A&A 664, A89 (2022)
https://doi.org/10.1051/0004-6361/202142431

Astronomy&
Astrophysics© ESO 2022

Supernova remnant G46.8–0.3: A new case of interaction withmolecular material
L. Supan1 , G. Fischetto2 , and G. Castelletti1
1

Instituto de Astronomía y Física del Espacio (IAFE, CONICET − UBA) CC 67, Suc. 28, 1428 Buenos Aires, Argentinae-mail: lsupan@iafe.uba.ar
2
Universidad Nacional del Sur (UNS), Av. L. N. Alem 1253, B8000CPB – Bahía Blanca, Argentina
Received 13 October 2021 / Accepted 8 March 2022
ABSTRACT

Although the Galactic supernova remnant (SNR) G46.8–0.3 has been known for more than 50 yr, no specific studies of this source
or its environment have been published to date. To make progress on this matter, we measured new flux densities from radio surveys
and combined them with previous estimates carefully collected from the literature to create an improved and fully populated versionof the integrated radio spectrum for G46.8–0.3. The resulting spectrum exhibits a featureless power-law form with an exponent α =−0.535 ± 0.012. The lack of a spectral turnover at the lowest radio frequencies, which is observable in many other SNRs, excludes
the presence of abundant ionised gas either proximate to the SNR itself or along its line of sight. The analysis of local changes in
the radio spectral index across G46.8–0.3 suggests a tendency to slightly steepen approximately at 1 GHz. Even if this steepening isreal, it does not impact the integrated spectrum of the source. Deeper imaging of the radio structures of G46.8–0.3 and spectral maps
constructed from matched raw data are needed to provide new insights into the local spectral properties of the remnant. On the basis of
the spectral properties of the atomic gas, we placed the remnant at 8.7 ± 1.0 kpc and we revisited the distance to the nearby H II region
G046.495−00.241 to 7.3 ± 1.2 kpc. From evolutionary models and our distance estimate, we conclude that G46.8–0.3 is a middleaged (∼1 × 104 yr) SNR. Furthermore, we recognise several 12 CO and 13 CO molecular structures in the proximity of the remnant.
We used combined CO-H I profiles to derive the kinematic distances to these features and characterise their physical properties. We
provide compelling evidence for environmental molecular clouds physically linked to G46.8–0.3 at its centre, on its eastern edge, andtowards the northern and southwestern rims on the far side of the SNR shell. Our study of the molecular matter does not confirm
that the remnant is embedded in a molecular cavity as previously suggested. G46.8–0.3 shows a line-of-sight coincidence with the
γ-ray source 4FGL J1918.1+1215c detected at GeV energies by the space telescope Fermi. A rough analysis based on the properties of
the interstellar matter close to G46.8–0.3 indicates that the GeV γ-ray photons detected in the direction to the SNR can be plausibly
attributed to hadronic collisions and/or bremsstrahlung radiation.
Key words. ISM: supernova remnants – ISM: individual objects: G46.8–0.3 – radio continuum: general – gamma rays: ISM

1. Introduction
The distribution and density of structures in the ambient medium
unquestionably represent key elements shaping the evolution ofsupernova remnants (SNRs). Atomic and molecular constituentsof the interstellar medium (ISM) have been robustly detected in
the vicinity of remnants from both core-collapse (CC, Type IIor Ib/c SNe; see, for instance, Kuriki et al. 2018; Supan et al.
2018; Sano et al. 2020a,b) and those created by the deflagration of an accreting white dwarf (two emblematic examples are
the Type Ia SNR Tycho in our Galaxy, Zhou et al. 2016, Chenet al. 2017, and N103B in the Large Magellanic Cloud, Sanoet al. 2018). Reciprocally, the physical properties of the interstellar material can be drastically modified by the passage oflong-lived SN shocks. However, this is not the complete story
because the synergy with the medium can begin even before theSN event. Indeed, several SNRs have been observed expanding
into shells and cavities carved by outflow phenomena in the progenitor stars. Examples of this are the CC SNRs Kes 75 (Su et al.
2009) and CTB 87 (Liu et al. 2018), as well as the SN Ia remnants Kepler (Chiotellis et al. 2012) and RCW 86 (Sano et al.
2019).

Observations also show that a considerable fraction of wellknown interacting SNRs are bright in γ rays at GeV energies (Tang 2019). Interpreted as a manifestation of accelerated
hadrons, this result helps to improve our understanding of
how the interplay between environmental properties and physical conditions at the SNR shock impacts the production andpropagation of cosmic rays.
In the present study, we look in detail at the relationshipbetween SNR G46.8–0.3 (also known as HC 30) and the interstellar matter around it. The earliest view at radio wavelengthsof this remnant was presented by Willis (1973). However, sincethen, no works devoted to the remnant or its immediate environment have been published. Some characteristics of the radio
emission from G46.8–0.3 are only recorded in papers dealing
with a sample of SNRs (e.g. Kassim 1989b; Dubner et al. 1996;
Sun et al. 2011; Ranasinghe & Leahy 2018). Concerning theambient medium, the only information presented to date comesfrom the atlas of CO-line structures compiled by Sofue et al.
(2021) for a large population of Galactic SNRs including G46.8–
0.3. These authors pointed out that a SNR-molecular cloud
scenario is feasible in direction to G46.8–0.3, as they found evidence suggesting it evolves in a molecular shell at 52 km s−1 .

Article published by EDP Sciences

A89, page 1 of 13

A&A 664, A89 (2022)

Adding to the picture described above, γ-ray emission has been
observed at GeV energies by the Fermi Gamma-ray Space Telescope in the direction of G46.8–0.3 (Abdollahi et al. 2020) but
its relationship with the remnant has not been studied to date.
This paper is organised as follows. A description of the data
used to analyse the radio structure of G46.8–0.3, as well as thosecorresponding to the atomic and molecular components of theinterstellar matter in the direction to the remnant is provided
in Sect. 2. Then, in Sect. 3, we examine the radio-continuum
spectral properties of the remnant. The analysis of the spatial distribution of the atomic material in the region of G46.8–0.3 alongwith a revised distance determination to our target source and thenearby H II region G046.495−00.241 are given in Sect. 4. Thissection also includes an estimation of the age of G46.8–0.3 madefrom standard evolutionary models. The spatial distribution of
the molecular gas in the G46.8–0.3 direction and the spectral
properties of the unveiled CO structures are discussed in Sect. 5.
In Sect. 6, we explore possible scenarios for the GeV γ-rays
detected in the region of the remnant. We close by summarising
our findings in Sect. 7.

2. Data
2.1. Radio continuum emission

We used continuum data from The HI/OH/Recombination Line
Survey of the Inner Milky Way (THOR1 , Beuther et al. 2016)
to trace the forward shock of G46.8–0.3 at 1.4 GHz. This image
represents the best view at radio wavelengths of the remnant presented to date. It includes interferometric observations from the
Karl G. Jansky Very Large Array combined with data from the
VLA Galactic Plane Survey (VGPS, Stil et al. 2006), which inturn includes short spacing data from the Effelsberg Radio Telescope. The angular resolution of the 1.4-GHz image is 2500 and
its rms noise level is 0.72 mJy beam−1 .
In order to track down possible changes in the radio spectral index as a function of frequency and position across theremnant, we constructed maps from the direct ratio between the
1.4-GHz THOR+VGPS image and the ones at 200 MHz (HPBW
2.0 9 × 2.0 4, rms ∼ 30 mJy beam−1 ) extracted from the Galactic
and Extragalactic All-Sky Murchison Widefield Array Survey(GLEAM2 , Wayth et al. 2015, Hurley-Walker et al. 2019), andat 4.8 GHz (HPBW 3.0 6 × 3.0 4, rms ∼ 9 mJy beam−1 ) from theGreen Bank Northern Sky Survey (GB6, Gregory et al. 1996).
Before combining the radio images, all of them were convolved
to a resolution of 40 and also aligned and interpolated to have apixel-by-pixel match to each other. Even though they were notmatched in the uv-plane, we highlight that the resulting maps
are still appropriate for revealing trends in the spectral index
distribution across G46.8–0.3.
2.2. Interstellar gas data and spectral analysis

Firstly, we notice that throughout this paper the velocities of theatomic and molecular line emissions are always referred to in thelocal standard of rest, LSR. The mean error in our radial velocity measurements is 7.6 km s−1 , estimated from the combined
effects of streaming motions, the spectral resolution of the data,
and the uncertainty in determining the central velocity. All ofthese contributions were added in quadrature.

Regarding the atomic component of the ISM, it was analysedthrough the neutral hydrogen (H I) 21 cm line emission. The data
used to this purpose were extracted from VGPS (Stil et al. 2006),
which combines observations carried out with the VLA and datafrom the 100-m single-dish Green Bank Telescope at the NRAO.
The beam size of the observations is ∼6000 . The spectral resolution and the typical rms noise level per channel are 1.56 km s−1
and ∼2 K, respectively.
To characterise the molecular environment of G46.8–0.3, weused observations in the rotational transition emission J = 1–
0 of the carbon monoxide isotopologues 12 CO and 13 CO takenfrom the publicly available FOREST Unbiased Galactic Plane
Imaging Survey constructed with the Nobeyama 45-m telescope(FUGIN3 , Umemoto et al. 2017). The angular resolution of thedata is ∼2000 , with a sensitivity of 0.24 K for 12 CO and 0.12 K
for 13 CO. For both CO isotopologue lines, the velocity resolutionof the data is 1.3 km s−1 , with a separation between consecutive
velocity channels of 0.65 km s−1 .
In our analysis, we used molecular line measurements of
12
CO to estimate the column density of the cold molecular hydrogen NH2 . We derived NH2 values using the conversion factor recommended by Bolatto et al. (2013), 2 ×1020 cm−2 (K km s−1 )−1 , between NH2R and the integrated 12 CO
J = 1–0 surface brightness W12 CO = T B (v) dv km s−1 , where
T B (v) is the 12 CO brightness temperature.
Using the estimation made for NH2 , the mass of the molecular features (assumed mostly consisting of molecular hydrogen)discovered in the surveyed region around G46.8–0.3 SNR comesfrom the relation M = µ mH d2 Ω NH2 . Here µ = 2.8 is the mean
molecular weight if a relative helium abundance of 25% isassumed, mH is the hydrogen mass, and Ω represents the solid
angle along the light of sight subtended by the molecular structure placed at a distance d. In addition, the number density ofeach cloud is obtained through the expression nH2 = NH2 /L,
where L is the depth of the molecular cloud in the line of sight,
which was assumed to be equal to its average size in the planeof the sky. The physical properties of the molecular features
identified in the direction of G46.8–0.3 are reported in Table 2.

3. An overview of the radio emission from
G46.8–0.3
The so-called THOR+VGPS image of the SNR G46.8–0.3 at
1.4 GHz is presented in Fig. 1. The mapped region also showsthe H II region G046.495−00.241 at RA ' 19h 17m 28s , Dec '11◦ 550 4500 (J2000) reported in the WISE Catalog of Galactic H II
Regions (Anderson et al. 2014)4 .
At radio wavelengths, G46.8–0.3 consists of an almost circular shell of ∼170 in diameter centred at RA ' 19h 18m 04s ,Dec ' 12◦ 090 3100 . The SNR surface brightness at 1 GHz isΣ1 GHz ' 8.3 × 10−21 W m−2 Hz−1 sr−1 . Although about 1700
times fainter than Cas A (the brightest SNR in the Galaxy),G46.8–0.3 still belongs to the large group of galactic SNRs
with intermediate values of Σ1 GHz (Green 2019). The brightness distribution in G46.8–0.3 is not uniform but presents several
spots and filamentary structures throughout the SNR shell. The
brightest parts of the remnant are localised towards two welldifferentiated regions in the north and the south of the shell
which have the appearance of “caps” at opposite sides of the3

https://nro-fugin.github.io/
Alternative names used in the literature to refer to this thermal source
are G46.5−0.2 (Kuchar & Bania 1990) and G46.495−0.25 (Quireza
et al. 2006).
41
2

https://www2.mpia-hd.mpg.de/thor/Overview.html
https://www.mwatelescope.org/gleam.

A89, page 2 of 13

The integrated spectrum of G46.8–0.3 at radio frequencies
that we created is plotted in Fig. 2. The weighted best fit to thedata corresponds to a single power-law slope

L. Supan et al.: Radio properties of the SNR G46.8–0.3 and its environs
Table 1. Integrated flux densities on SNR G46.8–0.3 used to construct
the radio continuum spectrum of the source presented in Fig. 2.

Fig. 1. Radio view of SNR G46.8–0.3 at 1.4 GHz from combined THOR
and VGPS continuum data (Beuther et al. 2016). The colour scale varies
linearly and is in units of mJy beam−1 . The angular resolution is 2500 andthe sensitivity is 0.72 mJy beam−1 . The bright source at the southwest
corner is the H II region named G046.495−00.241 in the WISE Catalog
of Galactic H II Regions (Anderson et al. 2014).

Frequency(MHz)

Scaled flux
(Jy)

Reference

30.9
38
83
88
111
118
155
178
200
400
408
610
610
960
1400
1400
1414
1420
1465
2695
2700
2730
3900
4800
4850
4850
5000
5000
10 300
11 20093.9 ± 18.8 (∗)
100 ± 15 (∗)
57.7 ± 9.9
70.9 ± 8.9
54.1 ± 9.8
55.0 ± 6.2
41.5 ± 4.6
42.6 ± 6.4
33.5 ± 3.4
28.0 ± 5.6 (†)
22.7 ± 4.5
22.9 ± 2.3 (†)
21.5 ± 3.2
16.0 ± 1.6
17.0 ± 3.4 (†)
16.7 ± 2.5
17.7 ± 1.8
15.9 ± 1.6
13.3 ± 0.1
11.0 ± 1.1
9.9 ± 1.5
10.0 ± 0.9
8.7 ± 0.9
6.9 ± 0.2
6.9 ± 0.4
7.2 ± 0.4
6.6 ± 0.7
5.6 ± 0.6
4.0 ± 0.8
5.1 ± 0.5

Kassim (1988)
Holden & Caswell (1969)
Kovalenko et al. (1994a)
This work (GLEAM)
Kovalenko et al. (1994a)
This work (GLEAM)
This work (GLEAM)
Holden & Caswell (1969)
This work (GLEAM)
Downes (1971)
Holden & Caswell (1969)
Moran (1965)
Holden & Caswell (1969)
Trushkin (1996)
Downes (1971)
Holden & Caswell (1969)
Altenhoff et al. (1970)
This work (THOR+VGPS)
Dubner et al. (1996)
Altenhoff et al. (1970)
Day et al. (1970)
Willis (1973)
Trushkin (1996)
Sun et al. (2011)
Condon et al. (1989)
This work (GB6)
Caswell & Clark (1975)
Angerhofer et al. (1977)
This work (NRO)
Trushkin (1996)

shell. The former is approximately 100 × 30 in size with two
spots at its sides towards RA ∼ 19h 18m 07s , Dec ∼ 12◦ 160 4200
and RA ∼ 19h 17m 45s , Dec ∼ 12◦ 150 0500 . The latter is ∼50 × 20
in size, with the brightest part towards RA ∼ 19h 18m 06s , Dec ∼12◦ 020 3900 . The filaments are mainly detected in the southernhalf of the source and are aligned approximately in the southwest
to the absolute scale of Perley & Butler (2017)
to northeast direction. A featureless band of weaker emission Notes. Fluxes were set (∗)(see text for details). No correction factor was applied to bringcrosses the remnant from east to west in the northern half of the measurement to the Perley & Butler (2017) scale because thethe shell. Additionally, a slight flattening of the shell boundary corresponding frequency is outside its 50 MHz–50 GHz validity range.
can be noted along the southwestern border, as well as a small (†) A correction to the absolute scale of Perley & Butler (2017) was notdent in the northwest rim coincident with the western spot of the applied due to the lack of information on the flux density calibrator
bright northern feature. These features could suggest a deceler- source.
ation of the shock due to interaction with the surroundings. The
correlations of the radio shell of G46.8−0.3 with the molecular
In our study, we also adopted the absolute flux density scale
component of the ISM is addressed in Sect. 5.
best-fit values.
presented by Perley & Butler (2017) to tied fluxes for frequenIn our study, we also adopted the absolute flux density scale cies between 50 MHz and 50 GHz, the range where the scale ispresented
by Perley& Butler(2017) to tied fluxes for frequen- accurate to 3% and up to 5% for measurements at the extreme
3.1.
Global radio
continuum
spectrumcies between 50 MHz and 50 GHz, the range where the scale frequency values. Table 1 contains all the flux density values
We
continuum
spectrum ofatG46.8–
is updated
accuratethe
to integrated
3% and upradio
to 5%
for measurements
the ex- for G46.8–0.3 covering about 2.5 decades in frequency from
0.3. For this purpose, we measured flux densities over the whole 30.9 MHz to 11.2 GHz. Only three fluxes in our list were notextension of the source using publicly available images at low- adjusted to this common scale because of the lack of information
radio frequencies from GLEAM, along with others at higher about the calibrator sources in the original publications.
frequencies from THOR+VGPS, and GB6 (details of these
The integrated spectrum of G46.8–0.3 at radio frequencies
observations are in Sect. 2.1), and the Radio Continuum Survey that we created is plotted in Fig. 2. The weighted best fit to theof the Galactic Plane at 10 GHz made with the Nobeyama Radio data corresponds to a single power-law slope α = −0.535 ± 0.012
Observatory (NRO, HPBW ∼2.0 7, rms ∼ 33 mJy beam−1 , Handa (S ν ∝ να ). This integrated spectral index is compatible withet al. 1987). We also combined our new flux determinations with what is expected for a middle-aged shell-type SNR accelerata carefully selected set of previously published estimates. While ing radio-emitting electrons via a first-order Fermi mechanism.
compiling these data, we rejected flux measurements previously Compared with previous determinations made for G46.8–0.3,
reported with error estimates greater than 20%, as well as data our result is largely consistent with those by Kovalenko et al.
points showing a large scatter beyond the range of 2σ best-fit (1994b; α = −0.53 ± 0.10), Dubner et al. (1996; α ∼ −0.53), andvalues.
Sun et al. (2011; α = −0.54 ± 0.02), but is flatter than the spectra
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A&A 664, A89 (2022)

Fig. 2. Updated integrated radio continuum spectrum of SNR G46.8–
0.3 constructed using fluxes listed in Table 1. Blue circles correspond tofluxes extracted from the literature, whereas the green squares are our
new measurements obtained from radio surveys (see text for details).
The solid line is the best-fitting curve to the weighted data and yields aspectral index α = −0.535 ± 0.012 (S ν ∝ να ). Shaded regions represent
the variation of the best-fit values at 1- and 2-σ levels.

measured by Kassim (1989a; α ∼ −0.6) and Taylor et al. (1992;
α = −0.72 ± 0.14, estimated using only flux density values at
327 MHz and 4.8 GHz). We consider that our spectrum notably
improves on the previous ones published for G46.8–0.3, most ofthem with larger relative error in the spectral index determination. This is essentially a consequence of the fact that we havefilled gaps along the radio frequency domain by compiling twice
as many flux density values as Kovalenko et al. (1994b) and Sun
et al. (2011), as well as setting the majority of the measurements
ensity scale
to the most accurate absolute flux density scale presented toor frequendate.

re the scales at the ex3.2. Local variations of the spectral index across G46.8–0.3

We present the spectral index maps between 200 MHz and
1.4 GHz and between 1.4 GHz and 4.8 GHz in Figs. 3a and b,
respectively. An explanation of how these maps were constructedis presented in Sect. 2.1. Uncertainties for the local spectral index
measurements on both maps are not higher than 0.1. We do not
attempt to quantify the remaining parameter errors because wehave no access to the collection of calibrated radio visibilities tomatch the data at each frequency before constructing the spectral
maps. Nevertheless, the spectral images we are presenting here
are still useful for tracing local trends in the radio domain. Some
contours of the 12 CO (J = 1–0) line emission are over-plotted on
the spectral maps for reference.
As revealed in Fig. 3a, the distribution of spectral index values in the 200 MHz–1.4 GHz map is essentially flat, with amean value α ∼ −0.3 over a horseshoe-like shape (in yellowgreenish shaded) that seems to align with the molecular material
into which the SNR shock front is running, which is an issue
we revisit in Sect. 5. The spectrum becomes slightly steeper(α ∼ −0.45) towards the north portion of the remnant. By comparison, the spectral indices between 1.4 and 4.8 GHz appear to
be more uniformly distributed and steeper than at lower frequencies ranging from 200 MHz to 1.4 GHz. A moderately flatter spot(α ∼ −0.5) is also noticeable in the 1.4/4.8 GHz map towards theeastern side of G46.8–0.3.
There seems to be a general trend in our spectral index mapsof a slight concave-down behaviour (i.e. a steeper spectrum tohigher radio frequencies). We recognise that the maps do nothave sufficient sensitivity to be able to decipher whether or not
A89, page 4 of 13

this signature primarily occurs in the brightest portions of theremnant. If so, it might be a consequence of a higher effective shock compression undergone by low-frequency electrons
in the interaction with adjacent clouds, which can cause local
increases in the magnetic field strength and/or the particle density. Even if this scenario is true for the case of G46.8–0.3
it has no measurable effect on the integrated spectrum of thesource, which as shown in Sect. 3.1 is well fitted by a single
power law (see Fig. 2). There are a few cases of SNRs showing a steepening at high radio frequencies in their integrated
spectra, with frequency cut values varying from at around afew MHz up to GHz (e.g. Galactic SNRs S147, Xiao et al.
2008 and HB 21, Pivato et al. 2013, and J0527–6549 in the
Large Magellanic Cloud, Bozzetto et al. 2010). All of them are
evolved SNRs for which the observed global spectral form was
explained in the standard diffusive acceleration shock theory by
the compression of the Galactic magnetic field in dense interstellar regions combined or not with a possible contribution ofsynchrotron losses of the high-energy electrons (see Urošević
2014 and Xiao et al. 2008 for a review of this subject). However,
we consider the occurrence of synchrotron losses in G46.8–0.3
unlikely. Indeed, the radiative cooling time of GeV electrons producing radio synchrotron emission in a magnetic field B scales
as tsyn ≈ 1.3 × 1010 (B/µG)−2 (E/GeV)−1 yr (Gaisser et al. 1998)
and in the case of G46.8–0.3 is found to be tsyn ∼ 5 × 106 yr.
In this calculation, we assume E = 3 GeV for the energy of theelectrons, along with an interstellar magnetic field strength in
the clouds of BISM ' n1/2 µG (Hollenbach & McKee 1989) and
B = 4BISM ∼ 30 µG in the upstream region because of the shockcompression in a gas with a mean molecular hydrogen density n ∼ 50 cm−3 (obtained from Table 2 presented below). We
notice that our tsyn estimate is absolutely comparable with typical values derived for SNRs evolving in relatively dense ambient
medium (see e.g. Fig. 5.3 in Aharonian 2004). However, as weshow in Sect. 4.2, it is much larger than the age of the SNR(tSNR ∼ 1 × 104 yr). More sensitive radio observations towards
G46.8–0.3, particularly at the low-frequency band, are required
to confirm the validity of our analysis of local variations in the
spectral index before coming to any firm conclusions.

4. Results of the H I data
Figure 4 displays the H I intensity in a velocity range from 10 to
66 km s−1 superposed on the 1.4 GHz radio continuum emission
from G46.8–0.3 and the nearby H II region G046.495−00.241.
The interstellar hydrogen appears to be distributed all aroundthe rim of G46.8–0.3 throughout the velocity interval presented
in Fig. 4. Over the SNR shell, the brightness temperature of the
H I decreases significantly as compared with values observed
outside the G46.8–0.3 boundary. There are H I local depressions,which are especially noticeable towards the northern and southern parts of the SNR shell. We checked that the spectra extracted
from these regions show clear signs of absorption. Due to theirmorphological correspondences with the brightness continuum
emission from the remnant, we argue that these features correspond to absorbing atomic gas located in front of G46.8–0.3
relative to the line of sight. There is also a faint belt-like featureof H I, seen for velocities greater than ∼30 km s−1 crossing theremnant from northeast to northwest. We find no signs of strong
absorption (only a 3σ signal) in the H I profiles (not presentedhere) obtained in this region. Of course, the low surface brightness of the remnant could be limiting the absorption, but alsomeans that most of the atomic gas forming the belt-like feature
could be placed on the far side of the SNR. In the southwest of

L. Supan&A
et al.:
Radio properties of the SNR G46.8–0.3 and its environs
proofs:Fig. 3. Spectral index maps for SNR G46.8–0.3 constructed between (a) 200 MHz and 1.4 GHz and (b) 1.4 and 4.8 GHz. The images used are from
GLEAM, THOR+VGPS, and GB6 surveys (see Sect. 2.1). Regions with flux densities lower than 4σ level of the respective rms noise in each image
were clipped. The colour scale displayed to the right indicates the spectral index measured (with a 40 resolution) over the SNR at both frequency
bands. Dark and light green contours represent the intensity of the FUGIN 12 CO (J = 1–0) molecular line emission (at a 20 spatial resolution)
integrated in the 30–50 km s−1 (levels: 500 and 800 K km s−1 ) and 50–66 km s−1 (levels: 1100 and 1400 K km s−1 ) ranges, respectively. A detailed&A
proofs: is presented in Sect. 5.
treatment of the molecular features associated with
G46.8–0.3

Fig. 4. Velocity channel maps of the H I emission in SNR G46.8–0.3 from VGPS spectral line data overplotted with the THOR+VGPS contours(levels: 70, 120, and 150 mJy beam−1 ) of the radio continuum radiation from the remnant and the H II region G046.495-00.241. For comparison,
the radio continuum image was smoothed to the 6000 spatial resolution of the H I data. Before constructing the maps presented in the figure, anappropriate mean background level was subtracted from each H I velocity channel. The integration velocity range is indicated in the bottom left
corner of each panel. All the velocity ranges are displayed with the linear colour scale in units of K km s−1 shown in panel a.

the remnant, the H I is seen in absorption against the continuum
radiation from the nearby H II region G046.495−00.241.
In summary, the distribution of the neutral gas towards
G46.8–0.3 does not show evidence for neighbouring structuresthat may have been impacted by the SN shock. In addition,
no signatures for material swept up due to the activity of theremnant or its progenitor are observed. If an expanding H I structure were present, it would be observed in a reduced velocity
interval of the atomic distribution as a region of low emission
surrounded by a shell completely or partially correlated withthe outer boundary of the SNR. A structure of this kind is
not observed in the case of G46.8–0.3. Our analysis based on
VGPS data confirms the previous result obtained by Park et al.
(2013) in the direction of G46.8–0.3 using lower spatial resolution (HPBW = 30 .35) 21 cm line data from the Inner-Galaxy
Arecibo L-band Feed Array (I-GALFA) H I survey (Gibson et al.
2012).
4.1. Newly derived distance for SNR G46.8–0.3 and thenearby H II region G046.495-00.241

There is no agreement on the estimates for the distance to G46.8–
0.3 reported in the literature. Sato (1979) suggested a distance
between 6.8 and 8.6 kpc from the analysis of H I absorption.

Later, Ranasinghe & Leahy (2018) from the analysis of H I spectral features, proposed lower and upper limits of the distance of
5.7 and 11.4 kpc. More recently, Liu et al. (2019) listed a distanceof 6.4 kpc through the detection of a hydrogen radio recombination line (RRL) using data from the Survey of Ionized Gas of
the Galaxy carried out with the Arecibo telescope (SIGGMA,
Liu et al. 2013).
In order to improve the distance estimate to our target and toobtain a more complete picture of the physical association withthe rich ambient medium – which we discuss later in Sect. 5 –,we begin with a discussion of the properties of the H I spectrum
seen against the remnant. To do so, we used the H I cube fromthe VGPS. Figure 5a shows H I emission and absorption profiles for G46.8–0.3. The emission spectrum was obtained in theline of sight directly towards the bright southern portion of thesource, while the absorption spectrum was constructed by subtracting the emission profile from a spectrum averaged over a
set of regions close to the remnant. Firstly, we mention that theabsence of any absorption feature at negative velocities in the
spectrum5 means that the SNR is inside the solar circle. Thus,
according to the circular rotation curve model of the Galaxy by
Reid et al. (2014), an upper limit on the distance of ∼11.4 kpc

5 We recall there is no kinematic distance ambiguity for the H I gas in
the first quadrant of the Galaxy at negative velocities.

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A&A 664, A89 (2022)

revealed in Fig. 5b, the H I spectrum exhibits absorption at the∼4σ level beyond the RRL velocity up to the velocity of thetangent point, which at the Galactic longitude of the H II regionis ∼65.6 km s−1 . Therefore, we placed the thermal source at thefar distance of 7.3 ± 1.2 kpc, in accordance with the distancethat we established in Sect. 5 to the molecular emission spatiallycoincident with it. We find that our distance determination ismarkedly consistent with the 7.8 kpc value reported by Quireza
et al. (2006) from measurements of intensity and width in helium
RRL data, but incompatible with those provided by Kuchar& Bania (1990) (d = 3.9 kpc) obtained using H I absorption
spectrum with data from the Boston University-Arecibo Galactic
H I Survey, Anderson & Bania (2009; d = 3.8 ± 0.6 kpc) basedon a combination of H I and CO sky surveys, and Anderson
et al. (2014; d = 4.7 ± 0.1 kpc) in their WISE census of H II.
4.2. Age estimate for G46.8–0.3

Fig. 5. H I line emission (green curve) and absorption (violet curve)
spectra against (a) G46.8−0.3 SNR and (b) G046.495−00.241 H II
region. The solid vertical line in each panel marks the velocity of thetangent point at the longitude of the sources according to the Reidet al. (2014) Galactic rotation curve, whereas the dashed vertical line inpanel b is for the hydrogen RRL at 58.4 km s−1 from G046.495−00.241
recorded in Liu et al. (2019). The horizontal shaded regions indicate therms noise level of ∼4 K in the H I absorption spectra.

(which approximately corresponds to a radial velocity of
0 km s−1 ) can be established. Secondly, we detect absorption up
to the velocity of the tangent point, which at the longitude of
the SNR is around 65 km s−1 (Reid et al. 2014). This represents
a lower limit on the distance of ∼5.7 kpc. The H I absorption beyond the tangent point velocity probably corresponds tovelocity perturbations near the tangent point, caused for instanceby expansion of the SNR. Additionally, we stress that significant and continuous absorption is observed from the velocity of∼ 40.4 km s−1 to that of the tangent point, which might even
imply that a better estimate of the minimum distance is the far
side distance for ∼40.4 km s−1 . Therefore, we assign a kinematic distance of 8.7 ± 1.0 kpc to the remnant. The error in our
measurement is primarily caused by uncertainties in determining
radial velocities along the line of sight, departures from circularmotions of the Galaxy, as well as uncertainties associated withthe selection of the rotation curve and solar motion. All of thesecontributions were added in quadrature in our calculations.
Now, we turn our attention to the H II region
G046.495−00.241 seen in the area mapped in Fig. 1 as abright source with an angular diameter of ∼70 . The centre ofthis source and that of the remnant are separated from eachother by approximately ∼160 .4. In order to revisit the kinematic
distance to G046.495−00.241, in Fig. 5b we present absorption
and emission profiles that we have constructed using spectralline VGPS observations directed to the central portion over
the thermal source. The velocity for the H II region deducedin Liu et al. (2019) from hydrogen RRL is 58.4 km s−1 . The
corresponding near and far distances inferred with the rotation
curve of Reid et al. (2014) are 4.2 and 7.3 kpc, respectively. As
A89, page 6 of 13

Taking into consideration the 8.7 ± 1.0 kpc distance to G46.8–
0.3 that we determined in this work and its angular radius '8.0 5
measured from the 1.4 GHz THOR+VGPS image, the physicalradius of the SNR is approximately RSNR ' 21.5 pc. We used thisresult to estimate the dynamical age of the remnant by employing
evolutionary models. To this end, we note that in surveys suchas VTSS6 (Dennison et al. 1998), DSS7 , APASS8 , or IPHAS9(Barentsen et al. 2014) there is no appreciable optical emission
in apparent association with G46.8–0.3, indicative of radiative
shocks. Also, no X-ray emission was detected associated with
the SNR in the only observation (∼9.9 ks) for this remnant available in the Chandra Data Archive10 . Nevertheless, although it is
not a strong constraint on age, the large size of G46.8–0.3 could
suggest that it is an evolved object. Under this interpretation, we
hypothesise that the remnant is in the later adiabatic expansion
stage of its evolution. We caution that, although it can be reliable in terms of the order of magnitude, our modest estimationof the age of G46.8–0.3 suffers from large uncertainties derived
from the assumed parameter values and the assumption of its
current evolutionary stage. According to the numerical approachof Cox (1972), for a Sedov-Taylor (ST) blast wave shock witha specific heat ratio γ = 5/3, the age for G46.8–0.3 given by
ST
tSNR = 14.5 (RSNR )5/2 (n0 /E51 )1/2 is about tSNR
∼ 1 × 104 yr. In
the calculations above, E51 is the kinetic energy of the explosion normalised to the canonical value 1051 erg, assumed to be
E51 = 1 in this work11 , while n0 is the pre-shock number density for which we adopted n0 ∼ 0.1 cm−3 , a value of reference
between a typical density inside a wind bubble excavated by thestellar progenitor (n0 ∼ 0.01 cm−3 ) and that of the diffuse intercloud gas (n0 ∼ 1 cm−3 ) (Inoue et al. 2012). In addition, theexpansion velocity of the SN shock during the adiabatic phase
S T −3/5
is vs ≈ 13.4 × 104 (tSNR)(E51 /n0 )1/5 km s−1 ∼ 845 km s−1 .
6

The Virginia Tech Spectral-Line Survey, http://www1.phys.vt.
edu/~halpha/#Images
7 The STScI Digitized Sky Survey, https://archive.stsci.edu/
cgi-bin/dss_form
8 The AAVSO Photometric All-Sky Survey, https://www.aavso.
org/apass
9 The INT Photometric Hα Survey of the Northern Galactic Plane,
https://www.iphas.org/
10 The non-detection of X-ray emission would support that the SNR has
significantly cooled.
11 By applying spherically symmetric SNR evolution models to a sample of 43 objects in the Galaxy with X-ray observations and reliable
distance estimates, Leahy et al. (2020) calculated a mean value ∼4 ×1050 erg for the energy released in a SN event.

L. Supan et al.: Radio properties of the SNR G46.8–0.3 and its environs

Fig. 6. Overall CO spectra covering the SNR G46.8–0.3 region. The
three shaded areas correspond to LSR velocity ranges for which thereis good spatial correspondence between the molecular matter and theremnant (see discussion in the text). The velocity of the tangent point(∼65 km s−1 , Reid et al. 2014) is marked with a black vertical line.

Following the approach presented by Blondin et al. (1998), thetransition time to the radiative phase will occur at ttr ≈ 2.9 ×
4/17 −9/17
104 E51n0
yr ∼ 1 × 105 yr, with a velocity of the shock wave
1/17 2/17
vs ≈ 260 E51 n0 km s−1 ∼ 200 km s−1 .

5. Results of the CO data
5.1. Morphology of the CO molecular gas components

Figure 6 depicts the average spectrum of the 12 CO and 13 CO (J =1–0) molecular lines over a circular region of radius 140 concentric with the remnant G46.8–0.3. Multiple velocity components
are evident towards the SNR in the range of ∼10–65 km s−1 .
In what follows, we investigate the spatial distribution of themolecular gas around G46.8–0.3 traced by CO isotopologues,leaving a discussion of the spectral characteristics of the molecular ambient matter to Sect. 5.2. We have omitted the analysisof the molecular gas responsible for the very narrow (FWHM∆v ' 2.5 km s−1 ) emission feature at ∼7 km s−1 because we findno evidence of association with the SNR.
In Fig. 7, we present three-colour integrated intensity mapsof the molecular environments in the direction of G46.8–0.3(12 CO J = 1–0 in red and 13 CO J = 1–0 in green) obtainedby FUGIN, together with the SNR radio synchrotron emission
at 1.4 GHz (in blue) as traced by THOR+VGPS data. In sucha representation, 12 CO observations typically delineate emission
mostly coming from low-density gas in the outer layers of thecloud, while the 13 CO data generally trace optically thin emission from denser regions. The mapped CO emissions correspondto molecular structures for which we consider there is a convincing match with the remnant. They are in three velocity intervals:
10–30 km s−1 , 30–50 km s−1 , and 50–66 km s−1 . The molecular matter in these velocity intervals is hereafter referred toas Regions A, B, and C, respectively. According to their kinematical properties, which are analysed in detail in Sect. 5.2, theregions are also separated into subareas or clouds. In the notation adopted throughout this work12 , each cloud is labelled witha capital letter (A, B, or C), a number referencing the distanceto the cloud, and the letters N, E, W, SW, or C depending onwhether the cloud is located to the north, east, west, or southwest of the remnant or at its centre, while the H II abbreviationis added for the gas immersed in the G046.495−00.241 region.
12 Our notation is similar to that adopted by Sano et al. (2017) in theirstudy of RCW 86 SNR.

Fig. 7. Colour representation of the molecular ambient gas towards SNR
G46.8–0.3 traced by the 12 CO J = 1–0 (in red) and the 13 CO J = 1–
0 (in green) rotational line emissions. Yellow regions are areas where
both CO isotope emissions overlap. The SNR radio continuum radiation at 1.4 GHz from THOR+VGPS surveys is represented in blue. The
velocity range used to integrate the CO intensity is shown in the bottom
left corner of each panel. The name of the discovered molecular features and their kinematic distances are also labelled. White continuous
polygons enclose structures associated with G46.8–0.3. Non-associatedclouds localised at the far distance of their central velocity are encircled
by green continuous polygons, whereas foreground molecular gas along
the line of sight is inside orange dashed polygons. Numbers from 1 to 9
spread out in panels a–c indicate the test regions used to construct the
spectra presented in Fig. 8.
A89, page 7 of 13

A&A 664, A89 (2022)

As shown in Fig. 7a, the gas in Region A at velocities in the
10–30 km s−1 range comprises two molecular components distributed around the SNR shell. These are: (i) the cloud named
A-10N and (ii) the cloud A-10SW. The former closely follows
the northern edge of the remnant. In addition, part of this gas
appears to be projectionally embedded in the synchrotron emission from the remnant encompassing a distortion observed in theouter envelope of the SNR shell at the position RA ∼ 19h 17m 45s ,Dec ∼ 12◦ 150 4000 . This correspondence is compatible with ashock wave of the SNR interacting with the ambient matter. The
molecular concentration also extends outside the northern limbof the remnant towards the southwest. The second molecularcomponent, labelled A-10SW, consists of a set of clumps concentrated on the southwest edge of the SNR, exactly at the sitewhere the shock front is flattened.
For intermediate velocities, 30–50 km s−1 , the ambient
medium in Region B can be divided into three main zones(Fig. 7b): (i) the molecular concentration towards the east,
including the clouds B-3E and B-8E, which correlates with thelimb of the remnant and extends out about 200 in the northeast direction beyond the shock front; (ii) the molecular gas
distributed in the interior of the SNR shell in a northeastsouthwest orientation, labelled cloud B-9C, which appears topass through bright patches of the radio continuum emission;and (iii) the western region comprising clouds B-3W and B-8W,
which seems to be part of a molecular wall in the environmentof our target source.
At the highest velocities, 50–66 km s−1 , there is molecular
gas seen in projection all around the remnant (Fig. 7c). The best
match is observed towards the east where the CO in the C-6E
region closely follows the edge of G46.8–0.3. The spatial correlation with the remnant decreases in the western half of themap, where the cloud C-4W is observed outside the SNR shock
front, but still around it. Towards the southwest is the molecular gas concentration named C-7.5HII, probably the birthplaceof the star-forming H II region G046.495−00.241.
Figure 7 also reveals that all the identified molecular regions
contain multiple enhancements where the 13 CO J = 1–0 line
emission is detected at levels higher than 5σ in coincidence withlocal maxima in the 12 CO emission. The 13 CO excess noticeable
in the clouds A-10N, A-10SW, B-8E, and B-9C seen in projection onto the radio shell of G46.8–0.3 may indicate strong
compression of the molecular gas due to interaction with the
SNR shock front.
At first glance, G46.8–0.3 seems to be embedded within adense molecular cavity. The existence of an almost complete COline cavity at 52 km s−1 associated with the remnant was alreadyproposed by Sofue et al. (2021) based on the FUGIN survey of
12
CO- and 13 CO (J = 1–0)-line channel maps, the same data weused in the current work. These authors did not determine thekinematic distance to the structure but mentioned possible nearand far values of 3.9 and 7.1 kpc, respectively. On the basis of
an CO–H I combined spectral analysis, in the following section
we determine the distance to each molecular structure unveiledin our work and hence we revisit the hypothesis of a cavity in themolecular medium around G46.8–0.3.
5.2. Kinematics of the CO molecular gas components

Here, we focus on determining the kinematic distances to theuncovered molecular structures comprising regions A-C shownin Fig. 7. To achieve this we extracted multiple H I and CO
emission and absorption spectra covering the complete extension of each cloud, some of which are presented in Fig. 8. The
A89, page 8 of 13

test areas corresponding to our examples are marked with numbers from 1 to 9 in Fig. 7. In Table 2, we report average physicalproperties (peak intensity, velocity, line-width, angular size, column density, mass, and number density) computed over each CO
structure unveiled in this work. Column densities and masses
were calculated from the expressions presented in Sect. 2.2 andusing the distance estimates provided here.
The sources of uncertainty in our distance results computedfrom CO–H I combined spectra are the same as those of our distance determination to the SNR and the H II region derived in
Sect. 4.1 from H I profiles only. As listed in Table 2, the final
error in the measured distances to the molecular clouds is better
than 10%, with the exception of those located at the near-side
or close to the tangent point, for which the mean error is ∼33%.
On the other side, the final error in the measured distances tothe molecular clouds combined with both the error of at least
30% in the CO-to-H2 conversion factor (Bolatto et al. 2013) andthe uncertainties in isolating overlapping regions of sky translate
into errors in the calculated properties (i.e. column densities ofeach velocity component and mass) of the interstellar gas. See
further details in Table 2.
5.2.1. Region A: 10–30 km s−1

Now, we analyse the molecular gas inside Region A, spatiallycorrelated with the northern and the southwestern edges ofG46.8–0.3 (Fig. 7a). The spectra corresponding to the selectedareas over this region appear in Figs. 8a, b. The excellent correspondence between the southern part of the molecular gas
emission from the cloud named A-10N and the 1.4 GHz brightest emission from G46.8–0.3 is consistent with an interaction
zone between the remnant and its immediate environment. If
so, the distance to the molecular region should agree with that
established for the SNR (8.7 ± 1.0 kpc, see Sect. 4.1). The 12 CO
spectrum over the A-10N cloud seen (in projection) immersedin the radio shell of the SNR exhibits a peak at ∼15 km s−1
with a 13 CO counterpart. Using the flat rotation curve modelof Reid et al. (2014), the kinematic distances associated withthis velocity are 1.0 and 10.2 kpc. In addition, the H I 21 cm
profile in the molecular component exhibits dips corresponding to CO emission lines at velocities up to the velocity ofthe tangent point, the maximum velocity that the foregroundclouds can have. The observed CO-H I anti-correlation can be
explained in terms of foreground molecular clouds along theline of sight absorbing the continuum radiation from the SNR(Roman-Duval et al. 2009). Therefore, we assigned a far side
distance of 10.2 ± 0.7 kpc to the investigated cloud. If the cloud
were located at the near position of its velocity, absorbed features
would be observed in the H I profile correlated with CO emission lines up to the velocity of the clump only. We also noticethe 12 CO line profiles in Region A deviate to higher velocities
relative to the 13 CO line maximum intensity. This is not surprising, as the latter is a more faithful tracer of the denser regions in
the molecular gas. At this stage, we note that the spectra (notpresented here) constructed over the molecular gas extending
beyond the SNR boundary confirm that the A-10N cloud is asingle structure placed at ∼10 kpc. Therefore, taking into consideration that we have established the location of the SNR at
8.7 ± 1.0 kpc, we interpret the asymmetry in the 12 CO line emission (line FWHM ∆v ' 6.5 km s−1 ) as a sign of perturbation
caused by the SNR shock wave penetrating into the most massive (∼214 × 103 M ) cloud interacting with G46.8–0.3, whichis physically located at the backside of the remnant. The striking morphological correspondence between the molecular gas

L. Supan et al.: Radio properties of the SNR G46.8–0.3 and its environs

Fig. 8. Collection of composite spectra of the CO (J = 1–0) and H I 21 cm line emissions towards test areas over the molecular structures discoveredin the direction of SNR G46.8–0.3, as shown in Fig. 7. The correspondences between the molecular and atomic emissions were used to determine
the kinematic distance to individual clouds (see analysis in Sect. 5.2). The shaded zones in yellow highlight the CO peak and the LSR velocity inthe region we are interested in. The blue vertical line in each spectrum indicates the velocity of the tangent point according to the rotation curvemodel of Reid et al. (2014). For the remaining spectra (not shown here) used in our analysis, we found approximately the same behaviour in eachregion as that exhibited by the spectra presented here.

and the radio boundary also further supports the shock–cloudinteraction idea.
Upon inspection of the molecular gas in Region A, wenoticed that the spectra constructed over the western extremeof the northern cloud, that is, where it curves to the south (atRA ∼ 19h 17m 15s , Dec ∼ 12◦ 150 2000 , see Fig. 7a), display a COemission peak at ∼16 km s−1 correlated with H I self-absorption
against warm 21 cm backgrounds (emitting at the same velocity
as the cloud). This behaviour implies that this part of the surveyed Region A, referred to as A-1N, is at the near distance of
1.1 ± 0.6 kpc. We estimated that the mass of this gas only represents a small fraction (<1%) of the molecular material in thenorth brightening in the velocity range 10–30 km s−1 .
As shown in Fig. 7a, the molecular gas in Region A also comprises bright clumps in the southwest (cloud A-10SW), where
the rim of G46.8–0.3 clearly deviates from a circular geometry.
The CO profile presented in Fig. 8b was obtained in a test area
just outside the SNR over the clump with a central velocity of∼19 km s−1 . The correlation of the CO peak with a maximum inthe H I emission indicates that this part of the molecular region
resides at the far position of 10.1 ± 0.6 kpc.
5.2.2. Region B: 30–50 km s−1

Over the CO material brightening between 30 and 50 km s−1
in the eastern area outside the remnant we find spectra showing H I self-absorption features at ∼41 km s−1 (Fig. 8c) andothers revealing CO peak intensities at ∼48 km s−1 coincident
with H I maxima (Fig. 8d). This means that the ambient matter eastwards of G46.8–0.3 results from a chance coincidenceof near-side (cloud B-3E at 2.8 ± 0.8 kpc) and far-side (cloud
B-8E at 8.2 ± 0.7 kpc) components superposed along the lineof sight. At RA ∼ 19h 18m 30s , Dec ∼ 12◦ 130 0000 , the molecularemission bends to the west and overlaps the eastern portion of
G46.8–0.3. The spectral properties over this zone (spectra notincluded in Fig. 8) indicate that it is an extension of the molecular material outside G46.8–0.3. Indeed, for the near-side gas, thecold H I embedded in the foreground molecular clouds absorbs
the continuum radiation from the SNR and produces spectral H I
21 cm absorption lines up to a velocity of around 41 km s−1 of thematerial we are interested in, while over the far-side, component
H I 21 cm valleys corresponding to CO emission line features
are observed up to the velocity of the tangent point. We also
estimated that the amount of gas in the near-side component constitutes ∼10% of the total material in the eastern region shown in
Fig. 7b. As opposed to the incidental near- and far-side overlap of
the CO structures in the east of the SNR, the spectral propertiesof the central B-9C cloud, which appears embedded in the SNR
shell (RA ∼ 19h 18m 00s , Dec ∼ 12◦ 060 1000 ) at a velocity of about
40 km s−1 , allow us to conclude that it corresponds to a single
cloud at the far kinematic distance of 8.7 ± 0.6 kpc. As an example, Fig. 8e presents a spectrum extracted over this central area.
The obtained distance to the cloud is fully compatible with the
distance we measured for G46.8–0.3 and therefore we interpretthe enhanced filaments of radio continuum emission as a traceof the SNR shock impacting on the relatively dense (∼40 cm−3 )B-9C cloud. It is worth mentioning that we find little evidence ofdisturbance in the profiles of the CO extracted over the molecular gas superimposed on the remnant. However, this result does
not rule out an association with G46.8–0.3.
Finally, we mention that near and far molecular components with line-of-sight coincidence were detected westwardsof G46.8–0.3 at a velocity of ∼48 km s−1 (spectra not shownhere). These cloud components are inside the regions we denote
A89, page 9 of 13

A&A 664, A89 (2022)
Table 2. Measured and derived properties for the molecular structures identified in the direction of SNR G46.8–0.3.

(1)
Velocityrange(km s−1 )(2)
Molecular
cloud
(MC)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

Distance

T peakvLSR∆vFWHM

Size

NH2

Mtot

nH2

(kpc)

(K)

(km s−1 )(km s−1 )(arcmin2 )(1021 cm−2 )(103 M )(cm−3 )

10 – 30

A-1 N
A-10 N
A-10SW

5.7
3.6
4.3

15.5
18.7
19.3

6.9
6.5
6.7

0.4
214
13

760
23
55

No
Yes
Yes

30 – 50

3.5
3.3
3.2
4.8
5.3

41.0
47.1
40.8
48.2
46.9

5.3
4.3
4.5
3.8
6.0

8×5
22 × 10
7×4

4.7
3.3
2.7

B-3 E
B-8 E
B-9 C
B-3 W
B-8 W

1.1 ± 0.6
10.2 ± 0.7
10.1 ± 0.6

2.7
5.4
2.5
3.1
4.7

10
106
23
2.7
107

70
52
40
175
50

No
Yes
Yes
No
No

50 – 66

C-6 E
C-4 W
C-7.5 HII

4.5
4.9
5.3

63.4
57.5
56.1

7.5
10.6
8.8

7.4
9.6
9.8

160
90
255

65
180
86

UncertainNoNo

2.8 ± 0.8
8.2 ± 0.7
8.7 ± 0.6
3.3 ± 0.9
8.2 ± 0.7
6.3 ± 2.0
4.2 ± 1.8
7.5 ± 1.2

19 × 1121 × 79×7
7×5
15 × 1033 × 820 × 927 × 7(11)
SNR/MC
association

Notes. The reported values are relative to the 12 CO J = 1–0 line transition and represent averages obtained over the entire extension of eachindividual cloud. The detailed analysis of the spectral characteristics of the molecular gas is presented in Sect. 5.2. Columns (1)–(2): velocity
intervals and names corresponding to the CO clouds identified in this work. For reference, see Fig. 7. Column (3): kinematic distances to eachcloud computed from CO and H I line emission and absorption spectra. The atomic and molecular data come from the VGPS and FUGIN surveys,
respectively. Columns (4)–(6): peak intensity (T peak ), line-of-sight velocity (vLSR ), and line width (full width at half maximum, ∆vFWHM ), respectively. Velocity measurements have a mean associated error of 7.6 km s−1 , as explained in Sect. 2.2. The reported estimates were calculated byfitting Gaussian functions to the CO-H I spectra. Column (7): average dimensions (height × width) of each CO cloud. Columns (8)–(9): molecularhydrogen column density (NH2 ) and total mass (Mtot ) of the clouds. Both magnitudes were calculated using expressions given in Sect. 2.2. The
mean percentage error in our estimates of NH2 and Mtot are, respectively, ∼35% and ∼50% (for the interacting clouds); see discussion in Sect. 5.2.
Column (10): number density of the molecular hydrogen computed from the expression nH2 = NH2 /L by assuming a path length along the line ofsight, L, equivalent to the mean linear size of each cloud listed in Col. (7). The error in nH2 is typically 40%. Column (11): classification of thephysical association between the SNR G46.8–0.3 and the discovered CO clouds based on the analysis presented in Sect. 5 of this work.

B-3W and B-8W, respectively. As shown in Fig. 7b, this ambient
medium was not overtaken by the SNR shock. We determinedthat approximately only 3% of the surveyed gas in the west is at
the near position of 3.3 ± 0.9 kpc forming the cloud B-3W, while
the remaining material is at 8.2 ± 0.7 kpc inside the cloud B-8W
(see Table 2).
5.2.3. Region C: 50–66 km s−1

To analyse the molecular material in Region C we divided it
into three areas of differing spectral properties. The first zone
comprises the gas forming the large eastwards structure (cloudC-6E). Towards the western half of the field is the second
molecular component (namely C-4W zone). In this region, at
a distance of up to ∼180 from G46.8–0.3, the CO (J = 1–0)line emission is significantly detected from RA ∼ 19h 17m 45s ,Dec ∼ 12◦ 270 0000 to RA ∼ 19h 17m 00s , Dec ∼ 12◦ 000 0000 (seeFig. 7c). Lastly, the third important area corresponds to the gas
seen projected onto the G046.495−00.241 H II region (the C7.5HII cloud). Before describing the spectra, we point out that
because of the small velocity difference measured in the profiles of the C-6E and C-4W areas between their CO peaks andthe tangent point (vTP ∼65 km s−1 , derived from the Reid et al.
2014 model) it is not simple to conclusively determine whether
the clouds are at the near or the far position. Even thougharguments based on morphological correlations and spectral correspondences between the CO and H I gases indeed may help toestimate their locations along the line of sight, the uncertainties are significantly larger than those associated with the A andB clouds. Therefore, our interpretation of the spectral features
obtained over the east and west areas in Region C should be
A89, page 10 of 13

regarded with care. As we discuss below, the situation is different
for the C-7.5HII zone because the H II region – whose distancewas determined in Sect. 4.1 – sits inside the molecular gas.
The CO profiles over the portion of the cloud C-6E superposed on the SNR shell show maxima at around 64 km s−1
correlated with H I absorption dips. This spectral condition(observed at almost the same velocity as that of the tangent point)
could indicate that the molecular emission occurs at a kinematic
distance of 6.3 ± 2.0 kpc associated with the mean central velocity measured in the region. This estimate is, within uncertainties,
in reasonable agreement with the distance to G46.8–0.3. A key
point to note is that many of the spectra we constructed overthe narrow portion of the C-6E cloud superposed on G46.8–
0.3 were found to be broadened (FWHM line ∆v ∼ 9 km s−1 ;see example in Fig. 8f). Analogous spectral features have beenfound in other known SNR–molecular cloud interactions andwere interpreted as the result of turbulent motions in the shockedgas (e.g. W44, Seta et al. 2004; G347.3–0.5, Moriguchi et al.
2005; Kes 69, Zhou et al. 2009; G357.7+0.3, Rho et al. 2017;
HB 3, Rho et al. 2021). We also found a similar line broadening in the CO spectra constructed over the portion of the C-6E
cloud immediately outside the SNR forward shock. In this case,the spectra exhibit strong CO emission lines with peak intensities at around ∼63 km s−1 without associated H I self-absorption
features (see the representative profiles in Fig. 8g). Therefore,
from our spectral analysis on the C-6E region we conclude that
it is a single cloud located at 6.3 ± 2.0 kpc. Taking into accountthe distance we measure to G46.8–0.3 and the large uncertainties in our distance determination to the molecular gas in the
C-6E zone, we cannot completely discard the possibility that thecloud is located in the immediate foreground relative to the SNR.

L. Supan et al.: Radio properties of the SNR G46.8–0.3 and its environs

The good spatial agreement of C-6E with the outer part of theremnant may be considered additional evidence for a possible
physical relationship between them.
Interestingly, we find that the spectral properties over thewestern part of Region C change from those observed in the east.
Indeed, the profiles extracted over the gas forming the C-4W
cloud display CO peaks at a mean central velocity of ∼58 km s−1 ,and all of them are correlated with H I absorption dips (Fig. 8h).
We interpret this result as indicative of cold H I gas within amolecular structure placed at the near distance of 4.2 ± 1.8 kpc,
which is absorbing the warmer H I emission from the ISM at thesame velocity as the CO cloud. Regarding the C-7.5HII zone,
where the CO emission is brightest, the spectra present a peakat ∼56 km s−1 with broad emission lines of ∼ 9 km s−1 (see e.g.
Fig. 8i). The position of the maximum CO intensity in accordance with both the radio recombination line velocity of the
H II region (58 km s−1 , Liu et al. 2019) and the atomic neutral gas seen in absorption against the background provided by
the H II region led us to conclude that the molecular gas in thisregion forms the natal cloud of the star formation activity in
G046.495−00.241.
Finally, we point out that our detailed analysis of the spatial distribution of the molecular structures forming Region C(including clouds located over a large distance interval ∼4–
8 kpc) is not in agreement with the existence of a partial
CO-line shell formed around G46.8–0.3 at the radial velocity of
52 km s−1 , as proposed by Sofue et al. (2021). In their study,
our target source is part of an extensive list made on the basis ofmorphological inspection, including the identification of approximately 60 candidates to molecular cavities and shells towards
Galactic SNRs.

6. GeV γ-rays in the field of G46.8–0.3
The latest version (Data Release 2) of the Fermi Large Area
Telescope Fourth Source Catalogue (4FGL)13 records an 8.1σ
detection of a γ-ray excess in the 100 MeV–1 TeV band, labelled
4FGL J1918.1+1215c, towards the eastern half of G46.8–0.3 (firstreport in Abdollahi et al. 2020). The spectrum of this source iswell fit by a power law dN/dE = φ0 E −Γ , with a differential flux
φ0 = (1.59 ± 0.25) × 10−12 cm−2 s−1 MeV−1 at E0 = 1099 MeV
and a photon index Γ = 2.65 ± 0.08, comparable to values measured in other SNRs with molecular cloud interactions (e.g.
Kes 79, Auchettl et al. 2014; G290.1−0.8, Auchettl et al. 2015;
W51C, Jogler & Funk 2016; W28, Cui et al. 2018).
Neither pulsars nor their associated wind nebulae aredetected towards the γ-ray region in G46.8–0.3, which could
explain the GeV emission. Motivated by the spatial correlation
projected in the sky between the molecular cloud–SNR interaction zone and the Fermi source, we tested the possibility thatthe observed γ-ray flux results from neutral pion decay emission
produced by collisions of hadronic cosmic rays with dense targets in the surrounding medium. To investigate this matter, given
the limited spatial resolution of the γ-ray observations, we consider the large region ∼300 × 190 in size corresponding to the 95%
confidence level (CL) of the GeV source location (for comparison, the diameter of the SNR is ∼170 ). As illustrated in Fig. 9
the area of interest comprises portions of the molecular material embedded in the A-10N, B-8E, and B-9C ambient clouds
unveiled in this work. To estimate the proton column density Np
we computed the integrated column density of H2 from the 12 CO
13 https://fermi.gsfc.nasa.gov/ssc/data/access/lat/

10yr_catalog/

Fig. 9. Grey-scale image (in mJy beam−1 ) corresponding to the radio
continuum emission from the shell of G46.8–0.3 overlaid with the 12 CO(J = 1–0) line emission from Regions A (in cyan, ∆v = 10–30 km s−1 ,levels: 40, 70, 100 K km s−1 ) and B (in yellow, ∆v = 30–50 km s−1 ,levels: 55, 80, 105 K km s−1 ) around the SNR. The velocity range ofthe plotted clouds is the same as in Fig. 7. The plus symbol and ellipses
illustrate the best localisation and the 95%/68% confidence contoursof the γ-ray emission detected by Fermi-LAT. The enclosed molecularareas corresponding to the A-10N, B-8E, and B-9C clouds were used in
Sect. 6 to calculate the total proton content.

line emission within the velocity ranges corresponding to theseinteracting clouds (see discussion in Sect. 5). Because of theuncertainty surrounding the association of G46.8–0.3 with themolecular material forming the C-6E region, we decided not toinclude any contribution from this matter in the following estimation. Using the relations presented in Sect. 2.2 we obtained
Np ' 2NH2 ' 2 × 1022 cm−2 . We then estimated the proton density np = Np /L by adopting a path length L ∼ 24.0 5 ' 64 pc
through the γ-ray emission, equivalent to the mean of the major
and minor axes of the 95% CL ellipse that we assumed to belocated at 9 kpc (i.e. the average distance for the interacting
A-10N, B-8E, and B-9C molecular clouds; see Table 2). The
resulting total proton content is therefore np ∼100 cm−3 . Regarding a possible contribution from the atomic gas phase, as theanalysis of the H I distribution around the location of G46.8–0.3
did not reveal convincing structures associated with the source(see Sect. 4), it was not incorporated into the calculation above.
For this reason, we are aware that the derived proton density
for the GeV region represents a lower limit. When taking into
account the contribution (integrated in the region of the molecular clouds) of the H I gas to the proton density, we find anincrease of 30%. Nevertheless, the exclusion of the atomic component does not significantly change our conclusion presented
below on the energetics of cosmic rays in the region of G46.8–
0.3. We also mention that our analysis does not incorporate
the contribution of the ionised hydrogen to the column density
because we do not find significant IR counterparts in the regionof interest.
The amount of energy injected into accelerated hadrons Wptot
can be estimated from the relation between the luminosity of the
GeV source Lγ and the characteristic cooling time of protons
through the neutral meson-production channel tpp→π0 , which inturn depends on the proton density (Aharonian et al. 2006). At
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A&A 664, A89 (2022)

the adopted distance d = 9 kpc, we obtain Lγ (E > 1 GeV) =4 π d2 wγ ' 5 × 1034 erg s−1 , where wγ ' 5 × 10−12 erg cm−2 s−1
is the γ-ray energy flux for E > 1 GeV. On the other hand, forthe total proton density we measure in the 95% CL region of
the Fermi detection, the cooling time of protons is found to be
tpp→π0 ≈ 1.4 × 108 (np /cm3 )−1 yr ∼ 1.4 × 106 yr. Therefore, thetotal energy of cosmic rays required to produce the observed γray flux in the region of SNR G46.8–0.3 is Wptot ∼ tpp→π0 Lγ '2.2 × 1048 erg. This value for Wptot corresponds to ∼ 0.2% of thecanonical E ' 1051 erg SN energy and provides support to theinterpretation that G46.8–0.3 could be the radio counterpart of
the Fermi source produced via a hadronic process. We notice thata similar condition remains valid for a less conservative case ofanalysis in a smaller region corresponding to the 68% CL of the
γ-ray detection.
Taking into account our estimation of the age of G46.8–0.3
and the properties of its environment revealed in this work, ourvalue for Wptot agrees well with those obtained for other interacting SNRs spatially correlated with γ-ray emission at GeV
and/or TeV energies. We encourage the reader to consult thedetailed information about this topic presented by Sano et al.
(2021). It is important to note that though relatively simplistic, our analysis suggests that G46.8–0.3 is outside of the linear
correlation proposed by these latter authors between the energyof cosmic-ray protons and the age of the accelerator. Certainly,
the existence of any evolutionary tendency is an important issue
that should be investigated on the basis of a larger sampleof SNRs. Additionally, a more comprehensive study involving
effects due to particle diffusion away from the SNR shell and/or
re-acceleration of pre-existing cosmic rays inside the shockcompressed clouds is required in order to obtain a clearer pictureof the γ-ray emission from G46.8–0.3.
Finally, with the abundant interstellar matter in the vicinityof G46.8–0.3 still in mind, we cautiously note that on the basisof the current information we have for the GeV γ-ray flux in the
direction to the remnant, a contribution to the high-energy emission from bremsstrahlung γ-rays of relativistic electrons could
also be likely. The critical aspect in modelling the occurrenceof this physical process is the requirement of high electron-toproton ratios of about 0.2, larger than the 0.01 value estimatedfrom cosmic-ray abundances (Gaisser et al. 1998). Even withthis caveat, there remain some examples in the literature where
electron–electron or electron–ion interactions partially explain
the γ-rays in the Fermi-LAT energy band via bremsstrahlung
radiation (see e.g. Kes 41, Supan et al. 2018; HB21, Ambrogi
et al. 2019). For the environment of the SNR G46.8–0.3, thecooling timescale due to bremsstrahlung losses is found to be
tbr ≈ 4 × 107 (np /cm3 )−1 yr ≈ 4 × 105 yr (Aharonian 2004). Thisis greater than the estimated age of the SNR (see Sect. 4.2)
and leads to a realistic total relativistic particle energy ofW ∼ 6.3 × 1047 erg.

7. Summary and conclusions
This work focuses on determining the properties of the radio
continuum emission from the SNR G46.8–0.3, along with thoseof the atomic and molecular constituents in the surrounding
medium. By assimilating new integrated flux densities that
we measure from recently published 88–200 MHz GLEAM
maps and others from surveys at 1.4, 4.8, and 10 GHz intoa collection of data carefully selected from the literature, wehave constructed the most complete version of the radio spectra for G46.8–0.3 available to date covering a broad range in
A89, page 12 of 13

frequency (30.9 MHz–11.2 GHz). A simple power law with an
α = −0.535 ± 0.012 slope provides the best fit to the set of fluxes
measured over the remnant in the radio domain. The straightnessof the spectrum implies there is no ionised gas localised either
inside the remnant, in its proximity, or in more distant ISM along
the line of sight able to absorb the radio emission from G46.8–
0.3, which would cause a deviation from a power law at thelowest frequencies. Although the integrated spectrum is well fit
by a simple power law, our analysis of the local variations in theradio spectral index with position and frequency over G46.8–0.3
suggests a possible steepening at approximately 1 GHz. To determine the reality of any potential spectral curvature within small
regions over G46.8–0.3, it is necessary to analyse high-quality
radio data over a large frequency range correctly matched in the
uv-domain.
No H I structures are found to be physically linked to G46.8–
0.3. On the basis of neutral hydrogen absorption and emission
spectra, we determine that the remnant is located at a distance of
8.7 ± 1.0 kpc, and from evolutionary models we also argue that
G46.8–0.3 was created in a stellar explosion, likely occurring∼1 × 104 yr ago.
Additionally, we provide for the first time a robust estimation of the distance to the molecular clouds lying in the directionof the SNR and present compelling signatures in the 12 CO and
13
CO (J = 1–0) line emissions of interaction between the forward shock of G46.8–0.3 and dense (with ∼100 × 103 M and∼50 cm−3 average mass and density, respectively) molecular
clouds distributed over the centre of the remnant, as well as along
its northern, eastern, and southeastern edges. According to ourfindings, G46.8–0.3 can be added to the list of evolved remnantsin our Galaxy with signs of interaction with molecular structures. Line emission observations of higher CO transitions withrespect to the J = 1–0 rotational line are of interest to quantitatively analyse variations in CO line ratios resulting from both
environmental and local shock conditions in the clouds. Unfortunately, at the time we made the analysis there were no images
available from The CO High-Resolution Survey (COHRS) 12 CO(J = 3–2) maps covering the complete extension of the remnant.
In line with the evidence we present in this work on the physical relationship between G46.8–0.3 and its surroundings, we also
discuss an explanation of the γ-ray flux at GeV energies via neutral pion decay after hadronic collisions, which we find to beplausible as this process requires only a few percent of the totalenergy released in the stellar explosion creating G46.8–0.3. Furthermore, although very simplistic, our analysis also indicatesthat electron-dense medium interactions resulting in high-energy
bremsstrahlung radiation cannot be conclusively ruled out at thisstage. Compilation of the radio fluxes that we present here isnecessary as an anchor at the lowest energies for the broadband spectral energy distribution of G46.8–0.3. Its inclusion
is crucial to characterising the physical process responsible forthe high-energy particle production in this remnant, which wedemonstrate belongs to the reduced group of evolved interacting
SNRs. Future γ-ray observations providing complementary datawith increased sensitivity and spatial resolution in the multi-GeV
Fermi energy range and at higher energies are also of interest inproperly determining the spectral behaviour for G46.8–0.3.
Acknowledgements. The authors acknowledge the anonymous referee for his/her
helpful comments. G. Castelletti and L. Supan are members of the Carrera delInvestigador Científico of CONICET, Argentina. This work was supported by
the ANPCyT (Argentina) research project with number BID PICT 2017-3320.
Nobeyama Radio Observatory is a branch of the National Astronomical Observatory of Japan, National Institutes of Natural Sciences. Part of the data wereretrieved from the JVO portal (http://jvo.nao.ac.jp/portal/) operated

L. Supan et al.: Radio properties of the SNR G46.8–0.3 and its environs
by ADC/NAOJ. The National Radio Astronomy Observatory is a facility of
the National Science Foundation operated under cooperative agreement by
Associated Universities, Inc.

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