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Observation Date (UT) Observation Lat

Canonical Name:Westerlund 2
TeVCat Name:TeV J1023-577
Other Names:WR20a
RCW 49
HESS J1023-575
1FGL J1023.0-5746
PSR J1022-5746
Source Type:Massive Star Cluster
R.A.:10 23 24 (hh mm ss)
Dec.:-57 47 24 (dd mm ss)
Gal Long: 284.22 (deg)
Gal Lat: -0.40 (deg)
Distance: 8 kpc
Flux:0.09 (Crab Units)
Energy Threshold:380 GeV
Spectral Index:2.58
Size (X):0.18 (deg)
Size (Y):0.18 (deg)
Discovery Date:2007-06
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 J1023-575
Source Class: Unid
Identified Object: None
R.A. (J2000): 155.81 deg (10 23 15)
Dec. (J2000): -57.78 deg (-57 46 34)
Positional uncertainty: 0.030 deg
Spatial Model: Gaussian
Size: 0.166 +/- 0.009 deg
Spectral Model: power law
Integral Flux > 1 TeV: 2.41e-12 +/- 1.34e-13 cm-2 s-1
Pivot Energy, E0: 1.87 TeV
Diff. Flux at E0: 7.50e-13 +/- 4.16e-14 cm-2 s-1 TeV-1
Spectral Index: 2.36 +/- 0.05
HGPS Source Notes:
This source is listed as unidentified in the HGPS. Five 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 J1023.1−5745 (3FGL)
- J1023−5746 (PSR)
- Westerlund 2 (EXTRA)
- 2FHL J1022.0−5750 (2FHL)
- 3FGL J1024.3−5757 (3FGL)
"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 Aharonian et al. (2007):
- RA (J2000): 10h 23m 18s +/- 12s(stat) +/-20s(syst)
- Dec (J2000): -57d 45' 50" +/- 1'30"(stat) +/- 20"(syst)
From HESS Collaboration (2011):
- RA (J2000): 10h 23m 24s +/- 7.2s (stat) +/- 20"(syst)
- Dec (J2000): -57d 47m 24s +/- 1m12s (stat) +/- 20"(syst)

Source Association:
From Asahina et al. (2017):
- "The formation mechanism of CO clouds observed with NANTEN2 and
Mopra telescope toward the stellar cluster Westerlund 2 is studied by
three-dimensional magnetohydrodynamic simulations taking into account
the interstellar cooling. These molecular clouds show a peculiar shape
composing of an arcshaped cloud in one side of a TeV gamma-ray source
HESS J1023-575 and a linear distribution of clouds (jet clouds) in
another side. We propose that these clouds are formed by the
interaction of a jet with interstellar neutral hydrogen (HI) clumps"
- "HESS J1023-575 is observed both at the high energy band (above 2.5
TeV) and the low energy band (0.7 − 2.5 TeV). The cooling by the
pp-interaction and diffusion time scales of the relativistic particles
are estimated to be about 5×10e6 yr and 2×10e4 yr, respectively, by
Furukawa et al. (2014). If the arc and the jet clouds and TeV
gamma-ray source are produced by a single object, a supernova remnant
or a pulsar wind nebulae is not likely as the origin for TeV gamma-ray
source since the diffusion time scale of the relativistic particles is
shorter than the age of the arc and jet clouds (∼ Myr). A microquasar
jet which is active over 1 Myr can explain both the age of the arc and
jet clouds and the lifetime of the relativistic particles."
From Furukawa et al. (2014):
- "We have made new CO observations of two molecular clouds, which we
call "jet" and "arc" clouds, toward the stellar cluster Westerlund 2
and the TeV gamma-ray source HESS J1023-575. ... In addition, we have
found a new counter jet cloud on the west. ... A sign of star
formation is found at the edge of the jet cloud and gives a constraint
on the age of the jet cloud to be ~Myrs. An analysis with the multi CO
transitions gives temperature as high as 20 K in a few places of the
jet cloud, suggesting that some additional heating may be operating
locally. The new TeV gamma-ray images by H.E.S.S. correspond to the
jet and arc clouds spatially better than the giant molecular clouds
associated with Westerlund 2. We suggest that the jet and arc clouds
are not physically linked with Westerlund 2 but are located at a
greater distance around 7.5 kpc. A microquasar with long-term activity
may be able to offer a possible engine to form the jet and arc clouds
and to produce the TeV gamma-rays, although none of the known
microquasars have a Myr age or steady TeV gamma-rays. Alternatively,
an anisotropic supernova explosion which occurred ~Myr ago may be able
to form the jet and arc clouds, whereas the TeV gamma-ray emission
requires a microquasar formed after the explosion."
From Acero et al. (2013):
- Analysis of the LAT data from this source leads to its
classification as a PWN candidate
From de Naurois (2011):
- due to the extension of this source and the lack of apparent variability,
mechanisms involving collective stellar wind effects are preferred. The
recent discovery of 2 neighbouring bright pulsars by Fermi motivate the
consideration of pulsar wind nebula emission
From HESS Collaboration (2011):
- "a clear identification of the object responsible for
the VHE emission from HESS J1023–575 is not yet possible, and
contribution from the nearby high-energy pulsar and/or the open
cluster remains a possibility"

Source Extent:
From Aharonian et al. (2007):
- radius: 0.18 +/- 0.02 deg
From HESS Collaboration (2011):
- radius: 0.18 +/- 0.02 deg

Spectral Properties:
From Aharonian et al. (2007):
- Spectral index: 2.53 +/- 0.16 (stat) +/- 0.1 (syst)
From HESS Collaboration (2011):
- Spectral Index: 2.58 +/- 0.19 (stat) +/- 0.2 (syst)

Seen by: H.E.S.S.
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