Optical Discovery of an Apparent Galactic Supernova Remnant G159.6+7.3

Deep Halpha images of portions of a faint 3 x 4 degree Halpha shell centered at l = 159.6 deg, b = 7.3 deg seen on the Virginia Tech Spectral Line Survey images revealed the presence of several thin emission filaments along its eastern limb. Low-dispersion optical spectra of two of these filaments covering the wavelength range of 4500 - 7500 Angstroms show narrow Halpha line emissions with velocities around -170 +/- 30 km/s. Both the morphology and spectra of these filaments are consistent with a Balmer dominated shock interpretation and we propose these optical filaments indicate that the large Halpha emission shell is a previously unrecognized supernova remnant. ROSAT All Sky Survey images indicate the possible presence of extremely faint, diffuse emission from the shell's central region. The shell's location more than seven degrees off the Galactic plane in a region of relatively low interstellar density may account for the lack of any reported associated nonthermal radio emissions. The rare discovery of a Galactic SNR at optical wavelengths suggests that additional high latitude SNRs may have escaped radio and X-ray detection.Comment: 6 pages, 6 figures, accepted to the Astronomical Journa

The Nature of the Ultraluminous Oxygen-Rich Supernova Remnant in NGC 4449

Optical images and spectra, both ground based and taken by Hubble Space Telescope (HST), of the young, luminous O-rich supernova remnant in the irregular galaxy NGC 4449 are presented. HST images of the remnant and its local region were obtained with the ACS/WFC using filters F435W, F555W, F814W (B, V, and I, respectively), F502N ([O III]), F658N (Halpha + [N II]), F660N ([N II]), and F550M (line-free continuum). These images show an unresolved remnant (FWHM < 0.05 arcsec) located within a rich cluster of OB stars which itself is enclosed by a nearly complete interstellar shell seen best in Halpha + [N II] emission approximately 8'' x 6'' (150 pc x 110 pc) in size. The remnant and its associated OB cluster are isolated from two large nearby H II regions. The ACS [O III] image shows the remnant may be partially surrounded by a clumpy ring of emission approximately 1'' (~20 pc) in diameter. Recent ground-based spectra of the remnant reveal (1) the emergence of broad, blueshifted emission lines of [S II] 6716, 6731, [Ar III] 7136, and [Ca II] 7291, 7324 which were not observed in spectra taken in 1978 -- 1980; (2) faint emission at 6540 -- 6605 A centered about Halpha and [N II] 6548, 6583 with an expansion velocity of 500 +/- 100 km/s; and (3) excess emission around 4600 -- 4700 A suggestive of a Wolf-Rayet population in the remnant's star cluster. We use these new data to re-interpret the origin of the remnant's prolonged and bright luminosity and propose that the remnant is strongly interacting with dense, circumstellar wind loss material from a ~20 Msolar progenitor star.Comment: 20 pages, 10 figures. Matches version published in Ap

A Detailed Kinematic Map of Cassiopeia A's Optical Main Shell and Outer High-Velocity Ejecta

We present three-dimensional kinematic reconstructions of optically emitting material in the young Galactic supernova remnant Cassiopeia A (Cas A). These Doppler maps have the highest spectral and spatial resolutions of any previous survey of Cas A and represent the most complete catalog of its optically emitting material to date. We confirm that the bulk of Cas A's optically bright ejecta populate a torus-like geometry tilted approximately 30 degrees with respect to the plane of the sky with a -4000 to +6000 km/s radial velocity asymmetry. Near-tangent viewing angle effects and an inhomogeneous surrounding CSM/ISM environment suggest that this geometry and velocity asymmetry may not be faithfully representative of the remnant's true 3D structure or the kinematic properties of the original explosion. The majority of the optical ejecta are arranged in several well-defined and nearly circular ring-like structures with diameters between approximately 30 arcsec (0.5 pc) and 2 arcmin (2 pc). These ejecta rings appear to be a common phenomenon of young core-collapse remnants and may be associated with post-explosion input of energy from plumes of radioactive 56Ni-rich ejecta that rise, expand, and compress non-radioactive material. Our optical survey also encompassed Cas A's faint outlying ejecta knots and exceptionally high-velocity NE and SW streams of S-rich debris often referred to as `jets'. These outer knots, which exhibit a chemical make-up suggestive of an origin deep within the progenitor star, appear to be arranged in opposing and wide-angle outflows with opening half-angles of approximately 40 degrees.Comment: 15 pages, 11 figures, accepted by ApJ, associated movie files can be found at https://www.cfa.harvard.edu/~dmilisav

A Comparison of X-ray and Optical Emission in Cassiopeia A

Broadband optical and narrowband Si XIII X-ray images of the young Galactic supernova remnant Cas A obtained over several decades are used to investigate spatial and temporal correlations on both large and small scales. The data consist of optical and near infrared ground-based and Hubble Space Telescope images taken between 1951 and 2011, and X-ray images from Einstein, ROSAT, and Chandra taken between 1979 and 2013. We find weak spatial correlations between the remnant's emission features on large scales, but several cases of good optical/X-ray correlations on small scales for features which have brightened due to recent interaction with the reverse shock. We also find instances where: (i) a time delay is observed between the appearance of a feature's optical and X-ray emissions, (ii) displacements of several arcseconds between a feature's X-ray and optical emission peaks and, (iii) regions showing no corresponding X-ray or optical emissions. To explain this behavior, we propose a inhomogeneous model for Cas A's ejecta consisting of small, dense optically emitting knots (n ~ 10^(2-3)/cm^(3)) and a much lower density (n ~ 0.1 - 1/cm^(3)) diffuse X-ray emitting component often spatially associated with optical emission knots. The X-ray emitting component is sometimes linked to optical clumps through shock induced mass ablation generating trailing material leading to spatially offset X-ray/optical emissions. A range of ejecta densities can also explain the observed X-ray/optical time delays since the remnant's 5000 km/s reverse shock heats dense ejecta clumps to temperatures around 3x10^4 K relatively quickly which then become optically bright while more diffuse ejecta become X-ray bright on longer timescales. Highly inhomogeneous ejecta as proposed here for Cas A may help explain some of the X-ray/opticalfeatures seen in other young core collapse SN remnants.Comment: 31 pages, 21 figures. Submitted to the Astrophysical Journal. Please contact the corresponding author for higher resolution postscript versions of the figures ([email protected]

First optical light from the supernova remnant G 17.4-2.3

Deep optical CCD images of the supernova remnant G 17.4-2.3 were obtained and faint emission has been discovered. The images, taken in the emission lines of Halpha+[N II], [S II] and [O III], reveal filamentary structures in the east, south-east area, while diffuse emission in the south and central regions of the remnant is also present. The radio emission in the same area is found to be well correlated with the brightest optical filament. The flux calibrated images suggest that the optical filamentary emission originates from shock-heated gas ([S II]/Halpha > 0.4), while the diffuse emission seems to originate from an HII region ([S II]/Halpha < 0.3). Furthermore, deep long-slit spectra were taken at the bright [O III] filament and clearly show that the emission originates from shock heated gas. The [O III] flux suggests shock velocities into the interstellar "clouds" greater than 100 km/s, while the [S II] 6716/6731 ratio indicates electron densities ~240 cm^{-3}. Finally, the Halpha emission has been measured to be between 7 to 20 x 10^{-17} erg s^{-1} cm^{-2} arcsec^{-2}.Comment: 6 pages, 5 figures, accepted for publication in A&

Discovery of Extensive Optical Emission Associated with the X-ray Bright, Radio Faint Galactic SNR G156.2+5.7

We present wide-field Halpha images of the Galactic supernova remnant G156.2+5.7 which reveal the presence of considerable faint Halpha line emission coincident with the remnant's X-ray emission. The outermost Halpha emission consists largely of long and thin (unresolved), smoothly curved filaments of Balmer-dominated emission presumably associated with the remnant's forward shock front. Patches of brighter Halpha emission along the western, south-central, and northeastern regions appear to be radiative shocked ISM filaments like those commonly seen in supernova remnants, with relatively strong [O I] 6300,6364 and [S II] 6716,6731 line emissions. Comparison of the observed Halpha emission with the ROSAT PSPC X-ray image of G156.2+5.7 shows that the thin Balmer-dominated filaments lie along the outermost edge of the remnant's detected X-ray emission. Brighter radiative emission features are not coincident with the remnant's brightest X-ray or radio regions. Areas of sharply weaker X-ray flux seen in the ROSAT image of G156.2+5.7 appear spatially coincident with dense interstellar clouds visible on optical and IRAS 60 and 100 micron emission images, as well as maps of increased optical extinction. This suggests significant X-ray absorption in these regions due to foreground interstellar dust, especially along the western and southern limbs. The close projected proximity and alignment of the remnant's brighter, radiative filaments with several of these interstellar clouds and dust lanes hint at a possible physically interaction between the G156.2+5.7 remnant and these interstellar clouds and may indicate a smaller distance to the remnant than previously estimated.Comment: To appear in Monthly Notices of the Royal Astronomical Societ

New optical filamentary structures in Pegasus

Deep Halpha+[N II] CCD images have been obtained in the area of the Pegasus Constellation. The resulting mosaic covers an extent of ~7.5 degr x ~8.5 degr and filamentary and diffuse emission is discovered. Several long filaments (up to ~1 degr) are found within the field, while diffuse emission is present mainly in the central and northern areas. The filaments show variations in the intensity along their extent suggesting inhomogeneous interstellar clouds. Faint soft X-ray emission is also detected in the ROSAT All-Sky Survey. It is mainly concentrated in the central areas of our field and overlaps the optical emission. The low ionization images of [S II] of selected areas mainly show faint diffuse emission, while in the medium ionization images of [O III] diffuse and faint filamentary structures are present. Spectrophotometric observations have been performed on the brightest filaments and indicate emission from photoionized or shock-heated gas. The sulfur line ratios indicate electron densities below ~600 cm^{-3}, while the absolute Halpha emission lies in the range of 1.1 - 8.8 x 10^{-17} erg s^{-1} cm^{-2} arcsec^{-2}. The detected optical line emission could be part of a single or multiple supernova explosions.Comment: 13 pages, 7 figures, accepted for publication in A&