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

TL;DR

This paper reports on a long-term observational campaign of the galaxy M87 in very high energy gamma rays during a low-emission state, providing spectral data that supports a spine layer jet emission model.

Contribution

First detailed VHE gamma-ray spectrum of M87 during a persistent low-emission state, combined with multiwavelength data, supporting a jet emission model.

Findings

No flaring activity observed during the campaign.

Spectrum between 100 GeV and 2 TeV fits a power law with photon index 2.21.

MAGIC spectrum matches Fermi-LAT data, covering four orders of magnitude.

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.