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the vela supernova remnant ( snr ) is a large ( diameter @xmath0 degrees ) , nearby supernova remnant associated with the vela pulsar and is one of the brightest objects in the x - ray sky .
recent work indicates the distance to vela is only @xmath1  pc ( cha   1999 and jenkins & wallerstein 1995 ) , making it an ideal candidate for resolving fine structure in x - rays with the _ chandra x - ray observatory_. _ rosat _ all - sky survey observations showed a complicated morphology and revealed the outer extent of the remnant for the first time .
aschenbach  ( 1995 ) identified several features protruding beyond what is believed to be the primary blastwave as `` explosion fragments '' .
the brightest of these features is the so - called `` bullet d '' .
tsunemi  1999 and miyata  2001 observed `` bullet a '' with _
asca _ and _ chandra _ respectively and detected strong si emission lines which they concluded was evidence that bullet  a was indeed composed of ejecta from the original explosion .
moriguchi  ( 2001 ) suggested that the bullet features were more likely the result of the interaction of the snr shock with an inhomogeneous medium as indicated by the numerous molecular clouds identified by a co survey of the region .
redman   ( 2000 ) noted the coincidence of the bright optical filamentary nebula rcw  37 and bullet  d. we proposed two _
observations , one at the head of the bullet and the other in the `` wake '' in order to examine the proposed explanations for bullet  d.
_ chandra _ observed the eastern limb of vela in september 2000 with the acis - i detector .
the exposure at the head of the bullet d was 31  ks and the exposure in the wake region was 52 ks .
figure  1 displays the image from the head of bullet d after selecting events in the 0.4 to 2.5  kev range , binning the data in @xmath2 pixels , and smoothing the data with a gaussian filter with a fwhm of 3 .
the data show a fragmented , filamentary structure consistent with a shock interacting with an inhomogeneous medium .
if this feature were a fragment of ejecta from the explosion , one would expect a larger contrast in intensity of the x - ray emission from the head of the feature to the trailing edges as seen in bullet  a.
the frontside - illuminated ( fi ) ccds on acis suffered radiation damage early in the mission which resulted in a large increase in the charge transfer - inefficiency ( cti ) and a significant reduction in the spectral resolution .
the observations of the bullet d are seriously impaired by this damage since the object covers all four ccds on the acis - i array .
we have used the cti - correction software and appropriate response matrices developed at penn state ( see townsley   2000 , townsley   2000a , and townsley   2000b ) .
the cti correction dramatically improves the quality of the data as demonstrated in our analysis of the spectra of the smc snr 1e0102.2 - 7219 ( see plucinsky   2001 ) .
fortunately , the spectrum of the bullet d is quite similar to that of 1e0102.2 - 7219 in that both have strong lines of o , ne , and mg with little or no fe .
we focus on the spectrum of the brightest part of the bullet d feature described by the ellipse in figure  1 .
the acis spectrum of this feature is shown in figure  2 .
the dominant features are the oviii  @xmath3 line at 654  ev , the neix triplet at @xmath4  ev , and the mgxi triplet at @xmath5  kev .
we fit these data with a collisional ionization equilibrium ( cie ) model and derived a best - fit temperature of @xmath6  kev after fixing the to be @xmath7 ( see dubner   1998 ) . at this temperature
, the cie model indicates that the o and ne abundances must be enhanced by several times over solar values .
next we fit the data with a model of a simple continuum plus gaussians to represent the line emission .
we used the ratios of ovii  triplet / oviii  @xmath3 and neix  triplet / nex  @xmath8 as non - equilibrium diagnostics .
both diagnostics indicated that temperatures below log(t)=6.4 are not consistent with the data .
both the o and ne line ratios are consistent with ionization timescales in the range @xmath9 .
we can impose an upper limit on the ionization timescale of @xmath10 by assuming an age of 12,000  yr and a maximum ambient density of @xmath11 .
we can then compare the emissivities of the oviii  @xmath3 , oviii  @xmath12 , neix  triplet , and nex  @xmath3 lines to the expected values .
the measured emissivities are plotted against the expected emissivities as a function of temperature and ionization timescale in figures  3 and  4 .
the measured ne emissivity is higher than the predicted emissivity for all values of the ionization timescale regardless of temperature , indicating that the abundance of ne must be higher than solar . in the preferred range of ionization timescales from @xmath13 ,
the ne abundance required to be consistent with the model predictions varies from 1.6 to @xmath14 solar .
the situation with o is significantly different . in the range of preferred ionization timescales from @xmath15 ,
the measured emissivity is lower than the expected emissivity for a large range of ionization timescales . over this range ,
the o abundance required to be consistent with the model predictions varies from 0.5 to @xmath16 solar .
the o and ne data are most consistent with each other for a temperature of log(t ) = 6.6 and an ionization timescale of @xmath17 . at these values
the ne abundance is @xmath18 twice solar and the o abundance is @xmath18 solar .
the morphology of the x - ray emission revealed by _ chandra _ is more consistent with a snr shock interacting with an inhomogeneous medium than with a discrete fragment of ejecta .
spectra indicate that ne is enhanced above solar values , but at a rather modest level , and o is consistent with the solar value .
we therefore conclude the _ chandra _ observations of the vela bullet d region are more consistent with a shock breakout hypothesis than with a bullet of ejecta hypothesis , although the _ chandra _ data can not rule out the bullet idea .
we thank all of the engineers , technicians , and scientists who have made the _ chandra x - ray observatory _ such a success .
ppp , rks , rje , tjg , and pos acknowledge support for this work from nasa contracts nas8 - 39703 and go0 - 1127 .
lkt and psb acknowledge support for this work from nasa contract nas8 - 38252 .
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we present results from two _ 
 chandra_/acis observations of the so - called vela `` bullet d '' region on the eastern limb of the vela supernova remnant . 
 the bullet d region is a bright x - ray feature , identified by aschenbach et al . 
 ( 1995 ) from the rosat all - sky survey , which protrudes beyond the blast wave on the eastern side of the remnant . 
 it has been suggested that this feature is a fragment of supernova ejecta which is just now pushing beyond the position of the main blast wave . 
 an alternate explanation is that the feature is a `` break - out '' of the shock in which inhomogeneities in the ambient medium cause the shock to be non - spherical . 
 the _ 
 chandra _ 
 image shows a fragmented , filamentary morphology within this region . 
 the _ chandra _ 
 spectra show strong emission lines of o , ne , and mg . 
 equilibrium ionization models indicate that the o and ne abundances are significantly enhanced compared to solar values . 
 however , non - equilibrium ionization models can fit the data with solar o abundances and ne abundances enhanced by only a factor of two . 
 the _ chandra _ 
 data are more consistent with the shock breakout hypothesis , although they can not exclude the fragment of ejecta hypothesis .    
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