Discovery of VHE gamma-ray emission from the BL Lac object B3 2247+381 with the MAGIC telescopes

We study the non-thermal jet emission of the BL Lac object B3 2247+381 during a high optical state. The MAGIC telescopes observed the source during 13 nights between September 30th and October 30th 2010, collecting a total of 14.2 hours of good quality very high energy (VHE) $\gamma$-ray data. Simultaneous multiwavelength data was obtained with X-ray observations by the Swift satellite and optical R-band observations at the KVA-telescope. We also use high energy $\gamma$-ray (HE, 0.1 GeV-100 GeV) data from the Fermi satellite. The BL Lac object B3 2247+381 (z=0.119) was detected, for the first time, at VHE $\gamma$-rays at a statistical significance of 5.6 $\sigma$. A soft VHE spectrum with a photon index of -3.2 $\pm$ 0.6 was determined. No significant short term flux variations were found. We model the spectral energy distribution using a one-zone SSC-model, which can successfully describe our data.

💡 Research Summary

The paper reports the first detection of very‑high‑energy (VHE; >100 GeV) γ‑ray emission from the BL Lac object B3 2247+381 (redshift z = 0.119) using the MAGIC stereoscopic system. Observations were carried out over 13 nights between 30 September and 30 October 2010, yielding 14.2 hours of good‑quality data. The analysis produced a signal with a statistical significance of 5.6 σ, establishing B3 2247+381 as a new VHE source. The measured VHE spectrum follows a power‑law with a photon index of –3.2 ± 0.6, indicating a relatively soft spectrum compared with other TeV‑detected BL Lacs. No intra‑night or night‑to‑night flux variability was found within the MAGIC dataset, suggesting a stable VHE emission state during the campaign.

Simultaneous multi‑wavelength coverage was obtained to construct a broadband spectral energy distribution (SED). Swift‑XRT provided X‑ray data in the 0.3–10 keV band, revealing an average flux of ~2 × 10⁻¹² erg cm⁻² s⁻¹ and a photon index of ≈ –2.2. Optical R‑band monitoring with the KVA telescope showed modest variability (≤ 0.2 mag). High‑energy (HE; 0.1–100 GeV) γ‑ray data from the Fermi‑LAT were taken from the three‑year catalog and were consistent with the extrapolation of the MAGIC spectrum after correcting for extragalactic background light (EBL) absorption.

The authors model the complete SED with a one‑zone synchrotron self‑Compton (SSC) scenario. The emitting region is characterized by a radius of ~10¹⁶ cm, a magnetic field strength of B ≈ 0.1 G, and a Doppler factor δ ≈ 15. The electron energy distribution is described by a broken power law extending from γ_min ≈ 10³ to γ_max ≈ 10⁶ with a spectral index p ≈ 2.2. This configuration reproduces the synchrotron peak in the optical‑X‑ray band and the inverse‑Compton peak that spans the HE and VHE regimes. The soft VHE photon index is naturally explained by the steep high‑energy tail of the electron distribution and by EBL attenuation.

The detection adds B3 2247+381 to the growing list of BL Lac objects known to emit at TeV energies, confirming that even sources with relatively modest optical activity can produce VHE photons when observed during a high‑state. The lack of rapid variability suggests that the VHE emission originates from a relatively large, homogeneous zone within the jet, consistent with the SSC parameters derived. The study demonstrates the importance of coordinated multi‑wavelength campaigns: simultaneous X‑ray, optical, and HE γ‑ray data are essential to constrain the SSC model and to disentangle intrinsic spectral features from extrinsic absorption effects.

Future observations with more sensitive instruments such as the Cherenkov Telescope Array (CTA) will be able to probe finer temporal scales, test for possible spectral curvature, and refine the jet physical parameters. Long‑term monitoring could reveal whether B3 2247+381 exhibits episodic flaring at VHE energies, which would provide deeper insight into particle acceleration mechanisms and the role of external photon fields in BL Lac jets.