Shell-type Supernova Remnants Research Articles

MHD lensing in inhomogeneous ISM for qualitative understanding of the morphology of supernova remnants

Morphological evolution of expanding shells of fast-mode magnetohydrodynamic (MHD) waves through an inhomogeneous ISM is investigated in order to qualitatively understand the complicated morphology of shell-type supernova remnants (SNR). Interstellar clouds with high Alfvén velocity act as concave lenses to diverge the MHD waves, while those with slow Alfvén velocity act as convex lenses to converge the waves to the focal points. By combination of various types of clouds and fluctuations with different Alfvén velocities, sizes, or wavelengths, the MHD-wave shells attain various morphological structures, exhibiting filaments, arcs, loops, holes, and focal strings, mimicking old and deformed SNRs.

The Gamma-Ray Origin of RX J0852.0-4622 Quantifying the Hadronic and Leptonic Components: Further Evidence for the Cosmic-Ray Acceleration in Young Shell-type SNRs

Fukui et al. quantified the hadronic and leptonic gamma-rays in the young TeV gamma-ray shell-type supernova remnant (SNR) RX J1713.7-3946 (RX J1713), and demonstrated that gamma rays are a combination of hadronic and leptonic gamma-ray components with a ratio of ∼6: 4 in gamma-ray counts N g. This discovery, which adopted a new methodology of multi-linear gamma-ray decomposition, was the first quantification of the two gamma-ray components. In the present work, we applied the same methodology to another TeV gamma-ray shell-type SNR RX J0852.0-4622 (RXJ0852) in 3D space characterized by (the interstellar proton column density N p)-(the nonthermal X-ray count N x)-[N g], and quantified the hadronic and leptonic gamma-ray components as having a ratio of ∼5:5 in N g. The present work adopted the fitting of two/three flat planes in 3D space instead of a single flat plane, which allowed suppression of the fitting errors. This quantification indicates that hadronic and leptonic gamma-rays are of the same order of magnitude in these two core-collapse SNRs, verifying the significant hadronic gamma-ray components. We argue that the target interstellar protons, in particular their spatial distribution, are essential in any attempts to identify the type of particles responsible for gamma-ray emission. The present results confirm that cosmic-ray (CR) energy ≲100 TeV is compatible with a scheme in which SNRs are the dominant source of these Galactic CRs.

G213.0−0.6, a true supernova remnant or just an H ii region?

Abstract G213.0−0.6 is a faint extended source situated in the anti-center region of the Galactic plane. It has been classified as a shell-type supernova remnant (SNR) based on its shell-like morphology, steep radio continuum spectrum, and high ratio of [H ii]/Hα. With new optical emission line data of Hα, [H ii], and [H ii] recently observed by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, the ratios of [H ii]/Hα and [H ii]/Hα are re-assessed. The lower values than those previously reported put G213.0−0.6 around the borderline of SNR-H ii region classification. We decompose the steep-spectrum synchrotron and the flat-spectrum thermal free-free emission in the area of G213.0−0.6 with multi-frequency radio continuum data. G213.0−0.6 is found to show a flat spectrum, in conflict with the properties of a shell-type SNR. Such a result is further confirmed by TT-plots made between the 863-MHz, 1.4-GHz, and 4.8-GHz data. Combining the evidence extracted in both optical and radio continuum, we argue that G213.0−0.6 is possibly not an SNR, but an H ii region instead. The VLSR pertaining to the Hα filaments places G213.0−0.6 approximately 1.9 kpc away in the Perseus Arm.

A Statistical Analysis of Galactic Radio Supernova Remnants

We present a revised table of 390 Galactic radio supernova remnants (SNRs) and their basic parameters. Statistical analyses are performed on SNR diameters, ages, spectral indices, Galactic heights, and spherical symmetries. Furthermore, the accuracy of distances estimated using the Σ–D relation is examined. The arithmetic mean of the Galactic SNR diameters is 30.5 pc with standard error 1.7 pc and standard deviation 25.4 pc. The geometric mean and geometric standard deviation factor of Galactic SNR diameters is 21.9 pc and 2.4, respectively. We estimate ages of 97 SNRs and find a supernova (SN) birth rate lower than, but within 2σ of, currently accepted values for the SN birth rate. The mean spectral index of shell-type SNRs is −0.51 ± 0.01 and no correlations are found between spectral indices and the SNR parameters of molecular cloud association, SN type, diameter, Galactic height, and surface brightness. The Galactic height distribution of SNRs is best described by an exponential distribution with a scale height of 48 ± 4 pc. The spherical symmetry measured by the ovality of radio SNRs is not correlated to any other SNR parameters considered here or to explosion type.

Cosmic-Ray Acceleration in Supernova Remnants

Supernova Remnants (SNRs) are generally believed to produce the cosmic rays in our Galaxy due to the powerful supernova blast waves generated by expanding SNRs. In contrast to the leptonic cosmic-ray component that is clearly seen by the SNR emission in a wide wavelength range, from radio to high-energy γ-ray, the hadronic cosmic-ray component can be detected only by very high energy γ-ray emission. Galactic SNRs of various ages have been intensively studied at very high energies. Among them are the shell-type SNRs: Tycho’s SNR, Cas A, IC 443, γCygni SNR, G166.0+4.3. The results of investigations of listed SNRs obtained in observations at 800 GeV–100 TeV energies by SHALON telescope are presented with spectral energy distribution and emission maps compared with experimental data from the wide energy range, from radio to high-energy gamma-rays. The TeV emission maps of supernova remnants obtained by SHALON are overlaid with ones viewed in radio- frequencies and X-rays to reveal SNR’s essential features which can lead to the effective generation of cosmic rays. The presented experimental data from high and very high energies are considered together with theoretical predictions to test the cosmic ray origin in these objects.

Revision of the γ-Ray Emission from SNR CTB 109 with the Fermi Large Area Telescope

CTB 109 is a middle-aged shell-type supernova remnant (SNR) with bright thermal X-ray emission. We reanalyze the GeV γ-ray emission from CTB 109 using 13 yr of Pass 8 data recorded by the Fermi Large Area Telescope. The γ-ray emission of CTB 109 shows a center bright morphology, which is well consistent with its thermal X-ray emission rather than the shell-type structure in the radio band. The spectral analysis shows an evident spectral curvature at ∼several GeV for the GeV γ-ray spectrum, which can naturally explain the lack of TeV γ-ray emission from CTB 109. Although either a leptonic or a hadronic model could fit the multiwavelength observations of CTB 109, the hadronic model is favored considering its γ-ray morphology and the spectral curvature of the GeV spectrum. The unusual γ-ray spectrum of CTB 109 with other SNRs and the luminosity-diameter squared relation make CTB 109 distinguished both from the young-aged SNRs with hard GeV γ-ray spectra and several old-aged SNRs interacting with molecular clouds.

Discovery of a Filamentary Synchrotron Structure Connected to the Coherent Magnetic Field in the Outer Galaxy

Using data from the Galactic Arecibo L-band Feed Array Continuum Transit Survey, we report the discovery of two previously unidentified, very compressed, thin, and straight polarized filaments approximately centered at Galactic coordinates, (l, b) = (182.°5, − 4.°0), which we call G182.5–4.0. Using data from the Isaac Newton Telescope Galactic Plane Survey, we also find straight, long, and extremely thin Hα filaments coincident with the radio emission. These filaments are positioned in projection at the edge of the Orion-Eridanus superbubble and we find evidence indicating that the filaments align with the coherent magnetic field of the outer Galaxy. We find a lower limit on the total radio flux at 1.4 GHz to be 0.7 ± 0.3 Jy with an average linearly polarized fraction of . We consider various scenarios that could explain the origin of these filaments, including a shell-type supernova remnant (SNR), a bow shock nebula associated with a pulsar, or relic fragments from one or more supernova explosions in the adjacent superbubble, with a hybrid scenario being most likely. This may represent an example of a new class of objects that is neither an SNR nor a bow shock. The highly compressed nature of these filaments and their alignment with the Galactic plane suggests a scenario where this object formed in a magnetic field that was compressed by the expanding Orion-Eridanus superbubble, suggesting that the object is related to this superbubble and implying a distance of ∼400 pc.

Peering into the Milky Way by FAST: IV. Identification of two new Galactic supernova remnants G203.1+6.6 and G206.7+5.9

A 5$^{\circ}$ $\times$ 7$^{\circ}$ sky area containing two large radio structures of G203.1+6.6 and G206.7+5.9 with a size of about 2.5$^{\circ}$ and 3.5$^{\circ}$ respectively is scanned by using the L-band 19-beam receiver of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The FAST L-band receiver covers a frequency range of 1.0-1.5 GHz. Commissioning of the receiving system, including the measurements of the half-power beam width, gain, and main-beam efficiency is made by observing the calibrators. The multi-channel spectroscopy backend mounted to the receiver allows an in-band spectral-index determination. The brightness-temperature spectral indices of both objects are measured to be $\beta$ $\sim$ $-$2.6 to $-$2.7. Polarized emission is detected from the archival Effelsberg $\lambda$11 cm data for all the shell structures of G203.1+6.6 and G206.7+5.9. These results clearly indicate a non-thermal synchrotron emitting nature, confirming that G203.1+6.6 and G206.7+5.9 are large shell-type supernova remnants (SNRs). Based on morphological correlation between the radio continuum emission of G206.7+5.9 and the HI structures, the kinematic distance to this new SNR is estimated to be about 440 pc, placing it in the Local Arm.

Diffuse GeV emission in the field of HESS J1912+101 revisited

We have analyzed 12 years of Fermi Large Area Telescope data toward the HESS J1912+101 region. With the latest source catalog and diffuse background models, a γ-ray excess is detected with a significance of ~8σ in the energy range of above 10 GeV. It has been argued that the diffuse GeV emission in the vicinity of HESS J1912+101 are from an extended pulsar wind nebula powered by PSR J1913+1011 and also that the hard GeV emission above 10 GeV stems from the shell-type supernova remnant and is connected with the TeV emissions. Different from previous works, our analysis indicates that the H2 spatial template is preferred over the other spatial templates, suggesting that the diffuse emission component spatially correlates with the dense molecular gas. This spatial correlation favors a hadronic emission scenario, although a leptonic origin cannot be ruled out. In the hadronic scenario, the parent proton spectrum can be described by a power-law function with an index of α = 2.36 ± 0.16. Above 50 GeV, there is no emission, and the upper limits reveal a spectral cutoff or break in the parent proton spectrum that can be explained as propagation effects of cosmic rays. We argue that the parent protons may come from the candidate supernova remnant HESS J1912+101 or the young massive star cluster Mc20.

The auto- and cross-angular power spectrum of the Cas A supernova remnant in radio and X-ray

ABSTRACT The shell type supernova remnant (SNR) Cas A exhibits structures at nearly all angular scales. Previous studies show the angular power spectrum (Cℓ) of the radio emission to be a broken power law, consistent with MHD turbulence. The break has been identified with the transition from 2D to 3D turbulence at the angular scale corresponding to the shell thickness. Alternatively, this can also be explained as 2D inverse cascade driven by energy injection from knot–shock interactions. Here we present Cℓ measured from archival VLA 5-GHz (C band) data, and Chandra X-ray data in the energy ranges ${\rm A}=0.6{-}1.0$ and ${\rm B} =4.2{-}6.0 \,{\rm keV}$, both of which are continuum dominated. The different emissions all trace fluctuations in the underlying plasma and possibly also the magnetic field, and we expect them to be correlated. We quantify this using the cross-Cℓ between the different emissions. We find that X-ray B is strongly correlated with both radio and X-ray A; however, X-ray A is only very weakly correlated with radio. This supports a picture where X-ray A is predominantly thermal bremsstrahlung, whereas X-ray B is a composite of thermal bremsstrahlung and non-thermal synchrotron emission. The various Cℓ measured here, all show a broken power-law behaviour. However, the slopes are typically shallower than those in radio and the position of the break also corresponds to smaller angular scales. These findings provide observational inputs regarding the nature of turbulence and the emission mechanisms in Cas A.

Angular power spectrum of supernova remnants: effects of structure, geometry and diffuse foreground

Abstract The study of the intensity fluctuation power spectrum of individual supernova remnants (SNRs) can reveal the structures present at sub-pc scales, and also constrain the physical process that generates those structures. There are various effects, such as the remnant shell thickness, projection of a three-dimensional structure onto a two-dimensional observational plane, and the presence of diffuse “foreground” emission, which causes the observed power spectrum to deviate from the intrinsic power spectrum of the fluctuations. Here, we report results from a systematic study of these effects, using direct numerical simulations, in the measured power spectrum. For an input power-law power spectrum, independent of the power-law index, we see a break in the observed power law at a scale which depends on the shell thickness of a shell-type SNR, and the three-dimensional turbulence changes to two-dimensional turbulence beyond that scale. We also report how the estimated power spectrum is expected to deviate from the intrinsic SNR power spectrum in the presence of additional diffuse Galactic synchrotron emission (DGSE) around the remnant shell. For a reasonable choice of the parameters, if the intrinsic SNR power spectrum is shallower than the DGSE power spectrum, the SNR contribution dominates at small angular scales of the estimated power spectra. On the other hand, if the SNR power spectrum is relatively steeper, the original power spectra is recovered only over a small window of angular scales. This study shows how detailed modeling may be used to infer the true power spectrum from the observed SNR intensity fluctuations power spectrum, which in turn can be used to constrain the nature of the turbulence that gives rise to these small scale structures.

High-energy gamma-ray study of the dynamically young SNR G150.3+4.5

Aims. The supernova remnant (SNR) G150.3+4.5 was recently discovered in the radio band; it exhibits a shell-like morphology with an angular size of ~ 3°, suggesting either an old or a nearby SNR. Extended γ-ray emission spatially coincident with the SNR was reported in the Fermi Galactic Extended Source Catalog, with a power-law spectral index of Γ = 1.91 ± 0.09. Studying particle acceleration in SNRs through their γ-ray emission is of primary concern to assess the nature of accelerated particles and the maximum energy they can reach. Methods. Using more than ten years of Fermi-LAT data, we investigate the morphological and spectral properties of the SNR G150.3+4.5 from 300 MeV to 3 TeV. We use the latest releases of the Fermi-LAT catalog, the instrument response functions and the Galactic and isotropic diffuse emissions. We use ROSAT all-sky survey data to assess any thermal and nonthermal X-ray emission, and we derive minimum and maximum distance to G150.3+4.5. Results. We describe the γ-ray emission of G150.3+4.5 by an extended component which is found to be spatially coincident with the radio SNR. The spectrum is hard and the detection of photons up to hundreds of GeV points towards an emission from a dynamically young SNR. The lack of X-ray emission gives a tight constraint on the ambient density n0 ≤ 3.6 × 10−3 cm−3. Since G150.3+4.5 is not reported as a historical SNR, we impose a lower limit on its age of t = 1 kyr. We estimate its distance to be between 0.7 and 4.5 kpc. We find that G150.3+4.5 is spectrally similar to other dynamically young and shell-type SNRs, such as RX J1713.7−3946 or Vela Junior. The broadband nonthermal emission is explained with a leptonic scenario, implying a downstream magnetic field of B = 5 μG and acceleration of particles up to few TeV energies.

Numerically investigating the peculiar periphery of a supernova remnant in the medium with a density gradient: the case of RCW 103

The young shell-type supernova remnant RCW 103 has peculiar properties in the X-ray morphology obtained with Chandra. The southeastern shell is brighter in the X-rays, and the curved border of the shell in this region is flatter than the other part.We investigate the formation of the peculiar periphery of the supernova remnant RCW 103 using 3D hydrodynamical simulation. Assuming that the supernova ejecta has been evolved in the medium with a density gradient, the detected shape of the periphery can be generally reproduced. For RCW 103, with the ejecta mass of 3.0 M ⊙, the density of the background material of 2.0 cm−3, and a gradient of 3.3 − 4.0 cm−3 pc−1, the X-ray periphery can be generally reproduced. The simulation turned out that the asymmetry of the SNR RCW 103 is mainly due to the inhomogeneous medium with a density gradient.

Prospects on high-energy source searches based on pattern recognition Object detection in the H.E.S.S. Galactic Plane Survey and catalogue cross-matches

The H.E.S.S. Galactic Plane Survey [HGPS, 1] represents one of the most sensitive surveys of the Galactic Plane at very high energies (VHE, 0.1 < E < 100 TeV). However the source detection algorithm of the HGPS pipeline is not well-suited for complex regions, including sources with shell-like morphologies. As an alternative and complementary approach, we have investigated blind search methods for VHE γ-ray source detection based on well-known and widely used image processing and pattern recognition techniques.Our goal is to build in a short amount of computational time a list of potentially valuable objects without prior case-specific morphological assumptions. We aim to classify and rank the detected objects in order to identify only the most promising source candidates for further multi-wavelength-association searches, dedicated analyses, or deeper observations.In the approach proposed, we extract sparse and pertinent structural information from the significance maps using a edge detection operator. We then apply a Hough circle transform and detect a collection of objects as local maxima in the Hough space. On the basis of morphological parameters we can characterize different object classes. Classification can be used to identify valuable source candidates sharing the characteristics of well-known sources.We show that using these pattern recognition techniques we can detect objects with partial circular symmetry irrespective of a morphological template (e.g. point-like, Gaussian-like, or shell-like). All the shell-type supernova remnants (SNRs) catalogued in the HGPS (from dedicated analyses) are associated with at least one detected object. Catalogue cross-matches indicate that several detected objects not catalogued in the HGPS are spatially coincident with multi-wavelength counterparts.This paper can be seen as a prospective study for the search of VHE γ-ray sources based on Hough transform and morphological classification. The algorithm have been tested on bootstrap simulations and applied to significance maps of the H.E.S.S. Galactic survey. Further investigation on the most promising candidates will be conducted in dedicated follow-up analyses.

Discovery of a new supernova remnant G21.8−3.0

ABSTRACT Sensitive radio continuum surveys of the Galactic plane are ideal for discovering new supernova remnants (SNRs). From the Sino-German λ6-cm polarization survey of the Galactic plane, an extended shell-like structure has been found at ℓ = 21${^{\circ}_{.}}$8, b = −3${^{\circ}_{.}}$0, which has a size of about 1°. New observations were made with the Effelsberg 100-m radio telescope at λ11 cm to estimate the source spectrum, together with Urumqi λ6-cm and Effelsberg λ21-cm data. The spectral index of G21.8−3.0 was found to be α = −0.72 ± 0.16. Polarized emission was mostly detected in the eastern half of G21.8−3.0 at both λ6 and λ11 cm. These properties, together with the Hα filament along its northern periphery and the lack of infrared emission, indicate that the emission is non-thermal, as is usual in shell-type SNRs.

Discovery of Shocked Molecular Clouds Associated with the Shell-type Supernova Remnant RX J0046.5−7308 in the Small Magellanic Cloud

RX J0046.5$-$7308 is a shell-type supernova remnant (SNR) in the Small Magellanic Cloud (SMC). We carried out new $^{12}$CO($J$ = 1-0, 3-2) observations toward the SNR using Mopra and the Atacama Submillimeter Telescope Experiment. We found eight molecular clouds (A-H) along the X-ray shell of the SNR. The typical cloud size and mass are $\sim$10-15 pc and $\sim$1000-3000 $M_{\odot}$, respectively. The X-ray shell is slightly deformed and has the brightest peak in the southwestern shell where two molecular clouds A and B are located. The four molecular clouds A, B, F, and G have high intensity ratios of $^{12}$CO($J$ = 3-2) / $^{12}$CO($J$ = 1-0) $> 1.2$, which are not attributable to any identified internal infrared sources or high-mass stars. The HI cavity and its expanding motion are found toward the SNR, which are likely created by strong stellar winds from a massive progenitor. We suggest that the molecular clouds A-D, F, and G and HI clouds within the wind-blown cavity at $V_\mathrm{LSR} = 117.1$-122.5 km s$^{-1}$ are to be associated with the SNR. The X-ray spectroscopy reveals the dynamical age of $26000^{+1000}_{-2000}$ yr and the progenitor mass of $\gtrsim 30$ $M_{\odot}$, which is also consistent with the proposed scenario. We determine physical conditions of the giant molecular cloud LIRS 36A using the large velocity gradient analysis with archival datasets of the Atacama Large Millimeter/submillimeter Array; the kinematic temperature is $72^{+50}_{-37}$ K and the number density of molecular hydrogen is $1500^{+600}_{-300}$ cm$^{-3}$. The next generation of $\gamma$-ray observations will allow us to study the pion-decay $\gamma$-rays from the molecular clouds in the SMC SNR.

Electron Acceleration in Middle-age Shell-type γ-Ray Supernova Remnants

Abstract Over the past decade, γ-ray observations of supernova remnants (SNRs) and accurate cosmic-ray (CR) spectral measurements have significantly advanced our understanding of particle acceleration in SNRs. In combination with multiwavelength observations of a large sample of SNRs, it has been proposed that the highest energy particles are mostly accelerated in young remnants, and the maximum energy that middle-age and old SNRs can accelerate particles to decreases rapidly with the decrease in shock speed. If SNRs dominate the CR flux observed at Earth, a large number of particles need to be accelerated in old SNRs for the soft CR spectrum even though they cannot produce very high-energy CRs. With radio, X-ray, and γ-ray observations of seven middle-age shell-type SNRs, we derive the distribution of high-energy electrons trapped in these remnants via a simple one-zone leptonic emission model and find that their spectral evolution is consistent with such a scenario. In particular, we find that particle acceleration by shocks in middle-age SNRs with age t can be described by a unified model with the maximum energy decreasing as t −3.1 and the number of GeV electrons increasing as t 2.5 in the absence of escape from SNRs.

Theoretical $$\Sigma $$ Σ -D relations for shell-type galactic supernova remnants

Relations between radio surface brightness ($\Sigma$) and diameter ($D$) of supernova remnants (SNRs) are important in astronomy. In this paper, following the work Duric \& Seaquist (1986) at adiabatic phase, we carefully investigate shell-type supernova remnants at radiative phase, and obtain theoretical $\Sigma$-$D$ relation at radiative phase of shell-type supernova remnants at 1 GHz. By using these theoretical $\Sigma$-$D$ relations at adiabatic phase and radiative phase, we also roughly determine phases of some supernova remnant from observation data.

Cosmic ray origin: Supernova remnants through the electromagnetic spectrum

Supernova Remnants have long been considered as unique candidates for cosmic-ray sources. Recent observations of several SNRs in X-rays and TeV gamma-rays will help in solving the problem of the origin of cosmic rays and are key to understanding the mechanism of particle acceleration at a propagating shock wave. The observation results of Galactic shell-type supernova remnants at different evolution stages Cas A, Tycho's SNR, γCygni SNR, IC 443 and G166.0+4.3 by the SHALON mirror Cherenkov telescope are presented. For each SNR the SHALON observation results are given with its spectral energy distribution compared with other experimental data and images by SHALON together with data from X-ray by Chandra and radio-data by Canadian Galactic Plane Survey DRAO (CGPS). The comparison of the source's morphology in different energy bands could reveal its essential features as a forward and reverse shock or the location of swept out dense molecular cloud. The experimental data presented here have confirmed the prediction of the theory about the hadronic generation mechanism of very high energy 800 GeV-100 TeV gamma-rays in Tycho's SNR, Cas A and IC 443. Also the collected experimental data help to make clear the origin of TeV gamma-ray emission in the SNRs like γCygni SNR and G166.0+4.3.

Spatially resolved spectroscopy of non-thermal X-rays in RX J1713.7−3946 with Chandra

Abstract The young shell-type supernova remnant (SNR) RX J1713.7−3946 has been studied as a suitable target to test the SNR paradigm for the origin of Galactic cosmic rays. We present a spatially resolved spectroscopy of the non-thermal X-ray emission in RX J1713.7−3946 with Chandra. In order to obtain X-ray properties of the filamentary structures and their surrounding regions, we divide the southeastern (SE), southwestern (SW), and northwestern (NW) parts of the SNR into subregions on the typical order of several 10" and extract spectra from each subregion. Their photon indices are significantly different among the subregions, with a range of 1.8 &amp;lt; Γ &amp;lt; 3. In the SE part, the clear filaments are harder (Γ ∼ 2.0) than the surrounding regions. This is a common feature often observed in young SNRs and naturally interpreted as a consequence of synchrotron cooling. On the other hand, the bright filamentary regions do not necessarily coincide with the hardest regions in the SW and NW parts. We also find the SW filamentary region is relatively rather soft (Γ ∼ 2.7). In addition, we find that hard regions with photon indices of 2.0–2.2 exist around the bright emission although the hard regions lie in the downstream region and the bright emission does not appear to be the blast wave shock front. Both the aforementioned characteristic regions in SW are located close to peaks of the interstellar gas. We discuss possible origins of the spatial variation of the photon indices, paying particular attention to the shock–cloud interactions.