MAGIC observations of the giant radio galaxy M87 in a low-emission state between 2005 and 2007

07/2012; DOI: 10.1051/0004-6361/201117827
Source: arXiv

ABSTRACT We present the results of a long M87 monitoring campaign in very high energy
$\gamma$-rays with the MAGIC-I Cherenkov telescope. We aim to model the
persistent non-thermal jet emission by monitoring and characterizing the very
high energy $\gamma$-ray emission of M87 during a low state. A total of 150\,h
of data were taken between 2005 and 2007 with the single MAGIC-I telescope, out
of which 128.6\,h survived the data quality selection. We also collected data
in the X-ray and \textit{Fermi}--LAT bands from the literature (partially
contemporaneous). No flaring activity was found during the campaign. The source
was found to be in a persistent low-emission state, which was at a confidence
level of $7\sigma$. We present the spectrum between 100\,GeV and 2\,TeV, which
is consistent with a simple power law with a photon index $\Gamma=2.21\pm0.21$
and a flux normalization at 300\,GeV of $(7.7\pm1.3) \times 10^{-8}$ TeV$^{-1}$
s$^{-1}$ m$^{-2}$. The extrapolation of the MAGIC spectrum into the GeV energy
range matches the previously published \textit{Fermi}--LAT spectrum well,
covering a combined energy range of four orders of magnitude with the same
spectral index. We model the broad band energy spectrum with a spine layer
model, which can satisfactorily describe our data.

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    ABSTRACT: We report the Fermi Large Area Telescope (LAT) discovery of high-energy (MeV/GeV) γ-ray emission positionally consistent with the center of the radio galaxy M87, at a source significance of over 10σ in 10 months of all-sky survey data. Following the detections of Cen A and Per A, this makes M87 the third radio galaxy seen with the LAT. The faint point-like γ-ray source has a >100 MeV flux of 2.45 (±0.63) × 10–8 photons cm–2 s–1 (photon index = 2.26 ± 0.13) with no significant variability detected within the LAT observation. This flux is comparable with the previous EGRET upper limit (<2.18 × 10–8 photons cm–2 s–1, 2σ), thus there is no evidence for a significant MeV/GeV flare on decade timescales. Contemporaneous Chandra and Very Long Baseline Array data indicate low activity in the unresolved X-ray and radio core relative to previous observations, suggesting M87 is in a quiescent overall level over the first year of Fermi-LAT observations. The LAT γ-ray spectrum is modeled as synchrotron self-Compton (SSC) emission from the electron population producing the radio-to-X-ray emission in the core. The resultant SSC spectrum extrapolates smoothly from the LAT band to the historical-minimum TeV emission. Alternative models for the core and possible contributions from the kiloparsec-scale jet in M87 are considered, and cannot be excluded.
    The Astrophysical Journal 11/2009; · 6.28 Impact Factor
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    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 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.
    The Astrophysical Journal 06/2010; 716(1). · 6.28 Impact Factor
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    ABSTRACT: The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10(12) electron volts and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.
    Science 08/2009; 325(5939):444-8. · 31.48 Impact Factor


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