Vela Supernova Remnant Research Articles

Hydrodynamic modelling of ejecta shrapnel in the Vela supernova remnant

Many supernova remnants (SNRs) are characterized by a knotty ejecta structure. The Vela SNR is an excellent example of remnant in which detached clumps of ejecta are visible as X-ray emitting bullets that have been observed and studied in great detail. We aim at modelling the evolution of ejecta shrapnel in the Vela SNR, investigating the role of their initial parameters (position and density) and addressing the effects of thermal conduction and radiative losses. We performed a set of 2-D hydrodynamic simulations describing the evolution of a density inhomogeneity in the ejecta profile. We explored different initial setups. We found that the final position of the shrapnel is very sensitive to its initial position within the ejecta, while the dependence on the initial density contrast is weaker. Our model also shows that moderately overdense knots can reproduce the detached features observed in the Vela SNR. Efficient thermal conduction produces detectable effects by determining an efficient mixing of the ejecta knot with the surrounding medium and shaping a characteristic elongated morphology in the clump.

Vela Jr.：超新星喷发团块与超新星遗骸的互动

重迭在船帆座超新星遗骸(Vela supernova remnant)视线上的ROSAT X射线源RX J0852.0-4622，或称Vela Jr.，因其在1.3千电子伏的波段上呈现圆壳外观和检测出44Ti的γ射线衰变线，所以被认为是新发现的年轻超新星遗骸。然而，观测证据指出Vela Jr.应属于船帆座超新星遗骸的一部份而非独立的物体。二维气体动力学模拟显示，超新星快速的高密度喷发气体物(子弹)可以影响遗骸的外围，产生类似于Vela Jr.的圆环结构。估计Vela Jr.的能量为 The ROSAT source RX J0852.0-4622, or Vela Jr., superposed on the Vela supernova remnant, has been identified as a separate young supernova remnant based on its shell-like appearance in X-ray > 1.3 keV and on its detection as a source of the 44Ti γ-ray decay line. However, a re-examination of the observational facts suggests that Vela Jr. should be part of the Vela remnant. We suggest that the shell X-ray feature is produced by a fast ejecta clump (bullet) from the Vela supernova that deposited its kinetic energy in the supernova remnant shell. The estimated energy < 0.3 × 1049 erg could be supplied by a <~0.1M⊙ clump moving at1700 km/s. Two-dimensional gas dynamic simulations show that a circular appearance can be produced in the supernova shell under proper conditions. We suggest that the nickel bubble effect is responsible for producing such heavy ejecta clumps in supernovae. There may be other similar X-ray features at a lower brightness level and there may be emission from heavy element ejecta in the object.

Suzaku View of the Supernova Remnant RCW 86: X-Ray Studies of Newly-Discovered Fe-Rich Ejecta

Abstract We report on results of imaging and spectral analysis of the supernova remnant (SNR) RCW 86 observed with Suzaku. The SNR is known to exhibit K-shell emission of low ionized Fe, possibly originating from supernova ejecta. We revealed the global distribution of the Fe-rich plasma in the entire remnant, for the first time; the Fe-K emission was clearly detected from the west, north, and south regions, in addition to the X-ray brighter shells of southwest and northeast, where the presence of the Fe-rich ejecta has already been reported. The spectrum of each region is well represented by a three-component model consisting of low- and high-temperature thermal plasmas and non-thermal emission. The lower-temperature component, with elemental abundances of near the solar values, likely originates from the forward shocked interstellar medium, while the Fe-rich ejecta is described by the hotter plasma. From the morphologies of the forward and reverse shocks in the west region, the total ejecta mass is estimated to be 1–2 $\ M_{\odot}$ for the typical explosion energy of $\sim\ $ 1 $\times$ 10 $^{51}\ $ erg. The integrated flux of the Fe-K emission from the entire SNR roughly corresponds to a total Fe mass of about 1 $\ M_{\odot}$ . Both of these estimates suggest a Type Ia supernova origin of this SNR. We also find possible evidence of an Fe-rich clump located beyond the forward-shock front in the north rim, which is reminiscent of ejecta knots observed in the Tycho and Vela SNRs.

Suzaku Detection of Diffuse Hard X-Ray Emission outside Vela X

Abstract Vela X is a large, 3 $^{\circ}$$\times$ 2 $^{\circ}$ , radio-emitting pulsar wind nebula (PWN) powered by the Vela pulsar in the Vela supernova remnant. Using four Suzaku/XIS observations pointed just outside Vela X, we have found hard X-ray emission extending throughout the fields of view. The hard X-ray spectra are well represented by a power-law. The photon index was measured to be constant at $\Gamma$$\sim$ 2.4, similar to that of the southern outer part of Vela X. The power-law flux decreases with increasing distance from the pulsar. These properties lead us to propose that the hard X-ray emission is associated with the Vela PWN. The larger X-ray extension found in this work strongly suggests that distinct populations of relativistic electrons form the X-ray PWN and Vela X, as was recently inferred from multiwavelength spectral modeling of Vela X.

FAR-ULTRAVIOLET EMISSION-LINE MORPHOLOGIES OF THE SUPERNOVA REMNANT G65.3+5.7

We present the first far-ultraviolet (FUV) emission-line morphologies of the whole region of the supernova remnant (SNR) G65.3+5.7 using the FIMS/SPEAR data. The morphologies of the C IV {\lambda}{\lambda}1548, 1551, He II {\lambda}1640, and O III] {\lambda}{\lambda}1661, 1666 lines appear to be closely related to the optical and/or soft X-ray images obtained in previous studies. Dramatic differences between the C IV morphology and the optical [O III] {\lambda}5007 image provide clues to a large resonant-scattering region and a foreground dust cloud. The FUV morphologies also reveal the overall distribution of various shocks in different evolutionary phases and an evolutionary asymmetry between the east and the southwest sides in terms of Galactic coordinates, possibly due to a Galactic density gradient in the global scale. The relative X-ray luminosity of G65.3+5.7 to C IV luminosity is considerably lower than those of the Cygnus Loop and the Vela SNRs. This implies that G65.3+5.7 has almost evolved into the radiative stage in the global sense and supports the previous proposal that G65.3+5.7 has lost its bright X-ray shell and become a member of mixed-morphology SNRs as it has evolved beyond the adiabatic stage.

Search for broad absorption lines in spectra of stars in the field of supernova remnant RX J0852.0-4622 (Vela Jr.)

Supernova remnant (SNR) RX J0852.0-4622 is one of the youngest and is most likely the closest among known galactic supernova remnants (SNRs). It was detected in X-rays, the 44Ti gamma-line, and radio. We obtain and analyze medium-resolution spectra of 14 stars in the direction towards the SNR RX J0852.0-4622 in an attempt to detect broad absorption lines of unshocked ejecta against background stars. Spectral synthesis is performed for all the stars in the wavelength range of 3740-4020AA to extract the broad absorption lines of Ca II related to the SNR RX J0852.0-4622. We do not detect any broad absorption line and place a 3-sigma upper limit on the relative depths of <0.04 for the broad Ca II absorption produced by the SNR. We detect narrow low and high velocity absorption components of Ca II. High velocity |V(LSR)|=100-140 km/s components are attributed to radiative shocks in clouds engulfed by the old Vela SNR. The upper limit to the absorption line strength combined with the width and flux of the 44Ti gamma-ray line 1.16 MeV lead us to conclude that SNR RX J0852.0-4622 was probably produced by an energetic SN Ic explosion.

A POSSIBLE SUPERNOVA REMNANT HIGH ABOVE THE GALACTIC DISK

We present the analysis of three Suzaku observations of a bright arc in the ROSAT All-Sky Survey 1/4 keV maps at $l \approx 247\degr$, $b \approx -64\degr$. In particular, we have tested the hypothesis that the arc is the edge of a bubble blown by an extraplanar supernova. One pointing direction is near the brightest part of the arc, one is toward the interior of the hypothesized bubble, and one is toward the bubble exterior. We fit spectral models generated from 1-D hydrodynamical simulations of extraplanar supernova remnants (SNRs) to the spectra. The spectra and the size of the arc ($\mathrm{radius} \approx 5\degr$) are reasonably well explained by a model in which the arc is the bright edge of a $\sim$100,000-yr old SNR located $\sim$1--2 kpc above the disk. The agreement between the model and the observations can be improved if the metallicity of the X-ray--emitting gas is $\sim$1/3 solar, which is plausible, as the dust which sequesters some metals is unlikely to have been destroyed in the lifetime of the SNR. The width of the arc is larger than that predicted by our SNR model; this discrepancy is also seen with the Vela SNR, and may be due to the 1-D nature of our simulations. If the arc is indeed the edge of an extraplanar SNR, this work supports the idea that extraplanar supernovae contribute to the heating of the $\sim$million-degree gas in the halo.

A new model for Vela Jr. Supernova Remnant

We consider Vela Jr. as being the old Supernova Remnant (SNR) at the beginning of the transition from adiabatic to radiative stage of evolution. According to our model, Vela Jr. is situated outside Vela SNR at the distance of ∼600 pc and its age is 17500 yr. We model the high energy fluxes from Vela Jr. and its broadband spectrum. We find our results compatible with experimental data in radio waves, X- and γ-rays. Our hydrodynamical model of Vela Jr. explains the observed TeV γ-ray flux by hadronic mechanism. The proposed model does not contradict to the low density environment of the SNR and does not need extreme fraction of the explosion energy to be transferred to Cosmic Rays.

Electron Spectrum in 10–1000 GeV from BETS &amp; PPB-BETS and Future Balloon Observation

We carried out electron observations with balloons to investigate acceleration and propagation of electrons in the universe. We developed BETS (Balloon-borne Electron Telescope with Scintillation fibers) for electron observations in Sanriku. We obtained electron energy spectrum from 10 to 100 GeV with it. At higher energies, the energy spectrum of electron might show some structure caused by nearby sources like the Vela supernova remnant. In order to observe the energy spectrum above 100 GeV, we needed a thicker calorimeter and a longer observation time. We developed PPB-BETS (Polar Patrol Balloon-BETS) by improving BETS to carry out a long duration ballooning in Antarctica. Energy spectrum of electrons from 100 to 1000 GeV was obtained by a successful fight for 13 days. The arrival direction of electrons seems to be consistent with isotropic distribution for the data taken with PPB-BETS, although we need better statistics to make it clear. We have a future plan to make an observation of electrons and gam...

Probing the Nature of the Vela X Cocoon

Vela X is a pulsar wind nebula (PWN) associated with the active pulsar B0833-45 and contained within the Vela supernova remnant (SNR). A collimated X-ray filament (cocoon) extends south-southwest from the pulsar to the center of Vela X. VLA observations uncovered radio emission coincident with the eastern edge of the cocoon and H.E.S.S. has detected TeV $\gamma$-ray emission from this region as well. Using XMM-\textit{Newton} archival data, covering the southern portion of this feature, we analyze the X-ray properties of the cocoon. The X-ray data are best fit by an absorbed nonequilibrium plasma model with a powerlaw component. Our analysis of the thermal emission shows enhanced abundances of O, Ne, and Mg within the cocoon, indicating the presence of ejecta-rich material from the propagation of the SNR reverse shock, consistent with Vela X being a disrupted PWN. We investigate the physical processes that excite the electrons in the PWN to emit in the radio, X-ray and $\gamma$-ray bands. The radio and non-thermal X-ray emission can be explained by synchrotron emission. We model the $\gamma$-ray emission by Inverse Compton scattering of electrons off of cosmic microwave background (CMB) photons. We use a 3-component broken power law to model the synchrotron emission, finding an intrinsic break in the electron spectrum at $\sim5 \times 10^{6}$ keV and a cooling break at $\sim$ 5.5 $\times 10^{10}$ keV. This cooling break along with a magnetic field strength of 5 $\times 10^{-6}$ G indicate that the synchrotron break occurs at $\sim$1 keV.

Spatial and temporal variations in interstellar absorption towards HD 72127AB

New optical spectra of Ca II and Na I towards HD 72127AB provide additional evidence for both spatial and temporal variations in the complex interstellar absorption along the two sightlines; archival ultraviolet spectra yield information on the abundances, depletions and physical conditions in the gas towards HD 72127A. Similarities in the strengths of various tracers of interstellar material in the two lines of sight suggest that the total hydrogen column densities (N ∼ 2.5 × 10 20 cm −2 ) and the depletions and ionization in the main components at low local standard of rest (LSR) velocities also are similar. Towards HD 72127A, the main components are relatively cool (T 900 K), but with depletions resembling those found in warm, diffuse disc clouds; the generally weaker components at higher velocities have much milder depletions, more like those found in halo clouds. Several trace neutral species – Ca I, Cr I and Fe I – are much stronger towards HD 72127B, however. The column density of Cr I ,f or example, is about 30 times the value determined towards ζ Oph (the only previous detection of that species in the ISM). Dielectronic recombination in warmer gas (T 5000 K) may be largely responsible for the enhanced abundances of those trace neutral species towards HD 72127B. If the main components towards HD 72127AB are associated with material in the Vela supernova remnant, the differences in abundances and physical conditions occur on scales of about 1100 au.

ChandraObservations 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 a ~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 ~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 ~5 that is not expected in the ISM around the Cygnus Loop. Introduction of nonthermal emission in our model fitting cannot naturally resolve the abundance-depletion problem. The origin of the depletion still remains as an open question.

First detection of a VHE gamma-ray spectral maximum from a cosmic source: HESS discovery of the Vela X nebula

The Vela supernova remnant (SNR) is a complex region containing a number of sources of non-thermal radiation. The inner section of this SNR, within 2 degrees of the pulsar PSR B0833-45, has been observed by the H.E.S.S. gamma-ray atmospheric Cherenkov detector in 2004 and 2005. A strong signal is seen from an extended region to the south of the pulsar, within an integration region of radius 0.8 deg. around the position (RA = 08h 35m 00s, dec = -45 deg. 36' J2000.0). The excess coincides with a region of hard X-ray emission seen by the ROSAT and ASCA satellites. The observed energy spectrum of the source between 550 GeV and 65 TeV is well fit by a power law function with photon index = 1.45 +/- 0.09(stat) +/- 0.2(sys) and an exponential cutoff at an energy of 13.8 +/- 2.3(stat) +/- 4.1(sys) TeV. The integral flux above 1 TeV is (1.28 +/- 0.17 (stat) +/- 0.38(sys)) x 10^{-11} cm^{-2} s^{-1}. This result is the first clear measurement of a peak in the spectral energy distribution from a VHE gamma-ray source, likely related to inverse Compton emission. A fit of an Inverse Compton model to the H.E.S.S. spectral energy distribution gives a total energy in non-thermal electrons of ~2 x 10^{45} erg between 5 TeV and 100 TeV, assuming a distance of 290 parsec to the pulsar. The best fit electron power law index is 2.0, with a spectral break at 67 TeV.

FIMS 관측 자료의 위치보정 알고리즘 개발

The FIMS(Far-ultraviolet IMaging Spectrograph), the main payload onboard the first Korean Science Technology SATellite, STSAT-1, has performed various astronomical observations, including the Cygnus Loop, Vela supernova remnants, LMC(Large Magellanic Cloud), since its launch on September 2003. It has been found that the attitude information provided by spacecraft bus system has the errors of more than about 10-15 arcmins due to the time offset problem and errors in attitude knowledge. We develop an algorithm for correction of position errors in FIMS data. The aspect for the FIMS data is determined by comparing the positions of observed bright stars with the Tycho-II and TD-1 catalogs. The position errors of the bright stars along the scanning (<TEX>${\gamma}$</TEX>) and spatial (<TEX>${\delta}$</TEX>) directions were considered as functions of <TEX>${\delta}$</TEX>, ignoring errors in position angle. The corrected positions of the bright stars coincided very well to their Tycho-II and TD-I positions. The correction algorithm is essential for the FIMS data analysis, and is being used for the FIMS data analysis.

Spatially Resolved Spectral Analysis of Vela Shrapnel D

Abstract 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 $\mathrm{O} \sim 5, \mathrm{Ne} \sim10 , \mathrm{Mg} \sim 10, \mathrm{Fe} \sim 1$. 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.

Two-component model for the interstellar scattering in direction to the Vela pulsar

This paper discusses a rigorous treatment of the refractive scintillation of pulsar PSR B0833-45 caused by a two-component interstellar scattering medium. It is assumed that the interstellar scattering medium is composed of a thin screen ISM and an extended interstellar medium. We consider that the scattering of the thin screen concentrates in a thin layer presented by a delta function distribution and that the scattering density of the extended irregular medium satisfies the Gaussian distribution. We investigate and develop equations for the flux density structure function corresponding to this two-component ISM geometry in the scattering density distribution and compare our result with that of the Vela pulsar observations. We conclude that the refractive scintillation caused by this two-component ISM scattering gives a more satisfactory explanation for the observed flux density variation of the Vela pulsar than does the single extended medium model. The level of refractive scintillation is strongly sensitive to the distribution of scattering material along the line of sight. The logarithmic slope of the structure function is sensitive to thin screen location and is relatively insensitive to the scattering strength of the thin screen medium. Therefore, the proposed model can be applied to interpret the structure function of flux density observed in pulsar PSR B0833-45. The result suggests that the medium consists of a discontinuous distribution of plasma turbulence embedded in the Vela supernova remnant. Thus our work provides some insight into the distribution of the scattering along the line of sight to the Vela pulsar.

Shocked Clouds in the Vela Supernova Remnant

Unusually strong high-excitation C I has been detected in 11 lines of sight through the Vela supernova remnant (SNR) by means of UV absorption line studies of IUE data. Most of these lines of sight lie near the western edge of the bright X-ray region of the SNR in a spatially distinct band approximately 1° by 4° oriented approximately north-south. The high-excitation C I (denoted C I* and C I**) is interpreted as evidence of a complex of shocked dense clouds interacting with the SNR, because of the high pressures indicated in this region. To further analyze the properties of this region of enhanced C I* and C I**, we present new HIRES-processed IRAS data of the entire Vela SNR. A temperature map calculated from the HIRES IRAS data, based on a two-component dust model, reveals the signature of hot dust at several locations in the SNR. The hot dust is anticorrelated spatially with X-ray emission, as would be expected for a dusty medium interacting with a shock wave. The regions of hot dust are strongly correlated with optical filaments, supporting a scenario of dense clouds interior to the SNR that have been shocked and are now cooling behind the supernova blast wave. With few exceptions, the lines of sight to the strong high-excitation C I pass through regions of hot dust and optical filaments. Possible mechanisms for the production of the unexpectedly large columns of high-excitation C I are discussed. Dense clouds on the back western hemisphere of the remnant may explain the relatively low X-ray emission in the western portion of the Vela SNR due to the slower forward shock velocity in regions where the shock has encountered the dense clouds. An alternate explanation for the presence of ground-state and excited-state neutrals, as well as ionized species, along the same line of sight is a magnetic precursor that heats and compresses the gas ahead of the shock.

FUSEObservations of Highly Ionized Gas in the Vela Supernova Remnant

We present Far Ultraviolet Spectroscopic Explorer (FUSE) data on O VI absorption lines observed toward nine stars behind the Vela supernova remnant (SNR). The O VI observations are compared with International Ultraviolet Explorer (IUE) observations of C IV absorption toward the same stars. All of these stars are B stars, which generally have complex continua, making the extraction of absorption lines difficult. However, four of the stars, HD 72088, HD 72089, HD 72350, and HD 72537, are rapid rotators (v sin i ≳ 150 km s-1), making the derivation of absorption column densities much easier. We present O VI and C IV column densities for the "main component" (i.e., the low-velocity component) for these stars. In the FUSE spectra of three of the rapid rotators (HD 72088, HD 72089, and HD 72537) we find high-velocity components of O VI at ~150 km s-1 that we attribute to the SNR shock. HD 72350 has very strong H2 absorption, which obscures any possible high-velocity O VI absorption. To detect the high-velocity O VI absorption in HD 72088 requires employing a technique for removing the H2 line at 1032.35 Å (121.6 km s-1 relative to O VI). The O VI data are consistent with steady radiative shock models with vs ≈ 150 km s-1, but we do not detect C IV at the same velocities. Instead we find high-velocity C IV at lower velocities (~100 km s-1). We discuss the implications of our results for models of the evolution of the Vela SNR.

Gamma-rays from the pulsar wind nebulae

We investigate the radiation processes inside supernova remnants which are powered by young pulsars. Using a recent model for particle acceleration by the pulsar wind nebulae (PWNe), in which positrons gain energy in the process of resonant scattering by the heavy nuclei, we construct a time-dependent radiation model for the PWNe. In this model, the spectra of relativistic particles, injected inside the nebula, depend on time due to the evolution of the pulsar parameters. Applying a simple model for the evolution of the PWNa, the equilibrium spectra of leptons and nuclei inside the nebula are determined as a function of time, taking into account the energy losses of particles on dierent processes and their escape from the nebula. We calculate the multiwavelength photon spectra produced by leptons and nuclei and compare them with the observations of the PWNe for which TeV-ray emission has been reported, i.e. the Crab Nebula, the Vela Supernova Remnant, and the nebula around PSR 1706-44. It is found that the emission from the Crab Nebula can be well fitted by the composition of the -ray emission produced by leptons (below10 TeV) and nuclei (10 TeV). The model is further tested by successful fitting of the high energy spectrum from the Vela SNR. In this case, the observed-ray emission is mainly due to leptons and the contribution of-rays from decay of neutral pions, produced in collision of nuclei, is significantly lower. However, the considered model does not give good fitting to the emission from PSR1706-44 for the likely parameters of this source, unless an additional target for relativistic leptons is present inside the nebula, e.g. the thermal infrared emission. Based on the knowledge obtained from these fittings, we predict the -ray fluxes in the TeV energy range from other PWNe, which are promissing TeV-ray sources due to their similarities to the -ray nebulae, i.e. MSH15-52 (PSR 1509-58), 3C58 (PSR J0205+6449), and CTB80 (PSR 1951+32). Possible detection of these sources by the new generation of Cherenkov telescopes is discussed.

Optical and Far‐Ultraviolet Spectroscopy of Knot D in the Vela Supernova Remnant

We present spectra of optical filaments associated with the X-ray knot D in the Vela supernova remnant. It has been suggested that knot D is formed by a bullet of supernova ejecta, that it is a breakout of the shock front of the Vela supernova remnant, and also that it is an outflow from the recently discovered remnant RX J0852.0� 4622. We find that knot D is a bow shock propagating into an interstellar cloud with normal abundances and typical cloud densities (nH � 4 11 cm � 3 ). Optical long-slit spectra show that the (S ii) �� 6716, 6731 to Hline ratio is greater than unity, proving that the optical filaments are shock excited. The analysis of far-ultraviolet spectra obtained with the Hopkins Ultraviolet Telescope and with the Far Ultravio- let Spectroscopic Explorer (FUSE) LWRS aperture show that slower shocks (� 100 km s � 1 ) produce most of the low-ionization lines such as O iii) � 1662, while faster shocks (� 180 km s � 1 ) produce the O vi �� 1032, 1038 and other high-ionization lines. C iii and O vi lines are also detected in the FUSE MDRS aperture, which was located on an X-ray-bright region away from the optical filaments. The lines have two velocity components consistent with � 150 km s � 1 shocks on the near and far sides of the knot. The driving pressure in the X-ray knot, P=kB � 1:8 � 10 7 cm � 3 K, is derived from the shock properties. This is over an order of mag- nitude larger than the characteristic X-ray pressure in the Vela supernova remnant. The velocity distribution of the emission and the overpressure support the idea that knot D is a bow shock around a bullet or cloud that originated near the center of the Vela remnant. Subject headings: ISM: individual (Vela Supernova Remnant) — shock waves — supernova remnants — ultraviolet: ISM