	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 J1825-137
	Source Class:	PWN
	Identified Object:	PSR J1826-1334
	R.A. (J2000):	276.26 deg (18 25 02)
	Dec. (J2000):	-13.97 deg (-13 57 57)
	Positional uncertainty:	0.125 deg
	Spatial Model:	3-Gaussian
	Size:	0.461 +/- 0.032 deg
	Spectral Model:	power law
	Integral Flux > 1 TeV:	1.92e-11 +/- 1.85e-12 cm-2 s-1
	Pivot Energy, E0:	1.16 TeV
	Diff. Flux at E0:	1.72e-11 +/- 5.73e-13 cm-2 s-1 TeV-1
	Spectral Index:	2.38 +/- 0.03
	HGPS Source Notes:

The following is the text from the HGPS paper:

"We note that this region-based spectral analysis method with a single integral flux correction factor assumes energyindependent source morphology. The spectra obtained for sources with energy-dependent morphology does not correspond to the correct total emission spectra of the sources." In the HGPS, energy-dependent morphology has been clearly established for two sources, including HESS J1825-137.

"HESS J1825-137 is a large PWN with a bright core surrounded by extended, asymmetric emission. The HGPS analysis finds it has a size of 0.46 +/- 0.03 deg , using three Gaussian components to model the VHE entire gamma-ray emission. This is significantly larger than the 0.24 +/- 0.02 deg obtained with a single symmetric Gaussian model or the 0.23 +/- 0.02 deg x 0.26 +/- 0.02 deg with a single asymmetric Gaussian in Aharonian et al. (2006). These models were stated to have rather poor chi2 goodness-of-fit values. The more complex approach taken for the morphology modeling in the HGPS improves the description of the gamma-ray emission from this PWN and accounts for the differences with respect to previous, simpler modeling."

"The HGPS catalog analysis reveals a distinct new source of VHE gamma rays, HESS J1826-130, in what was previously considered extended emission from the nearby PWN HESS J1825-137 (Aharonian et al. (2006)). "

This is one of the 31 firmly-identified objects among the HGPS sources. Four 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 J1824.5-1351e (3FGL)
- 2FHL J1824.5-1350e (2FHL)
- G18.0-0.7 (PWN)
- B1823-13 (PSR)

From Mitchell et al. (2017):
- "We find that HESS J1825-137 is not only the brightest source in
that region above 32 TeV, but is also one of the most luminous of all
firmly identified pulsar wind nebulae in the Milky Way"

Source position and its uncertainty:

On 181107 the source position was updated from that from Aharonian et al. (2006b) to that of
H.E.S.S. Collaboration et al. (2018).

From Abeysekara et al. (2020):
- We found the PWN (named VER J1825-138) to be centered at: - R.A. (J2000): 276.37 deg +/- 0.02deg (stat) +/- 0.01deg (syst) = 18h 25m 28.8 s
- Dec. (J2000): -13.83 deg +/- 0.02deg (stat) +/- 0.01deg (syst) = -13d 29' 48''

From H.E.S.S. Collaboration et al. (2018):
- "The peak of the nebula emission is found to be at:"
- R.A. (J2000): 18h 25m 49s +/- 14s (stat) +/- 5s (syst)
- Dec. (J2000): -13d 46' 35'' +/- 14'' (stat) +/- 1' (syst)

From Aharonian et al. (2006b):
- R.A. (J2000): 18 25 41 +/- 4s(stat)
- Dec. (J2000): -13 50 20 +/- 40''(stat)

From Aharonian et al. (2006a):
- l: 17.82 +/- 0.03 (deg)
- b: -0.74 +/- 0.03 (deg)
- R.A. (deg): 276.51 (Convert to HMS: 18 26 02.4)
- Dec. (deg): -13.76 (Convert to HMS: -13 45 36)

From HESS online catalog:
- R.A. (J2000): 18h 26m 02s
- Dec. (J2000): -13d 45m 36s
- l: 17.82
- b: -0.74

Source Extent:

From H.E.S.S. Collaboration et al. (2018):
- "The nebula emission is revealed to extend out to 1.5 degrees from
the pulsar, approx. 1.5 times further than previously seen, making
HESS J1825-137, with an intrinsic diameter of approx. 100 pc,
potentially the largest gamma-ray PWN currently known."
- the angular extent of the source was determined using two different datasets:
- dataset A (data from HESS I, CT 1-4; 387 hours of data from 2004-2016)
- dataset B (data from HESS II, CT 1-5; 136 hours of data from 2012-2016)
- for dataset A the major axis of the emission was found to be oriented along 208.0deg +/- 0.6deg (stat) +/- 10deg (syst)
- for dataset B the major axis of the emission was found to be oriented along 211.0deg +/- 1.0deg (stat) +/- 6deg (syst)
- "This angle was used to define the major axis of the emission
emanating South of the pulsar. The minor axis of the emission was
subsequently defined to be perpendicular to this. The orientation
angle of 17deg +/- 12deg (stat) (measured anticlockwise from North)
found by Aharonian et al. (2006) using a 2D Gaussian morphological fit
is in agreement within errors of the major axis found above,
corresponding to a orientation angle of 28.0deg +/- 0.6deg stat +/- 10.0deg (syst),
taking into account the 180 deg change of reference."
- "Slices were taken from the excess map along the major and minor
axes of the emission (208deg and 118deg) as determined from the
azimuthal profiles, with dimensions 3.5 deg x 0.5 deg"
- "The rms of the distribution along the excess slices were
0.554deg +/- 0.005deg (stat) +/- 0.05deg (syst) and
0.512deg +/- 0.005deg (stat) +/- 0.05deg (syst) along
the North-South and East-West directions respectively, a factor of 1.2
- 1.3 larger than the Gaussian widths"
- "The radial extent of the nebula was measured using the radial
profile of the emission in the Southern half of the nebula .."
- over the full energy range this method yielded a characteristic of
0.66deg +/- 0.03deg (stat) +/- 0.04(syst) for dataset A and
0.72deg +/- 0.03deg (stat) +/- 0.04(syst) for dataset B.
- "which adopting a distance of 4 kpc to the nebula, corresponds to a
physical extent of 46 pc."
- the gamma-ray emission from the nebula was analysed in different
energy bands and the angular extent was found to be energy-dependent.
- full details of the angular extent as a function of energy can be
found in Table 3 of the paper. To give an idea of the range:
Dataset A:
... 250 - 500 GeV: 0.76deg +/- 0.03deg (stat) +/- 0.2deg (syst)
... > 32 TeV: 0.14deg +/- 0.1deg (stat) +/- 0.05deg (syst)
Dataset B:
... < 125 GeV: 0.37deg +/- 0.15deg (stat) +/- 0.3deg (syst)
... 500 GeV - 1 TeV: 0.72deg +/- 0.05deg (stat) +/- 0.2deg (syst)
... > 16 TeV: 0.22deg +/- 0.12deg (stat) +/- 0.2deg (syst)

From Mitchell et al. (2017):
- the angular extent of the source was determined using two different datasets:
- dataset A (data from HESS I telescopes; 387 hours of data)
- dataset B (data from 2016 using HESS II; 101 hours of data)
- Extent (dataset A): r50 (equiv. to half width at half maximum) = 0.53deg +/- 0.02deg (stat) +/- 0.06deg (syst)
- Extent (dataset B): r50 (equiv. to half width at half maximum) = 0.51deg +/- 0.04deg (stat) +/- 0.06deg (syst)
- The major axis of the emission was found to be oriented along 214deg +/- 4deg (stat) +/- 4deg (syst)

From Mitchell et al. (2016):
- "Given its large angular extent, despite its 4~kpc distance, it may
have the largest intrinsic size of any TeV PWN so far detected."

From Aharonian et al. (2006a):
- radius: 0.16 +/- 0.02 deg

From Aharonian et al. (2006b):
- major axis: 0.26 +/- 0.02 deg (diameter)
- minor axis: 0.23 +/- 0.02 deg (diameter)
- angle: 17 +/- 12 deg
... measured counterclockwise from the North

From HESS online catalog:
- radius: 9.6' = 0.16deg

A TeV Halo?

From Sudoh et al. (2019):
- "In addition to HAWC, imaging air Cherenkov telescopes like HESS,
MAGIC and VERITAS have detected a number of extended TeV gamma-ray
sources that are associated with pulsars or PWNe observed at other
wavelengths."
- "These systems are called “TeV PWN,” but many of them have an
extension exceeding ∼10 pc (H.E.S.S. Collaboration 2018a, 2018b),
while hydrodynamical simulations predict a typical PWN size on the
order of 1 pc [e.g. Gaensler & Slane 2006; see Sudoh et al. 2019 for
other references]."
- "More pointedly, they are usually much more extended than the size
of X-ray PWN observed from the same system (Kargaltsev & Pavlov 2010,
Kargaltsev et al. 2013)."
- "These observations suggest that some of these gamma-ray sources may
be interpreted as TeV halos, instead of emission from confined
particles inside PWNe."
- "In particular, HESS J1825-137 has the largest radius (∼50 pc H.E.S.S. Collaboration 2019)
among “TeV PWN” [8]. A TeV halo explanation for this source is already
discussed in Aharonian 2004, 2013."

Source Association and Properties:

From Cao et al. (2021):
- HESS J1825-137 may be associated with LHAASO J1825-1326

From Principe et al. (2019)
- "We have analyzed 10 years of Fermi-LAT data in the energy range
between 1 GeV and 1 TeV and we have performed, for the first time with
Fermi-LAT data, an energy dependent analysis of the extension of the
PWN HESS J1825-137. The nebula HESS J1825-137 presents a strong energy
dependent morphology. The spectral index was seen by Aharonian et al. (2006)
to soften with increasing distance from the PSR, this implies that the
population of electrons in the nebula had travelled and cooled out to
large distances. The fact that the low-energy electrons at large
distances from the pulsar produce the softest spectrum was interpreted
to mean that these are the oldest electrons in the system. In this
work, the observation was extended low to few GeV confirming the
emission at large distances by the lowest energetic particles"

From Araya et al. (2019):
- "HESS J1825-137 is a bright very-high-energy (VHE) gamma-ray source
that has been firmly established as a pulsar wind nebula (PWN), and
one of the most extended gamma-ray objects within this category. The
progenitor supernova remnant (SNR) for this PWN has not been firmly
established. We carried out an analysis of gamma-ray observations in
the region of HESS J1825-137 with the Fermi-LAT which reveal emission
in the direction away from the Galactic plane. The region lies beyond
the PWN and reaches a distance from the pulsar compatible with the
supposed location of the SNR shock front. The spectrum of the gamma
rays is hard with a photon index of approx. 1.9 in the 10-250 GeV
range. Several scenarios for the origin of the emission are discussed,
including the SNR as a source of high-energy particles and the
"leakage"' of leptons from the PWN."

From Duvidovich et al. (2019):
- "The study of HESS J1825-137 was carried out on the basis of new
radio observations centred at the position of PSR J1826-1334 performed
with the Karl G. Jansky Very Large Array at 1.4 GHz in configurations
B and C."
- "The new radio continuum image towards PSR J1826-1334 reveals a
bright radio source, with the pulsar located in its centre, which
suggests that this feature could be the radio counterpart of the
compact component of the PWN detected at high energy. The new 1.4 GHz
radio data do not reveal emission with an extension comparable with
that observed in gamma-rays for the HESS J1825-137 source."

From H.E.S.S. Collaboration et al. (2018):
- "Interestingly, as part of the second HAWC catalogue, it was found
that the confused source associated with the combined emission from
the region, 2HWC J1825-134, is brighter than the Crab Nebula at 7 TeV
(Abeysekara et al. 2017). This point is included in figure 10 and is
compatible within the HAWC systematic errors of the total nebula
spectrum from HESS J1825–137."

From Khangulyan et al. (2017):
- "The very high energy gamma-ray emission reported from a number of
pulsar wind nebulae (PWNe) is naturally explained by the inverse
Compton scattering of multi-TeV electrons. However, the physical
dimensions of some gamma-ray-emitting PWNe significantly exceed the
scales anticipated by the standard hydrodynamical paradigm of PWN
formation. The most “disturbing” case in this regard is HESS
J1825-137, which extends to distances of r ≈ 70 pc from the central
pulsar PSR J1826-1334."
- The authors discuss different possibilities to explain this:
- "If the gamma-ray emission is indeed produced inside the PWN, but
not by electrons that escaped the nebula and diffuse in the
interstellar medium, the formation of such an anomalously extended
plerion could be realized, in a diluted environment ..."
- "... we explore an alternative scenario assuming that the pulsar
responsible for the formation of the nebula initially had a very short
rotation period. In this case, the sizes of both the PWN and the
surrounding supernova remnant depend on the initial pulsar period, the
braking index, and the ISM density. ... We show that this demand can
be achieved if the braking index is small, and the pulsar birth period
is short... "

From Abeysekara et al. (2017):
- this is one of three sources potentially associated with 2HWC J1825-134

From Abeysekara et al. (2015):
- "1HWC J1825-133 has a post-trials significance of 5.4sigma. It is
approx. 0.5deg to the south of the HESS J1825-137 centroid position,
which is an extended PWN with spectral softening as a function of
distance from the pulsar towards a southeast direction. The simple
power -law flux derived from this dataset is lower than the flux
extrapolated from the simple power-law assumption measured by
H.E.S.S."

From Acero et al. (2013):
- Analysis of the LAT data from this source leads to its
classification as a clearly identified PWN

Spectral and Flux Information:

From H.E.S.S. Collaboration et al. (2018):
- "The total gamma-ray flux of the nebula above 1 TeV is found to be
(1.12 +/- 0.03 (stat) +/- 0.25 (sys)) x 10e-11 cm-2 s-1, corresponding
to approx. 64% of the flux of the Crab Nebula."
- the nebula was divided into a grid of 38 boxes each comprising
0.26deg x 0.26deg and the spectrum within each of these boxes was
derived. Full details can be found in Table 5 of the paper but a
sample of the spectral indices and integrated fluxes between 1 - 5
TeV, in units of 10e-13 cm-2 s-1 TeV-1, are provided here to show the
range:
Box id, distance, spectral index, flux - with statistical and systematic uncertainties
- box 9, 0 pc from the centre: 2.00 +/- 0.03 +/- 0.02, 8.7 +/- 0.3 +/- 0.3
- box 8, 18 pc from the centre: 2.06 +/- 0.03 +/- 0.01, 10.4 +/- 0.3 +/- 0.3
- box c, 93 pc from the centre: 3.3 +/- 0.3 +/- 0.3, 0.5 +/- 0.2 +/- 0.3
The spectrum for the entire nebula was fit using three different spectral models
(power law, power law with exponential cut off and log parabola) for the two
different data sets and for different angular extents:
E0 is 1 TeV for all spectra and the flux at this energy is given in units of 10e-12 cm-2 s-1 TeV-1
The statistical and systematic uncertainties are presented in that order.
These data come from Table 2 of the paper.
Dataset A:
- Region of 0.4 deg radius:
- Power law: spectral index: 2.28 +/- 0.01 +/- 0.02, flux: 6.81 +/- 0.07 +/- 0.2
- Power law with exponential cut off: spectral index: 2.13 +/- 0.02 +/- 0.03, flux: 7.20 +/- 0.09 +/- 0.2, Ecut = 19 +/- 3 +/- 0.8 TeV
- Log parabola: spectral index: 2.26 +/- 0.01 +/- 0.02, flux: 7.4 +/- 0.1 +/- 0.1, beta = 0.078 +/- 0.008 +/- 0.01
- Region of 0.8 deg radius:
- Power law: spectral index: 2.33 +/- 0.01 +/- 0.01, flux: 17.9 +/- 0.2 +/- 0.4
- Power law with exponential cut off spectral index: 2.18 +/- 0.02 +/- 0.02, flux: 18.8 +/- 0.2 +/- 0.3, Ecut = 19 +/- 3 +/- 2 TeV
- Log parabola spectral index: 2.31 +/- 0.01 +/- 0.01, flux: 19.3 +/- 0.3 +/- 0.2, beta = 0.076 +/- 0.009 +/- 0.008
Dataset B: - Region of 0.8 deg radius:
- Power law: spectral index: 2.23 +/- 0.02 +/- 0.04, flux: 15.0 +/- 0.5 +/- 2
- Power law with exponential cut off: spectral index: 2.06 +/- 0.05 +/- 0.08, flux: 16.1 +/- 0.6 +/- 2, Ecut = 15 +/- 5 +/- 6TeV
- Log parabola: spectral index: 2.21 +/- 0.03 +/- 0.04, flux: 16.5 +/- 0.6 +/- 2, beta = 0.08 +/- 0.02 +/- 0.03
- "It is found that curved spectral models are favoured over a power
law model for all three spectra shown in table 2, at the 7 sigma level
for analysis A and 3 sigma for analysis B"
- "The total flux above 300 GeV was found to be
6.07 +/- 0.13 (stat) +/- 0.07 (syst) ×10e-11 cm-2 s-1,
corresponding to ∼ 54% of the flux of the Crab nebula. Above 1 TeV, a
flux of
1.12 +/- 0.03 (stat) +/- 0.25 (syst) x10e-11 cm-2 s-1 was obtained,
such that the proportion relative to the Crab nebula increases
slightly to approx. 64% of the Crab nebula flux. The source remains
strong at approx. 61% of the Crab nebula flux above 10 TeV. It is
impressive that the gamma-ray flux is so strong given that the
distance to HESS J1825-137 is approximately twice that of the Crab
nebula (at 2 kpc), despite (or perhaps because of) the significantly
older age of the system."

From Mitchell et al. (2017):
- The spectrum was fit using three different spectral models (power
law, power law with exponential cut off and log parabola) for two
different data sets:
- Region of 0.8 deg radius:
- spectral index (power law): 2.33 +/- 0.01
- spectral index (power law with exponential cut off): 2.17 +/- 0.02, Ecut = 19 +/- 3 TeV
- spectral index (log parabola): 2.31 +/- 0.01, beta = 0.076 +/- 0.009
- Region of 0.4 deg radius:
- spectral index (power law): 2.28 +/- 0.01
- spectral index (power law with exponential cut off): 2.13 +/- 0.02, Ecut = 19 +/- 2 TeV
- spectral index (log parabola): 2.26 +/- 0.01, beta = 0.078 +/- 0.008

From Aharonian et al. (2006b):
- Spectral index (power law): 2.38 +/- 0.02(stat) +/- 0.15(sys)
- Spectral index (power law with exponential cut off): 2.26 +/- 0.03(stat) +/- 0.15(sys), Ecut = 24.8 +/) 7.2 TeV
- Spectral index (log parabola): 2.29 +/- 0.02(stat) +/- 0.15(sys), beta = -0.17 +/ 0.04
- Spectral indices (broken power law): 2.26 +/- 0.03(stat) +/- 0.15(sys), 2.63 +/- 0.07(stat) +/- 0.15(sys), Ebreak = 2.7 +/- 0.5 TeV

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