Fast-moving Knots Research Articles

H.E.S.S. and MAGIC observations of a sudden cessation of a very-high-energy γ-ray flare in PKS 1510−089 in May 2016

The flat spectrum radio quasar (FSRQ) PKS 1510−089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE, E > 100 GeV) γ rays. The VHE γ-ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHE γ-ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of ∼20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHE γ-ray spectrum of PKS 1510–089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical R-band observations with ATOM revealed a counterpart of the γ-ray flare, even though the detailed flux evolution differs from the VHE γ-ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE γ-ray flare. In the high-energy (HE, E > 100 MeV) γ-ray band, only a moderate flux increase was observed with Fermi-LAT, while the HE γ-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the γ-ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located ∼50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHE γ rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.

A Deep Near-infrared [Fe ii]+[Si i] Emission Line Image of the Supernova Remnant Cassiopeia A

Abstract We present a long-exposure (∼10 hr), narrowband image of the supernova (SN) remnant Cassiopeia A (Cas A) centered at 1.644 μm emission. The passband contains [Fe ii] 1.644 μm and [Si i] 1.645 μm lines, and our “deep [Fe ii]+[Si i] image” provides an unprecedented panoramic view of Cas A, showing both shocked and unshocked SN ejecta, together with shocked circumstellar medium at subarcsecond (∼0.″7 or 0.012 pc) resolution. The diffuse emission from the unshocked SN ejecta has a form of clumps, filaments, and arcs, and their spatial distribution correlates well with that of the Spitzer [Si ii] infrared emission, suggesting that the emission is likely due to [Si i] not [Fe ii] as in shocked material. The structure of the optically invisible western area of Cas A is clearly seen for the first time. The area is filled with many quasi-stationary flocculi (QSFs) and fragments of the disrupted ejecta shell. We identified 309 knots in the deep [Fe ii]+[Si i] image and classified them into QSFs and fast-moving knots (FMKs). The comparison with previous optical plates indicates that the lifetime of most QSFs is ≳60 yr. The total H+He mass of QSFs is ≈0.23 M ⊙, implying that the mass fraction of dense clumps in the progenitor’s mass ejection immediately prior to the SN explosion is about 4%–6%. FMKs in the deep [Fe ii]+[Si i] image mostly correspond to S-rich ejecta knots in optical studies, while those outside the southeastern disrupted ejecta shell appear Fe-rich. The mass of the [Fe ii] line emitting, shocked dense Fe ejecta is ∼3 × 10−5 M ⊙.

The Temperature and Ionization of Unshocked Ejecta in Cas A

Abstract The supernova remnant Cassiopeia A (Cas A) is one of the few remnants in which it is possible to observe unshocked ejecta. A deep 1.64 μm image of Cas A shows a patch of diffuse emission from unshocked ejecta, as well as brighter emission from fast-moving knots and quasi-stationary flocculi. Emission at 1.64 μm is usually interpreted as [Fe ii] emission, and spectra of the bright knots confirm this by showing the expected emission in other [Fe ii] lines. We performed near-infrared spectroscopy on the diffuse emission region and found that the unshocked ejecta emission does not show those lines, but rather shows the [Si i] 1.607 μm line. This means that the 1.64 μm line from the unshocked ejecta may be the [Si i] 1.645 line from the same upper level, rather than [Fe ii]. We find that the [Si i] line is formed by recombination, and we use the [Si i] to [Si ii] ratio to infer a temperature of about 100 K, which is close to the value assumed for an analysis of low-frequency radio absorption and that can be inferred from emission by cool dust. Our results constrain estimates of Cas A’s total mass of unshocked ejecta that are extremely sensitive to temperature assumptions, but they do not resolve the ambiguity due to clumping.

Near-Infrared Knots and Dense Fe Ejecta in the Cassiopeia A Supernova Remnant

Abstract We report the results of broadband (0.95–2.46 μm) near-infrared spectroscopic observations of the Cassiopeia A supernova remnant. Using a clump-finding algorithm in two-dimensional dispersed images, we identify 63 “knots” from eight slit positions and derive their spectroscopic properties. All of the knots emit [Fe ii] lines together with other ionic forbidden lines of heavy elements, and some of them also emit H and He lines. We identify 46 emission line features in total from the 63 knots and measure their fluxes and radial velocities. The results of our analyses of the emission line features based on principal component analysis show that the knots can be classified into three groups: (1) He-rich, (2) S-rich, and (3) Fe-rich knots. The He-rich knots have relatively small, , line-of-sight speeds and radiate strong He i and [Fe ii] lines resembling closely optical quasi-stationary flocculi of circumstellar medium, while the S-rich knots show strong lines from O-burning material with large radial velocities up to indicating that they are supernova ejecta material known as fast-moving knots. The Fe-rich knots also have large radial velocities but show no lines from O-burning material. We discuss the origin of the Fe-rich knots and conclude that they are most likely “pure” Fe ejecta synthesized in the innermost region during the supernova explosion. The comparison of [Fe ii] images with other waveband images shows that these dense Fe ejecta are mainly distributed along the southwestern shell just outside the unshocked 44Ti in the interior, supporting the presence of unshocked Fe associated with 44Ti.

INTERACTION BETWEEN CASSIOPEIA A AND NEARBY MOLECULAR CLOUDS

We present spectroscopy of the supernova remnant Cassiopeia A (Cas A) observed at infrared wavelengths from 10 - 40 micron with the Spitzer Space Telescope and at millimeter wavelengths in 12CO and 13CO J = 2 - 1 (230 and 220 GHz) with the Heinrich Hertz Submillimeter Telescope. The IR spectra demonstrate high-velocity features toward a molecular cloud coincident with a region of bright radio continuum emission along the northern shock front of Cas A. The millimeter observations indicate that CO emission is broadened by a factor of two in some clouds toward Cas A, particularly to the south and west. We believe that these features trace interactions between the Cas A shock front and nearby molecular clouds. In addition, some of the molecular clouds that exhibit broadening in CO lie 1 - 2' away from the furthest extent of the supernova remnant shock front. We propose that this material may be accelerated by ejecta with velocity significantly larger than the observed free-expansion velocity of the Cas A shock front. These observations may trace cloud interactions with fast-moving outflows such as the bipolar outflow along the southwest to northeast axis of the Cas A supernova remnant, as well as fast-moving knots seen emerging in other directions.

Modeling of Highly-Excited Atomic Level Populations in Astrophysical Plasmas

Abstract Recombination lines arising in transitions between highly-excited atomic levels are becoming increasingly used for the plasma diagnostics, as the sensitivity of astronomical instrumentation increases. This paper describes a model used to compute the level population of the Rydberg levels and gives a collection of hydrogenic approximation expressions that can be used to estimate the relevant atomic data. The Rydberg level populations may also be used to estimate level-specific recombination rate for lower atomic levels accounting for the electron cascade. The expressions are applied to predict the radio recombination line spectrum of the fast-moving knots of the Cassiopeia A supernova remnant.

SpitzerIMAGING AND SPECTRAL MAPPING OF THE OXYGEN-RICH SUPERNOVA REMNANT G292.0+1.8

We present mid-infrared continuum and emission line images of the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8, acquired using the MIPS and IRS instruments on the Spitzer Space Telescope. The MIPS 24 micron and 70 micron images of G292.0+1.8 are dominated by continuum emission from a network of filaments encircling the SNR. The morphology of the SNR, as seen in the mid-infrared, resembles that seen in X-rays with the Chandra X-ray Observatory. Most of the mid-infrared emission in the MIPS images is produced by circumstellar dust heated in the non-radiative shocks around G292.0+1.8, confirming the results of earlier mid-IR observations with AKARI. In addition to emission from hot dust, we have also mapped atomic line emission between 14 micron and 36 micron using IRS spectral maps. The line emission is primarily associated with the bright oxygen-rich optical knots, but is also detected from fast-moving knots of ejecta. We confirm our earlier detection of 15-25 micron emission characteristic of magnesium silicate dust in spectra of the radiatively shocked ejecta. We do not detect silicon line emission from any of the radiatively shocked ejecta in the southeast of the SNR, possibly because that the reverse shock has not yet penetrated most of the Si-rich ejecta in that region. This may indicate that G292.0+1.8 is less evolved in the southeast than the rest of the SNR, and may be further evidence in favor of an asymmetric SN explosion as proposed in recent X-ray studies of G292.0+1.8.

THE THREE-DIMENSIONAL STRUCTURE OF CASSIOPEIA A

We used the Spitzer Space Telescope's Infrared Spectrograph to map nearly the entire extent of Cassiopeia A between 5-40 micron. Using infrared and Chandra X-ray Doppler velocity measurements, along with the locations of optical ejecta beyond the forward shock, we constructed a 3-D model of the remnant. The structure of Cas A can be characterized into a spherical component, a tilted thick disk, and multiple ejecta jets/pistons and optical fast-moving knots all populating the thick disk plane. The Bright Ring in Cas A identifies the intersection between the thick plane/pistons and a roughly spherical reverse shock. The ejecta pistons indicate a radial velocity gradient in the explosion. Some ejecta pistons are bipolar with oppositely-directed flows about the expansion center while some ejecta pistons show no such symmetry. Some ejecta pistons appear to maintain the integrity of the nuclear burning layers while others appear to have punched through the outer layers. The ejecta pistons indicate a radial velocity gradient in the explosion. In 3-D, the Fe jet in the southeast occupies a "hole" in the Si-group emission and does not represent "overturning", as previously thought. Although interaction with the circumstellar medium affects the detailed appearance of the remnant and may affect the visibility of the southeast Fe jet, the bulk of the symmetries and asymmetries in Cas A are intrinsic to the explosion.

Fine-structure infrared lines from the Cassiopeia A knots

Aims: Archival observations of infrared fine-structure lines of the young Galactic supernova remnant Cassiopeia A allow us to test existing models and determine the physical parameters of various regions of the fast-moving knots (FMKs), which are metal-dominated clouds of material ejected by the supernova explosion. Methods: The fluxes of the far-infrared [O i] and [O iii] lines are extracted from the previously unpublished archival ISO data. The archival Spitzer data are used to determine the fluxes of the O, Ne, Si, S, Ar and Fe ion fine-structure lines originating in the FMKs. The ratios of these line fluxes are used for the plasma diagnostics. We also determine the infrared line flux ratios to the optical [O iii] 5007 A line in the knots having previously measured reddening. Additionally, we analyze several optical and near-infrared observations of the FMKs to obtain clearer insight into the post-shock photoionized region (PIR) structure. Results: We show that the infrared oxygen line flux predictions of all existing theoretical models are correct only to within a factor of several. For the models to reproduce the observations it is essential to include the electron conductivity and effects of the dust. Detailed analysis of the diagnostic line flux ratios allows us to qualitatively confirm the general model of the FMK emission and to determine observationally the physical conditions in the PIR after the shock front. We infer that the pre-shock cloud densities most probably constitute several hundred particles per cm^3. We also determine the Cas A luminosities in the infrared continuum and lines. We show that accounting for the charge exchange processes in the post-shock PIR allows us to reproduce most of the relevant spectral line ratios even in the frame of a single-temperature model of this region. We also estimate its plasma parameters, thickness, and carbon abundance.

A Catalog of Outer Ejecta Knots in the Cassiopeia A Supernova Remnant

Hubble Space Telescope images of the core-collapse supernova remnant Cassiopeia A are used to identify high-velocity knots of ejecta located outside the remnant's main emission shell of expanding debris. These ejecta fragments are found near or ahead of the remnant's forward shock front and mostly lie from 12000 to 30000 in radial distance from the remnant's center of expansion. Filter flux ratios when correlated with published spectra show that these knots can be divided into three emission classes: (1) knots dominated by [N II] lambda lambda 6548, 6583 emissions, ( 2) knots dominated by [O II] kk7319, 7330 emissions, and (3) knots displaying filter flux ratios suggestive of [ S II], [ O II], and [Ar III] lambda 7135 emission line strengths similar to the fast-moving knots'' (FMKs) found in the remnant's bright main shell. Of 1825 knots identified, 444 are strong [N II] emission knots, 192 are strong [O II] emission knots, and 1189 are FMK-like knots. In terms of location around the remnant, 972, 207, and 646 knots are found in the remnant's northeast jet, southwest jet, and non-jet regions, respectively. Assuming a distance of 3.4 kpc, derived knot transverse velocities based on proper motion measurements spanning a 9 month interval indicate maximum transverse expansion velocities for these three knot classes of 14,500, 13,500, and 11,500 km s(-1), respectively. We present a catalog of these outlying ejecta clumps comprising finding charts, epoch 2004.2 knot positions, proper motions, photometric filter fluxes, and estimated knot emission type, along with cross-references to previous knot identifications and data. This compilation represents a nearly tenfold increase in the number of outlying, high-velocity ejecta knots identified around the Cassiopeia A remnant.

Optical and near-infrared recombination lines of oxygen ions from Cassiopeia A knots

Context. Fast-moving knots (FMK) in the Galactic supernova remnant Cassiopeia A consist mainly of metals and allow us to study element production in supernovae and to investigate shock physics in great detail.Aims. We discuss and suggest observations of a previously unexplored class of spectral lines, the metal recombination lines in optical and near-infrared bands, emitted by the cold ionized and cooling plasma in fast-moving knots.Methods. By tracing ion radiative and dielectronic recombination, collisional l -redistribution and radiative cascade processes, we compute resulting oxygen, silicon and sulphur recombination line emissivities. This allows us to determine the oxygen recombination line fluxes, based on a fast-moving knot model that predicts the existence of highly-ionized ions from moderate to very low plasma temperatures.Results. The calculations predict oxygen ion recombination line fluxes detectable with modern optical telescopes in the wavelength range from 0.5 to 3 μ m. Recombination line flux ratios to collisionally-excited lines will allow us to probe in detail the process of rapid cloud cooling after the passage of a shock front, to test high abundances of O4+ , O5+ and O6+ ions at low temperatures and measure them, to test existing theoretical models of FMK and to build more precise ones.

Discovery of Outlying High‐Velocity Oxygen‐Rich Ejecta in Cassiopeia A

Analysis of broadband HST ACS and WFPC2 images of the young Galactic supernova remnant Cassiopeia A reveals a far larger population of outlying, high-velocity knots of ejecta with a broader range of chemical properties than previously suspected. In this paper, we concentrate on a similar or equal to 1.5 arcmin(2) region located along the eastern limb of the remnant where we identify three main classes of outer ejecta: (1) knots dominated by [N(II)] lambda lambda 6548, 6583 emission, (2) knots dominated by oxygen emission lines, especially [O(II)] lambda lambda 7319, 7330, and (3) knots with emission-line strengths similar to the [S(II)]-strong fast-moving knot (FMK) ejecta commonly seen in the main emission shell. Mean transverse velocities derived from observed proper motion for N-rich, O-rich, and FMK-like knots identified in this region were found to be 8100, 7900, and 7600 km s(-1), respectively. The discovery of a significant population of O-rich ejecta situated between the suspected N-rich outer photospheric layer and S-rich FMK-like ejecta suggests that the progenitor's chemical layers were not completely disrupted by the supernova explosion outside of the remnant's northeast and southwest high-velocity jet'' regions. In addition, we find the majority of O-rich outer ejecta at projected locations out beyond the remnant's fastest moving Fe-rich X-ray emission material seen in Chandra and XMM-Newton data along the eastern limb, suggesting that penetration of Fe-rich material up through the S-and Si-rich mantle did not extend past the progenitor's N- or O-rich outer layers for this section of the remnant.

Exploring the Kinematics of the Oxygen‐rich Supernova Remnant G292.0+1.8: Ejecta Shells, Fast‐moving Knots, and Shocked Circumstellar Material

We present the results of an in-depth optical study of the core-collapse supernova remnant G292.0+1.8 using the Rutgers Fabry-Perot (RFP) imaging spectrometer. Our observations provide a detailed picture of the supernova remnant in the emission lines of [O III] λ5007, Hα, and [N II] λ6548. The [O III] Fabry-Perot scans reveal a bright crescent-shaped spur of previously known high-velocity (Vradial ~ 1500 km s-1) O-rich ejecta located on the eastern side of the remnant. The spur consists of a semicoherent structure of mostly redshifted material, along with several clumps that have apparently broken out of the more orderly shell-like expansion. The high-velocity (600 km s-1) component of the spur also displays a scalloped morphology characteristic of Rayleigh-Taylor instabilities. We also find a large number of fast-moving knots (FMKs) of O-rich ejecta undetected in prior photographic plate images and similar to features seen in Cas A. The FMKs are distributed sparsely in the interior of G292.0+1.8 and are seen mostly in blueshifted emission out to Vradial ≈ -1700 km s-1. The position-velocity distribution of the FMKs can be kinematically described as a shell 34 in radius expanding at a velocity of 1700 km s-1. Another feature apparent in the [O III] scans is an equatorial belt consisting of both a barlike structure at zero radial velocity and a clumpy, high-velocity ejecta component seen in projection along the line of sight. Portions of the zero-velocity bar are spatially well correlated with a similar structure seen in the Chandra X-ray image of G292.0+1.8. The bar is also detected in our Hα RFP images at zero radial velocity, providing further evidence that this structure is of circumstellar origin. We find that the optical and X-ray properties of the bar are consistent with incomplete (partially radiative) shocks in material of moderate densities. There are also a number of faint, elongated structures seen in Hα at zero radial velocity across the interior of G292.0+1.8 that lack [O III] and X-ray counterparts. These filaments may be low-density H I clouds photoionized by hard radiation from the interior of the remnant. Overall, these results suggest that G292.0+1.8 is currently interacting with a low-density environment. We find no evidence for high-velocity Hα or [N II] emission over the dynamical range sampled by the RFP. Assuming a distance of 6 kpc for G292.0+1.8, we estimate a kinematic age of (3000-3400)d6 yr for this remnant.

Location of the Optical Reverse Shock in the Cassiopeia A Supernova Remnant

We use two epochs of Hubble Space Telescope WFPC2 images separated by 2 yr to determine the location and propagation of the reverse shock (RS) in the young supernova remnant Cassiopeia A (Cas A). The images trace optical line emission from fast-moving knots and filaments of highly processed ejecta as they cross the RS, become heated and compressed, and radiatively cool. At numerous positions around the optical shell, new emission features are seen in the 2002 images that were not yet visible in the 2000 exposures. In a few instances emission features seen in the first epoch have completely disappeared in the second epoch. We concentrate on two regions along the rim of the main emission shell in Cas A for close inspection: one in the northwestern part of the shell and another along the southwestern part of it. In these regions the RS is viewed almost edge-on, and its precise position has been measured. The RS is coherent in these regions over arcminute (~1 pc) scales but is highly distorted perpendicular to the direction of expansion. We find the RS to be generally expanding at 50%-60% of the ~5500 km s-1 bulk velocity of the optical ejecta. We present shock models for the ejecta that are consistent with the high densities and short cooling times observed in the optical knots of the Cas A remnant.

Imaging of the Supernova Remnant Cassiopeia A with the Multiband Imaging Photometer for Spitzer (MIPS)

We present new images of the supernova remnant (SNR) Cas A observed in the 24 and 70 μm bands of the Spitzer Space Telescope (Spitzer). The IR emission correlates well with the Si X-ray and optical [S II] emission but poorly with either the synchrotron-dominated radio structure or the continuum X-ray emission. The IR is therefore dominated by thermal emission from dust within the SNR and associated with emission-line gas inside the reverse shock region, confirming earlier IRAS and Infrared Space Observatory results. Supplemented by new photometric measurements from archived Midcourse Space Experiment images, we suggest stochastic heating to model the overall mid- to far-IR spectral energy distribution. The 24 and 70 μm images also reveal a counterjet to the well-known northeast jet feature imaged previously at X-ray, optical, and radio wavelengths. This IR counterjet corresponds well with (optical) fast-moving knots confirming its outflow nature. The opposing jetlike features define a symmetry axis that bisects the SNR and suggest that the supernova explosion was axisymmetric. The IR images also show a region in which the SNR forward shock appears to be propagating into a ~650 M⊙ molecular cloud. The new images also show other details of the surrounding ISM structure, including two groups of knots extending ~6'-12' on either side of the SNR.

Faint hard X-ray sources in the Galactic Center region: Supernova ejecta fragments population

Long Chandra observations of field in the Galactic Center (GC) region reported by Muno et al. ([CITE]) have revealed a population of about 2000 hard faint X-ray sources limited by luminosities . We show that fast moving knots (FMKs) of supernovae ejecta could comprise a sizeable fraction of the sources. Each supernova event can produce hundreds of FMKs. The presence of ~3 supernova remnants of age ∼103 in the GC region could provide the required number of the FMKs. Simulated X-ray spectra of the FMKs contain both thermal and nonthermal components. The nonthermal spectra are power-laws with hard photon indexes and prominent lines of some metals (e.g. Fe). The – distribution and the hardness ratios of the FMKs in the GC region are consistent with the Chandra findings. Spatial distribution of the FMKs in the GC region should appear as an ensemble of coherent shell and jet-like structures of a few arcmin-scale.

An Optical Survey of Outlying Ejecta in Cassiopeia A: Evidence for a Turbulent, Asymmetric Explosion

A deep optical survey of the Cassiopeia A supernova remnant has revealed dozens of new emission-line ejecta knots out beyond the remnant's bright nebular shell. Most of the newly detected knots exhibit a 4500-7500 A spectrum dominated by [N II] λλ6548,6583 line emissions. After accounting for possible decelerations, the estimated space velocities for about four dozen of these [N II] knots suggest a nearly isotropic 10,000 km s-1 ejection velocity. However, a small group along the southwestern limb show significantly higher velocities of up to 12,000 km s-1. Over 20 outlying O + S emission knots were also discovered, mostly along the remnant's western limb. These knots have optical spectral properties like those seen in the main shell's metal-rich fast-moving knots but with much higher estimated space velocities of between 7600 and 12,600 km s-1. Discovery of these knots means that the remnant's highest-velocity, O + S debris are not confined to just the remnant's northeast jet. [S II] λλ6716,6731 emissions dominate the spectra of these knots above an expansion velocity of 11,000 km s-1. A few mixed emission knots, which show both strong nitrogen and sulfur line emissions, were also detected along the remnant's western rim. The properties of these outlying debris knots suggest a turbulent supernova expansion in which the innermost S-rich layers were ejected up through overlying material in certain regions, attaining final outward velocities greater than the star's N and He-rich surface layers. The detection of such high-velocity, sulfur-rich ejecta only along the remnant's northeast and southwest limbs further suggests an asymmetric expansion, possibly bipolar. A turbulent expansion may help explain the creation of the observed mixed emission knots. It is unclear, however, if mixed knots represent truly microscopically mixed debris or are simply small, comoving clusters of chemically distinct ejecta.

Newly Synthesized Elements and Pristine Dust in the Cassiopeia A Supernova Remnant

Spectroscopic observations at 2.4-45 μm of the young supernova remnant Cas A with the Infrared Space Observatory (ISO) Short Wavelength Spectrometer reveal strong emission lines of O, Ne, Si, S, and Ar. These lines are observed at high velocities (several 103 km s-1) and are therefore associated with the supernova ejecta known as the fast-moving knots (FMKs). Continuum emission from dust is also seen in the Cas A spectrum. The continuum strength is spatially well correlated with the O and Ar line strengths, which indicates that the dust emission also arises from the FMKs. The dust continuum has an emission feature at ~22 μm which cannot be fitted by typical astronomical silicates but can be fitted with a particular class of silicate minerals. This suggests that the dust in Cas A is silicate material that has freshly condensed from the Cas A ejecta into a mineral form that is uncharacteristic of typical ISM dust grains.

The X-Ray Expansion of the Supernova Remnant Cassiopeia A

We present the first X-ray expansion measurements of the young, bright supernova remnant Cas A. We have acquired a deep (232 ks) ROSAT high-resolution image of Cas A that is compared with archival images of similar quality taken 17 yr earlier with the Einstein Observatory. The average expansion rate between the two epochs is found to be 0.20%±0.01% yr-1, twice as fast as the expansion of the bright radio ring and two-thirds as fast as the ensemble of optical fast-moving knots. This argues that we are observing a fundamentally different hydrodynamical structure in each band, although all of these components are located on the same patchy ring. In addition, we detect significant variations in the expansion rate as a function of azimuth around the bright X-ray ring. These results are discussed in the context of two classes of models, one involving a preexisting circumstellar shell and the other having a uniform or gradiated density profile that decreases monotonically away from the center.

Reddening Measurements and Physical Conditions for Cassiopeia A from Optical and Near-Infrared Spectra

view Abstract Citations (52) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Reddening Measurements and Physical Conditions for Cassiopeia A from Optical and Near-Infrared Spectra Hurford, Alan P. ; Fesen, Robert A. Abstract Spectra of five fast-moving knots (FMKs) and two quasi-stationary flocculi (QSFs) in Cas A, covering the wavelength regions 4000-10,500 Å and 4300-6800 Å, respectively, are presented. FMK extinction values derived from observed [S ii] auroral/transauroral line ratios indicate an optical reddening range of Av ≃ 4.6-5.4 mag. Estimates of QSF extinctions obtained assuming an intrinsic Balmer ratio of Hα/Hβ = 3.0 are larger, lying in the range Av ≃ 5.3-6.2 mag. While these FMK and QSF reddening measurements are higher than Searle's value of Av = 4.3 mag, they are consistent with the radio-inferred extinction values of Troland et al. for their various locations in Cas A. The FMK reddening values indicate Av variations of at least ≃0.5 mag, and possibly up to ≃1 mag in Cas A's bright northern and northeastern shell emission. The higher QSF reddening values may be due to line-of-sight extinction variations or collisionally excited Hα emission. Adopting a reddening value ≃1 mag larger than Searle's value brings the dereddened oxygen line ratios of Cas A FMKs into better agreement with other young, oxygen-rich remnants and the 150-200 km s-1 shock/ionization front models of Sutherland and Dopita. Spectra of a faint FMK exhibiting weak oxygen emission but strong forbidden and permitted calcium lines are also presented. This FMK may represent a rare clump of relatively unmixed material originating from a thin (≃0.05 to 0.10 Msun), silicon-rich zone in the Cas A progenitor. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177775 Bibcode: 1996ApJ...469..246H Keywords: ISM: INDIVIDUAL NAME: CASSIOPEIA A; ISM: SUPERNOVA REMNANTS; ISM: DUST; EXTINCTION; ISM: STRUCTURE full text sources ADS | data products SIMBAD (24)