NGC1275 - Upper limits on VHE emission from a radio galaxy PDF Print

 

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VERITAS upper limit on the differential flux at the decorrelation energy 338 GeV. See text and Figure 3 below for more details.

VERITAS upper limit on the very high energy emission from the radio galaxy NGC1275

 

 

Reference:  V. A. Acciari et al. (The VERITAS Collaboration), Astrophysical Journal, 706: L275, 2009

Full text version

ArXiv version: ArXiV:0911.0740

Contact person: Nicola Galante

 

NGC 1275 (Perseus A, 3C 84) is a radio galaxy located in the center of the Perseus cluster and is one of the most unusual early-type galaxies in the nearby universe (z = 0.018). Its radio emission is core dominated, but it also has strong emission lines. In addition, the emission line system shows a double structure, corresponding to both a high-velocity and a low-velocity system. The puzzling nature of NGC 1275 makes it difficult to definitively classify it in a standard AGN subclass. Its spectral energy distribution (SED) extends from radio to HE gamma-rays. In particular, the Fermi gamma-ray Space telescope discovered in 2008 and reported a gamma-ray flux described by a power law up to an energy of 25 GeV. VHE (E>100 GeV) observation by a sensitive instrument like VERITAS can provide crucial information about the highest energy accelerated particles in radio-galaxy jets.

VERITAS observed the core region of NGC 1275 for about 8 hr good quality data between 2009 January 15 and February 26 at an average zenith angle of 30 degrees. No significant excess is detected by VERITAS in the NGC 1275 data sample. A 99% confidence level upper limit is calculated using the method described by Rolke et al. (2005) and assuming a Gaussian background. A differential flux upper limit of 2.1% of the Crab nebula flux level is calculated at the decorrelation energy Edecorr = 340 GeV. We define the decorrelation energy as the energy that minimizes the effect of the photon index choice on the flux upper limit calculation.

Since the Fermi gamma-ray Space Telescope operates in survey mode, contemporaneous data from the Fermi-LAT exist. Simultaneous Fermi-LAT data to the VERITAS observational campaign on NGC 1275 show a 14-σ significance detection at the source location, with a spectral energy distribution described by the power law

dN/dE = (1.74 ± 0.45) × 10-9 (E/100 MeV)(-2.15±0.11) cm-2 s-1 MeV-1

The VERITAS upper limit is clearly incompatible with a power-law extrapolation of the Fermi-LAT measured spectrum, with a normalized χ2 probability P(χ2norm) = 3.6×10-4. The VERITAS upper limit together with the Fermi-LAT measured SED shows for the first time a HE-VHE emission from a radio galaxy that does not follow a power law. This has implications in the expected electron-positron energy distribution in the radio-galaxy jet. Three possible scenarios are considered to explain the deviation from the power-law regime, all under the assumption of a homogeneus single-zone relativistic jet in the radio galaxy:

  •  An electron-positron energy distribution in the relativistic jet that follows a power law with a rather sharp cutoff. In such a case, the expected γ-ray energy disctribution would follow a power law with an exponential cutoff. In such a case, the VERITAS upper limit would be compatible with the Fermi-LAT data with an exponential cutoff at about 20 GeV.
  • An electron-positron energy distribution in the relativistic jet that follows a power law with an exponential cutoff. In such a case, the expected γ-ray energy distribution would follow a power law with a sub-exponential cutoff. In such a case, the VERITAS upper limit would be compatible with the Fermi-LAT data with a sub-exponential cutoff at about 120 GeV.
  • An electron-positron energy distribution in the relativistic jet produced by a constant injection of particles following a power law but with a constant and energy-independent leakage of particles. In this case, the resulting γ-ray energy distribution would be described by two smoothly connected power laws. In this case, the VERITAS upper limit would be compatible with the Fermi-LAT data with a break energy of about 16 GeV.

The figure above shows the VERITAS flux upper limit together with the Fermi-LAT data and the fits under
the three different assumptions.

 

 

 

Figures from paper (click to get full size image): 

 

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Figure 2: Decorrelation energy for the standard-cuts analysis on VERITAS data. Three different photon indices, -2.17, -3.0 and -4.0, are assumed for the power law used to calculate the flux upper limit. The central value of the interval where the three normalized power laws intersect is taken as decorrelation energy. The dependency of the differential flux upper limit on the assumed photon index is strongly reduced at the decorrelation energy. Assuming a photon index -2.5, the effect of the photon index change within the values here considered at the decorrelation energy is about 15%.
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Figure 3 (shown above): VERITAS upper limit on the differential flux at the decorrelation energy 338 GeV. The solid circles with error bars are the measurement by the Fermi gamma-ray space telescope during the VERITAS observation campaign. Empty circles with error bars are the measurement presented in Abdo, A. A., et al. 2009a, ApJ, 699, 31 from the energy-binned analysis. The solid line is the power-law fit to the Fermi data. The dashed line is the extrapolation of the power-law. The dotted-dashed line is the fit of a power law with an exponential cutoff at 18 GeV. The double-dotted dashed line is the fit of a power law with a sub-exponential cutoff at 120 GeV and the dotted line is the smooth broken power law fit of a break energy at 16 GeV. All fits are done on the Fermi data analyzed in this work (solid circles).

 

Last Updated on Thursday, 27 May 2010 02:26
 

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