MAGIC observations of the distant quasar 3C279 during an optical outburst in 2007

The flat-spectrum radio-quasar 3C279 (z=0.536) is the most distant object detected at very high energy (VHE) gamma-rays. It is thus an important beacon for the study of the interaction of the VHE gamma-rays with the Extra-galactic Background Light (EBL). Previous observations by EGRET showed a highly variable flux that can differ up to a factor of 100. In this paper results from an observation campaign with the MAGIC telescope during an optical flare in January 2007 will be presented and previous MAGIC results from 2006 will be summarized.

💡 Research Summary

The paper reports on very‑high‑energy (VHE; >100 GeV) gamma‑ray observations of the flat‑spectrum radio quasar 3C 279 (z = 0.536) performed with the MAGIC‑I imaging atmospheric Cherenkov telescope. 3C 279 is the most distant VHE source known and the only flat‑spectrum radio quasar (FSRQ) detected at these energies, making it a crucial probe of the extragalactic background light (EBL) attenuation.

Observations were carried out in two campaigns: February 2006 and January 2007. Both campaigns were triggered by strong optical outbursts monitored in the R‑band by the Tuorla Observatory and the KVA telescope. The 2006 data were analyzed with the standard MAGIC pipeline, while the 2007 data benefited from an upgraded 2 GHz read‑out and a timing‑based analysis that improves sensitivity near the threshold. All data were taken in On/Off mode to allow reliable background subtraction.

In 2006, a clear VHE signal was detected on the night of 23 February with a pre‑trial significance of 6.2 σ (5.8 σ after accounting for ten trial nights). The differential spectrum is well described by a power law with photon index α = 4.1 ± 0.7 (stat) ± 0.2 (syst). The integrated flux above 100 GeV for that night is (5.15 ± 0.82 stat ± 1.50 syst) × 10⁻¹⁰ cm⁻² s⁻¹. A χ² test shows the probability of zero flux over the whole 10‑night run to be 2.3 × 10⁻⁷ (≈5 σ).

In 2007, a VHE flare was discovered on 16 January with a pre‑trial significance of 5.6 σ (5.2 σ after trials). This flare occurred a few days after the peak of the optical outburst, suggesting a possible lag between the low‑energy and VHE emission. The analysis of the 2007 spectrum is still ongoing, but the preliminary light curve indicates a short‑duration flare, consistent with the highly variable nature of 3C 279 in the VHE band.

To interpret the observed spectra, the authors de‑absorbed the data using two extreme EBL models: a low‑density model (Primack et al.) close to galaxy‑count lower limits, and a high‑density “fast‑evolution” model (Stecker et al.). The low‑EBL scenario yields an intrinsic photon index α_int ≈ 2.9 ± 0.9 (stat) ± 0.5 (syst), which is physically plausible. The high‑EBL model would require α_int ≈ 0.5 ± 1.2, an unrealistically hard spectrum that is difficult to reconcile with EGRET measurements and standard emission physics. Consequently, the results favor an EBL density near the lower limits set by galaxy counts, in agreement with other constraints from lower‑redshift VHE sources.

Regarding the emission mechanism, simple one‑zone synchrotron‑self‑Compton (SSC) models cannot simultaneously reproduce the optical/X‑ray and VHE data. External‑Compton (EC) models that rely on seed photons from the broad‑line region (BLR) are also challenged if the gamma‑ray emitting region lies outside the BLR, as suggested by the observed variability timescales. The authors discuss alternative possibilities, including SSC models with a narrow electron distribution, internal absorption that hardens the spectrum, and multi‑zone scenarios. A one‑zone SSC+EC model fitted to the de‑absorbed 2006 data (using the low‑EBL correction) provides a reasonable representation, but still leaves tensions with the full spectral energy distribution.

The detection of VHE emission from 3C 279 at such a large distance confirms that gamma‑rays can survive propagation through the EBL, and it provides a valuable, independent probe of the EBL density in the 0.2–2 µm range. The short, intense VHE flares observed in both years underline the extreme variability of the source and the importance of rapid optical triggers for ground‑based gamma‑ray observatories. The paper concludes that further multi‑wavelength campaigns, especially with simultaneous X‑ray and optical coverage, together with refined timing analyses, will be essential to disentangle the location and physics of the VHE emission region in 3C 279 and to tighten constraints on the EBL.