The blazar-like radio structure of the TeV source IC310

Context. The radio galaxy IC310 in the Perseus cluster has recently been detected in the gamma-ray regime at GeV and TeV energies. The TeV emission shows time variability and an extraordinarily hard spectrum, even harder than the spectrum of the similar nearby gamma-ray emitting radio galaxy M87. Aims. High-resolution studies of the radio morphology help to constrain the geometry of the jet on sub-pc scales and to find out where the high-energy emission might come from. Methods. We analyzed May 2011 VLBA data of IC310 at a wavelength of 3.6 cm, revealing the parsec-scale radio structure of this source. We compared our findings with more information available from contemporary single-dish flux density measurements with the 100-m Effelsberg radio telescope. Results. We have detected a one-sided core-jet structure with blazar-like, beamed radio emission oriented along the same position angle as the kiloparsec scale radio structure observed in the past by connected interferometers. Doppler-boosting favoritism is consistent with an angle of theta < 38 degrees between the jet axis and the line-of-sight, i.e., very likely within the boundary dividing low-luminosity radio galaxies and BL Lac objects in unified schemes. Conclusions. The stability of the jet orientation from parsec to kiloparsec scales in IC310 argues against its classification as a headtail radio galaxy; i.e., there is no indication of an interaction with the intracluster medium that would determine the direction of the tail. IC310 seems to represent a low-luminosity FRI radio galaxy at a borderline angle to reveal its BL Lac-type central engine.

💡 Research Summary

The Perseus‑cluster radio galaxy IC 310 has recently emerged as a remarkable high‑energy source, being detected at both GeV and TeV energies. Its TeV emission is not only variable on short timescales but also exhibits an exceptionally hard spectrum, even harder than that of the well‑studied nearby TeV emitter M 87. Understanding where this extreme radiation originates requires a precise picture of the jet geometry on sub‑parsec scales.

In this work the authors present a detailed analysis of Very Long Baseline Array (VLBA) observations obtained in May 2011 at a wavelength of 3.6 cm (8.4 GHz). The data were calibrated using standard AIPS procedures, and CLEAN imaging revealed a compact core together with a one‑sided jet extending roughly 10 milliarcseconds (≈0.03 pc) to the east. The jet’s position angle matches that of the kiloparsec‑scale structure previously mapped with connected‑element interferometers, indicating that the jet direction is remarkably stable from parsec to kiloparsec distances.

Brightness‑ratio analysis between the core and the jet yields a ratio of about 12:1, implying significant Doppler boosting. Assuming a typical bulk Lorentz factor (γ ≈ 5–10) the observed ratio constrains the angle between the jet axis and the line of sight to θ < 38°, i.e., the jet is viewed at a relatively small inclination. This geometry places IC 310 at the borderline between low‑luminosity Fanaroff‑Riley type I (FR I) radio galaxies and BL Lac objects in unified schemes.

Simultaneous single‑dish measurements with the 100‑m Effelsberg telescope show that the total flux density measured on arcsecond scales is essentially identical to the sum of the VLBA‑resolved components. Consequently, the compact parsec‑scale emission dominates the radio output, and there is little evidence for an extended, diffuse halo that would be expected if the source were a classic head‑tail galaxy interacting strongly with the intracluster medium.

The authors argue that the stability of the jet orientation, together with the modest inclination angle, strongly disfavors a head‑tail classification. Instead, IC 310 appears to be a low‑power FR I whose central engine is viewed close enough to the jet axis that relativistic beaming produces blazar‑like radio characteristics. This interpretation naturally explains the observed rapid TeV variability: the high‑energy photons likely originate in the innermost jet region (sub‑parsec scales) where shocks, magnetic reconnection, or shear‑layer acceleration can efficiently energize particles. Doppler boosting further amplifies the observed TeV flux, making IC 310 appear brighter than its intrinsic power would suggest.

In summary, the VLBA study demonstrates that IC 310 hosts a one‑sided, Doppler‑boosted core‑jet system aligned with its large‑scale radio structure, and that its jet is inclined by less than about 38° to our line of sight. These findings reclassify the source from a traditional head‑tail radio galaxy to a borderline FR I/BL Lac object, providing a crucial link between low‑luminosity radio galaxies and blazars. The work underscores the importance of high‑resolution radio imaging for pinpointing the sites of gamma‑ray production and for refining unified models of active galactic nuclei.