We present new images and continuum spectral analysis for 14 resolved Galactic supernova remnants (SNRs) selected from the 74 MHz Very Large Array Low-Frequency Sky Survey Redux (VLSSr). We combine new integrated measurements from the VLSSr with, when available, flux densities extracted from the Galactic and Extragalactic All-Sky Murchison Widefield Array Survey and measurements from the literature to generate improved integrated continuum spectra sampled from ~15 MHz to ~217 GHz. We present the VLSSr images. When possible we combine them with publicly available images at 1.4 GHz, to analyse the resolved morphology and spectral index distribution across each SNR. We interpret the results and look for evidence of thermal absorption caused by ionised gas either proximate to the SNR itself, or along its line of sight. Three of the SNRs, G4.5+6.8 (Kepler), G28.6−0.1, and G120.1+1.4 (Tycho), have integrated spectra which can be adequately fit with simple power laws. The resolved spectral index map for Tycho confirms internal absorption which was previously detected by the Low Frequency Array, but it is insufficient to affect the fit to the integrated spectrum. Two of the SNRs are pulsar wind nebulae, G21.5−0.9 and G130.7+3.1 (3C 58). For those we identify high-frequency spectral breaks at 38 and 12 GHz, respectively. For the integrated spectra of the remaining nine SNRs, a low frequency spectral turnover is necessary to adequately fit the data. In all cases we are able to explain the turnover by extrinsic thermal absorption. For G18.8+0.3 (Kes 67), G21.8−0.6 (Kes 69), G29.7−0.3 (Kes 75), and G41.1−0.3 (3C 397), we attribute the absorption to ionised gas along the line of sight, possibly from extended H II region envelopes. For G23.3−0.3 (W41) the absorption can be attributed to H II regions located in its immediate proximity. Thermal absorption from interactions at the ionised interface between SNR forward shocks and the surrounding medium were previously identified as responsible for the low frequency turnover in SNR G31.9+0.0 (3C 391); our integrated spectrum is consistent with the previous results. We present evidence for the same phenomenon in three additional SNRs G27.4+0.0 (Kes 73), G39.2–0.3 (3C 396), and G43.3–0.2 (W49B), and derive constraints on the physical properties of the interaction. This result indicates that interactions between SNRs and their environs should be readily detectable through thermal absorption by future low frequency observations of SNRs with improved sensitivity and resolution.

Aims. G15.4+0.1 is a faint supernova remnant (SNR) that has recently been associated with the γ-ray source HESS J1818-154. We investigate a hadronic scenario for the production of the γ-ray emission. Methods. Molecular 13 CO (J = 1-0) taken from the Galactic Ring Survey (GRS) and neutral hydrogen (HI) data from the Southern Galactic Plane Survey (SGPS) have been used in combination with new 1420 MHz radio continuum observations carried out with the Giant Metrewave Radio Telescope (GMRT). Results. From the new observations and analysis of archival data we provided for the first time a reliable estimate for the distance to the SNR G15.4+0.1 and discovered molecular clouds located at the same distance. On the basis of HI absorption features, we estimate the distance to G15.4+0.1 in 4.8 ± 1.0 kpc. The 13 CO observations clearly show a molecular cloud about 5 in size with two bright clumps, labeled A and B, clump A positionally associated with the location of HESS J1818-154 and clump B in coincidence with the brightest northern border of the radio SNR shell. The HI absorption and the 13 CO emission study indicates a possible interaction between the molecular material and the remnant. We estimate the masses and densities of the molecular gas as (1.2 ± 0.5) × 10 3 M and (1.5 ± 0.4) × 10 3 cm −3 for clump A and (3.0 ± 0.7) × 10 3 M and (1.1 ± 0.3) × 10 3 cm −3 for clump B. Calculations show that the average density of the molecular clump A is sufficient to produce the detected γ-ray flux, thus favoring a hadronic origin for the high-energy emission.

Aims. We report on the first detailed multiwavelength study of the radio source G29.37+0.1, which is an as-yet-unclassified object linked to the very-high-energy γ-emitting source HESS J1844−030. The origin of the multiwavelength emission toward G29.37+0.1 has not been clarified so far, leaving open the question about the physical relationship between these sources. Methods. Using observations carried out with the Giant Metrewave Radio Telescope (GMRT), we performed high-quality fullsynthesis imaging at 610 MHz of the field containing G29.37+0.1. The obtained data, combined with observations at 1400 MHz from The Multi-Array Galactic Plane Imaging Survey (MAGPIS) were used to investigate in detail the properties of its radio emission. Additionally, we reprocessed archival data obtained with the XMM-Newton and Chandra observatories in order to get a multiwavelength view of this unusual source. Results. The radio source G29.37+0.1 mainly consists of a bright twisted structure, named the S-shaped feature. The high sensitivity of the new GMRT observations allowed the identification of potential lobes, jets, and a nuclear central region in the S-shaped morphology of G29.37+0.1. We also highlight the detection of diffuse and low surface brightness emission enveloping the brightest emitting regions. The brightest emission in G29.37+0.1 has a radio synchrotron spectral index α=0.59±0.09. Variations in the spectral behaviour are observed across the whole radio source with the flattest spectral features in the central nuclear and jets components (α∼0.3). These results lead us to conclude that the brightest radio emission from G29.37+0.1 likely represents a newly recognized radio galaxy. The identification of optical and infrared counterparts to the emission arising from the core of G29.37+0.1 strengthens our interpretation of an extragalactic origin of the radio emission. We performed several tests to explain the physical mechanism responsible for the observed X-ray emission, which appears overlapping the northeastern part of the radio emission. Our spectral analysis demonstrated that a non-thermal origin for the X-ray emission compatible with a pulsar wind nebula is quite possible. The analysis of the spatial distribution of the CO gas revealed the presence of a complex of molecular clouds located in projection adjacent to the radio halo emission and probably interacting with it. We propose that the faint halo represents a composite supernova remnant with a pulsar powered component given by the diffuse X-ray emission superimposed along the line of sight to the radio galaxy. Further broadband observations of HESS J1844−030 are needed to disentangle its origin, although its shape and position suggest an extragalactic origin connected to G29.37+0.1.

Aims. The goal of this paper is to detect synchrotron emission from the relic electrons of the crushed pulsar wind nebula (PWN) HESS J1825−137 and to investigate the origin of the γ-ray emission from HESS J1826−130. Methods. The study of HESS J1825−137 was carried out on the basis of new radio observations centred at the position of PSR J1826−1334 performed with the Karl G. Jansky Very Large Array at 1.4 GHz in configurations B and C. To investigate the nature of HESS J1826−130, we reprocessed unpublished archival data obtained with XMM-Newton. Results. The new radio continuum image towards PSR J1826−1334 reveals a bright radio source, with the pulsar located in its centre, which suggests that this feature could be the radio counterpart of the compact component of the PWN detected at high energy. The new 1.4 GHz radio data do not reveal emission with an extension comparable with that observed in γ-rays for the HESS J1825−137 source. On the other hand, the XMM-Newton study of the region including PSR J1826−1256 reveals an elongated non-thermal X-ray emitting nebula with the pulsar located in the northern border and a tail towards the peak of the very high energy source. The spectrum is characterized by a power law with a photon index going from 1.6 around the pulsar to 2.7 in the borders of the nebula, a behaviour consistent with synchrotron cooling of electrons. From our X-ray analysis we propose that HESS J1826−130 is likely produced by the PWN powered by PSR J1826−1256 via the inverse Compton mechanism.

The supernova remnant (SNR) G338.3−0.0 spatially correlates with HESS J1640−465, which is considered the most luminous γ-ray source associated with a SNR in our Galaxy. The X-ray pulsar PSR J1640−4631 has been recently discovered within the SNR shell, which could favor a leptonic origin for the detected very-high-energy (VHE) emission. In spite of this, the origin of the VHE radiation from HESS J1640−465 has not been unambiguously clarified so far. Indeed, a hadronic explanation cannot be ruled out by current observations. On the basis of atomic (HI) and molecular ( 12 CO) archival data, we determine, for the first time, the total ambient density of protons in the region of the G338.3−0.0/HESS J1640−465 system, a critical parameter for understanding the emission mechanisms at very high energies. The value obtained is in the 100−130 cm −3 range. Besides this, we developed a new hadronic model to describe the spectral energy distribution (SED) of the HESS J1640−465 source, which includes the latest total γ-ray cross-section for protonproton collisions available in the literature. By using the assessed ambient proton density, we found that the total energy in accelerated protons required to fit the data is 5.4 +4.7 −2.3 × 10 49 erg and 1.6−0.7 × 10 50 erg for a source distance of 8.5 and 13 kpc, respectively. The case where the source distance is 8.5 kpc agrees with the typical scenario in which the energy released is on the order of 10 51 erg and ∼10% of that energy is transferred to the accelerated protons, whereas the case corresponding to a source distance of 13 kpc requires either a higher value of the energy released in the explosion or a larger energy fraction to accelerate protons.

Aims. The supernova remnant (SNR) G15.4+0.1 is considered to be the possible counterpart of the γ-ray source HESS J1818−154. With the goal of getting a complete view of this remnant and understanding the nature of the γ-ray flux, we conducted a detailed radio study that includes the search for pulsations and a model of the broadband emission for the SNR G15.4+0.1/HESS J1818−154 system. Methods. Low-frequency imaging at 624 MHz and pulsar observations at 624 and 1404 MHz towards G15.4+0.1 were carried out with the Giant Metrewave Radio Telescope (GMRT). We correlated the new radio data with observations of the source at X-ray and infrared wavelengths from XMM-Newton and Herschel observatories, respectively. To characterize the neutral hydrogen (HI) medium towards G15.4+0.1, we used data from the Southern Galactic Plane Survey. We modelled the spectral energy distribution (SED) using both hadronic and leptonic scenarios. Results. From the combination of the new GMRT observations with existing data, we derived a continuum spectral index α = −0.62 ± 0.03 for the whole remnant. The local synchrotron spectra of G15.4+0.1, calculated from the combination of the GMRT data with 330 MHz observations from the Very Large Array, tends to be flatter in the central part of the remnant, accompanying the region where the blast wave is impinging molecular gas. No spectral index trace was found indicating the radio counterpart to the pulsar wind nebula proposed from X-ray observations. In addition, the search for radio pulsations yielded negative results. Emission at far-infrared wavelengths is observed in the region where the SNR shock is interacting with dense molecular clumps. We also identified HI features forming a shell that wraps most of the outer border of G15.4+0.1. Characteristic parameters were estimated for the shocked HI gas. We found that either a purely hadronic or leptonic model is compatible with the broadband emission known so far.

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 version of 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 is real, 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 middle-aged (~1 × 104 yr) SNR. Furthermore, we recognise several 12CO and 13CO 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, and towards 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.

We have identified a new supernova remnant (SNR), G51.04+0.07, using observations at 74 MHz from the Very Large Array Low-Frequency Sky Survey Redux. Earlier, higher frequency radio continuum, recombination line, and infrared data had correctly inferred the presence of nonthermal radio emission within a larger, complex environment including ionised nebulae and active star formation. However, our observations have allowed us to redefine at least one SNR as a relatively small source (7 ′ .5×3 ′ in size) located at the southern periphery of the originally defined SNR candidate G51.21+0.11. The integrated flux density of G51.04+0.07 at 74 MHz is 6.1±0.8 Jy, while its radio continuum spectrum has a slope α=−0.52±0.05 (S ν ∝ ν α ), typical of a shell-type remnant. We also measured spatial variations in the spectral index between 74 and 1400 MHz across the source, ranging from a steeper spectrum (α=−0.50±0.04) coincident with the brightest emission to a flatter component (α=−0.30±0.07) in the surrounding fainter region. To probe the interstellar medium into which the redefined SNR is likely evolving, we have analysed the surrounding atomic and molecular gas using the 21 cm neutral hydrogen (HI) and 13 CO (J=1−0) emissions. We found that G51.04+0.07 is confined within an elongated HI cavity and that its radio emission is consistent with the remains of a stellar explosion that occurred ∼6300 yr ago at a distance of 7.7±2.3 kpc. Kinematic data suggest that the newly discovered SNR lies in front of HII regions in the complex, consistent with the lack of a turnover in the low frequency continuum spectrum. The CO observations revealed molecular material that traces the central and northern parts of G51.04+0.07. The interaction between the cloud and the radio source is not conclusive and motivates further study. The relatively low flux density (∼1.5 Jy at 1400 MHz) of G51.04+0.07 is consistent with this and many similar SNRs lying hidden along complex lines of sight towards inner Galactic emission complexes. It would also not be surprising if the larger complex studied here hosted additional SNRs.