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Observation Date (UT) Observation Lat

Canonical Name:W 51
TeVCat Name:TeV J1923+141
Other Names:HESS J1923+141
SNR G49.2-0.7
0FGL J1923.0+1411
2HWC J1922+140
1LHAASO J1922+1403
Source Type:SNR/Molec. Cloud
R.A.:19 22 55.2 (hh mm ss)
Dec.:+14 11 27.6 (dd mm ss)
Gal Long: 49.12 (deg)
Gal Lat: -0.36 (deg)
Distance: 5.4 kpc
Flux:0.03 (Crab Units)
Energy Threshold:1000 GeV
Spectral Index:
Extended:Yes
Size (X):0.12 (deg)
Size (Y):0.12 (deg)
Discovery Date:2008-10
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 J1923+141
Source Class: Unid
Identified Object: None
R.A. (J2000): 290.75 deg (19 22 59)
Dec. (J2000): 14.14 deg (14 08 34)
Positional uncertainty: 0.061 deg
Spatial Model: Gaussian
Size: 0.120 +/- 0.019 deg
Spectral Model: power law
Integral Flux > 1 TeV: 6.90e-13 +/- 1.14e-13 cm-2 s-1
Pivot Energy, E0: 1.40 TeV
Diff. Flux at E0: 4.52e-13 +/- 7.08e-14 cm-2 s-1 TeV-1
Spectral Index: 2.55 +/- 0.17
HGPS Source Notes:
From the HGPS paper:

- "The VHE gamma-ray source HESS J1923+141 (preliminary H.E.S.S. results published in Fiasson et al. 2009) is spatially coincident with the W 51 region, studied in detail with the MAGIC IACT (Aleksic et al. 2012). The HGPS results are generally compatible with those from MAGIC. However, the latter shows evidence for a gamma-ray source composed of two components above 1 TeV, which cannot yet be confirmed by H.E.S.S. One component is coincident with the interaction region between W 51C and W 51B, while the other is coincident with the potential PWN CXOU J192318.5+140305 (Koo et al. 2005), suggesting that HESS J1923+141 may be a composite of VHE emission of different astrophysical origins."

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":
- 3FGL J1923.2+1408e (3FGL)
- 2FHL J1923.2+1408e (2FHL)

Source Position:

From Aleksic et al. (2012):
- R.A. (J2000): 19.382 +/- 0.001 h (= 19h 22m 55.2s)
- Dec. (J2000): 14.191 +/- 0.015 deg (= 14d 11' 27.6")

From Krause et al. (2011):
- R.A. (J2000): 19.387 +/- 0.002h (= 19h 23m 13.2s)
- Dec. (J2000): +14.18 +/- 0.02d (= +14d 10' 48")

Distance:

From Ranasinghe & Leahy (2018):
- "We carry out a comprehensive study of H I 21 cm line observations
and 13CO line observations of 21 supernova remnants (SNRs). The aim of
the study is to search for H I absorption features to obtain kinematic
distances in a consistent manner."
- "We conclude that the SNR and the HII regions are likely associated
with each other and place the SNR and the HII regions at a distance of
5.4 kpc."
- in Table 1 of the paper, which summarises the results, the distances
found in the literature for this SNR are listed as 4.3 kpc and 6 kpc
whilst the distance derived from the analysis presented in this paper
is quoted as 5.4 +/- 0.6 kpc

From Tian & Leahy (2013):
- "W51C has a distance of about 4.3 kpc, smaller than the tangent
point distance of 5.5 kpc in that direction, but still in the
Sagittarius spiral arm."

Source Extent:

From Aleksic et al. (2012):
- extension 0.12 +/- 0.02(stat) +/- 0.02(syst) deg

From Krause et al. (2011):
- extension (sigma of 2-d Gaussian fit) of 0.16 +/- 0.02deg

From Feinstein et al. (2008)
- "source extends more than PSF"

Flux:

From Aleksic et al. (2012):
- the integrated flux above 1TeV is equivalent to ~3% of the Crab Nebula

From Krause et al. (2011):
- the integrated flux above 1TeV corresponds to 3.8% of the Crab Nebula

From Abdo et al. (2009)
- 0.24 Crab > 35 TeV
- This Milagro flux is based on a comparison of the quoted flux
at 35 TeV of 39.4e-17 /TeV /s /cm2 with that from the Crab
(162.6e-17 /TeV /s /cm2) also quoted at 35 TeV

From Feinstein et al. (2008)
- 3% Crab

Spectral Properties:

From Aleksic et al. (2012):
- Spectral index: 2/58 +/- 0.07(stat) +/- 0.22(syst)

From Krause et al. (2011):
- Spectral index: 2.40 +/- 0.12(stat)

Special Notes:

- HESS announced a detection at 4.4 sigma Feinstein et al. (2008)
at 6th Workshop on Science with the New Generation of High Energy
Gamma-Ray Experiments (link below).

- it is the May 2009 HESS source of the month

Source Classification:

From LHAASO Collaboration (2024):
- Abstract:
- "The gamma -ray emission from the W51 complex is widely acknowledged
to be attributed to the interaction between the cosmic rays (CRs)
accelerated by the shock of supernova remnant (SNR) W51C and the
dense molecular clouds in the adjacent star-forming region,
W51B. However, the maximum acceleration capability of W51C for CRs
remains elusive."
- "Based on observations conducted with the Large High Altitude Air
Shower Observatory (LHAASO), we report a significant detection of gamma
rays emanating from the W51 complex, with energies from 2 TeV to 200
TeV. The LHAASO measurements, for the first time, extend the
gamma-ray emission from the W51 complex beyond 100 TeV and reveal a
significant spectrum bending at tens of TeV."
- "By combining the pi-0-decay bump featured data from Fermi-LAT, the
broadband gamma-ray spectrum of the W51 region can be
well-characterized by a simple pp-collision model. The observed
spectral bending feature suggests an exponential cutoff at ∼400 ~TeV
or a power-law break at ∼200 ~TeV in the CR proton spectrum, most
likely providing the first evidence of SNRs serving as CR
accelerators approaching the PeV regime. Additionally, two young star
clusters within W51B could also be theoretically viable to produce
the most energetic gamma rays observed by LHAASO. Our findings
strongly support the presence of extreme CR accelerators within the
W51 complex and provide new insights into the origin of Galactic
CRs."

From Abeysekara et al. (2017):
- 2HWC J1922+140 is associated with W 51
- "Source 2HWC J1922+140 is detected by HAWC in the point source
search. However, the residual map exhibits various excess around the
position of the source once the point source modeled has been
subtracted. This indicates that there may be additional emission
farther away from W51C than previously reported. Given the source
location, there may be a significant contribution from Galactic
diffuse emission to this extended emission. The spectrum fit is thus
performed both using a point source model and an extended source
model, with radius 0.9 deg. The spectrum measurement reported here
under the point source hypothesis appears to be in agreement with the
MAGIC and Fermi-LAT results, whereas the one performed with the
extended hypothesis is larger by about a factor 3."

From Brogan et al. (2013):
- "We present a comprehensive view of the W51B HII region complex and
the W51C supernova remnant (SNR) using new radio observations from the
VLA, VLBA, MERLIN, JCMT, and CSO along with archival data from
Spitzer, ROSAT, ASCA, and Chandra. ... The 400 cm image shows
non-thermal emission surrounding the G49.2-0.3 HII region, and a
compact source of non-thermal emission (W51B_NT) coincident with the
previously-identified OH (1720 MHz) maser spots, non-thermal 21 and 90
cm emission, and a hard X-ray source. W51B_NT falls within the region
of high likelihood for the position of TeV gamma-ray emission. ... The
multiwavelength data demonstrate that the northern end of the W51B HII
region complex has been partly enveloped by the advancing W51C SNR and
this interaction explains the presence of W51B_NT and the OH
masers. ... Confirmation of the W51B/W51C interaction provides
additional evidence in favor of this region being one of the best
candidates for hadronic particle acceleration known thus far. "

From Tian & Leahy (2013):
- "TeV emission from several middle-aged SNRs interacting with adjacent
molecular clouds has been considered to originate from hadronic
processes (e.g. Ohira et al. 2011). However for W51C, we have found it
not interacting with any HV HI. Also we find that the observed nearby
OH masers from dense molecular gas are likely associated with the star
forming regions in W51B and not W51C. These new results show clear
absence of evidence of molecular or atomic gas interacting with W51C,
thus casting doubt on a hadronic origin for the GeV/TeV emission from
W51C. We think W51C is similar to the case of Tycho SN 1572 (Tian & Leahy
2011), where the SNR was found not to interact with molecular or HI clouds."

From Aleksic et al. (2012):
- "The main part of the emission coincides with the shocked cloud region,
while we find a feature extending towards the pulsar wind nebula. The possible
contribution of the pulsar wind nebula, assuming a point-like source, shows no
dependence on energy and it is about 20% of the overall emission. The broad band
spectral energy distribution can be explained with a hadronic model that implies
proton acceleration above 100 TeV. This result, together with the morphology of
the source, tentatively suggests that we observe ongoing acceleration of ions in
the interaction zone between supernova remnant and cloud. These results shed
light on the long-standing problem of the origin of galactic cosmic rays."

From Krause et al. (2011):
- "... the bulk of the VHE gamma-ray emission comes from the shocked
molecular cloud located where the SNR shock engulfs a large molecular
cloud, creating a shocked gas region distinguishable in the radio data."

From de Naurois (2011):
- categorised as an old SNR in the vicinity of a molecular cloud

From Abdo et al. (2009):
- report a source associated with 0FGL J1923.0+1411 with a statistical
significance of 3.4 sigma

From Fiasson et al. (2009):
- the presence of a PWN candidate provides a competitive leptonic scenario

From Feinstein et al. (2008):
- An association with shocked molecular clouds in the vicinity of
SNR G49.2-0.7 is also postulated



Seen by: Milagro, H.E.S.S., MAGIC, HAWC, LHAASO
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