Cygnus Loop Research Articles

Simulating the peculiar periphery of the Cygnus Loop

Abstract The middle-aged Galactic supernova remnant (SNR) the Cygnus Loop (CL) displays a peculiar morphology in X-rays, featuring a blowout in the southern region. The underlying process accounting for the formation of the peculiar periphery remains a mystery. To this end, we conduct hydrodynamical simulations to investigate the SNR evolution coupled with a tailored stellar-wind model: a bipolar stellar wind emanating from a runaway red supergiant progenitor, excavating a wind-blown cavity elongated along the $-z$-direction. Our simulation results reveal that the forward shock of the consequent SNR sweeps up the modified ambient media, shaping the overall morphology with a blowout comparable to that of CL. Besides, a series of simulation runs are performed to assess the impacts of different model parameters and the projection effect (observational angle $\theta_{\mathrm{obs}}$) on the final SNR profile. Three physical quantities are extracted from simulation results to characterize the simulated SNR and make a direct comparison with the X-ray observations of CL. We find that the final SNR morphology is sensitive to both stellar-wind properties and $\theta_{\mathrm{obs}}$. A Cygnus-Loop-like SNR could be reproduced under appropriate parameter combinations at $\theta_{\mathrm{obs}}=0^{\circ}$. While for $\theta_{\mathrm{obs}}\lesssim30^{\circ}$, the projected morphology akin to CL could be also generated under specific conditions.

The Rockets for Extended-source X-ray Spectroscopy Instrument Design

The Rockets for Extended-source X-ray Spectroscopy (tREXS) are a series of suborbital rocket payloads designed to collect spectral emission from extended astronomical sources of soft X-rays. The tREXS spectrograph uses mechanical beam-shaping modules and reflection gratings to passively focus and then disperse incident X-rays to an array of Teledyne/e2v CIS 113 CMOS X-ray sensors. Designed to achieve a moderate spectral resolution, R ⪆ 50, from ≈15 to 40 Å over a large field of view (>10 deg2), the tREXS instrument is sensitive to line emission from key ion species over the soft-X-ray band. Here we present the complete end-to-end design of the tREXS spectrograph and discuss its significance and expected performance, using simulated observations of the Cygnus Loop supernova remnant and an enhanced region in the soft X-ray background.

A Monte Carlo Simulation on Resonant Scattering of X-Ray Line Emission in Supernova Remnants

Resonant scattering (RS) of X-ray line emission in supernova remnants (SNRs) may modify the observed line profiles and fluxes and has a potential impact on estimating the physical properties of the hot gas and hence on understanding the SNR physics, but has not been theoretically modeled ever. Here we present our Monte Carlo simulation of the RS effect on X-ray resonant-line emission, typified by the O vii Heα r line, from SNRs. We employ the physical conditions characterized by the Sedov–Taylor solution and some basic parameters similar to those in Cygnus Loop. We show that the impact of the RS effect is most significant near the edge of the remnant. The line profiles are predicted to be asymmetric because of different temperatures and photon production efficiencies of the expanding gas at different radii. We also predict the surface brightness of the line emission would decrease in the outer projected region but is slightly enhanced in the inner. The G-ratio of the O vii Heα triplet can be effectively elevated by RS in the outer region. We show that the RS effect of the O vii Heα r line in the southwestern boundary region of Cygnus Loop is nonnegligible. The observed O vii G-ratio of ∼1.8 of the region could be achieved with RS taken into account for properly elevated O abundance from the previous estimates. Additional simulation performed for the SNRs in ejecta-dominated phases like Cas A shows that RS in the shocked ejecta may have some apparent effects on the observational properties of oxygen-resonant lines.

A refined search for high-velocity gas in the Cygnus Loop supernova remnant

ABSTRACT We present the results of a sensitive search for high-velocity gas in interstellar absorption lines associated with the Cygnus Loop supernova remnant (SNR). We examine high-resolution high signal-to-noise ratio optical spectra of six stars in the Cygnus Loop region with distances greater than ∼700 pc. All stars show low-velocity Na i and Ca ii absorption. However, only one star, HD 198301, exhibits high-velocity Ca ii absorption components, at velocities of +62, +82, and +96 km s−1. The distance to this star of ∼870 pc helps to constrain the distance to the receding edge of the Cygnus Loop’s expanding shock front. One of our targets, HD 335334, was previously thought to exhibit high positive and high negative velocity interstellar Na i and Ca ii absorption. This was one factor leading Fesen et al. to derive a distance to the Cygnus Loop of 725 ± 15 pc. However, we find that HD 335334 is in fact a double-line spectroscopic binary and shows no evidence of high-velocity interstellar absorption. As such, the distance to HD 335334 cannot be used to constrain the distance to the Cygnus Loop. Our detection of Ca ii absorption approaching 100 km s−1 toward HD 198301 is the first conclusive detection of high-velocity absorption from a low-ionization species associated with the Cygnus Loop SNR. A large jump in the Na i column density toward BD+31 4218, a star located beyond the northwestern boundary of the Cygnus Loop, helps to constrain the distance to a large molecular cloud complex with which the Cygnus Loop is evidently interacting.

QUIJOTE scientific results – XIII. Intensity and polarization study of the microwave spectra of supernova remnants in the QUIJOTE-MFI wide survey: CTB 80, Cygnus Loop, HB 21, CTA 1, Tycho, and HB 9

ABSTRACT We use the new QUIJOTE-MFI wide survey (11, 13, 17, and 19 GHz) to produce spectral energy distributions (SEDs), on an angular scale of 1°, of the supernova remnants (SNRs) CTB 80, Cygnus Loop, HB 21, CTA 1, Tycho, and HB 9. We provide new measurements of the polarized synchrotron radiation in the microwave range. The intensity and polarization SEDs are obtained and modelled by combining QUIJOTE-MFI maps with ancillary data. In intensity, we confirm the curved spectra of CTB 80 and HB 21 with a break frequency νb at 2.0$^{+1.2}_{-0.5}$ and 5.0$^{+1.2}_{-1.0}$ GHz, respectively; and spectral indices above the break of $-0.6^{+0.04}_{-0.05}$ and $-0.86^{+0.04}_{-0.05}$. We provide constraints on the Anomalous Microwave Emission, suggesting that it is negligible towards these SNRs. From a simultaneous intensity and polarization fit, we recover synchrotron spectral indices as flat as −0.24, and the whole sample has a mean and scatter of −0.44 ± 0.12. The polarization fractions have a mean and scatter of 6.1 ± 1.9 per cent. When combining our results with the measurements from other QUIJOTE (Q-U-I JOint TEnerife CMB experiment) studies of SNRs, we find that radio spectral indices are flatter for mature SNRs, and particularly flatter for CTB 80 ($-0.24^{+0.07}_{-0.06}$) and HB 21 ($-0.34^{+0.04}_{-0.03}$). In addition, the evolution of the spectral indices against the SNRs age is modelled with a power-law function, providing an exponent −0.07 ± 0.03 and amplitude −0.49 ± 0.02 (at 10 kyr), which are conservative with respect to previous studies of our Galaxy and the Large Magellanic Cloud.

The Cygnus Loop: Shock Precursors and Electron–Ion Equilibration

The outermost edges of some supernova remnants are marked by filaments of pure Balmer line emission produced by nonradiative shock fronts. The Hα profiles of these filaments provide the shock speed and electron–proton temperature ratio in the shock. We have used the Hectochelle multiobject spectrograph on the MMT telescope to observe nine positions along the eastern nonradiative filaments of the Cygnus Loop, thereby extending the studies of Medina et al. to lower shock speeds. We complement the spectra with images from 1993 to 2020 to measure proper motions to further constrain the parameters. We also present observations of the photoionization precursor to show that the gas is strongly heated, and that the Hα emission from the precursor is dominated by collisional excitation rather than recombination.

On the Hα faintness of the North Polar Spur

ABSTRACT The ratio of Hα intensity to 1.4-GHz radio continuum intensity in the North Polar Spur (NPS) is measured to be ≲ 50, two orders of magnitude smaller than the values of ∼104 observed in the typical shell-type old supernova remnants of the Cygnus Loop and S147. The extremely low Hα-to-radio intensity ratio favours the Galactic Centre explosion model for the NPS, which postulates a giant shock wave at a distance of several kiloparsecs in the hot and low-density Galactic halo with a low hydrogen recombination rate, over the local supernova(e) remnant model.

Third Epoch HST Imaging of a Nonradiative Shock in the Cygnus Loop Supernova Remnant

We present new HST/WFC3 optical images of a region in the northeastern part of the Cygnus Loop supernova remnant, which includes a well-studied Balmer-line filament. These data represent the third epoch of HST Hα imaging and a second epoch of [O iii] λ5007 imaging of that particular filament. The Hα images were used to measure the proper motions at various locations along the shock front, and the values ranged from 55 to 85 mas yr−1 with a median value of 70 mas yr−1, which corresponds to a shock velocity of 240 km s−1. The proper motions between epochs 1 and 2 were the same as between epochs 2 and 3, implying that there has been no measurable deceleration of the shock in the 22 yr period between the first and third epochs. The range of proper motions (and therefore shock velocities) along the filament indicate variations of over a factor of two in the preshock density. The [O iii] emission is prominent toward one end of the filament where the shock has transitioned from nonradiative to radiative. The proper motion is smaller than for the Hα filaments, and it corresponds to a shock velocity of about 155 km s−1. The images obtained about 18 yr apart show that the [O iii] morphology has not changed, which places limits on any short-timescale variations due to catastrophic cooling or thermal instabilities. We find that the effective shock age is less than the eddy turnover timescale, which implies that turbulence has not yet influenced the dynamics of the shock.

Proper motion of Cygnus Loop shock filaments

We determined the shock speed in the Galactic supernova remnant Cygnus Loop, using the proper motion of its optical filaments and the latest estimate of its distance. The proper motion was measured by comparing H? images of the remnant observed in two epochs: 1993 (Kitt Peak National Observatory) and 2018/2019 (National Astronomical Observatory Rozhen and Astronomical Station Vidojevica). We derived shock speed for 35 locations along different filaments, which is twice as much as in earlier studies of the north-eastern part of the Cygnus Loop. For the first time, we have measured the shock speed of the radiative filaments in this region. Three of the analyzed locations where we measured the proper motion of filaments are radiative, based on their presence in [SII] images from the second epoch. The other filaments are non-radiative. The speed we obtained for the non-radiative filaments is in the range of 240{650 km s-1, with an estimate for the uncertainty of 70 km s-1. These values are mostly in agreement with previous studies. The radiative filaments have lower speeds of 100-160 ?70 km s-1, which is in agreement with the assumption that they are older in evolutionary terms. This clear distinction between the speed of the two types of filaments proves that [SII] emission can be used for identifying radiative filaments in supernova remnants.

First Wide Field-of-view X-Ray Observations by a Lobster-eye Focusing Telescope in Orbit

As a novel X-ray focusing technology, lobster-eye micropore optics (MPO) feature both a wide observing field of view and true imaging capability, promising sky monitoring with significantly improved sensitivity and spatial resolution in soft X-rays. Since first proposed by Angel, the optics have been extensively studied, developed and trialed over the past decades. In this Letter, we report on the first-light results from a flight experiment of the Lobster Eye Imager for Astronomy, a pathfinder of the wide-field X-ray telescope of the Einstein Probe mission. The piggyback imager, launched in 2022 July, has a mostly unvignetted field of view of 18.°6 × 18.°6. Its spatial resolution is in the range of 4′–7′ in FWHM and the focal spot effective area is 2–3 cm2, both showing only mild fluctuations across the field of view. We present images of the Galactic center region, Sco X-1, and the diffuse Cygnus Loop nebular taken in snapshot observations over 0.5–4 keV. These are truly wide-field X-ray images of celestial bodies observed, for the first time, by a focusing imaging telescope. Initial analyses of the in-flight data show excellent agreement between the observed images and the on-ground calibration and simulations. The instrument and its characterization are briefly described, as well as the flight experiment. The results provide a solid basis for the development of the present and proposed wide-field X-ray missions using lobster-eye MPO.

New Continuum and Polarization Observations of the Cygnus Loop with FAST. II. Images and Analyses

We present total-intensity and polarized-intensity images of the Cygnus Loop supernova remnant (SNR) observed by the Five-hundred-meter Aperture Spherical radio Telescope. The high angular-resolution and high-sensitivity images enable us to thoroughly compare the properties of the northern part with the southern part of the SNR. The central filament in the northern part and the southern part have a similar foreground rotation measure, meaning their distances are likely similar. The polarization analysis indicates that the random magnetic field is larger than the regular field in the northern part, but negligible in the southern part. The total-intensity image is decomposed into components of various angular scales, and the brightness-temperature spectral index of the shell structures in the northern part is similar to that in the southern part in the component images. All the evidence suggests that the northern and southern parts of the Cygnus Loop are situated and thus evolved in different environments of interstellar medium, while belonging to the same SNR.

On the Measurement of Vorticity in Astrophysical Fluids

Vorticity is central to the nature of, and dynamical processes in turbulence, including turbulence in astrophysical fluids. The results of Raymond et al. on vorticity in the post-shock fluid of the Cygnus Loop supernova remnant are therefore of great interest. I consider the degree to which spectroscopic measurements of an optically thin line, the most common type of astronomical velocimetry, can yield unambiguous measurements of the vorticity in a fluid. I consider an ideal case of observations in the plane of a flow which possesses vorticity in a direction perpendicular to the plane of observations. The observationally deduced vorticity (referred to as the pseudovorticity) is a reasonable match for the true vorticity. However, the pseudovorticity of an irrotational model flow is also nonzero, and comparable in magnitude to that for a vortical flow. Astronomical spectroscopic observations may yield a good estimate of the vorticity, but its robustness cannot be insured.

Hot spots in the neutrino flux created by cosmic rays from Cygnus and Vela

An analysis of 7.5 years of data in the high-energy starting event sample has been recently published by the IceCube collaboration. The hottest spot in a search for neutrino sources was found far above the Galactic plane and is thus, at first sight, difficult to reconcile with a Galactic origin. In this work, we calculate the cosmic ray (CR) density around nearby, young supernova remnants assuming anisotropic diffusion. Combining the obtained CR densities with the matter distribution deduced from extinction maps, we find two prominent hot spots: The one close to the most significant point in the IceCube search for point sources is created by CRs from the Cygnus loop and has an intensity corresponding to two to four neutrino events. Another, more extended one may be caused by CRs from Vela if CR trajectories are sufficiently disturbed by the magnetic field in the shell around the superbubble Loop I.

Multiple accelerated particle populations in the Cygnus Loop with Fermi-LAT

Context. The Cygnus Loop (G74.0-8.5) is a very well-known nearby supernova remnant (SNR) in our Galaxy. Thanks to its large size, brightness, and angular offset from the Galactic plane, it has been studied in detail from radio to γ-ray emission. The γ-rays probe the populations of energetic particles and their acceleration mechanisms at low shock speeds. Aims. We present an analysis of the γ-ray emission detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope over 11 yr in the region of the Cygnus Loop. Methods. We performed detailed morphological and spectral studies of the γ-ray emission toward the remnant from 100 MeV to 100 GeV and compared it with X-ray, UV, optical, and radio images. The higher statistics with respect to the previous studies enabled us to decompose the emission from the remnant into two morphological components to model its nonthermal multiwavelength emission. Results. The extended γ-ray emission is well correlated with the thermal X-ray and UV emission of the SNR. Our morphological analysis reveals that a model considering two contributions from the X-ray and the UV emission regions is the best description of the γ-ray data. Both components show a curved spectrum, but the X-ray component is softer and more curved than the UV component, suggesting a different physical origin. The multiwavelength modeling of emission toward the SNR suggests that the nonthermal radio and γ-ray emission associated with the UV component is mostly due to the reacceleration of preexisting cosmic rays by radiative shocks in the adjacent clouds, while the nonthermal emission associated with the X-ray component arises from freshly accelerated cosmic rays.

New continuum and polarization observations of the Cygnus Loop with FAST I. Data processing and verification

We report on the continuum and polarization observations of the Cygnus Loop supernova remnant (SNR) conducted by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). FAST observations provide high angular resolution and high sensitivity images of the SNR, which will help to disentangle its nature. We obtained Stokes I, Q and U maps over the frequency range of 1.03 – 1.46 GHz split into channels of 7.63 kHz. The original angular resolution is in the range of , and we combined all the data at a common resolution of . The temperature scale of the total intensity and the spectral index from the in-band temperature-temperature plot are consistent with previous observations, which validates the data calibration and map-making procedures. The rms sensitivity for the band-averaged total-intensity map is about 20 mK in brightness temperature, which is at the level of confusion limit. For the first time, we apply rotation measure (RM) synthesis to the Cygnus Loop to obtain the polarization intensity and RM maps. The rms sensitivity for polarization is about 5 mK, far below the total-intensity confusion limit. We also obtained RMs of eight extragalactic sources, and demonstrate that the wide-band frequency coverage helps to overcome the ambiguity of RM determinations.

An updated distance to the Cygnus Loop based on Gaia Early DR3

ABSTRACT We present a revised distance to the Cygnus Loop supernova remnant of 725 ± 15 pc based on Gaia Early Data Release 3 parallax measurements (EDR3) for several stars previously found to be located either inside or behind the supernova based on the presence of high-velocity absorption lines in their spectra. This revised distance estimate and error means the Cygnus Loop remnant now has an estimated distance uncertainty comparable to that of its ≃18 pc radius.

New high-frequency radio observations of the Cygnus Loop supernova remnant with the Italian radio telescopes

ABSTRACT Supernova remnants (SNRs) represent a powerful laboratory to study the cosmic ray acceleration processes at shocks, and their relation to the properties of the circumstellar medium. With the aim of studying the high-frequency radio emission and investigating the energy distribution of accelerated electrons and the magnetic field conditions, we performed single-dish observations of the large and complex Cygnus Loop SNR from 7.0–24.8 GHz with the Medicina and Sardinia Radio Telescopes, focusing on the northern filament (NGC 6992) and the southern shell. Both regions show a spectrum well fitted by a power-law function (S ∝ ν−α), with spectral index α = 0.45 ± 0.05 for NGC 6992 and α = 0.49 ± 0.01 for the southern shell and without any indication of a spectral break. The spectra are significantly flatter than the whole Cygnus Loop spectrum (α = 0.54 ± 0.01), suggesting a departure from the plain shock acceleration mechanisms, which for NGC 6992 could be related to the ongoing transition towards a radiative shock. We model the integrated spectrum of the whole SNR considering the evolution of the maximum energy and magnetic field amplification. Through the radio spectral parameters, we infer a magnetic field at the shock of 10 μG. This value is compatible with purely adiabatic compression of the interstellar magnetic field, suggesting that the amplification process is currently inefficient.

Turbulence and Energetic Particles in Radiative Shock Waves in the Cygnus Loop. II. Development of Postshock Turbulence

Abstract Radiative shock waves in the Cygnus Loop and other supernova remnants show different morphologies in [O iii] and Hα emission. We use HST spectra and narrowband images to study the development of turbulence in the cooling region behind a shock on the west limb of the Cygnus Loop. We refine our earlier estimates of shock parameters that were based upon ground-based spectra, including ram pressure, vorticity, and magnetic field strength. We apply several techniques, including Fourier power spectra and the Rolling Hough Transform, to quantify the shape of the rippled shock front as viewed in different emission lines. We assess the relative importance of thermal instabilities, the thin shell instability, upstream density variations, and upstream magnetic field variations in producing the observed structure.

Turbulence and Energetic Particles in Radiative Shock Waves in the Cygnus Loop. I. Shock Properties

Abstract We have obtained a contiguous set of long-slit spectra of a shock wave in the Cygnus Loop to investigate its structure, which is far from the morphology predicted by 1D models. Proper motions from Hubble Space Telescope images combined with the known distance to the Cygnus Loop provide an accurate shock speed. Earlier analyses of shock spectra estimated the shock speed, postshock density, temperature, and elemental abundances. In this paper we determine several more shock parameters: a more accurate shock speed, ram pressure, density, compression ratio, dust destruction efficiency, magnetic field strength, and vorticity in the cooling region. From the derived shock properties we estimate the emissivities of synchrotron emission in the radio and pion decay emission in the gamma-rays. Both are consistent with the observations if we assume simple adiabatic compression of ambient cosmic-rays as in the van der Laan mechanism. We also find that, although the morphology is far from that predicted by 1D models and the line ratios vary dramatically from point to point, the average spectrum is matched reasonably well by 1D shock models with the shock speed derived from the measured proper motion. A subsequent paper will analyze the development of turbulence in the cooling zone behind the shock.

Unbiased Spectroscopic Study of the Cygnus Loop with LAMOST. I. Optical Properties of Emission Lines and the Global Spectrum

Abstract We present an unbiased spectroscopic study of the Galactic supernova remnant (SNR) Cygnus Loop using the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) DR5. LAMOST features both a large field of view and a large aperture, which allow us to simultaneously obtain 4000 spectra at ∼3700–9000 Å with R ≈ 1800. The Cygnus Loop is a prototype of middle-aged SNRs, which has the advantages of being bright, large in angular size, and relatively unobscured by dust. Along the line of sight to the Cygnus Loop, 2747 LAMOST DR5 spectra are found in total, which are spatially distributed over the entire remnant. This spectral sample is free of the selection bias of most previous studies, which often focus on bright filaments or regions bright in [O iii]. Visual inspection verifies that 368 spectra (13% of the total) show clear spectral features to confirm their association with the remnant. In addition, 176 spectra with line emission show ambiguity of their origin but have a possible association to the SNR. In particular, the 154 spectra dominated by the SNR emission are further analyzed by identifying emission lines and measuring their intensities. We examine distributions of physical properties such as electron density and temperature, which vary significantly inside the remnant, using theoretical models. By combining a large number of the LAMOST spectra, a global spectrum representing the Cygnus Loop is constructed, which presents characteristics of radiative shocks. Finally, we discuss the effect of the unbiased spectral sample on the global spectrum and its implication to understand a spatially unresolved SNR in a distant galaxy.