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Register a new userA Distance Estimate to the Cygnus Loop Based on the Distances to Two Stars Located Within the Remnant
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Underlying nearly every quantitative discussion of the Cygnus Loop supernova remnant is uncertainty about its distance. Here we present optical images and spectra of nebulosities around two stars whose mass-loss material appears to have interacted with the remnants expanding shock front and thus can be used to estimate the Cygnus Loops distance. Narrow passband images reveal a small emission-line nebula surrounding an M4 red giant near the remnants eastern nebula NGC 6992. Optical spectra of the nebula show it to be shock-heated with significantly higher electron densities than seen in the remnants filaments. This along with a bow-shaped morphology suggests it is likely red giant mass-loss material shocked and accelerated by passage of the Cygnus Loops blast wave. We also identify a B7 V star located along the remnants northwestern limb which also appears to have interacted with the remnants shock wave. It lies within a small arc of nebulosity in an unusually complex region of highly curved and distorted filaments along the remnants northern shock front suggestive of a localized disturbance of the shock front due to the B stars stellar winds. Based on the assumption that these two stars lie inside the remnant, combined with an estimated distance to a molecular cloud situated along the remnants western limb, we propose a distance to the Cygnus Loop of 1.0 +/- 0.2 kpc. Although larger than several recent estimates of 500 - 800 pc, a distance ~1 kpc helps resolve difficulties with the remnants postshock cosmic ray and gas pressure ratio and estimated supernova explosion energy.
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We present a revised distance to the Cygnus Loop supernova remnant of $725pm15$ 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 $simeq$18 pc radius.
We present here the observation of the Cygnus Superbubble (CSB) using the Solid-state slit camera (SSC) aboard the Monitor of All-sky X-ray Image. The CSB is a large diffuse structure in the Cygnus region with enhanced soft X-ray emission. By utilizing the CCD spectral resolution of the SSC, we detect Fe, Ne, Mg emission lines from the CSB for the first time. The best fit model implies thin hot plasma of kT ~ 0.3 keV with depleted abundance of 0.26 +/- 0.1 solar. Joint spectrum fitting of the ROSAT PSPC data and MAXI/SSC data enables us to measure precise values of NH and temperature inside the CSB. The results show that all of the regions in the CSB have similar NH and temperature, indicating that the CSB is single unity. The energy budgets calculation suggests that 2-3 Myrs of stellar wind from the Cyg OB2 is enough to power up the CSB, whereas due to its off center position, the origin of the CSB is most likely a Hypernova.
Supernova remnants (SNRs) represent a powerful laboratory to study the Cosmic-Ray acceleration processes at the shocks, and their relation to the properties of the circumstellar medium. With the aim of studying the high-frequency radio emission and investigating the energy distribution of accelerated electrons and the magnetic field conditions, we performed single-dish observations of the large and complex Cygnus Loop SNR from 7.0 to 24.8 GHz with the Medicina and the Sardinia Radio Telescope, focusing on the northern filament (NGC 6992) and the southern shell. Both regions show a spectrum well fitted by a power-law function ($Spropto u^{-alpha}$), with spectral index $alpha=0.45pm0.05$ for NGC 6992 and $alpha=0.49pm0.01$ for the southern shell and without any indication of a spectral break. The spectra are significantly flatter than the whole Cygnus Loop spectrum ($alpha=0.54pm0.01$), 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 $mu$G. This value is compatible with a pure adiabatic compression of the interstellar magnetic field, suggesting that the amplification process is currently inefficient.
Charge exchange (CX) is an important process in shock physics since it indicates an interaction between downstream ions and ambient neutral hydrogen, suggesting a presence of a collisionless shock. We present a high-resolution spectroscopy of an X-ray bright spot in a nearby supernova remnant (SNR), the Cygnus Loop, with the Reflection Grating Spectrometer (RGS) onboard XMM-Newton. The target is a compact knotty structure called southwestern knot (SW-K) located at the outer edge of the shell, where the blast wave is likely interacting with dense surrounding materials. The RGS spectrum of the SW-K shows details of the line features below ~ 1 keV, where we discover a high forbidden-to-resonance line ratio of OVII He$alpha$. The soft-band (10-35 AA) spectrum is well explained by a thermal component with a CX X-ray emission obscured by neutral and ionized absorbers. The presence of the CX X-ray emission will provide new insights into the shock physics of SNRs. The high-resolution spectroscopy also reveals that the CNO, Ne and Fe abundances are truly lower than the solar values (0.2-0.4 solar) at the SW-K region . Our result gives a clue to solving the previously known low-abundance problem reported from a number of evolved SNRs.
We use new and archival Chandra observations of Cygnus A, totalling $sim$1.9 Ms, to investigate the distribution and temperature structure of gas lying within the projected extent of the cocoon shock and exhibiting a rib-like structure. We confirm that the X-rays are dominated by thermal emission with an average temperature of around 4 keV, and have discovered an asymmetry in the temperature gradient, with the southwestern part of the gas cooler than the rest by up to 2 keV. Pressure estimates suggest that the gas is a coherent structure of single origin located inside the cocoon, with a mass of roughly $2times10^{10} M_{odot}$. We conclude that the gas is debris resulting from disintegration of the cool core of the Cygnus A cluster after the passage of the jet during the early stages of the current epoch of activity. The 4 keV gas now lies on the central inside surface of the hotter cocoon rim. The temperature gradient could result from an offset between the centre of the cluster core and the Cygnus A host galaxy at the switch-on of current radio activity.
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