Very Long Baseline Interferometry Search for the Radio Counterpart of Hess J1943+213

Abstract

HESS J1943+213, a TeV point source close to the Galactic plane recently discovered by the H.E.S.S. Collaboration, was proposed to be an extreme BL Lacertae object, though a pulsar wind nebula (PWN) nature could not be completely discarded. To investigate its nature, we performed high-resolution radio observations with the European Very Long Baseline Interferometry Network (EVN) and reanalyzed archival continuum and H i data. The EVN observations revealed a compact radio counterpart of the TeV source. The low brightness temperature and the resolved nature of the radio source are indications against the beamed BL Lacertae hypothesis. The radio/X-ray source appears immersed in a ∼1 elliptical feature, suggesting a possible galactic origin (PWN nature) for the HESS source. We found that HESS J1943+213 is located in the interior of a ∼1 • diameter H i feature and explored the possibility of them being physically related.

VERITAS and VLBA observations of HESS J1943+213

“…These values are well within the range for blazars. We do not confirm the significantly lower brightness temperature measurement of T B = 7.7×10 7 K, which was based on the EVN 1.6 GHz image [10] and which has subsequently been reanalyzed showing higher brightness temperature, T B > 1.8×10 9 [12].…”

“…Based on our position measurement of the HESS J1943+213 counterpart and the position reported from the Gabányi et al [10] EVN detection, we see a change in position of 1.1 mas. This is consistent with zero, given the ∼2.5 mas uncertainty of the position measurements.…”

“…The radio counterpart to HESS J1943+213 was observed with the Very Long Baseline Array (VLBA) on 2015 August 11 (Project ID: BS246) as part of a request to follow up on the European VLBA Network (EVN) detection and characterization of the source [10] extended, jet-like emission in the 1.6 GHz, 4.3 GHz, and 7.6 GHz images. This is the first detection of the extended milliarcsecond-scale structure of this source in 4.3 GHz and 7.6 GHz bands, allowing multifrequency exploration of the source properties in VLBI.…”

“…Despite the favored blazar classification from the time of the discovery [9], there were initial challenges primarily from 1.6 GHz VLBI observations with EVN measuring a brightness temperature too low for an AGN [10]. The blazar classification was solidified by the combination of the discovery of a potential host galaxy in the near-IR [11], a comparative study of the source SED with another extreme HBL [6] and follow up VLBI observations that measured a brightness temperature within the blazar range, a jet-core structure, and overall radio properties consistent with a blazar classification [12].…”

Supernova Remnants Interacting with Molecular Clouds: X-Ray and Gamma-Ray Signatures

“…Some examples are the results presented on the Lupus Loop (Colomb and Dubner 1982), G296.5+10.0 (Dubner et al 1986), Vela (Dubner et al 1998b), W50 -SS 443 (Dubner et al 1998a), Tycho (Reynoso et al 1999), SN 1006 , W28 (Velázquez et al 2002), Puppis A (Reynoso et al 2003), Kes 75 (Leahy and Tian 2008), IC 443 (Lee et al 2008), and several Southern Galactic SNRs (Koo et al 2004). Recent observations also indicate the existence of shells and super-shells around SNRs (Park et al 2013), which are particularly useful as they permit the reconstruction of the history of the SNR progenitor, providing hints on the nature of of "dark" γ-ray sources (e.g., Gabányi et al 2013). …”