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Observation Date (UT) Observation Lat

Canonical Name:HESS J1943+213
TeVCat Name:TeV J1943+213
Other Names:
Source Type:HBL
R.A.:19 43 55 (hh mm ss)
Dec.:+21 18 08 (dd mm ss)
Gal Long: 57.76 (deg)
Gal Lat: -1.29 (deg)
Flux:0.015 (Crab Units)
Energy Threshold:470 GeV
Spectral Index:3.1
Discovery Date:2010-11
Discovered By: H.E.S.S.
TeVCat SubCat:Default Catalog

Source Notes:

H.E.S.S. Galactic Plane Survey (HGPS, 2018):
A selection of information for each of the 78 sources in the HGPS is provided in TeVCat. For full details, visit the HGPS website.

Name: HESS J1943+213
Source Class: Unid
Identified Object: None
R.A. (J2000): 296.00 deg (19 43 59)
Dec. (J2000): 21.31 deg (21 18 50)
Positional uncertainty: 0.032 deg
Spatial Model: Gaussian
Size: 0.120 +/- 0.019 deg
Spectral Model: power law
Integral Flux > 1 TeV: 3.94e-13 +/- 7.59e-14 cm-2 s-1
Pivot Energy, E0: 1.54 TeV
Diff. Flux at E0: 2.12e-13 +/- 4.05e-14 cm-2 s-1 TeV-1
Spectral Index: 2.83 +/- 0.22
HGPS Source Notes:
The following is the text from the HGPS paper:
We note that one source in HGPS, HESS J1943+213, is likely an extragalactic object. It has no measured extension and a radio counterpart that many recent studies tend to classify as a BLLac object ( Peter et al. 2014; Straal et al. 2016; Akiyama et al. 2016). However, its VHE flux has not revealed any variability so far, which is unusual for such an object ( Shahinyan & VERITAS Collaboration 2017).

Two possible associations are listed in Table A.9. "This is a list of astronomical objects, extracted from catalogs of plausible counterparts, which are are found to be spatially coincident with the HGPS source":
- IGR J19443+2117 (EXTRA)
- 2FHL J1944.1+2117 (2FHL)
"EXTRA associations: For completeness, in addition to the associations obtained through the catalog-based, automatic procedure, we add a list of 20 extra associated objects that are plausible counterparts for some HGPS sources and are not covered by the limited set of catalogs we use."

Source position and its uncertainty:

From Abramowski et al. (2011):
- R.A. (J2000): 19h 43m 55s +/- 1s(stat) +/- 1s(syst)
- Dec. (J2000): +21d 18' 08" +/- 17"(stat) +/- 20"(syst)

Source Class:

From VERITAS Collaboration (2018):
- "Studies exploring the classification of the source are converging
towards its identification as an extreme synchrotron BL Lac object."
- "Here we present 38 hours of VERITAS observations of HESS J1943+213
taken over two years. The source is detected with approx. 20 standard
deviations significance, showing a remarkably stable flux and spectrum
in VHE gamma rays. Multi-frequency very-long-baseline array (VLBA)
observations of the source confirm the extended, jet-like structure
previously found in the 1.6 GHz band with European VLBI Network and
detect this component in the 4.6 GHz and the 7.3 GHz bands. The radio
spectral indices of the core and the jet and the level of polarization
derived from the VLBA observations are in a range typical for
blazars. Data from VERITAS, Fermi-LAT, Swift-XRT, FLWO 48'' telescope,
and archival infrared and hard X-ray observations are used to
construct and model the spectral energy distribution (SED) of the
source with a synchrotron-self-Compton model."

From Shahinyan et al. (2016):
"Monitoring observations of HESS J1943+213 show a remarkably stable
flux and spectrum in VHE gamma rays. Studies exploring the
classification of HESS J1943+213 are converging towards accepting the
source as an extreme synchrotron BL Lac object. Specifically, overall
SED characteristics of the source, the detection of a potential host
galaxy in near-IR imaging, and VLBI observations of the HESS J1943+213
radio counterpart showing extended jet-like emission at milliarcsecond
scale and core flux density variability establish the source as a
blazar. Recent Very Long Baseline Array (VLBA) observations of the
source (shown here for the first time) confirm the extended structure
found in the 1.6 GHz band and detect the jet-like component in the 4.6
GHz and the 7.3 GHz bands. The spectral indices of the core and the
jet-like components derived from the VLBA observations are in a range
typical for blazars. HESS J1943+213 fits the extreme HBL description;
however, with variability detected only at milliarcseconds scales in
radio, it appears to be an abnormally stable VHE gamma-ray blazar."

From Straal et al. (2016):
"HESS J1943+213 is an unidentified TeV source that is likely a
high-frequency-peaked BL Lac (HBL) object but also compatible with a
pulsar wind nebula (PWN) nature. Each of these enormously different
astronomical interpretations is supported by some of the observed
unusual characteristics. In order to finally classify and understand
this object we took a three-pronged approach, through time-domain,
high angular resolution, and multi-frequency radio studies."
"... we rule out the PWN hypothesis and conclude the source is a BL
Lac object. The consistently high fraction (70%) of the flux density
from the extended structure then leads us to conclude that HESS
J1943+213 must be a non-classical HBL object."

From Kazunori et al. (2016):
- VLBI observations with a nearly full EVN array (1.6 GHz) were performed
"The milliarcsecond-scale structure of HESS J1943+213 has a clear
asymmetric morphology, consisting of a compact core and a diffuse
jet-like tail. The core component is characterized by the brightness
temperature ... which is typical for low-luminosity blazars in
general. Overall, radio properties of HESS J1943+213 are consistent
with the source classification as an "extreme high-frequency-peaked BL
Lac object".

From Gabanyi et al. (2015):
"The source is point-like at the high resolution of EVN (10- mas
scale) and also with the e-MERLIN (100-mas scale), according to our
new L- and C-band data. The detected compact radio feature has a flat
spectrum and in L-band showed flux density variability thus consistent
with the proposed BL Lac nature of the source. Compared to the lower
resolution VLA and WSRT observations at L band, significant flux
density (≈ 60 mJy) is missing from the high-resolution data. Whether
this large-scale radio structure is related to the TeV-emitting BL Lac
object or associated with the Galactic material in the line of sight
towards the source is yet unclear."

From Shahinyan et al. (2015):
- "The agreement between fluxes measured approximately five years
apart with VERITAS and H.E.S.S. and a lack of a variability detection
in other energy bands is surprising if HESS J1943+213 is a blazar."
- "The VERITAS-measured spectrum of HESS J1943+213, albeit
preliminary, exhibits a harder index than the H.E.S.S. spectrum. The
soft spectral index from H.E.S.S. constitutes one of the key pieces of
evidence against the PWN hypothesis. Thus, in conjunction with the
lack of detectible variability in the VHE regime, the harder VERITAS
spectrum may be counted in favor of the PWN hypothesis."

From Abramowski et al. (2011):
- Three possible explanations for the nature of the source are considered:
gamma-ray binary, PWN or BL Lac object
- Conclusion:
Based on its point-like nature, its lack of variability and the non-detection of an
optical counter-part (as would be expected if it were a binary system), it is postulated
that this source is extragalactic but shining through the galactic plane. Its X-ray
spectrum extends above 100 keV so it could be an extreme HBL.

From Gabanyi et al. (2011):
- The nature of this radio source reported as the counterpart for the VHE source
was investigated. Conclusions: "The estimated brightness temperature of the radio
point source counterpart of HESS J1943+213 is well below the value expected from
the Doppler-boosted radio emission of a BL Lacertae object. This fact and the
discovery of traces of a distant supernova explosion around the location of the
TeV source lead us to conclude that the most likely origin of the high-energy emission
is a remote pulsar wind nebula. If this scenario is true, then the HI shell around
HESS J1943+213 may represent a population of hitherto missing Galactic SNRs."

From Leahy & Tian:
- "We have analyzed the same archival data to obtain a reliable HI absorption
spectrum. We find a different distance of >= 17 kpc. This results disarms
Gabanyi et al. (2011)'s conclusion and strongly supports that HESS J1943+213
is an extragalactic source, consistent with preferred counterpart of the HESS


​From Zahoor et al. (2021):
- In this work, the effect of the absorption of the extragalactic
galactic background light was used to place constraints on the
redshifts of gamma-ray sources.
- "The very high energy (VHE) gamma-ray spectral indices of blazars
show strong correlation with the source redshift. Absence of any such
correlation in low energy gamma rays and X-rays indicate the presence
of Extragalactic Background Light (EBL) induced absorption of VHE
gamma rays. By employing a linear regression analysis, this
observational feature of blazars is used to constrain the redshift of
BL Lac objects which was unknown/uncertain earlier."
- Two different regression relations were used to constrain the source
redshift and the following values were found for HESS J1943+213:
- z = 0.20 +/- 0.06
- z = 0.21 +/- 0.06

From Sahu et al. (2019):
- "Using the photohadronic model and performing a statistical analysis for different
redshifts, we were able to constrain the redshift in the range 0.14 ≤ z ≤ 0.19."

From VERITAS Collaboration (2018):
- "The well-measured gamma-ray peak of the SED with VERITAS and
Fermi-LAT provides constraining upper limits on the source redshift. "
- "Assuming a model of the EBL by Franceschini et al. (2008), the 68%
upper bound of the Fermi-LAT spectrum is extrapolated into the VHE
regime and absorbed for a range of redshift values. The upper bound of
the Fermi-LAT spectrum is used in order for the upper limit on the
redshift to be conservative"
- "The 95% upper limit on the redshift derived from the chi2
distribution is z < 0.23, which is significantly more constraining
than the existing z < 0.45 95% upper limit from Peter et al. (2014)."

From Farina et al. (2018):
- "Deep H-band imaging collected during the ARGOS (Advanced Rayleigh
guided Ground layer adaptive Optics System at the Large Binocular
Telescope) commissioning allowed us to separate the contribution of
the nuclear emission and to unveil the properties of the host galaxy
with unprecedented detail. The host galaxy is well fitted by a Seersic
profile with index of n approx. 2 and total magnitude of H
approx. 16.15 mag. Under the assumption that BL Lac host galaxies are
standard candles, we infer a redshift of z approx. 0.21. In the
framework of the current model for the EBL, this value is in agreement
with the observed dimming of the VHE spectrum due to the scatter of
energetic photons on the EBL."
- the authors state in the conclusions: "we locate HESS J1943+213 at z
approx. 0.21, or, more conservatively, in the redshift range 0.14 < z < 0.30
(considering variation of the typical luminosity of BL Lac host
galaxies and uncertainties in the Galactic extinction)."

From Peter et al. (2014):
- "The source is most likely located at a redshift between 0.03 and
0.45 according to extension and EBL attenuation arguments."

From Abramowski et al. (2011):
- "All available observations favor an interpretation
as an extreme, high-frequency peaked BL Lac object with a
redshift z>0.14."

From Cerruti et al. (2011):
- "The lower limit on the redshift, based on the expected flux from
the host galaxy, is z>0.14

Spectral Index:

From Abramowski et al. (2011):
- 3.1 +/- 0.3(stat) +/- 0.2(syst)

From Cerruti et al. (2011):
- 3.1 +/- 0.3(stat) between 0.470-6 TeV

Seen by: H.E.S.S., VERITAS
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