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Observation Date (UT) Observation Lat

Canonical Name:Vela X
TeVCat Name:TeV J0835-456
Other Names:HESS J0835-455
Source Type:PWN/TeV Halo
R.A.:08 35 00 (hh mm ss)
Dec.:-45 36 00 (dd mm ss)
Gal Long: 263.86 (deg)
Gal Lat: -3.09 (deg)
Distance: 0.29 kpc
Flux:0.75 (Crab Units)
Energy Threshold:450 GeV
Spectral Index:1.32
Size (X):0.48 (deg)
Size (Y):0.36 (deg)
Discovery Date:2006-03
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 J0835-455
Source Class: PWN
Identified Object: Vela X
R.A. (J2000): 128.89 deg (08 35 32)
Dec. (J2000): -45.66 deg (-45 39 32)
Positional uncertainty: 0.057 deg
Spatial Model: 3-Gaussian
Size: 0.585 +/- 0.052 deg
Spectral Model: power law
Integral Flux > 1 TeV: 1.74e-11 +/- 1.40e-12 cm-2 s-1
Pivot Energy, E0: 3.02 TeV
Diff. Flux at E0: 1.83e-12 +/- 6.99e-14 cm-2 s-1 TeV-1
Spectral Index: 1.89 +/- 0.03
HGPS Source Notes:
"This source (Vela X) exhibits complex morphology, and the HGPS analysis best models the VHE emission as a superposition of three Gaussian components with an average size 0.58 deg +/- 0.052 deg. This value is somewhat larger than the value published first in Aharonian et al. (2006), where it was modeled as a single asymmetric Gaussian of size 0.48 deg +/- 0.03 deg x 0.36 deg +/- 0.03 deg. However, a more recent H.E.S.S. publication (Abramowski et al. 2012) studied the complex emission more thoroughly. It fit profiles of the emission along two perpendicular axes, the main one aligned with the primary orientation of the emission. Along the major axis, the study measured a Gaussian size 0.52 deg +/- 0.02 deg, and along the minor axis, two Gaussians (sizes 0.12 deg +/- 0.02 deg and 0.60 deg +/- 0.04 deg) were required to best fit the emission. The HGPS model of the emission from HESS J0835−455 is thus largely compatible with the most recent dedicated study of the VHE emission, and the apparent discrepancy is simply a result of comparing two different multi-component models with our general outlier criterion (Eq. (29))."

Source position and its uncertainty:
From H.E.S.S. Collaboration (2012):
- R.A. (J2000): 08h 35m 00s
- Dec. (J2000): -45d 36' 00"
- No information on the positional uncertainty
From Aharonian et al. (2006):
- R.A. (J2000): 08h 35m 00s
- Dec. (J2000): −45d 36'
- statistical error of 2'

Source Extent:
From H.E.S.S. Collaboration (2012):
- the emission is statistically significant over a region of radius
From H.E.S.S. Collaboration (2006):
- semi-major axis: 0.48 +/- 0.03 deg
- semi-minor axis: 0.36 +/- 0.03 deg
- major axis of fitted distribution is at position angle of 41 +/- 7 deg

From HESS online catalog:
- radius: 26' (=0.43 deg)

Spectral Information:
From H.E.S.S. Collaboration (2019):
- In this paper archival Suzaku data are combined with contemporary
H.E.S.S data. The Suzaku observations were centred on three different
regions within the PWN.
- Three regions were thus defined for spectral analysis. These
corresponded with the Suzaku pointings.
- "For Pointings 1 and 2 we use circular spectral extraction regions
with a radius of 7.5' centred in the middle of the Suzaku XIS field of
- "For Pointing 0 we define a spectral extraction region such that we
avoid the region immediately adjacent to the pulsar."
- Pointing O: R.A. = 128.81 deg, Dec. = -45.286 deg
... rectangle with major edge of 17.4' and minor edge of 4.2'
... tilted by 157 deg with respect to the R.A. axis
- Pointing 1: R.A. = 128.7666 deg, Dec. = -45.4581 deg (7.5' circle)
- Pointing 2: R.A. = 128.6368 deg, Dec. = -45.8007 deg (7.5' circle)
- "The gamma-ray spectra are best modelled by power laws with
exponential cutoffs in pointings 1 and 2 (farther from the
pulsar). For pointing 0 (closer to the pulsar) the best-fit
exponential-cutoff power law has a cutoff energy consistent within
statistical uncertainties with the other two regions, although the
presence of a cutoff is not statistically significant (< 3 sigma)."
- The spectral parameters for power-law (PL) and exponentially cutoff
power-law (ECPL) fits are listed below for each of the pointings. The
fluxes are given in units of 10e-8 cm-2 s-1 sr-1
Pointing 0:
- PL: Flux (> 1TeV) = 3.6 +/- 0.4 (stat) +/- 0.7 (sys), Index = 1.75 +/- 0.10(stat) +/- 0.14(sys)
- ECPL: Flux (> 1TeV) = 3.5 +/- 0.5(stat) +/- 0.7(sys), Index= 1.2 +/- 0.3(stat) +/- 0.1(sys), E_cut = 16 +/- 7(stat) +/- 6(sys) TeV
Pointing 1:
- PL: Flux (> 1TeV) = 4.1 +/- 0.2(stat) +/- 1.0(sys), Index = 1.92 +/- 0.06(stat) +/- 0.10(sys)
- ECPL: Flux (> 1TeV) = 4.1 +/- 0.3(stat) +/- 1.5(sys), Index = 1.47 +/- 0.16(stat) +/- 0.16(sys), E_cut = 14 +/- 5(stat) +/- 6(sys) TeV
Pointing 2:
- PL: Flux (> 1TeV) = 4.6 +/- 0.3(stat) +/- 0.9(sys), Index = 1.84 +/- 0.06(stat) +/- 0.11(sys)
- ECPL: Flux (> 1TeV) = 4.6 +/- 0.3(stat) +/- 1.4(sys), Index = 1.27 +/- 0.14(stat) +/- 0.10(sys), E_cut = 12 +/- 3(stat) +/- 6(sys) TeV

From H.E.S.S. Collaboration (2012):
- spectral index: 1.32 +/- 0.06(stat) +/- 0.12(sys)
- exponential cutoff at an energy of 14.0 +/- 1.6 (stat) +/- 2.6 (sys) TeV

A TeV halo?
From Fang, Xiao-Jun & Peng-Fei (2019):
- "Vela X, as the PWN of Vela pulsar, is close to the center of
Vela SNR (Sushch et al. 2011). H.E.S.S has detected an extended TeV
structure around Vela pulsar with a scale of ∼ 6 pc (Aharonian et al. 2006;
Abramowski et al. 2012), which is considered to be correlated with the
X-ray filament (Hinton et al. 2011). The TeV halo is more extended
than the X-ray filament, and the derived magnetic field is only ∼ 4 microG,
much smaller than that close to the pulsar (Hinton et al. 2011). So
it is possible that the TeV structure is produced by the escaping
electrons that are wandering in the turbulent environment inside the
Vela SNR. Huang et al. (2018) also indicates that Vela X should be
surrounded by a slowdiffusion environment, so that its lepton flux at
the Earth will not conflict with the current experiments."
From Huang et al. (2018):
- "In this paper, we have shown that recent HESS data of the CR
electron flux at ∼10 TeV place interesting constraints on the
diffusion coefficient around Vela X. We find that a highly inefficient
diffusion region in the immediate vicinity of Vela X must be present,
with D(10 TeV) approx <= 10e28 cm2 s-1. The result is consistent with
the recent finding that there are inefficient diffusion regions
surrounding the Geminga and PSR B0656+14 PWNe, suggesting that such
inefficient diffusion regions may be common around PWNe of various

Source Association
From Tibaldo et al. (2018):
- In this paper, 9.5 years of LAT data are used to measure the
gamma-ray properties of this source between 10 GeV and 2 TeV.
- " An extended 2deg x 3deg nebula is seen in radio and GeV gamma
rays. An elongated cocoon prevails in X-rays and TeV gamma rays. We
use 9.5 years of data from the Fermi Large Area Telescope (LAT) to
disentangle gamma-ray emission from the two components in the energy
range from 10 GeV to 2 TeV, bridging the gap between previous
measurements at GeV and TeV energies."
- "The best fit to the LAT data is provided by the combination of the
two components derived at energies < 100 GeV and > 100 GeV. The first
component has a soft spectrum, spectral index 2.19 +/- 0.16 +0.05 −0.22,
and extends over a region of radius 1.36deg +/- 0.04deg, consistent
with the radio nebula. The second component has a harder spectrum,
spectral index 0.9 +/- 0.3 +0.3 −0.1, and is concentrated over an area
of radius 0.63deg +/- 0.03deg, coincident with the X-ray cocoon that
had already been established to account for the bulk of the emission
at TeV energies."
From H.E.S.S. Collaboration (2012):
- "Compared to the previous H.E.S.S. observations of Vela X the new
analysis confirms the general spatial overlap of the bulk of the VHE
gamma-ray emission with the X-ray cocoon, while its extent and
morphology appear more consistent with the (more extended) radio
emission, contradicting the simple correspondence between VHE γ-ray
and X-ray emissions. Morphological and spectral results challenge the
interpretation of the origin of γ-ray emission in the GeV and TeV
ranges in the framework of current models."

Seen by: H.E.S.S.
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