document: within a decade of being identified as a supernova remnant ( snr ) , g65.2@xmath05.7  ( also called g65.3 + 5.7 ) was the subject of several observational papers published in quick succession . 
   however , once its radio and optical images and its optical and x - ray spectra were recorded , the remnant was ignored . 
   now , a quarter of a century later , g65.2@xmath05.7  has once again become interesting , this time because it helps to explain an unusual class of supernova remnants called thermal composite snrs ( also known as mixed morphology snrs ) . 
   both of these terms describe the combination of a shell - like radio continuum morphology with a centrally - bright x - ray morphology in which the x - ray emission is due to thermal , rather than synchrotron , processes . 
   compared with shell - type and plerionic snrs , these remnants have more mysterious origins . 
   various workers have suggested that thermal composite snrs lack x - ray shells because they have evolved into the radiative phase ( and thus have cool , x - ray dim shells ) , or that their centers are x - ray bright because thermal conduction , other forms of entropy mixing , or cloudlet evaporation has enhanced the central densities . 
   ejecta enrichment or dust destruction could increase the metallicities in the centers , also enhancing the x - ray emission . 
   alternatively , a thermal composite morphology may be due to collisions with molecular clouds . 
   interested readers are directed to @xcite , @xcite , @xcite , @xcite , @xcite , @xcite , @xcite , and @xcite for additional discussions of these processes 
   .    currently , there are few observations of the `` smoking gun '' for the radiative phase evolution hypothesis : h  shells . 
   the h  shell on w44 is the only possible example ( @xcite , with interpretations in @xcite and @xcite ) . here 
   we demonstrate that another thermal composite supernova remnant , g65.2@xmath05.7 , is clearly in the radiative phase . 
   ironically , this demonstration is not performed by discovering that a previously known thermal composite snr is in the radiative phase , but is done by demonstrating that a previously known radiative phase snr is a thermal composite . in section 2 , we discuss published optical observations , indicating that g65.2@xmath05.7  has entered the shell formation phase ; we also discuss published radio and low spatial resolution x - ray observations , showing that g65.2@xmath05.7  has a shell - like radio continuum morphology and emits thermal x - rays . with these characteristics , 
   g65.2@xmath05.7  meets two of the three requirements for the thermal composite classification . the third requirement , 
   that the remnant has a centrally filled x - ray morphology , is established in section 3 , through the analysis of archival _ rosat _  observations . 
   furthermore , model fits to the _ rosat _  data imply that the temperature peaks in the center and decreases mildly with radius , as is found for other thermal composite supernova remnants . in section 4 
   , we summarize the results . 
   the first observations of g65.2@xmath05.7 , those made with the red prints of the palomar sky survey @xcite , only found two bright filaments , s91 and s94 , but failed to recognize the entire outline of the remnant . 
   the true extent of the remnant remained unknown until @xcite s optical emission line survey of the galactic plane revealed filaments circumscribing a 4@xmath5@xmath6 3.3@xmath5  ellipse . 
   @xcite s radio frequency data also suggested a similarly sized shell - type remnant .    even before its true extent was revealed 
   , @xcite used their h@xmath7 , [ n@xmath8 , [ s  ]  ( 6717   ) , and [ s  ]  ( 6731   )  measurements to show that the s91 and s94 filaments were dense ( @xmath9 @xmath10 ) shell fragments belonging to a large supernova remnant . 
   @xcite made additional optical emission line measurements of the s91 filament and of one of the brightest [ o@xmath11  filaments . 
   both regions have large [ o / h@xmath12  ratios , which , when compared with the shock models of @xcite , indicate that the photons were emitted by thick ( @xmath13 @xmath14 ) postshock cooling regions behind moderate velocity ( @xmath15 to 120 km s@xmath16 ) shocks 
   . given its [ o / h@xmath12  ratio , s91 s relatively modest [ o@xmath11/h@xmath12  ratio indicates that the cooling and recombination behind the shock is `` complete '' @xcite . 
   the cooling zone along @xcite s second pointing direction appears to be well developed , but incomplete . 
   given that the second pointing direction was chosen for its anomalously large [ o@xmath11/h@xmath12  ratio , this portion may be less complete than most of the remnant s shell . 
   similarly , @xcite found regions of incomplete cooling as well as regions of nearly complete cooling . 
   the shell temperature found from the older [ o@xmath11  and [ s  measurements is @xmath1738,000  k ( fesen , blair & kirshner 1985 ; sabbadin & dodorico 1976 ) . 
   @xcite calculated shock velocities of 90 to 140 km sec@xmath16 , which are roughly consistent with @xcite s kinematically determined expansion velocity of @xmath18 km s@xmath16 . in summary , 
   g65.2@xmath05.7  is sufficiently evolved for the shock to be weak and the gas behind it to be relatively cool and recombined . 
   multiple techniques have been employed to estimate the remnant s size and distance . 
   @xcite applied the @xmath19-d relation to their 1420 mhz data obtaining a diameter of @xmath20 pc and a distance of @xmath21 pc . 
   @xcite applied the galactic kinematic relationship to her fabry - perot interferometric h@xmath7  data in order to estimate the distance as 800 pc and the minor and major axes of this elliptically shaped remnant as 56 and 64 pc , respectively . with a distance of @xmath21 pc and galactic latitude of 5.7@xmath5 , 
   the remnant lies @xmath22  pc above the galactic disk . 
   furthermore , g65.2@xmath05.7 s age has been estimated as @xmath23  yr ( @xcite , using radiative phase snr relations ) . 
   + while others were observing the remnant with optical and radio - frequency detectors , @xcite and @xcite were observing it with x - ray detectors . 
   @xcite s _ heao  1 _ ( bandpass = 0.2 to 2.5 kev , fwhm = 3@xmath5 ) spectrum exhibits line emission , the hallmark of thermal emission from hot plasma . the model which best fits the _ heao  1 _ 
   spectrum is a raymond and smith thermal model with a temperature between @xmath24 and @xmath25  k @xcite . 
   although they were not able to resolve the emission spatially with the _ heao  1 _ data , they were able to show that the emission region is not point - like . in the following section , 
   we extend upon this work . 
   we use archival _ rosat _  data to map the remnant , confirming that the emitting gas is extended and finding that it fills the remnant s outline , lacks a bright shell , and exhibits a slowly decreasing temperature gradient . 
   between august 1995 and september 1997 , the remnant was mapped with a series of overlapping _ rosat _ 
   pspc observations . 
   we processed the _ rosat _  pspc data ( removed the contamination , scattered solar x - rays , long - term enhancements , after - pulses , particle background , and bright point sources ) and mosaiced the pointings according to the extended object analysis procedures described in @xcite and @xcite . 
   the resulting images in the _ rosat _  1l2 band ( sensitive to 0.11 to 0.284 kev photons ) , 45 band ( sensitive to 0.44 to 1.21 kev photons ) , and 67 band ( sensitive to 0.73 to 2.04 kev photons ) are displayed in figure  [ rosatimages.fig ] , along with a 1l2 band image overlayed with contours from the 67 band map . with the exception of a ridge of band 1l2 emission along the northeast edge and roughly coincident with bright optical filaments , 
   the periphery is x - ray dim . 
   most of the projected interior shows x - ray emission . because of 
   its generally `` centrally filled '' x - ray morphology , thermal x - ray emission , and `` shell - type '' radio continuum morphology , the remnant should now be classified as a thermal composite . 
   comparison of the soft and hard band images shows that the x - ray emission originating in the snr interior is slightly harder than that originating nearer to the edges . 
   we quantify this trend with a spectral model . 
   the gas is assumed to be near collisional ionizational equilibrium , as is observed both in similar remnants such as w44 ( harrus et al . 
   1997 ; shelton , kuntz & petre 2004 ) , 3c391 ( chen & slane 2001 ) , and 0045 - 734 in the smc ( yokogawa et al 2002 ) , and in simulated remnants which have evolved long enough to have centrally - peaked x - ray morphologies @xcite . given 
   s spectral resolution , the differences between equilibrium and nearly equilibrium spectra are inconsequential , as are differences between newer and older versions of the @xcite spectral code . in general , the model spectra are calculated and handled according to the techniques described in @xcite . 
   thus , we used the @xcite spectral code to create a suite of collisional ionizational equilibrium spectral models . 
   the electron temperature ( @xmath26 ) ranged from @xmath27  k to @xmath28  k , with increments in log(@xmath26 ) of 0.05 . in order to account for the absorption by the interstellar medium , we multiplied our suite of model spectra by @xmath29 , where @xmath30 is the product of the absorption column density ( @xmath31 ) and the absorption cross section @xcite . 
   @xmath31 ranged from @xmath32 @xmath14 to @xmath33 @xmath14 , with increments in log(@xmath31 ) of 0.10 . 
   the resulting spectra were convolved with the _ rosat _  pspc response matrix , yielding the countrates in each _ rosat _  pspc band . 
   we prepared the data for comparison with the calculated models by excluding counts from regions of the x - ray image that were contaminated by bright point sources or dominated by the galactic background emission , dividing the the remaining snr area into ten , concentric , nearly equal area annuli . 
   these annuli were centered on ra = 19@xmath2 33@xmath3 41@xmath4 , dec = 31@xmath5 15 57  in j2000 coordinates and had outer radii of 26 , 39 , 48 , 56 , 64 , 71 , 79 , 86 , 94 , and 122 arcmin , respectively . 
   regions beyond the snr and regions contaminated by point sources ( including the slightly extended hard source centered on ra = 19@xmath2 36@xmath3 46@xmath4 , dec = 30@xmath5 40 07  ) were excluded from the annuli areas . as a result , in order to create approximately equal area masks , the outermost annuli were extended to relatively large radii . 
   we then calculated the countrates in each of the _ rosat _  pspc bands for each annulus . 
   the background countrate for each band was calculated from the cleaned data for the portion of the observed sky that lies outside the snr s footprint and subtracted from the countrate for each annulus , resulting in the `` observed countrates '' . 
   the standard chi - squared test was used to compare the observed countrates in the _ rosat _  1l , 4 , 5 , and 6 bands for each annulus with the countrates predicted for the spectral models . 
   band 2 was omitted , owing to complications of an absorption edge in the detector and band 7 was omitted owing to its low countrate . 
   the best fitting models were then selected for each annulus . 
   figure  [ spectrum.fig ] displays the observed and model spectra for the seventh annulus ( spanning the region between 79 and 86 arcmin from the remnant s center ) . 
   the best - fit temperatures range from 2.5 to @xmath34  k and decrease slightly with radius ( see figure  [ temp.fig ] ) . 
   the shallow temperature gradient is a common feature of thermal composite snrs ( i.e. , 3c400.2 @xcite , kes 27 @xcite , w44 @xcite ) . in the best fit models for the various regions , the column densities ranged from @xmath35 to @xmath36  @xmath14 , but did not exhibit a radial trend . 
   the snr s apparent flux of 0.11 to 2.04 kev photons is @xmath37 ergs @xmath14 s@xmath16 . 
   if there had been no interstellar scattering , the flux would have been @xmath38 ergs @xmath14 s@xmath16 . assuming that the remnant is 900 pc distant , 
   its luminosity is @xmath39 ergs s@xmath16 and the electron density of the x - ray emitting gas is @xmath40 @xmath10 . 
   based on the theoretical predictions and hydrocode simulations of the archetypical thermal composite supernova remnant , w44 ( @xcite , @xcite ) , we expect the ambient density to be about 10 times the density within the remnant s center . 
   thus , the ambient density would be roughly 0.1 @xmath10 , a reasonable value at g65.2@xmath05.7 s location 90 pc above the galactic plane . 
   the electron density is lower than that of w44 , causing the luminosity to be lower as well . 
   in addition to displaying a centrally concentrated snr morphology , the band 45 and band 67 x - ray images also reveal a `` slightly extended point source '' centered on ra = 19@xmath2 36@xmath3 46@xmath4 , dec = 30@xmath5 40 07  and extending approximately 6.5 arcmin in radius in the harder band map and less far in the band 45 map . 
   the 1l2 band map does not show excess emission at or near this location . 
   the observed spectrum is too noisy for meaningful fitting to model spectra and there is no known pulsar at this location . 
   although the princeton - arecibo survey for millisecond pulsar s did not find a pulsar coincident with the observed bright spot , it did find a pulsar , j1931 + 30 , within the remnant s outline @xcite . 
   we did not find excess x - ray emission at its location , ra = 19@xmath2 31@xmath3 28@xmath4 , dec = 30@xmath5 35. 
   these _ rosat _  observations , in combination with existing radio observations , establish g65.2@xmath05.7 s membership in the thermal composite ( mixed morphology ) class of supernova remnants . 
   the radio continuum shells and thermal x - ray bright centers of this puzzling class of remnants have inspired several theoretical explanations . in one scenario , 
   these remnants have evolved beyond the adiabatic phase into the radiative phase . as a result , their peripheries are now cool and not x - ray emissive . 
   this scenario can explain why these remnants lack x - ray bright shells , while other explanations are required in order to account for the extreme brightnesses of these remnants centers . 
   here , we note that crucial support for the evolutionary hypothesis is provided by optical evidence of nearly complete cooling behind g65.2@xmath05.7 s relatively slow shockfront . 
   + if @xcite s age estimate is correct , then g65.2@xmath05.7  is the oldest known thermal composite supernova remnant . 
   the fact that several other thermal composite supernova remnants neighbor molecular clouds has led to the suggestion that collisions with molecular clouds cause the characteristic thermal composite morphology . in order to test this suggestion on g65.2@xmath05.7 , we searched the @xcite co survey data and the @xcite h  survey data near @xmath41 at the remnant s systemic velocity ( @xmath42  km s@xmath16 , @xcite ) . 
   confusion with galactic material made the search inconclusive . 
   the possibility of a snr - cloud interaction could potentially be addressed with a targeted search for co emission features outlining the remnant . 
   +      r.l.s . wishes to thank the national research council and nasa s long term space astrophysics grant ( nag5 - 10807 ) for financial support and the anonomous referee for his / her helpful comments . 
   k.d.k . wishes to thank gsrp for financial support . 
   this research has made use of data obtained from the high energy astrophysics science archive research center ( heasarc ) , provided by nasa s goddard space flight center .
summary: this paper presents archival _ rosat _  pspc observations of the g65.2@xmath05.7  supernova remnant ( 
   also known as g65.3 + 5.7 ) . 
   little material obscures this remnant and so it was well observed , even at the softest end of _ rosat _ s bandpass ( @xmath1 to 0.28  kev ) . 
   these soft x - ray images reveal the remnant s centrally - filled morphology which , in combination with existing radio frequency observations , places g65.2@xmath05.7  in the thermal composite ( mixed morphology ) class of supernova remnants . 
   not only might g65.2@xmath05.7  be the oldest known thermal composite supernova remnant , but owing to its optically revealed cool , dense shell , this remnant supports the proposal that thermal composite supernova remnants lack x - ray bright shells because they have evolved beyond the adiabatic phase . 
   these observations also reveal a slightly extended point source centered on ra = 19@xmath2 36@xmath3 46@xmath4 , dec = 30@xmath5 40 07  and extending 6.5 arcmin in radius in the band 67 map . 
   the source of this emission has yet to be discovered , as there is no known pulsar at this location .