EVN observations of HESS J1943+213: evidence for an extreme TeV BL Lac object
- Massachusetts Institute of Technology, Haystack Observatory, Route 40, Westford, MA 01886 (United States)
- Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków (Poland)
- Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526 (Japan)
- National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)
- INAF Istituto di Radioastronomia, via Gobetti 101, I-40129 Bologna (Italy)
We report on the 1.6 GHz (18 cm) very long baseline interferometry (VLBI) observations of the unresolved, steady TeV source HESS J1943+213 located in the Galactic plane, performed with the European VLBI Network (EVN) in 2014. Our new observations with a nearly full EVN array provide the deepest image of HESS J1943+213 at the highest resolution ever achieved, enabling us to resolve the long-standing issues of the source identification. The milliarcsecond-scale structure of HESS J1943+213 has a clear asymmetric morphology consisting of a compact core and a diffuse jet-like tail. This is broadly consistent with the previous e-EVN observations of the source performed in 2011 and re-analyzed in this work. The core component is characterized by the brightness temperature of ≳1.8×10{sup 9} K, which is typical for low-luminosity blazars in general. Overall, the radio properties of HESS J1943+213 are consistent with the source classification as an “extreme high-frequency-peaked BL Lac object.” Remarkably, we note that because HESS J1943+213 does not reveal any optical or infrared signatures of the active galactic nucleus activity, it would never be recognized and identified as a BL Lac object if not for its location close to the Galactic plane where the High Energy Stereoscopic System surveyed for and the follow-up dedicated X-ray and radio studies triggered by the source detection in the TeV range. Our results suggest, therefore, a presence of an unrecognized, possibly very numerous population of particularly extreme HBLs and simultaneously demonstrate that the low-frequency VLBI observations with high angular resolution are indispensable for a proper identification of such objects.
- OSTI ID:
- 22869065
- Journal Information:
- Astrophysical Journal Letters, Vol. 823, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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