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

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.

💡 Research Summary

The paper reports on a dedicated long‑term monitoring campaign of the nearby giant radio galaxy M87 with the single‑dish MAGIC‑I Imaging Atmospheric Cherenkov Telescope, carried out between 2005 and 2007. A total of 150 h of observations were taken, of which 128.6 h survived strict quality cuts (weather, hardware stability, and trigger performance). The data were processed with the standard MAGIC analysis chain: image cleaning, Hillas parameter extraction, γ/hadron separation using a Random Forest classifier, and energy reconstruction calibrated with Monte‑Carlo simulations.

No significant flaring activity was found during the entire campaign. The nightly fluxes are consistent with a steady, low‑state emission at a level of ≈2.5 × 10⁻¹² cm⁻² s⁻¹ above 300 GeV, i.e. an order of magnitude below the bright flares reported in later years. The detection significance of the integrated signal reaches 7 σ, confirming that M87 was in a persistent low‑emission state throughout the three‑year period.

A differential energy spectrum was derived in the 100 GeV–2 TeV range. The spectrum is well described by a simple power law:

dN/dE = (7.7 ± 1.3) × 10⁻⁸ (E/300 GeV)⁻²·²¹ TeV⁻¹ s⁻¹ m⁻²,

with a photon index Γ = 2.21 ± 0.21. The χ² per degree of freedom is close to unity, indicating an excellent fit. When extrapolated down to the GeV band, the MAGIC spectrum aligns perfectly with the contemporaneous Fermi‑LAT measurement (0.1–100 GeV) published earlier, which also shows a photon index of ≈2.2. Consequently, the combined GeV–TeV spectral energy distribution (SED) of M87 spans four orders of magnitude in energy with a single, unbroken power‑law shape.

To place the VHE results in a broader context, the authors collected archival X‑ray (Chandra) and radio (VLBI) fluxes that are either simultaneous or closely spaced in time. The X‑ray core flux (2–10 keV) remained stable at (1.1 ± 0.2) × 10⁻¹² erg cm⁻² s⁻¹, and no correlated variability with the VHE band was observed. This lack of variability suggests that the same particle population is responsible for the emission across the entire high‑energy range.

The broadband SED was modeled using a “spine‑layer” jet structure, a two‑component scenario in which a fast inner spine (bulk Lorentz factor Γ_spine ≈ 15, magnetic field B_spine ≈ 0.1 G) is surrounded by a slower outer layer (Γ_layer ≈ 3, B_layer ≈ 0.3 G). Relativistic electrons in the spine emit synchrotron radiation that serves as seed photons for inverse‑Compton scattering by electrons in the layer (external Compton). Both electron populations are described by power‑law energy distributions; the spine has an index α ≈ 2.2, while the layer is slightly steeper (α ≈ 2.5). The model reproduces the MAGIC‑I VHE spectrum, the Fermi‑LAT GeV spectrum, and the X‑ray and radio data simultaneously. The key parameters governing the VHE output are the magnetic field strength and the maximum electron energy in the layer (E_max ≈ 10 TeV).

The authors discuss the implications of detecting a steady VHE component in the absence of flares. The spine‑layer configuration naturally explains how a relatively modest jet can sustain a persistent γ‑ray output: the continuous interaction between the two flow components provides a steady supply of seed photons for inverse‑Compton scattering, while the layer’s lower bulk speed reduces Doppler de‑boosting, allowing the VHE flux to be observable from Earth. This result challenges the view that VHE emission from radio galaxies is exclusively associated with transient events and supports the idea that complex internal jet structures can maintain high‑energy radiation over long periods.

In conclusion, the MAGIC‑I observations have firmly established a low‑state VHE γ‑ray signal from M87 with high statistical significance, measured a simple power‑law spectrum extending from 100 GeV to 2 TeV, and demonstrated that a spine‑layer jet model can successfully account for the entire multi‑wavelength SED. The study provides a crucial benchmark for theoretical models of particle acceleration and radiation in misaligned active galactic nuclei and highlights the importance of long‑term monitoring to capture the full dynamical range of these powerful extragalactic jets.