High-Resolution X-Ray Spectroscopy of Supernova Remnants

Thermal X-ray spectra from supernova remnants (SNRs) are dominated by a number of line emission from various elements. Resolving the individual lines is critically important for a variety of scientific topics such as diagnosing high-temperature and low-density non-equilibrium plasmas, identifying spectral features like charge exchange and resonance line scattering, revealing kinematics and elemental abundances of SN ejecta and the circumstellar medium, and studying the interstellar medium or planets' atmospheres from extinction features seen in X-ray spectra of very bright SNRs. This chapter reviews high-resolution X-ray spectroscopy of SNRs obtained so far. Most results were derived with dispersive spectrometers aboard Einstein, Chandra, and XMM-Newton satellites. Because these dispersive spectrometers were slitless, one has to select small objects with angular sizes less than a few arcminutes to successfully perform high-resolution spectroscopy. Despite this limitation, the three satellites delivered fruitful scientific results in the last few decades. Arrays of low-temperature microcalorimeters offer excellent opportunities for high-resolution X-ray spectroscopy of SNRs, as they are non-dispersive spectrometers that work for largely extended sources as well as point-like sources. The microcalorimeter aboard the Hitomi satellite already delivered pioneering results during its short lifetime. The upcoming X-Ray Imaging and Spectroscopy Mission, which is a recovery mission of Hitomi, will truly open the new discovery window to high-resolution X-ray spectroscopy of SNRs.Comment: Updates of a chapter of the book "High-Resolution X-Ray Spectroscopy" (Springer) - Editors: Cosimo Bambi and Jiachen Jian

High-Resolution X-Ray Spectroscopy of Galactic Supernova Remnants

High-resolution X-ray spectroscopy of Galactic supernova remnants (SNRs), based on grating spectrometers onboard XMM-Newton and Chandra, has been revealing a variety of new astrophysical phenomena. Broadened oxygen lines for a northwestern compact knot in SN 1006 clearly show a high oxygen temperature of ~300 keV. The high temperature together with a lower electron temperature (kTe ~ 1 keV) can be reasonably interpreted as temperature non-equilibration between electrons and oxygen behind a collisionless shock. An ejecta knot in the Puppis A SNR shows blueshifted line emission by ~ 1500kms-1. The line widths are fairly narrow in contrast to the SN 1006's knot; an upper limit of 0.9 eV is obtained for O VIII Lyα, which translates to an oxygen temperature of kTO < 30 keV. The low temperature suggests that the knot was heated by a reverse shock whose velocity is 4 times slower than that of a forward shock. Anomalous intensity ratios in O VII Heα lines, i.e., a stronger forbidden line than a resonance line, is found in a cloud-shock interaction region in Puppis A. The line ratio can be best explained by the charge-exchange emission that should arise at interfaces between the cold/warm clouds and the hot plasma. There are several other targets for which we plan to analyze high-quality grating data prior to the operation of the soft X-ray spectrometer onboard Astro-H

Spatially Resolved Spectral Analysis of Vela Shrapnel D

The ROSAT all-sky survey discovered several `shrapnels', showing boomerang structures outside the Vela supernova remnant. We observed shrapnel D with the XMM-Newton satellite. There is an X-ray bright ridge structure in our FOV running from north to south. Applying the VNEI model to X-ray spectra of various regions, we find that the plasma in the eastern part from the X-ray ridge is significantly different from that in the western part. The X-ray spectra in the western part can be represented by a single-temperature component. The abundances of heavy elements are almost uniform, whereas they are heavily overabundant, except for Fe; the relative abundances to the solar values are O about 5, Ne about 10, Mg about 10, Fe about 1. This indicates that shrapnel D originated from the ejecta of the supernova. We find that the plasma in the eastern part from the ridge consists of two components with different temperatures; the hot component comes from the ejecta, while the cold component comes from the interstellar matter. These two components are considered to be in contact with each other, forming a contact discontinuity. Around the northern part of the contact discontinuity, we find wave-like structures of which the typical scale are comparable with that of the Rayleigh--Taylor instability.Comment: 15 pages, 14 figure

The Ejecta Distributions of the Heavy Elements in the Cygnus Loop

We analyzed the metal distribution of the Cygnus Loop using 14 and 7 pointings observation data obtained by the \textit{Suzaku} and the \textit{XMM-Newton} observatories. The spectral analysis shows that all the spectra are well fitted by the two-$kT_e$ non-equilibrium ionization plasma model as shown by the earlier observations. From the best-fit parameters of the high-$kT_e$ component, we calculated the emission measures about various elements and showed the metal distribution of the ejecta component. We found that the distributions of Si and Fe are centered at the southwest of the geometric center toward the blow-out region. From the best-fit parameters, we also estimated the progenitor mass of the Cygnus Loop from our field of view and the metal rich region with a radius of 25 arcmin from the metal center. The result from the metal circle is similar to that from our entire FOV, which suggests the mixing of the metal. From the results, we estimated the mass of the progenitor star at 12-15\MO.Comment: 15 pages, 5 figures, PASJ, Vol.61, No.

Constraining the Age and Distance of the Galactic Supernova Remnant G156.2+5.7 by H-alpha Expansion Measurements

We present deep H-alpha images of portions of the X-ray bright but optically faint Galactic supernova remnant G156.2+5.7, revealing numerous and delicately thin nonradiative filaments which mark the location of the remnant's forward shock. These new images show that these filaments have a complex structure not visible on previous lower resolution optical images. By comparing H-alpha images taken in 2004 at the McDonald Observatory and in 2015-2016 at the Kiso Observatory, we set a stringent 1-sigma upper limit of expansion to be 0.06 arcsec/yr. This proper motion, combined with a shock speed of 500 km/s inferred from X-ray spectral analyses, gives a distance of > 1.7 kpc. In addition, a simple comparison of expansion indices of several SNRs allows us to infer the age of the remnant to be a few 10,000 yr old. These estimates are more straightforward and reliable than any other previous studies, and clearly rule out a possibility that G156.2+5.7 is physically associated with part of the Taurus-Auriga cloud and dust complex at a distance of 200-300 pc.Comment: 16 pages, 5 figures, accepted for publication in The Astrophysical Journa

The Plasma Structure of the Southwestern Region of the Cygnus Loop with the XMM-Newton Observatory

We observed the southwestern region of the Cygnus Loop in two pointings with \textit{XMM-Newton}. The region observed is called the "blow-out" region that is extended further in the south. The origin of the "blow-out" is not well understood while it is suggested that there is another supernova remnant here in radio observation. To investigate the detail structure of this region in X-ray, we divided our fields of view into 33 box regions. The spectra are well fitted by a two-component nonequilibrium ionization model. The emission measure distributions of heavy elements decrease from the inner region to the outer region of the Loop. Then, we also divided our fields of view into 26 annular sectors to examine the radial plasma structure. Judging from metal abundances obtained, it is consistent with that the X-ray emission is the Cygnus Loop origin and we concluded that high-$kT_{e}$ component ($\sim$0.4 keV) originates from the ejecta while low-$kT_{e}$ component ($\sim$0.2 keV) is derived from the swept-up interstellar medium. The flux of low-$kT_{e}$ component is much less than that of high-$kT_{e}$ component, suggesting the ISM component is very thin. Also, the relative abundances in the ejecta component shows similar values to those obtained from previous observations of the Cygnus Loop. We find no evidence in X-ray that the nature of the "blow-out" region originated from the extra supernova remnant. From the ejecta component, we calculated the masses for various metals and estimated the origin of the Cygnus Loop as the core-collapse explosion rather than the Type Ia supernova.Comment: 10 pages, 28 figures, accepted for publication in the Astrophysical Journa

Chandra Observations of the Northeastern Rim of the Cygnus Loop

We present results from spatially resolved spectral analyses of the northeastern (NE) rim of the Cygnus Loop supernova remnant (SNR) based on two Chandra observations. One pointing includes northern outermost abundance-enhanced regions discovered by recent Suzaku observations, while the other pointing is located on regions with "normal" abundances in the NE rim of the Cygnus Loop. The superior spatial resolving power of Chandra allows us to reveal that the abundance-enhanced region is concentrated in an about 200"-thickness region behind the shock front. We confirm absolute metal abundances (i.e., relative to H) as well as abundance ratios between metals are consistent with those of the solar values within a factor of about 2. Also, we find that the emission measure in the region gradually decreases toward the shock front. These features are in contrast with those of the ejecta fragments around the Vela SNR, which leads us to believe that the abundance enhancements are not likely due to metal-rich ejecta. We suggest that the origin of the plasma in this region is the interstellar medium (ISM). In the "normal" abundance regions, we confirm that abundances are depleted to the solar values by a factor of about 5 that is not expected in the ISM around the Cygnus Loop. Introduction of non-thermal emission in our model fitting can not naturally resolve the abundance-depletion problem. The origin of the depletion still remains as an open question.Comment: 18 pages, 6 figure