osti_21451112,
title = {VERITAS 2008-2009 MONITORING OF THE VARIABLE GAMMA-RAY SOURCE M 87},
author = {Acciari, V. A. and Benbow, W. and Aliu, E. and Arlen, T. and Chow, Y. C. and Aune, T. and Beilicke, M. and Buckley, J. H. and Bugaev, V. and Dickherber, R. and Boltuch, D. and Bradbury, S. M. and Byrum, K. and Cannon, A. and Cesarini, A. and Ciupik, L. and Cogan, P. and Cui, W. and Finley, J. P. and Duke, C., E-mail: cmhui@physics.utah.ed},
abstractNote = {M 87 is a nearby radio galaxy that is detected at energies ranging from radio to very high energy (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 M 87 in 2008 and 2009. In the 2008 observing season, VERITAS detected an excess with a statistical significance of 7.2 standard deviations ({sigma}) from M 87 during a joint multi-wavelength monitoring campaign conducted by three major VHE experiments along with the Chandra X-ray Observatory. In 2008 February, VERITAS observed a VHE flare from M 87 occurring over a 4 day timespan. The peak nightly flux above 250 GeV was (1.14 {+-} 0.26) x 10{sup -11} cm{sup -2} s{sup -1}, which corresponded to 7.7% of the Crab Nebula flux. M 87 was marginally detected before this 4 day flare period, and was not detected afterward. 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 M 87 at a historical maximum, while the flux from the nearby knot HST-1 remained quiescent. Acciari et al. presented the 2008 contemporaneous VHE gamma-ray, Chandra X-ray, and Very Long Baseline Array 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 M 87 and marginally detected a 4.2{sigma} excess corresponding to a flux of {approx}1% of the Crab Nebula. No VHE flaring activity was observed in 2009.},
doi = {10.1088/0004-637X/716/1/819},
journal = {Astrophysical Journal},
number = 1,
volume = 716,
place = {United States},
year = {Thu Jun 10 00:00:00 EDT 2010},
month = {Thu Jun 10 00:00:00 EDT 2010}
}
  • The giant elliptical radio galaxy M 87 is the closest known extragalactic object emitting very-high-energy (VHE){gamma}-rays. The prominent jet of M 87 was extensively studied throughout the electromagnetic spectrum in the past revealing a complex structure resolved at radio, optical and X-ray energies. Knots in the jet indicate active regions, possibly associated with particle acceleration to ultra-relativistic energies. However, the origin of the measured VHE {gamma}-rays is still unknown. No clear correlation of the VHE {gamma}-rays with other wavelengths was found so far, whereas the size of the VHE emitting region is strongly constrained by the detection of variability onmore » time-scales of days in 2005 and 2008. In a joint effort, H.E.S.S., MAGIC and VERITAS performed an intensive, coordinated monitoring campaign on M 87 in 2008 with a total coverage of >120 h of observation time. The motivation, coordination, results and implications for future campaigns are discussed.« less
  • 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 sourcemore » is detected at 8.7 standard deviations (σ) and is found to be extended and asymmetric with a width of 9.'5 ± 1.'2 along the major axis and 4.'0 ± 0.'5 along the minor axis. The spectrum is well described by a differential power law with an index of 2.10 ± 0.14{sub stat} ± 0.21{sub sys} and a normalization of (9.5 ± 1.6{sub stat} ± 2.2{sub sys}) × 10{sup –13} TeV{sup –1} cm{sup –2} s{sup –1} at 1 TeV. We interpret these results in the context of multiwavelength scenarios which particularly favor the pulsar wind nebula interpretation.« less
  • VERITAS has been monitoring the very-high-energy (VHE; > 100 GeV) gamma-ray activity of the radio galaxy M 87 since 2007. During 2008, flaring activity on a timescale of a few days was observed with a peak flux of (0.70 {+-} 0.16) Multiplication-Sign 10{sup -11} cm{sup -2} s{sup -1} at energies above 350 GeV. In 2010 April, VERITAS detected a flare from M 87 with peak flux of (2.71 {+-} 0.68) Multiplication-Sign 10{sup -11} cm{sup -2} s{sup -1} for E > 350 GeV. The source was observed for six consecutive nights during the flare, resulting in a total of 21 hrmore » 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{sup +0.22}{sub -0.15} days. The shortest detected exponential rise time is three times as long, at 2.87{sup +1.65}{sub -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 M 87 and the constraints they provide on the physical size of the emitting region.« less
  • The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 - 6) Multiplication-Sign 10{sup 9} M{sub Sun }) provides a unique opportunity to investigate the origin of very high energy (VHE; E > 100 GeV) {gamma}-ray emission generated in relativistic outflows and the surroundings of supermassive black holes. M 87 has been established as a VHE {gamma}-ray emitter since 2006. The VHE {gamma}-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC andmore » VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz Very Long Baseline Array, VLBA). The excellent sampling of the VHE {gamma}-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times of {tau}{sup rise}{sub d} = (1.69 {+-} 0.30) days and {tau}{sup decay}{sub d} = (0.611 {+-} 0.080) days, respectively. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales ({approx}day), peak fluxes ({Phi}{sub >0.35TeV} {approx_equal} (1-3) Multiplication-Sign 10{sup -11} photons cm{sup -2} s{sup -1}), and VHE spectra. VLBA radio observations of 43 GHz of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken {approx}3 days after the peak of the VHE {gamma}-ray emission reveal an enhanced flux from the core (flux increased by factor {approx}2; variability timescale <2 days). The long-term (2001-2010) multi-wavelength (MWL) light curve of M 87, spanning from radio to VHE and including data from Hubble Space Telescope, Liverpool Telescope, Very Large Array, and European VLBI Network, is used to further investigate the origin of the VHE {gamma}-ray emission. No unique, common MWL signature of the three VHE flares has been identified. In the outer kiloparsec jet region, in particular in HST-1, no enhanced MWL activity was detected in 2008 and 2010, disfavoring it as the origin of the VHE flares during these years. Shortly after two of the three flares (2008 and 2010), the X-ray core was observed to be at a higher flux level than its characteristic range (determined from more than 60 monitoring observations: 2002-2009). In 2005, the strong flux dominance of HST-1 could have suppressed the detection of such a feature. Published models for VHE {gamma}-ray emission from M 87 are reviewed in the light of the new data.« less