# Revisiting Gamma-Ray Emission of the Supernova Remnant RCW 103

Yi Xing  Key Laboratory for Research in Galaxies and Cosmology, Shanghai
Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road,
Shanghai 200030, China<EMAIL_ADDRESS><EMAIL_ADDRESS>Zhongxiang
Wang Department of Astronomy, School of Physics and Astronomy, Yunnan
University, Kunming 650091, China  Key Laboratory for Research in Galaxies and
Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80
Nandan Road, Shanghai 200030, China<EMAIL_ADDRESS><EMAIL_ADDRESS>Dong Zheng  Department of Astronomy, School of Physics and Astronomy, Yunnan
University, Kunming 650091, China

###### Abstract

We analyze more than 15 years of the $\gamma$-ray data, obtained with the
Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope
(Fermi), for the region of the young supernova remnant (SNR) RCW 103, since
the nearby source 4FGL J1616.2$-$5054e, counterpart to HESS J1616$-$518 and
$\simeq$13 arcmin away from the SNR, is determined to be extended in the more
recent Fermi-LAT source catalog. Different templates for 4FGL J1616.2$-$5054e
and RCW 103 are tested, and we find that a point source with a power-law (PL)
spectrum at the southern limb of the SNR best describes the detected
$\gamma$-ray emission. The photon index of the PL emission is $\Gamma\simeq
2.31$ (or $\alpha\simeq 2.4$ in a Log-Parabola model), softer than the
previously reported $\Gamma\simeq 2.0$ when the counterpart to HESS
J1616$-$518 was considered to be a point source (which likely caused mis-
identification of extended emission at RCW 103). In order to produce the
$\gamma$-ray emission in a hadronic scenario, we estimate that protons with an
index$\sim$2.4 PL energy distribution are needed. These results fit with those
from multi-wavelength observations that have indicated the remnant at the
southern limb is interacting with a molecular cloud.

Gamma-rays (637); Supernova remnants (1667)

##  1 Introductions

The supernova remnant (SNR) RCW 103 (or G332.4$-$0.4; Rodgers et al. 1960) has
been extensively studied at multi-wavelengths. Its role as a valuable sample
of a young SNR, as well as its product, 1E 161348$-$5055 as a likely magnetar
(or previously known as a central compact object; Tuohy & Garmire 1980; D’Aì
et al. 2016; Rea et al. 2016), have attracted much attention to it. Adopting
its most recently derived distance of 3.1 kpc (Reynoso et al., 2004), its age
has been estimated to be $\sim$2 kyr (Carter et al., 1997) or $\sim$4 kyr
(Braun et al., 2019; Leahy et al., 2020). The SNR appears in the sky as a
nearly circular, shell-like structure, with a diameter of 10′. Notably, its
southern part is brighter than the regions of other directions (Figure 1),
which has been seen at multi-wavelengths from radio frequencies to X-rays
(e.g., Pinheiro Gonçalves et al. 2011).

SNRs are primary $\gamma$-ray sources in the Galaxy (e.g., Zeng et al. 2019;
Ballet et al. 2023). Cosmic-ray particles are produced from their shock fronts
through the diffuse shock acceleration process (e.g., Drury 1983). The
particles emit $\gamma$-rays via the hadronic or/and leptonic processes; the
former involves proton-proton collisions and the latter electron radiation
processes including synchrotron and inverse Compton scattering (ICS).

Previously, Xing et al. (2014) has reported likely detection of extended
$\gamma$-ray emission from RCW 103 in the 1–300 GeV data obtained with the
Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope
(Fermi). At the time, only 5 years of the Fermi-LAT data were collected and
the source model for the data analysis was based on the Fermi-LAT second
source catalog (Nolan et al., 2012). In addition, the SNR is located in a
crowded region, with two very-high-energy (VHE) sources nearby. They are HESS
J1616$-$508 and HESS J1614$-$518, which were detected in the High Energy
Stereoscopic System (HESS) survey (Aharonian et al., 2005, 2006; H. E. S. S.
Collaboration et al., 2018). In particular, the first source is $\sim$13
arcmin away from RCW 103, considered to be very close given the spatial
resolutions of Fermi-LAT and VHE detectors. In the previous analysis, the GeV
counterpart to HESS J1616$-$508 was taken as a point source (2FGL
J1615.0$-$5051), but according to the latest Fermi LAT source catalog (4FGL-
DR4; Ballet et al. 2023), both GeV counterparts to HESS J1616$-$508 and HESS
J1614$-$518 are extended sources, named as 4FGL J1616.2$-$5054e and 4FGL
J1615.3$-$5146e respectively (see Figure 1). With the updated source
information taken into consideration, we re-analyzed $>$15 years of the Fermi-
LAT data for RCW 103, and obtained new results. In this paper, we report the
results.

Figure 1: Top: TS maps for the region of HESS J1616$-$508 and RCW 103 in the
energy ranges of $\geq$0.3 GeV and $\geq 10$ GeV (left and right panel,
respectively). The known Fermi-LAT sources in the field (marked with plus
signs) are removed (i.e., included in the source model and thus not shown in
the TS maps), and the extended source 4FGL J1615.3$-$5146e (marked with a
dashed circle) is also removed. The solid circles mark the regions of the VHE
sources HESS J1616$-$508 and HESS J1614$-$518\. The region of RCW 103 and its
CCO are marked with a small cyan circle and a cyan cross, respectively.
Bottom: MOST radio map of the region. The same symbols as in top panels are
used to mark the region of RCW 103 and its CCO, while the cyan plus marks the
brightest position in the radio map. The white solid and dashed circles mark
the 1$\sigma$ and 2$\sigma$ error circles of the PS source in RCW 103 detected
in Fermi-LAT data.

##  2 Data Analysis and Results

###  2.1 FERMI-LAT data and source model

We selected the LAT events in energy range of from 0.1 to 500 GeV from the
updated Fermi LAT Pass 8 database (P8R3). The region of interest (RoI) was set
to be centered at HESS J1616$-$508 with a size of 20∘ $\times$ 20∘. The start
and end time of the events were August 4 2008, 15:43:39 (UTC) and December 11
2023, 23:26:35 (UTC), respectively. We included the events in the SOURCE event
class with zenith angles less than 90 degree to avoid the Earth’s limb
contamination, and excluded events with ‘bad’ quality flags. Both selections
are recommended by the LAT
team111http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/.

A source model was constructed by including all sources within a 20-degree
radius of HESS J1616$-$508 based on 4FGL-DR4 (Ballet et al., 2023). The
positions and spectral parameters for these sources provided in the catalog
were adopted. For sources within 5-degree radius of HESS J1616$-$508, we set
their spectral parameters free, and for the remaining sources, their spectral
parameters were fixed at their catalog values. In this latest source catalog,
the counterpart of HESS J1616$-$508 is an extended source described by a
uniform disk with a radius of 0$\fdg$32, and its $\gamma$-ray spectrum is
modeled with a power law (PL). The spectral model gll_iem_v07.fit and spectral
file iso_P8R3_SOURCE_V3_v1.txt were included as the Galactic and extragalactic
diffuse emission, respectively. The normalizations of the two diffuse-emission
components were always set as free parameters in our analysis.

In constructing 4FGL catalogs (Abdollahi et al., 2020), the LAT events below
316 MeV were given low weights, especially in the vicinity of the Galactic
plane. In addition, only Front events were utilized in the low energy band
below 316 MeV (Abdollahi et al., 2020) in order to avoid the relatively poor
point spread function (PSF) and the contamination from the Earth limb. Given
these, we did not include the events below 300 MeV in our analysis since the
target is located in the Galactic plane with a few nearby sources around.

###  2.2 Maximum Likelihood Analysis

With the source model, we performed the standard binned likelihood analysis to
the LAT data in 0.3–500 GeV. A PL photon index $\Gamma$=2.02$\pm$0.03 and a
0.3–500 GeV photon flux $F_{0.3-500}\simeq(3.5\pm 0.2)\times 10^{-8}$ photon
cm-2 s-1 were obtained for HESS J1616$-$508, and the Test Statistic (TS) value
was 1640.

To examine the source field, we calculated TS maps centered at HESS
J1616$-$508 in different energy ranges. All sources in the source model,
except the counterpart of HESS J1616$-$508, were included and thus removed in
the TS maps. In Figure 1, we showed TS maps in the energy ranges of 0.3–500
GeV and 10–500 GeV as the representative ones, where the latter helps reveal
the emission in a high, $\lesssim 0\fdg 2$ spatial
resolution222https://www.slac.stanford.edu/exp/glast/groups/canda
/lat_Performance.htm (smaller than the separation between HESS J1616$-$508 and
RCW 103). As can be seen in the low-energy TS map, possibly extended emission
within the source region of HESS J1616$-$508 (H. E. S. S. Collaboration et
al., 2018) is detected, which is enclosed by the extended source 4FGL
J1616.2$-$5054e given in 4FGL-DR4. We also noted that the other extended
source 4FGL J1615.3$-$5146e, corresponding to HESS J1614$-$518, is cleanly
removed. However, the position of the highest TS values within 4FGL
J1616.2$-$5054e is off the center towards the south-east direction, where RCW
103 is located. In the high-energy TS map, the high TS-value region is
possibly composed of two components, one slightly off the center of 4FGL
J1616.2$-$5054e and the other close to the location of RCW 103. Thus, it is
likely that emission from RCW 103 was detected even though it is close to an
extended source.

Table 1: Comparison of maximum likelihood values Source Model | 2$\times(\log L-\log L_{0}$)
---|---
HESS0.32 | 0
HESS0.40 | 43.2
PS $+$ HESS0.32 | 57.1
Radiodisk $+$ HESS0.32 | 63.5
Radioprofile $+$ HESS0.32 | 71.3
Radiops $+$ HESS0.32 | 82.1
Radiops $+$ HESS0.38 | 106.4

###  2.3 Spatial Distribution Analysis and Source Identification

In order to fully probe the $\gamma$-ray emission in the region, we ran a
series of spatial analysis with different spatial templates considered. The
base model was that given in 4FGL-DR4, which sets a 0$\fdg$32 uniform disk
template for the GeV emission of HESS J1616$-$508 (HESS0.32 model). All the
likelihood results were compared to the likelihood $L_{0}$ of this base model.

We first tested a uniform disk with radius ranging from 0$\fdg$02 to 0$\fdg$50
(with a step of 0$\fdg$02) centered at HESS J1616$-$508 as a check for the
source’s GeV $\gamma$-ray emission. A simple PL was used to model the
emission. We found that when radius was 0$\fdg$40 (HESS0.40 model), a maximum
$\log L$ value was obtained, which indicates a fit improvement at a
6.6$\sigma$ significance level (calculated from $\sqrt{2\times(\log L-\log
L_{0})}$; Table 1) compared to the HESS0.32 model.

Figure 2: TS maps for the region of HESS J1616$-$508 and RCW 103 in the energy
range of 5–500 GeV, with the GeV counterpart to HESS J1616$-$508 kept (left)
and removed (right). The symbols are the same as in Figure 1.

We then added different model templates in the source model at the position of
the central compact object (CCO) of RCW 103. The templates included a point
source (PS; PS model), a uniform disk with a radius of 5$\arcmin$ (given RCW
103’s size of 10$\arcmin$ in diameter detected at radio frequencies; Radiodisk
model), and an emission template derived from the Molonglo Observatory
Synthesis Telescope (MOST) radio map333http://snrcat.physics.umanitoba.ca/
SNRrecord.php?id=G332.4m00.4 (Radioprofile model). For HESS J1616$-$508, the
catalog HESS0.32 template was still used. Performing the maximum likelihood
analysis to the data with each of the models, PS $+$ HESS0.32, Radiodisk $+$
HESS0.32, and Radioprofile $+$ HESS0.32, the likelihood values were obtained
and compared to that of the base model (see Table 1). The model of
Radioprofile $+$ HESS0.32 provided the best fit, as it induced a 8.4$\sigma$
fit improvement compared to only HESS0.32. In addition, we noted that the fit
improvement was 5.3$\sigma$ when the non-catalog model HESS0.40 we obtained
above was considered for comparison. These results indicate that extra
emission is present at the position of RCW 103.

We further removed the counterpart of HESS J1616$-$508 from TS maps to reveal
the residual emission in the RCW 103 region. Two models for the HESS
J1616$-$508 source were considered, one the HESS0.32 model and the other a
uniform disk with a radius of 0$\fdg$38 (HESS0.38 model). The latter was used
because of the results obtained in the following analysis. As a check, we
first calculated two TS maps in 0.3–5 GeV respectively with the two models and
verified that no significant differences between the two models were found. We
then calculated TS maps in 5–500 GeV without and with HESS0.38 being removed
(Figure 2). Comparing the two TS maps, it is clear to see the removal of the
emission from HESS J1616$-$508 and the presence of the emission at RCW 103.
However, the latter did not exactly match the SNR’s circular region,
suggesting further analysis investigation would be needed.

We ran gtfindsrc in the Fermitools to the $>$5 GeV data and determined the
position of the residual emission. We obtained R.A.=244$\fdg$427,
Decl.=$-$51$\fdg$119 (equinox J2000.0) with a 1$\sigma$ nominal uncertainty of
0$\fdg$047\. The CCO is 0$\fdg$081 away from this position and outside of the
2$\sigma$ error circle (0$\fdg$077). Matching the position and
1$\sigma$/2$\sigma$ error circles to the MOST radio map of RCW 103 (Figure 1),
it can be seen that the source region is actually over the brightest part of
the SNR’s radio shell. Given this, we set a model for the residual emission
which is at the position of the brightest part of the shell (Radiops model),
and performed the likelihood analysis. The used $\gamma$-ray template for HESS
J1616$-$508 was HESS0.32. Comparing the $\log L$ values, we found that the
residual emission is more likely a point source located at the bright radio
shell, as the fit improvement of the model Radiops $+$ HESS0.32 is at
4.3$\sigma$ and 3.3$\sigma$ significance levels with respect to the models of
Radiodisk $+$ HESS0.32 and Radioprofile $+$ HESS0.32, respectively (see Table
1).

Figure 3: $\gamma$-ray spectra of RCW 103 and HESS J1616$-$508 and their
respective spectral models obtained from the likelihood analysis. For RCW 103,
we also show a Log-Parabola fit obtained in Section 2.3 (red dashed curve).

To be as complete as possible, we re-checked the extension of the counterpart
of HESS J1616$-$508 by setting Radiops for RCW 103. The uniform disk with a
radius of 0$\fdg$02–0$\fdg$50 (in a 0$\fdg$02 step) was tested again. We found
that a 0$\fdg$38 radius (thus the HESS0.38 model used above) resulted in a
maximum $\log L$ value and the fit improvement of the model of Radiops $+$
HESS0.38 is at a 4.9$\sigma$ significance level with respect to that of
Radiops $+$ HESS0.32 (Table 1). We thus considered this Radiops $+$ HESS0.38
model as the final one and used it in the following analysis.

Based on this model, we obtained $\Gamma=1.98\pm$0.03,
$F_{0.3-500}\simeq(3.1\pm 0.3)\times 10^{-8}$ photon cm-2 s-1 for the
counterpart of HESS J1616$-$508 (TS=1233) and $\Gamma=2.31\pm$0.07,
$F_{0.3-500}\simeq 1.1\pm 0.2\times 10^{-8}$ photon cm-2 s-1 for RCW 103
(TS=165). Compared to the results previously reported in Xing et al. (2014),
in which a point source was considered for HESS J1616$-$508, the emission has
a higher $\Gamma$ value (or softer). The $\gamma$-ray luminosity is 2.4$\times
10^{34}$ erg s-1 at a distance of 3.1 kpc.

We also tested a Log-Parabola model, $dN/dE=N_{0}(E/E_{b})^{[-\alpha-\beta
log(E/E_{b})]}$, for the emission of RCW 103. The parameters $\alpha$ and
$\beta$ were set free, and $E_{b}$, a scale parameter, was always fixed.
Different $E_{b}$ values were tested. If we fixed it at 5 GeV, the resulting
likelihood value indicated the fitting was better than that with the PL at a
$\sim$2$\sigma$ significance level, where $\alpha=2.4+/-0.6$,
$\beta=0.11+/-0.04$, and $F_{0.3-500}\simeq(9.5\pm 3.6)\times 10^{-9}$ photon
cm-2 s-1 (TS $\simeq 166$). The results are similar to those with the PL (note
that the curvature parameter $\beta$ is very small), but the uncertainties are
larger.

###  2.4 Spectral Analysis

We extracted the $\gamma$-ray spectra of HESS J1616$-$508 and RCW 103 by
performing the maximum likelihood analysis of the LAT data in 10 evenly
divided energy bands in logarithm from 0.1 to 500 GeV. In the extraction, the
spectral normalizations of the sources within 5 degree of the center of HESS
J1616$-$508 were set as free parameters, while all the other parameters were
fixed at the values obtained from the maximum likelihood analysis (the Radiops
$+$ HESS0.38 model). Emissions of HESS J1616$-$508 and RCW 103 were
respectively described by a PL, with $\Gamma=2$. For the obtained spectral
data points, we kept those with TS$\geq$9 and used the derived 95% flux upper
limits instead when TS$<$9 (Table 2).

We also evaluated the systematic uncertainties induced by the uncertainties of
the Galactic diffuse emission. Following the commonly-used method (e.g., Abdo
et al. 2009, 2010), we repeated the likelihood analysis in each energy band
with the normalizations of the Galactic diffuse component artificially fixed
to the $\pm$6% deviation values from the best-fit values. This $\pm$6%
deviation represents the local departure from the best-fit diffuse model
obtained from analyzing the source-free regions in the Galactic plane.

The obtained spectra with both statistical and systematic uncertainties are
shown in Figure 3, while the flux and TS values of the spectral data points
are provided in Table 2. The PL spectral models obtained for the two sources
from the likelihood analysis match the spectral data points. However for RCW
103, the two upper limits in 17–91 GeV (Table 2) are at the model-fit line,
showing a slight inconsistency. Thus, its emission may be better described
with the Log-Parabola model we obtained in Section 2.3, which is also shown in
Figure 3.

Table 2: Flux Measurements

Band (GeV) | $G_{\rm HESS}$ | TS | $G_{\rm RCW}$ | TS
---|---|---|---|---
0.15 (0.1–0.2) | 1.43 | 0 | 1.73 | 0
0.36 (0.2–0.5) | 2.11 | 0 | 0.77$\pm$0.26 | 44
0.84 (0.5–1.3) | 1.17$\pm$0.41 | 119 | 0.72$\pm$0.63 | 67
1.97 (1.3–3.0) | 1.67$\pm$0.34 | 294 | 0.33$\pm$0.12 | 26
4.62 (3.0–7.1) | 1.43$\pm$0.26 | 198 | 0.37$\pm$0.07 | 47
10.83 (7.1–16.6) | 1.66$\pm$0.21 | 210 | 0.34$\pm$0.07 | 41
25.37 (16.6–38.8) | 1.63$\pm$0.21 | 146 | 0.18 | 1
59.46 (38.8–91.0) | 1.66$\pm$0.25 | 89 | 0.15 | 0
139.36 (91.0–213.3) | 1.56$\pm$0.33 | 44 | 0.28 | 0
326.60 (213.3–500.0) | 1.02$\pm$0.47 | 10 | 1.31 | 6

Note: $G$ is the energy flux ($E^{2}dN/dE$) in 10-11 erg cm-2 s-1. Fluxes
without uncertainties are the 95$\%$ upper limits.

###  2.5 Variability Check

We checked the $\gamma$-ray emission of RCW 103 for any long-term variability.
Following the procedure introduced in Nolan et al. (2012), the variability
index TSvar of a 60-day binned light curve (94 time bins) in 0.3–500 GeV was
calculated, and we obtained TS${}_{var}\simeq 62.8$. Given that a variable
source would be identified at a 99% confidence level when
TS${}_{var}\geq$127.6 (for 93 degrees of freedom), the emission did not show
significant long-term variability.

##  3 Discussion

Using the latest Fermi-LAT source catalog, we have revisited the detection of
$\gamma$-ray emission of RCW 103 by analyzing the data collected for $>15$
years. The SNR’s emission is resolved at high-energy bands (Figures 1 & 2),
even though the nearby source 4FGL J1616.2$-$5054e, the counterpart to the VHE
source HESS J1616$-$508, has been determined to be extended. Differing from
the previous result reported in Xing et al. (2014) that the emission was
extended, we have found that it is consistent with being a PS. It has a PL
spectrum with photon index $\Gamma\simeq 2.31$ or a Log-Parabola one with
slope $\alpha\simeq 2.4$, and the spectrum is softer than the previously
reported ($\Gamma\simeq 2.0$ in Xing et al. 2014). We note that this spectrum
is very similar to that of the SNR Kes 17, which had $\Gamma\simeq 2.39$
(Supán et al., 2023).

The determined error-circle region of the PS emission coincides with the
southern limb of RCW 103 (Figure 1), the brightest part of the SNR. No non-
thermal X-ray emission has been detected from any part of RCW 103 (e.g., Braun
et al. 2019), which otherwise may provide information for the origin of the
$\gamma$-ray emission. On the other hand, multi-wavelength observations have
shown different pieces of evidence for the interaction of the remnant with a
molecular cloud (MC) in the vicinity of the southern limb (see, e.g., Oliva et
al. 1999; Paron et al. 2006; Reach et al. 2006; Pinheiro Gonçalves et al.
2011; see also Braun et al. 2019 and references therein). Considering these,
the positional coincidence thus strongly suggest a hadronic origin for the
$\gamma$-ray emission.

Figure 4: SED of RCW 103, including radio fluxes (blue data points), an X-ray
flux upper limit (green arrow), and $\gamma$-ray fluxes (red data points).
Electrons with an index$\sim 2.3$ PL energy distribution produce the pink and
yellow spectra through synchrotron radiation and ICS, respectively, and
protons with an index$\sim 2.4$ PL energy distribution produce the grey
spectrum through the hadronic proton-proton collisions.

We studied the broadband spectral energy distribution (SED) of the southern
limb of RCW 103. The radio flux measurements of the whole SNR were provided in
Beard (1966), Goss & Shaver (1970), Shaver & Goss (1970), Caswell et al.
(1980), and Dickel et al. (1996), and we divided them by 2 to approximate the
radio fluxes of the southern part. We analyzed archival X-ray data from one
Chandra observation of RCW 103 (Obsid:18459), and obtained the X-ray flux in
0.5–7 keV from a region just outside the southern limb as the upper limit on
non-thermal X-ray emission, where the value (unabsorbed) was $\simeq 1.7\times
10^{-11}$ erg s-1 cm-2. The radio fluxes and the X-ray flux upper limit, as
well as the $\gamma$-ray fluxes, are shown in Figure 4.

To approximately fit the SED, we assumed that the particles accelerated in the
SNR have a power-law form in energy with a high-energy cutoff:

$dN_{i}/dE=A_{i}E^{-\alpha_{i}}\mathrm{exp}(-E/E_{i,c})\ ,$ (1)

where $i=e/p$, the electrons or protons, $A_{i}$ is the normalization, and
$\alpha_{i}$ and $E_{i,c}$ are the power-law index and the cutoff energy,
respectively. The total energy ($W_{i}$) is obtained from the integral of the
equation. We used the Python package naima (Zabalza, 2015) to find reasonable
fits to the SED.

By fitting the radio data with synchrotron radiation of the electrons, we
obtained $\alpha_{e}=2.28\pm 0.01$, magnetic field strength $B=24.9\pm 0.3$
$\mu$G, and $W_{e}=3.8\pm 0.1\times 10^{47}$ erg, where $E_{e,c}$ was fixed at
1 TeV. The same population of the electrons also upper-scatter the low-energy
photons in the field and produce $\gamma$-ray emission (i.e., the ICS
process). Assuming the cosmic microwave background and the interstellar
radiation field with a temperature of 30 K and an energy density of 0.5 eV
cm-3 (Porter et al., 2006) at the source position, the produced $\gamma$-ray
spectrum from ICS is shown in Figure 4, which is low compared to the observed
$\gamma$-ray fluxes. It should noted that in this fitting practice, the
allowed parameter ranges are much larger than those reported above, mainly
because of the lack of a tight constraint in X-rays. Different combinations of
the parameters can provide adequate fits to the radio data. However, we also
noted that none of the combinations can simultaneously fit the $\gamma$-ray
data. Because of the following hadronic model results (which have
$\alpha_{p}\simeq 2.4$), we chose to report the above parameters because of a
similar electron PL index.

In order to fit the $\gamma$-ray data points with a hadronic model, in which
$\gamma$-ray emission primarily arises from proton-proton collisions, we
obtained $\alpha_{p}=2.4\pm 0.1$ and $W_{p}=2.4^{+0.9}_{-0.5}\times 10^{49}$
erg, where $E_{p,c}=1$ TeV was fixed and the target density was assumed to be
$n_{0}=10\ {\rm cm^{-3}}$ (also adopted in Xing et al. 2014). As shown in
Figure 4, the model spectrum can adequately describe the $\gamma$-ray data
points. In addition, the spectrum drops to low flux values above $\sim 0.1$
TeV, which may explain the non-detection of this SNR in the TeV band (e.g., H.
E. S. S. Collaboration et al. 2018). The hadronic-model fitting results, in
combination with the positional coincidence of the $\gamma$-ray emission with
the southern limb, thus point to the remnant-MC interaction as the likely
origin for the high-energy emission from RCW 103.

This research is supported by the Basic Research Program of Yunnan Province
(No. 202201AS070005), the National Natural Science Foundation of China
(12273033), and the Original Innovation Program of the Chinese Academy of
Sciences (E085021002). D.Z. acknowledges the support of the science research
program for graduate students of Yunnan University (KC-23234629).

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