the supernova remnant cta1 (g119.5+10.3): a study of the...

Astron. Astrophys. 324, 1152–1164 (1997) ASTRONOMYAND

ASTROPHYSICS

The supernova remnant CTA1 (G 119.5+10.3): a studyof the breakout phenomenonS. Pineault1, T.L. Landecker2, C.M. Swerdlyk2, and W. Reich3

1 Departement de physique et Observatoire du Mont Megantic, Universite Laval, G1K 7P4 Ste-Foy (Quebec), Canada2 Dominion Radio Astrophysical Observatory, National Research Council of Canada, P.O. Box 248, Penticton, B.C., Canada3 Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn 1, Germany

Received 5 November 1996 / Accepted 21 February 1997

Abstract. The supernova remnant (SNR) CTA1 has been ob-served with the Dominion Radio Astrophysical ObservatorySynthesis Telescope, at 1420 MHz and 408 MHz, and with theEffelsberg 100 m telescope at 1420 MHz. Angular resolution ofthe final maps is 1′ and 3.5′ at 1420 and 408 MHz respectively.New HIRES infrared observations with a resolution of about 1′

are also presented. Using those new observations, together witha new flux density measured from the 22.25 MHz DRAO arrayand previously published values for the integrated flux density,we deduce an integrated spectral index of α = 0.57 ± 0.006(where Sν ∝ ν−α). The new high-resolution radio maps con-firm that, in addition to the bright radio arcs visible to the southand east, the SNR has a very substantial extension to the north-west, interpreted as the breakout of the SNR blast wave intoa medium of lower density. Spatial variations are found in thespectral index distribution over the SNR, the diffuse emissionto the northwest being generally of a steeper spectral index thanemission associated with the brighter regions. Although this canbe explained within the context of diffusive shock accelerationtheory by variations in the Mach number of the SNR shock,it is not clear whether sufficiently large changes can occur fol-lowing breakout. Faint emission is also present past the southernlimb-brightened radio filaments. This could be the result of pro-jection effects or due to electrons diffusing upstream ahead ofthe shock front with a mean free path of order 0.02 pc. A breakin the southeastern part of the radio shell is best explained bya density enhancement or cloud which has caused a drastic re-duction in shock velocity possibly resulting in a decrease in theacceleration efficiency.

Key words: acceleration of particles – ISM: cosmic rays – ISM:individual objects: CTA1 – ISM: supernova remnants – radiocontinuum: interstellar

Send offprint requests to: S. Pineault

1. Introduction

The extended radio source CTA1 (G 119.5+10.3) was first pro-posed as a supernova remnant (SNR) by Harris and Roberts(1960). Observations at radio wavelengths now cover 38 MHz to2695 MHz (Caswell 1967; Sieber et al. 1979, 1981; Rees 1990).Pineault et al. (1993, Paper I) presented a continuum image at1420 MHz with angular resolution of order 1′, the best resolu-tion achieved to date. The radio images showed a morphologytypical of a shell SNR, an incomplete shell of filaments and ex-tended emission roughly defining a circular object of diameter90′. However, the boundary of the SNR to the northwest wasnot clearly established by any of these observations, and opticalobservations in the O iii line (Fesen et al. 1981, 1983) stronglysuggested that the SNR extends beyond the circle. In Paper Iwe argued that CTA1 is a “breakout” remnant, one in which theblast wave, initially expanding in a medium of average density,has abruptly broken out into a region of quite low density. H iobservations, presented in Paper I, supported this interpretation.

Breakout of a SN blast wave into a region of lower densitymay be a more frequent phenomenon than suggested by the (sofar) relatively small number of known unambiguous cases, thecanonical example being that of VRO 42.05.01 (Landecker etal. 1982; Pineault et al. 1987). Maybe CTA1 should be com-pared to objects like the Cygnus Loop (Green 1990) or 3C 391(Moffett & Reynolds 1994), both of which, like CTA1, showstrong limb-brightened arcs and, in the opposite quadrant, morediffuse extended emission. The implied non-uniformity of theneighbouring interstellar medium (ISM) is likely to have strongimplications, not only on the resulting morphology of the SNR,but also on the spectral properties of its radio emission.

Indeed SNRs are believed to be the primary accelerationsites for cosmic rays of energies up to 1015 GeV, a suggestionwhich has been considerably strengthened recently by the dis-covery (Koyama et al. 1995) of non-thermal X-ray emission inthe young SNR 1006, probably synchrotron radiation from elec-trons accelerated to energies of order 1014 GeV within the shockregion. One of the favoured particle acceleration mechanisms is

S. Pineault et al.: The supernova remnant CTA1 (G 119.5+10.3) 1153

the first- or second-order Fermi process (Bell 1978a,b; Bland-ford & Ostriker 1978; Droge et al. 1987; Schlickeiser & Furst1989). Although many details of this mechanism are not entirelyestablished, its successes in explaining major features observedin environments as diverse as SNRs and active galactic nuclei arenoteworthy. Since the efficiency of the acceleration mechanismand probably also of other factors relevant to the mechanism,such as the injection efficiency, depend on a number of physicalparameters, including shock velocity, local density or magneticfield (intensity and orientation), it is likely that the emissivity re-sulting from the accelerated particles will show variations fromobject to object or spatially within the same object.

The increase in the number of high-quality radio maps ofSNRs has shown that this is indeed the case, and spectral indexvariations have been suggested in a number of objects, including(this is not meant to be an exhaustive list) the Cygnus Loop (De-Noyer 1974; Dickel & Willis 1982, Green 1990), S 147 (Furst& Reich 1986), G39.2-0.3 and G41.1-0.3 (Anderson & Rud-nick 1993), Cas A (Anderson et al. 1991; Anderson & Rudnick1996). These SNRs are at various ages and one must be care-ful in comparing apparently similar properties (for example,in the case of Cas A, which is a very young remnant, Fermiacceleration alone may be unable to explain all the variationsobserved). However it is interesting that both the Cygnus Loopand S 147 (a middle-aged and old SNR, respectively) share thesame property that the strongest limb-brightened radio featureshave a flatter spectral index than the regions of diffuse emission.

Because CTA1 is located at a high galactic latitude (10◦) andat a relatively close distance (about 1.4 kpc, Paper I), confusionby foreground or background emission is minimized and goodspatial resolution is possible. High resolution multifrequencyradio observations of CTA1 thus offer a unique opportunityto study both the morphology of this SNR and possible spa-tial variations of its spectral properties. We report here on newobservations of CTA1 specifically made to investigate furtherthe breakout hypothesis. We present high-sensitivity images ofCTA1 and we investigate its extent, the polarization propertiesof the emission as well as the spectral index of the emission andits variation across the remnant. In addition, we have obtainedan integrated flux density for CTA1 at 22.25 MHz. This fluxdensity helps establish the spectrum of the SNR emission.

2. Observations and data reduction

2.1. Observations at 1420 and 408 MHz

The region around CTA1 was observed with the Synthe-sis Telescope at the Dominion Radio Astrophysical Observa-tory (DRAO) in 1993. The telescope, decribed by Roger etal. (1973b), has recently been expanded to seven antennas.Since the 1420 MHz observations presented in Paper I weremade in 1983, the sensitivity of the telescope to 1420 MHz con-tinuum emission has been enhanced by a factor of ∼ 4, a 408MHz continuum channel has been added (Veidt et al. 1985),and polarimetry at 1420 MHz has become available (Smegal etal. 1997). For full coverage at 1420 MHz of the region in which

Fig. 1. A map of the continuum emission from CTA1 at 1420 MHzobtained with the 100 m Effelsberg Telescope. Resolution is 9.35 ′.Contour levels are at 4.6-5.0 K in steps of 0.1 K, 5.2-6.0 K in steps of0.2 K, 6.3 and 6.6 K. Grayscale steps occur at 4.7, 5.8, 5.0, 5.4, 5.8 and6.6 K.

we believed emission from CTA1 might be found, we madeobervations on three field centres. Details of the observationsare given in Table 1. Observations of H i line emission fromatomic hydrogen were made simultaneously; results of theseobservations will be presented in a subsequent paper.

Reduction of data from the Synthesis Telescope followedconventional lines, including application of the CLEAN andself-calibration algorithms. After these processes were com-plete, the three individual 1420 MHz images were combinedinto one. At any given point, the intensity from each map wasweighted by a factor which took into account the value of theprimary antenna polar diagram in the individual observations ofthat point and the relative map noise (in this case the three mapshad identical noise).

The DRAO Synthesis Telescope cannot observe on base-lines shorter than 13 m, and it therefore loses sensitivity tostructure of large angular scale. Single-antenna observationswere used to provide information on such structure. Observa-tions, at a central frequency of 1.40 GHz and a bandwidth of20 MHz, were made with the Effelsberg 100-m Radiotelescopeon 1-2 April 1993, scanning a region 3.5◦ × 3.5◦. Scans weremade in two orthogonal directions at a spacing of 4 ′. Scanningspeed was 3 ′ per second, and two complete coverages of thearea were obtained. Calibration was established by observingthe unresolved source 3C286, whose flux density was taken tobe 14.4 Jy. The observations were converted to brightness tem-perature using a conversion factor TB/S = 2.05 K Jy−1 (Reichet al. 1992). The half-power beamwidth was 9.35 ′± 0.15 ′ andnoise on the final map was about 6 mJy/beam, close to the con-

1154 S. Pineault et al.: The supernova remnant CTA1 (G 119.5+10.3)

Table 1. Observational parameters for the DRAO observations

Field centres (J2000) α1 = 0 h8 m40 s δ1 = 72◦30′

α2 = 23 h52 m50 s δ2 = 73◦20′

α3 = 0 h11 m10 s δ3 = 74◦3′

Date of observation 29 January –15 March 1994Frequency 408 MHz 1420 MHzContinuum bandwidtha 4 MHz 30 MHzPolarization right circular Stokes I, Q, UField of view (to 50%) 5.3◦ 1.75◦

Field of view (to 20%) 8.1◦ 2.6◦

Synthesized beam (NS × EW) 3.5′ × 3.5′ 1.0′ × 1.0′

Grating ring radii (NS × EW) 9.9◦ × 9.9◦ 2.8◦ × 2.8◦

Observed rms noise (field centre) 5 mJy/beam 0.28 mJy/beam0.84 K 0.047K

Calibrators and assumed flux densities b

3C147 48 Jy 22.0 Jy3C295 54 Jy 21.0 Jy3C48 38.4 Jy 15.3 Jy3C286 Position angle 33.5◦ East of North

a The 1420-MHz continuum bandpass excludes H i emissionb Flux densities consistent with the scale of Baars et al. (1977)

fusion level for this size beam. The observations were processedin the NOD2 package (Haslam 1974). Orthogonal scans werereconciled and a map produced showing brightness tempera-tures relative to the map edges which were assumed to be zero.This map was fitted onto an image derived from the 1420 MHzStockert survey (Reich 1982) to provide a final image wherethe broadest structure is derived from the Stockert data and thesmaller-scale structure from the Effelsberg data.

The Effelsberg/Stockert data and the data from the DRAOSynthesis Telescope were combined to produce the final 1420MHz image. Spatial frequencies present in the individual datasets were filtered, and the maps were added with a smooth tran-sition over the range of baselines from 15 to 50 metres. Noaccount was taken of the small difference in centre frequency,because the intensity change (≈ 2%) was considered smallerthan the probable difference in the calibration of the two datasets.

A similar procedure was followed to produce the 408 MHzimage. Only two out of the three Synthesis Telescope imageswere used; the third image was of lower quality because ofinterference. Discarding the third image made little differenceto the noise level on the map, because this is mostly determinedby confusion, or to the area covered, since the individual mapcentres were very close together compared to the primary beamof the antennas at 408 MHz. Information on the largest structuresat 408 MHz was obtained from the data of Haslam et al. (1982).

2.2. Observations at 22.25 MHz

The 22.25 MHz Telescope at DRAO (Costain et al. 1969) wasused in the period 1965 to 1969 to measure emission from dis-crete sources and to map the galactic background emission. The

telescope formed a beam of 1.1◦ × 1.7◦ sec z (EW × NS),where z is the zenith angle. The beamwidth at the declinationof CTA1 was 1.1◦ × 1.85◦. The beam could be steered alongthe meridian, and drift scans were made at a set of standarddeclinations spaced by less than half the beamwidth. The fluxdensity of CTA1 was measured from archival data by integra-tion over several adjacent scans. Calibration was based on anassumed flux density of 29100 ± 1500 Jy for Cyg A (on thescale of Roger et al. 1973a).

3. Results

3.1. Radio continuum emission

Fig. 1 shows the map obtained from the Effelsberg observations.The bright partial shell of CTA1 is clearly seen in the south andthe east. A low-level region extends to the north and northwest,without distinct edge, and there is also a low-level extensionbeyond the southern sharp rim.

Figs. 2 and 3 show the full-resolution images of the CTA1region at 1420 and 408 MHz respectively. Because of the over-lapping coverage of the Synthesis Telescope fields, the noiseacross a large part of the maps is uniformly low; the noise rises,of course, at the edges of the region where the primary beamis falling to low levels. Fig. 4 is a grayscale-only version of the1420 MHz map from which the brightest point radio sourceshave been removed.

At 1420 MHz, the high sensitivity and general high qual-ity of the new Effelsberg image contribute significantly to thequality of the new image. The map published in Paper I hasthe same angular resolution, but the increase in sensitivity fromthe earlier image has made apparent several new features and


Fig. 2. Continuum emission from CTA1 at 1420 MHz. The map is acombination of three separate fields (see text) observed with the DRAOSynthesis Telescope to which short-spacing data have been added (aftercorrecting for primary beam attenuation of individual fields). Blackcontour levels are at 3.5, 3.7 and 3.9 K, white ones at 4.1 to 6.1 Kby steps of 0.4 K. Grayscale transitions occur at 3.3, 3.5, 3.7, 3.9,4.1 and 4.3 K. Resolution is 1 arcmin. The small dash-dotted box inthe southeastern part of the shell outlines an area shown enlarged inFig. 10.

considerably clarified the nature of others. In particular, the pre-vious map did suggest that the northeast shell segment appearedto split into two (at α ≈ 0 h 16 m and δ ≈ 73◦ 0 ′ ), the most in-tense emission connecting with the hook feature (at α ≈ 0 h 7 m

and δ ≈ 73◦ 20 ′ ) and a fainter one extending, initially, nearlystraight north. The current observations not only confirm theexistence of this fainter filament but clearly trace it along an arccurving to the northwest with a radius of curvature slightly largerthan that of the brightest parts of the remnant. The emission be-tween the central emission bridge (a nearly vertical feature atα ≈ 0 h 5 m) and the eastern boundary of the SNR is revealed inconsiderable detail.

As remarked in Paper I, the southeastern part of the SNRmerges into a semi-circular shell-like feature concave to thesouth (with centre of curvature near α ≈ 0 h 12.5 m, δ ≈72◦ 10 ′ ), opposite to the curvature of the main outline of theSNR. Although a point source appears at both 1420 and 408MHz near the centre of curvature of this feature, which we willterm the reverse shell, we prefer an interpretation (discussedin Sect. 4) in terms of an overdense region which has caused adrastic decrease in the shock velocity (and particle accelerationefficiency) at this position.

Fig. 3. Continuum emission at 408 MHz, where only the best two ofthe three fields observed were added. Black contour levels are at 43,47 and 51 K, white ones at 55, 63, 72 and 90 K. Grayscale transitionsoccur at 35, 37.5, 40, 43, 47, 51, 55, 63, 72 and 90 K. Resolution is 3.5arcmin.

The 408 MHz map (Fig. 3), although of lower angular res-olution, reveals almost all the features seen at 1420 MHz. Thisimage, with its large angular extent, conveys at a glance thegeneral morphology of the remnant, the classical SNR shell,incomplete, giving way to a large, low-brightness extension,probably the result of a breakout in the northwest.

It is clear from our 1420 and 408 MHz images, especiallyFigs 1 and 3, that extended galactic emission features are presentto the south of CTA1. Our method of measuring spectral indexeffectively discriminates against this emission (see below) andour conclusions concerning the SNR are not affected by it.

Fig. 5 shows the polarized emission from CTA1 at 1420MHz. The results are compatible with those obtained by Sieberet al. (1979, 1981) based on their 1720 MHz and 2695 MHzobservations. The rotation measure is very close to zero overthe whole of the eastern rim of the SNR (Sieber et al. 1981)and here the observed e-vector orientation indicates that themagnetic field is closely tangential to the rim. With their angularresolution of 4.4 ′ at 2695 MHz, Sieber et al. found fractionalpolarization up to 45% on the eastern rim and 15 to 25% onthe southern rim. Our observing frequency is lower, but ourangular resolution is higher, and we find very similar levelsof fractional polarization. We find some small patches on thecentral emission bridge (running north at α ≈ 0 h 5 m) wherethe polarized fraction is 50%.

Sieber et al. (1979) remarked on the fact that polarizedemission falls to a very low level around the position of the


Fig. 4. Grayscale map of the 1420 MHz continuum emission afterremoval of the strongest point sources. Resolution is 1 ′. Shadings runsmoothy from 3.2 to 6.0 K. A galactic coordinate grid is superimposedon the map.

small-diameter source at α = 0 h 13 m 0 s, δ = 72◦ 31 ′ 22 ′′ (theplanetary nebula NGC 40). They suggested that depolarizationoccurs through Faraday rotation in ionized gas in and aroundNGC 40, indicating that the planetary nebula is closer than theSNR. Sabbadin (1986) gives a distance to NGC 40 of 0.9± 0.2kpc, derived using various methods. This is consistent with ourdistance to CTA1, derived in Paper I, of 1.4± 0.3 kpc. At 1420MHz a larger area, about 30 ′ across, has very low polarization.There are areas on the southern rim where the brightness is highin Stokes I , but the polarized fraction is less than 2%. Thislarger area is not centred on NGC 40, and we cannot reach firmconclusions about the location of the depolarizing material.

Fig. 6 shows the radio emission at 22.25 MHz (contours)superposed onto the 408 MHz map (shadings). Clearly CTA1is the dominant source in this part of the sky. Within the limi-tations imposed by the much lower resolution and instrumentalproblems (e.g., ionospheric refraction), the 22.25 MHz emissiondoes appear to be compatible with the larger extension clearlyseen at both 1420 and 408 MHz.

3.2. Integrated radio spectrum and spectral index variations

Many of the integrated flux densities for CTA1 available in theliterature are presented in Table 2 and are displayed in Fig. 7.Some of the measured values appear low by comparison withother measurements at adjacent frequencies; this is undoubtedlydue to lack of sensitivity, particularly to extended emission.An example is provided by a comparison of the 151.5 MHzmap of Sieber et al. (1981) with the new images presented in

Fig. 5. Linearly polarized emission from CTA1 at 1420 MHz. Resolu-tion is 1 ′. The grayscale shows the intensity of the polarized emission;the scale from white to black corresponds to 0 to 0.6 K. The vectors,parallel to the electric field, indicate the angle and intensity of thepolarized emission. A vector of length 6 ′ corresponds to a polarizedintensity of 1 K.

this paper. In fact, Sieber et al. (1981) remarked that extendedstructure may have been removed by their data processing, andconcluded that the spectral index of CTA1 at low frequenciesremained ill-determined. Other than our new 1420 and 408 MHzimages, those images which seem best to represent the wholeSNR, including the extended emission, are the 1720 and 2695MHz maps of Sieber et al. (1978,1981), the 102.5 MHz map ofGlushak et al. (1987), and the 22.25 MHz map of Fig. 6. Fig. 7shows a spectrum fitted to the flux densities obtained from thesemeasurements, together with our new data; this fit correspondsto a spectral index of 0.57 ± 0.006, significantly steeper thanprevious estimates. The low-frequency flux densities are crucialin establishing this spectral index; the factor which limits itsaccuracy is the difficulty of separating this large diffuse SNRfrom its background.

Our new observations are of sufficient resolution and sensi-tivity to allow us to carry out a search for spectral index varia-tions across the SNR. We have used a method which producesmaps of differential spectral index, but we are acutely awarethat all methods of calculating spectral index maps are fraughtwith pitfalls, and must be used with great care.

As a first step, all point sources more intense than 25 mJyat 1420 MHz and 60 mJy at 408 MHz were subtracted from theimages. The 1420 MHz map was then convolved to the resolu-tion of the 408 MHz map (3.′5×3.′5). Differential spectral indexwas measured by constructing T-T plots, in which the brightnesstemperature at one frequency is plotted point-by-point againstthe brightness temperature at the other frequency, and the spec-tral index is determined as the slope of the straight line fitted by


Fig. 6. A map of CTA1 showing the radio continuum emission at 22.25MHz (contours, resolution 1.1◦× 1.85◦) superposed on the radio con-tinuum emission at 408 MHz (shadings, resolution 3.5 ′). Point sourceshave been removed. Contour values are at 85.0, 87.5, 90.0, 92.5 and95.0 kK. Grayscale transitions occur at 35, 38, 41, 45, 48, 52, 60, 65 and90 K. Note that the flux density quoted in Table 2 was not integratedfrom this map.

regression. A map of spectral index was produced by perform-ing such a calculation within an analysis box of 0.◦5× 0.◦5, andsliding this aperture over the full extent of the input images. Atthe same time, the error in each fit was computed point-by-point;this error “map” is an essential tool for assessing the value ofthe spectral index map. The aperture size was chosen as a com-promise between minimizing errors while retaining the abilityto detect small-scale spectral variation. The aperture must belarge enough that there is significant brightness variation withinit. This method, the “convolution differential spectral index”technique, is described in more detail by Zhang et al. (1997).

Fig. 8 shows the two images which were the inputs to thiscomputation, together with the resulting spectral index map andthe spectral index error map. Points with an error exceeding 1have been masked in the spectral index map. Small-scale vari-ations in spectral index are apparent, but we are not convincedthat the accuracy justifies a discussion of these features. FromFig. 8 we simply draw the conclusion that the spectral indexacross the bright parts of the SNR is 0.55 and that in the “halo”is 0.80. The error on these values is about 0.125.

The convolution differential spectral index method is supe-rior in a number of ways to the simpler technique of dividingone map by the other after background subtraction and thenevaluating the spectral index at each point. In particular, the T-Tplot method is immune from errors in the zero-level of the indi-vidual maps, while the technique of simply dividing one map bythe other is extremely sensitive to such errors. Furthermore, the

Fig. 7. The spectrum of CTA1 from the integrated flux density valuesSν shown in Table 2. The results of observations described in this paperare shown as filled squares. All other data points are shown as opensquares. The straight line is a least-square fit to the data at 22.25, 102.5,408, 1420, 1720 and 2695 MHz. It corresponds to α = 0.57 ± 0.006where Sν ∝ ν−α.

Table 2. Measured flux densities of CTA1

Freq. Beam size Flux density Reference(MHz) (′) (Jy)

22.25 66× 110 632± 200 This paper81.5 10× 10 67.7± 8.6a Branson (1966)102.5 48× 25 141± 20b Glushak et al. (1987)151.5 42× 44 62.6± 6 Sieber et al. (1981)178 20× 21 55.5± 8.9c Caswell (1967)408 3.5× 3.7 70± 2 This paper1420 1.0× 1.1 35± 2 This paper1720 7.9 31.6± 2.5 Sieber et al. (1981)2695 4.4 23.6± 2.8 Sieber et al. (1978)

a Scaled by 1.074 (Sieber et al. 1981)b New value integrated from the map published by Glushak etal. (1987), including the full extent of the SNR. Original valuepublished by those authors is 120 Jy

c Scaled by 1.11 (Sieber et al. 1981)

method we have described discriminates against variations inbackground or foreground emission, as long as those variationshave angular structure larger than the aperture in which the com-putation is made. If there are two emission components present,this method will compute a spectral index dominated by that ofthe component whose brightness is varying most rapidly in theaperture. It is apparent from Figs 1, 2 and 3 that CTA1 is seenagainst a varying background, and we must apply our techniquecarefully. In general, the background falls off with increasing


Fig. 8a–d. Spectral index (α) variationsacross CTA1. The dashed circle outlines thearea within which the analysis was limited.The top two panels show the two radio con-tinuum maps after removal of the strongestpoint sources: a 408 MHz (left) with shad-ings running smoothly from 35.5 to 70 Kand b 1420 MHz (right) to the same reso-lution (3.5 arcmin) with shadings runningsmoothly from 3.2 to 4.5 K. The bottom twopanels show: c on the left, the spectral indexmap for which the lightest shading corre-sponds to α in the range 0.0 to 0.3 and suc-cessive ones to intervals of 0.1 in α (endingwith α > 1.0, darkest shading) and wherevalues with an error greater than 1 have beenmasked (blanked), and d on the right, thespectral index error map where each shad-ing corresponds to a range of 0.125 in the er-ror (lightest shading starts at 0.0 and darkestone corresponds to an error greater than 1.0– note that the white areas within the darkones are not areas where the error is zero,but areas of undefined data where no statis-tically significant analysis could be carriedout).

galactic latitude, on a scale larger than the 2 degree size of theSNR, and there appears to be very little small-scale structurein the background. We feel that our technique has effectivelyseparated the SNR emission from its surroundings.

However, the galactic background can be expected to havea spectral index of about 0.8 in the frequency range 408 to1420 MHz (Salter & Brown 1988), very close to the spectralindex we have determined for the diffuse component of theSNR. Consequently, before accepting our conclusion, we haveapplied two other tests.

First, there is evidence from the integrated spectral indexthat the diffuse low-brightness component of CTA1 has a higherspectral index than the bright shell component. When spectralindex is computed from images which miss the extended com-ponent of the SNR, a spectral index of 0.30 is obtained (Sieberet al. 1981). When measurements which include all SNR emis-sion are used, as in Fig. 7, a steeper index (0.57) results. We notethat “background” emission has been subtracted, more or lesssuccessfully, in deriving this value. Separation of the SNR fromits background should be more successful at low frequencies,where the spectral index of the galactic emission is about 0.4between 10 and 100 MHz (Salter & Brown 1988). We there-fore believe that we have accurately established the integratedspectral index of the SNR.

As a second test, we made a separate calculation of spec-tral index within three boxes on the SNR. After first removingthe smoothly-varying background (by fitting a twisted plane,outside the SNR boundary, to the smoothed maps in galacticcoordinates), boxes, of size between 20 and 40 smoothed beamareas, were chosen. Resulting spectral indices between 408 and1420 MHz were: on the southern shell, 0.58, on the eastern shell,0.51, and on the brightest part of the breakout region, 0.90. Sub-stantial errors arise from difficulty in unambiguously eliminat-ing the background. However, the results once again supportour conclusion that spectral index is significantly higher in thebreakout region.

3.3. Emission in other wavebands

Our new 1420 MHz and 408 MHz images are sufficient to elab-orate a nearly complete and self-consistent model of CTA1 as abreakout SNR, however, in addition to the neutral hydrogen datadiscussed in Paper I, crucial additional information has since be-come available in the infrared and at X-ray wavelengths.


3.3.1. Neutral hydrogen data

One of the key H i features present in the data of Paper I is thepresence of a partial H i shell to the northwest. In Paper I, it wasargued that it more or less completed the SNR outline seen inthe continuum emission and marked the extent of the SNR inthis direction. This shell describes a partial arc running roughlyfrom α ≈ 23 h 50 m, δ ≈ 72◦ 47 ′ through α ≈ 23 h 57 m, δ ≈73◦ 17 ′ to α ≈ 0 h 5 m, δ ≈ 73◦ 41 ′ . Figs. 2 to 4 clearly showthat the faint continuum radio emission extends past the H i shellto the northwest suggesting that the H i gas lies ahead or behind,being seen in projection.

3.3.2. CTA1 at X-ray wavelengths

The first detailed map (Seward 1990) showed the X-ray emis-sion to be more or less centrally concentrated, correspondingwith the central radio emission bridge and the region betweenthe latter and the bright eastern radio emission. However cover-age was far from complete allowing no definite measurement ofthe actual extent of the SNR at X-ray wavelengths. New ROSATobservations by Seward et al. (1995) have considerably clarifiedthe picture and add support to the breakout model. Firstly, thetongue of emission to the southwest, clearly visible on the radiomaps, also has an X-ray counterpart extending west to approx-imately α ≈ 23 h 53 m and δ ≈ 72◦ 40 ′ . Secondly, the X-rayemission appears to extend north into the proposed breakoutregion (up to a declination of δ ≈ 73◦ 50 ′ ). This emission doesnot extend as far as the radio continuum emission, but roughlycoincides with the H i shell discussed above. In the south andeast the X-ray emission is confined within the bright outline ofthe radio SNR. Confirming the earlier Einstein results, the X-rayemission is seen to be centrally peaked, the maximum occuringat α ≈ 0 h 7 m and δ ≈ 73◦ 0 ′ i.e., slightly east of the hook-likefeature which joins the central emission bridge to the northeastsegment on the radio maps. It is at the same declination as the22.25 MHz maximum, although at a different right ascension.

Seward et al. (1995) note that the X-ray emission from thecentre of the remnant is considerably brighter than that fromthe rim, in sharp contrast to the radio observations, and thatacceptable fits cannot be obtained to the X-ray spectra withsingle-temperature models. However, they obtain equally goodfits with two different types of two-component models. Thefirst is a two-temperature thermal model, corresponding to alow-temperature shell surrounding a warmer interior. As a re-alization of this model, they consider a two-phase ISM consist-ing of discrete cool clouds embedded in a uniform intercloudmedium, such that the evaporation of clouds (Cowie & McKee1977) causes an increase in the density of the central regions ofthe SNR. Using the results of numerical simulations by White &Long (1991), they conclude that the cloudy ISM can produce acentre-filled morphology, although considerable differences ex-ist between the model and the observed morphology. The effectof significant density variations (obviously a key factor in thebreakout model) is, however, not included. The second modelassumes that CTA1 is an older Crab-like remnant in which a

Fig. 9. A 60µm HIRES map of CTA1 (grayscale) on which a fewcontours of the radio emission at 1420 MHz have been superposed.Resolution is 2.5 arcmin in both cases. Shadings run smoothly from-0.6 to 0.6 MJy/sr. Contour values are at 3.6, 4.2 and 4.7 K.

central non-thermal synchrotron nebula (presumably due to anunobserved pulsar) is surrounded by a low-temperature thermalshell. However, the X-ray synchrotron nebula is of low surfacebrightness and would contribute negligible flux at radio wave-lengths. Although our radio data do not allow us to discriminatebetween these two models, we feel that the cloudy ISM modelprovides a simpler framework, and it is this model which willform the basis of our discussion (Sect. 4).

3.3.3. Infrared data

The initial coadded IRAS data provided evidence for the pres-ence of two cavities of low infrared emissivity, in support ofthe breakout idea, but the low-resolution and sensitivity of thedata could not allow a more detailed comparison. As a nextstep, we obtained high-resolution-processed (HIRES) (Fowler& Aumann 1994) IRAS images produced at the Infrared Pro-cessing and Analysis Center (IPAC)1. The images used are theresult of 20 iterations of the algorithm, giving an approximateresolution at 60µm of 1.3 ′ × 0.85 ′ at a position angle (eastof north) of 19◦. However, the beam is not strictly gaussianand varies slightly over the map. Because the full-resolutionmap shows considerable detail, but often at rather low values ofsurface brightness, we experimented smoothing it to differentresolutions in order to increase the signal-to-noise ratio. A good

1 The Infrared Processing and Analysis Center (IPAC) is funded byNASA as part of the Infrared Astronomical Satellite (IRAS) extendedmission under contract to the Jet Propulsion Laboratory (JPL)


compromise between good resolution and increased sensitivitywas found with a circular beam of 2.5 ′. Fig. 9 shows the 60µmmap at this resolution with a few contours of the radio emissionat 1420 MHz superposed. Some point sources appear slightlyelongated because of the varying resolution across the map.

In addition to the main features already discussed in PaperI (the enhanced band of IR emission running diagonally fromwest to north and coincident with the H i shell, the prominentvoids), there is evidence that the splitting of the eastern radioshells into two curving arcs in the northeast has a corresponding,though weak, IR counterpart. These faint features are about30 ′ long and originate near α ≈ 0 h 15 m, δ ≈ 73◦ 0 ′ , onearc running in a mostly north direction closely following thelast (outer) radio contour shown, the other one running in anorthwest direction nearly coincident with a “finger” of radioemission. The same is true of the hook-like feature near α ≈0 h 6 m and δ ≈ 73◦ 30 ′ and of the central emission bridge,although the IR emission appears more patchy in the latter case.The enhanced IR emissivity of these features is most likely theresult of the increased density resulting from the compression bythe SNR blast wave (which a significant fraction of dust grainshave apparently survived).

As for the bright IR extended source at α ≈ 0 h 13 m andδ ≈ 72◦ 15 ′ , we will argue in the next section that this is not aregion which has been compressed by the shock wave, but rathera denser cloud pre-dating the SN explosion, whose density isso much higher than the mean ISM density that the shock wavehas significantly slowed down, creating the “reverse” shell ap-pearance so obvious at radio wavelengths, particularly at 1420MHz. Fig. 10 shows an enlarged view of this part of the SNRshell at 1420 MHz, with some contours of the IR emission su-perposed. Although the extended IR feature is not spherical inshape, it is seen to be wholly contained within the 19 ′ diametercircle shown in Fig. 10. The bright, nearly circular, IR emissionfeature to the north of it is the planetary nebula NGC 40.

4. Discussion

In Paper I, we showed evidence for the breakout nature of theSNR and presented a semi-quantitative model of the breakoutevolution consistent with the then available observations. Thepresent observations strengthen the case for a breakout and al-low us to address a number of important issues quantitatively.Concerning the real extent of the SNR to the northwest, we con-sider it to extend from α ≈ 23 h 48 m to α ≈ 0 h 18 m and fromδ ≈ 71◦ 42 ′ to δ ≈ 74◦ 30 ′ , and all our determinations ofintegrated flux density are based on this boundary. The remain-ing outstanding issues to discuss are the nature of the spectralindex variations, the presence of faint emission extending pastthe bright radio shell edge in the south and the nature of the“reverse” shell.

We adopt the picture of a cloudy ISM of background par-ticle density no, as suggested by Blandford and Cowie (1982),and assume that the SNR dynamics are adequately describedby the Sedov self-similar solution (the discussion could eas-ily be modified to include a late isothermal phase). The time

Fig. 10. An enlarged view of the “reverse” shell area showing the con-tinuum 1420 MHz radio emission a (top) including all point sourcesand b (bottom) after removal of the brightest point radio sources. Res-olution is 1 ′. Shadings run smoothly from 3.2 to 4.5 K. The smallcircle shown in the top panel has a diameter of about 19 arcmin or 3.8pc for a distance of 1.4 kpc. Contours of the 60 µm IR emission (at aresolution of 2.5 ′) are shown in the bottom panel, at values of 0.5, 0.7,1.0 and 32 MJy/sr.

variation of the shock radius Rs is thus described by Rs =0.34 (E51/µno)1/5 t

2/5yr pc, where E51 is the supernova energy

in units of 1051erg, µ the mean molecular weight and tyr theSNR age in year. The observed SNR angular radius θ = 50′

and assumed distance d = 1.4 kpc thus correspond to anage, in units of 104 yr, of τ4 = 2.7 (µno/E51)1/2 ξ5/2, whereξ = (d/1.4 kpc) (θ/50′). The current shock velocity v and post-shock temperature T of the SNR shell are thus:

v7 = 3 (E51/µno)1/2 ξ−3/2, (1)

T6 = 2 (E51/no) ξ−3, (2)

in units of of 107 cm s−1 and 106K respectively. For the sake ofthe discussion, we shall take E51 = 1 and no = 0.1 cm−3 for theparticle number density in which the SNR is propagating, a value


intermediate between that proposed by Blandford and Cowie(0.01 cm−3) and that used in Paper I (1 cm−3) to discuss theSNR evolution before the breakout. The clouds immersed in thisdiffuse medium have a much larger density (typically 10 cm−3,say) and are assumed to be crushed and partly evaporated in thehot post-shock diffuse medium.

4.1. The breakout to the northwest

Despite the slightly different values assumed for the backgroundnumber density no, the discussion of Paper I for the SNR evo-lution before and after breakout remains generally valid, themain change being that dynamical timescales are all shorter ina medium of lower no. The shock breaking out into the lowerdensity cavity to the northwest (assumed to have a density of or-der 0.01 cm−3) would thus cross the cavity in a correspondinglyshorter time. The lack of significant radio emission correspond-ing with the IR feature running diagonally from the southwestto the northeast of the SNR (which presumably marks the endof the cavity opposite CTA1) means that either the shock hasnot yet crossed the cavity (unlikely) or that it has become tooweak to cause significant particle acceleration. The H i shell tothe northwest, observed by Pineault et al. (1993), is a clear indi-cation that the shock has indeed traversed the cavity and beguncompressing the wall material.

The most interesting aspect of the breakout, however, is thefact that the diffuse radio emission to the northwest seems tohave a steeper index than the brighter south and east regionsof the SNR. Significant spectral index variations are hard todemonstrate unambiguously, but they have been suggested ina number of SNR cases (as noted in Sect. 1). Fermi acceler-ation in shocks (or difussive shock acceleration (DSA); Bell1978a,b; Blandford & Ostriker 1978) is believed to be the mainmechanism producing a distribution of non-thermal particlesin a variety of astrophysical environments. The question is thuswhether DSA can explain spectral index variations of order 0.25as found in CTA1. The spectral index α predicted by DSA inadiabatic shocks is given by:

α =3

2(r − 1)=

M 2 + 32(M 2 − 1)

, (3)

where r = (γ + 1)/(γ − 1 + 2/M 2) is the shock compressionratio, γ the ratio of specific heats, M = u1/cs the shock Machnumber (u1 is the shock speed and cs the sound velocity in thepre-shock gas). The second equality is forγ = 5/3. If the densityis large enough, there will be further energy increase associatedwith the compression that accompanies radiative cooling, butthis will not affect the electron spectrum (e.g., Blandford &Cowie 1982). For strong shocks in a γ = 5/3 gas, theory pre-dictsα = 0.5, the value observed for the brighter shell. Howevera spectral index α ≈ 0.75, as observed in the diffuse northwestregion, would require a shock with M 2 ≈ 9, correspondingto a compression ratio r ≈ 3. As an estimate of the condi-tions likely to be encountered upon breakout, let us ask whatthe Mach number would be if the SNR shell component was

to break out now into a tunnel or cavity. This will underesti-mate the shock velocity at breakout (which occurred sometimein the past) but will nevertheless provide useful constraints. Un-less the magnetic energy density exceeds the thermal energydensity (unlikely), the current Mach number of the shell flowis M = 30 (E51/µno)1/2 T

−1/24 ξ−3/2. It is to be noted that

M ∝ p−1/2 where p is the gas pressure so that one wouldnot expect a large change in Mach number to occur for shockspropagating in a multiphase medium in approximate pressureequilibrium. However numerical studies of the breakout processby Falle & Garlick (1982) and Tenorio-Tagle et al. (1985) (seealso Pineault et al. 1987 for an application to VRO 42.05.01)have painted a slightly more complex picture indicating that,upon crossing into a lower density medium, acceleration willindeed take place but not to the full extent naively suggested bythe simple n−1/2 estimate. In other words, the shock velocitymay not increase as much as expected from simple density scal-ing arguments whereas the much higher temperature prevailingin the low-density cavity (e.g., Cox & Smith 1972; Blandford &Cowie 1982) dictates a substantial increase in sound speed andthe net result is a decrease in Mach number. Despite numerousuncertainties, it thus seems possible that an SNR shock, whenbreaking out into a lower density environment, could result in asteeper radio spectral index if DSA continues to operate and ifthe shock Mach number has sufficiently weakened. An alterna-tive (unsatisfactory) explanation for the differing spectral indexobserved from the diffuse breakout region is that DSA is not asimportant as in the bright shell region and that emission sim-ply originates from a different population of particles having asteeper energy distribution.

Other means of obtaining different spectral indices have alsobeen investigated. For example, Droge et al. (1987) and Schlick-eiser & Furst (1989) have considered the effect of includingsecond-order Fermi acceleration in adiabatic shocks. Howeverthey conclude that flatter (i.e., α < 0.5) spectra may be pro-duced in plasmas with a low value of the parameter β = c2

s/v2A,

where cs and vA are the sound speed and Alfven velocity respec-tively, and that steeper spectral indices can only be produced bydifferent compression ratios. Although our own observationsdo not show the presence of flatter spectrum regions in CTA1,it appears clear that current and future observations of SNRswill increasingly constrain the particle acceleration processesoperating in SNRs. Numerical modelling of the breakout pro-cess taking into account the particle acceleration mechanismwill also undoubtedly help explain the spectral index variationsfound.

Whether the breakout phenomenon plays a role in explain-ing spectral index variations or not, it is of interest to placeour own observations of CTA1 in the broader context of spec-tral variations suggested for other SNRs. Anderson & Rudnick(1993) present a convenient summary of the most commonlydiscussed models and review the state of the observations. Inaddition to their own observations of G39.2–0.3 and G41.1–0.3, they identify seven shell SNRs which have been reportedto exhibit spectral variations. Grouping (in a somewhat non-standard fashion) remnants into “younger” and “older” ones,


they conclude that, whereas the situation is not clear for theyounger ones, older remnants have a tendency to show flatterspectral indices where the emissivity is the highest, a statementcompatible with our observations.

4.2. Emission beyond the bright southern shell edge

Our 1420 and 408 MHz maps both show the presence of faintradio emission beyond the bright shell to the south. The simplestexplanation is that this emission does not physically extend pastthe shell edge but that it is simply the result of a projectioneffect: the outflow of hot gas and relativistic particles has acomponent towards or away from us and some of the materialappears projected outside the bright radio edge. The fact that theH i shell discussed in Paper I does not coincide with the end ofthe diffuse radio emission to the northwest supports that view.

However there is also the possibility that diffusion of rel-ativistic electrons ahead of the shock accounts for this effect(completely or partly). The possible existence of radio ha-los around SNRs has been discussed by Reynolds (1994) andthe absence of obvious halos around a number of SNRs hasbeen used by Achterberg et al. (1994) to deduce that magneto-hydrodynamic (MHD) turbulence must be enhanced outsideSNR shocks (otherwise the electrons would diffuse much far-ther ahead). On the other hand, Moffett & Reynolds (1994) haveexplained small scale extensions past the bright edge of the SNR3C391 as due to relativistic electrons diffusing upstream alongthe magnetic field with a mean free path (mfp) larger than thatcharacteristic of the general ISM. If this process is operating inCTA1, we can estimate the characteristic mfp of the relativis-tic electrons. If θBn represents the angle between the magneticfield and shock normal, then the mfp for diffusion ahead ofthe shock is given by (Reynolds 1994; Achterberg et al. 1994)λ = λ‖ cos2 θBn, where λ‖ ≈ rg(δB/B)2, rg is the gyroradiusof the particle and (δB/B)2 the relative intensity of the magneticturbulence. The relativistic electrons will thus diffuse a typicaldistanceL = λ c/3 vs ahead of the shock (vs is the shock speed).Let Lobs = aL, where Lobs is the observed “precursor” size anda is a parameter of order unity which depends on the particu-lar injection model (a varies typically between ln 2 and 3 forexponential and Bell-type models; see Reynolds 1994). If ∆is the precursor size in arcmin and d the distance in kpc, then(Achterberg et al. 1994):

λ = 9.6× 104( vs

100 km s−1

) ∆ d

a cos2 θBncm, (4)

= 2× 1015( E51

µno

)1/2 ∆ d1.4

cos2 θBncm, (5)

where a = 2 has been used in the second equality, taken toapply to the specific case of CTA1. From Fig. 4, ∆ ≈ 5 ′

and, using (somewhat arbitrarily) cos2 θBn = 0.5, we find λ ≈0.02 pc, an entirely reasonable value between λISM ≈ 0.2 pcand λSNR ≈ 0.003 pc, the values deduced by Achterberg etal. (1994) for the general ISM and young SNRs respectively.Since λ = rg (δB/B)−2 cos2 θBn, the above value for λ can betranslated into an estimate for δB/B. Electrons radiating at 1

GHz in a field of 3µG have a typical gyroradius of∼ 3×1013 cmwhich means that δB/B ' 0.015 (using again cos2 θBn = 0.5).Because the emission to the east is less sharply defined than inthe south, we feel a similar calculation is not warranted for theeastern filaments.

4.3. The “reverse” shell

The bright southern radio shell shows evidence of a break atα ≈ 0 h 13 m, δ ≈ 72◦ 12 ′ . This is most obviously seen on the1420 MHz maps of CTA1 (Figs 4 and 10). Such a feature couldbe formed if, for example, a massive wind-blowing star waslocated at the centre of this reverse shell. Many point sourcesare present on our maps, one of which is indeed located near thecentre of curvature at α = 0 h 12 m 47.1 s, δ = 72◦ 13 ′ 5.7 ′′. Themeasured flux densities of this source are 79.6±15.4 mJy (peakflux of 79.7 mJy) and 30.9 ± 1.5 mJy (peak flux of 23.8 mJy)at 408 and 1420 MHz respectively. The spectral index of thissource (0.76 from integrated values, 0.96 from peak fluxes) is ofno particular interest and certainly not characteristic of a stellarwind (α ≈ −0.7). No HIRES source is seen at this positioneither (Fig. 9). This reverse shell is therefore unlikely to resultfrom a stellar wind object although one cannot completely ruleout other less obvious stellar-type candidates.

We believe that the simplest explanation for this feature isthat it is the result of a dense cloud which has been overtakenby the SNR blast wave. This is supported by the IR data, whichshow enhanced emissivity at this position, and also by the H idata of Paper I where an H i cloud can be seen at an LSR velocityof about−6 to−9 km s−1. If that explanation holds, the fact thatthe shock has progressed very little inside the cloud allows us toestimate the density contrast between the background mediumin which the blast wave is propagating (no) and the cloud (n1).The ratio of the shock velocities is v1/vo = ∆r/∆R where ∆ris the distance travelled inside the cloud and ∆R that travelledby the unimpeded shock wave in the nearby medium. FromFig. 4, we see that ∆R is comparable to the thickness of thebright southern shell (∼ 8′) and that ∆r is of the order of theachieved resolution at 1420 MHz (1 ′). Adopting ∆R/∆r ≈10 and using the fact that shock velocities scale as n−1/2, weconclude that n1/no ≈ 100. Such an increase in density wouldindeed be expected to perturb the shape and velocity of the shocksignificantly. Calculations of H i cloud evaporation in SNR blastwaves (e.g., Tsunemi & Inoue 1980) show that the resultingevaporated cloud skin would initially be thin.

However the hypothesis that the SNR blast wave has runinto a density enhancement poses a problem. A currently heldview is that SNRs appear brighter where the blast wave is mov-ing against dense regions of the ISM (also the explanation forsharp brightness gradients often observed) and we would there-fore naively expect the “reverse” shell to stand out as one ofthe brightest parts of the SNR. We now argue that the lackof brightness is related to the reduced efficiency of the ac-celeration mechanism as the shock encounters an overdensecloud. The first possibility is that, as a result of the reducedshock velocity, the particles have not yet had time to be ac-


celerated to sufficiently high energies (we are grateful to thereferee for pointing this out). The acceleration time τacc of anelectron to energy E is of order (Ellison et al. 1990, Reynolds1996) τacc = 6.25 × 105 (η/B) v−2

7 E s, where η is a factorwhich depends on the turbulence level and magnetic field ori-entation, and B is the preshock magnetic field strength. We usev7 = 0.3 (Eq. (1) withno = 100 cm−3),B = 3µG,E = 10−2 erg(corresponding to electrons radiating at ν ≈ 1 GHz in an as-sumed 10µG postshock magnetic field) and, following Reynolds(1996), η = 10, which yields τacc ≈ 6000 yr. An estimate of theavailable time for acceleration can be obtained by evaluating thetime t∆ it would have taken the unimpeded shock wave to travela distance ∆R (of order the southern shell thickness) at a veloc-ity given by Eq. (1). This time is of order t∆ ≈ 104 n

1/2o yr, so

that τacc can indeed be larger than t∆ for small enough valuesof no (a similar but more uncertain estimate could have beenobtained using the thickness ∆r of the reverse shell component).

Another possible explanation for the low brightness is thatthe acceleration mechanism has simply broken down because ofthe enhanced density. Diffusive shock acceleration requires thatthe relativistic particles repeatedly scatter back and forth acrossthe shock. The scattering centres are usually taken to be Alfvenwaves produced by the particles themselves. Bell (1978a) hasshown that the growth and decay rates of these waves are givenby:

Γ+ = 6.2× 10−9 (100 vs/c)(δf/fgal)n−1/2e E

−3/2GeV s−1, (6)

Γ− = 3.29× 10−9 nH s−1, (7)

where the latter has a slight dependence on temperature. InEq. (6), δf/fgal = (f (0, p) − fo(p))/fgal, where f (x, p) is theusual phase space distribution function, p being the momentumand x a distance measured perpendicular to the shock front.Since f (0, p) is the distribution at the shock (after acceleration),f0(p) the background distribution in the undisturbed mediuminto which the shock is propagating and fgal a typical distri-bution for the galactic cosmic ray background (we may takefgal = fo(p) here), the quantity δf/fgal is essentially a mea-sure of the enhancement in the number density of relativisticparticles near the shock as compared to the average value inthe nearby galactic medium. On one hand, the growth rate de-creases as the shock slows down and, on the other hand, thedecay rate Γ− depends on the first power of the number densityof H atoms. It is thus possible that at high enough densities,the rate will be severely damped ahead of the shock, allowingthe relativistic particles to freely escape. The quantity (δf/fgal)can be estimated by comparing the observed average brightnesstemperature within the SNR shell with that of a neighboringpatch outside the SNR and using the standard theory of op-tically thin synchrotron radiation. The brightness temperatureTB within the beam is given by TB ∝ KLB

(γ+1)/2⊥ , where

the constants K and γ relate to the particle energy distributionN (E) dE = KE−γdE and L is a depth along the line of sight.By comparing the excess brightness (near the reverse shell) tothat in a nearby patch of sky, we can determine KSNR/Ko and

thus estimate δf/fgal. We assume γ = 2 and a compressionratio of 3 for the magnetic field (in adiabatic shocks, this ra-tio is between 1 and 4, e.g. Reynolds 1994). We have assumedLSNR ≈ 0.1R and Lo ≈ 1.5 d (probably a reasonable esti-mate at the high galactic latitude of CTA1). With these param-eters and the measured brightness temperatures, we estimateδf/fgal ≈ 200. Given the uncertain parameters, it is reason-able to assume that δf/fgal lies somewhere between 100 and1000. This compares with a value of 104 used by Bell (1978a)who was considering Cas A, a considerably younger SNR. Us-ing Eq. (1) for the current shell shock velocity and an ionizedfraction x = 0.99 (i.e, corresponding to the preshock gas beingnearly completely pre-ionized even within the cloud), we obtaina critical number density noc between 40 and 130E−3/4

GeV cm−3,for values of δf/fgal of 100 and 1000 respectively. For a nearlycompletely neutral medium with x = 0.01, the correspondingnumbers would be 13 and 41. Although these numbers shouldnot be taken at face value (among other things, the energy of therelativistic electrons radiating at about 1 GHz will depend onthe unknown magnetic field), they nevertheless indicate that, ina sufficiently old SNR (i.e., one where the shock velocity hasdecreased significantly and hence in which the growth rate ofAlfven waves is hampered), the acceleration mechanism maybe quenched when encountering sufficiently dense clouds, themore so the more neutral the cloud. However, this does not en-tirely solve the low-brightness cloud problem as the particlesaccelerated prior to the encounter should still be available toradiate: it may be that the reduced MHD turbulence in the cloudmakes it an efficient region where the local relativistic particlescan leak out.

5. Conclusions

We have presented the most accurate (up to 120 beams per diam-eter at 1420 MHz) radio maps of CTA1, showing all structuresdown to the resolution limit. Whereas the most conspicuouslimb-brightened radio emission is found in the south and east,diffuse low-level emission is seen to extend considerably to thenorthwest. This is interpreted as a breakout of the SNR blastwave into a region of lower density. The radio spectrum in themore diffuse region appears steeper than in the brighter regions.An interpretation in terms of weaker shocks propagating into thebreakout region cannot be ruled out although it requires ratherspecific conditions to be met following breakout. The faint ra-dio emission seen extending past the southern bright filamentsmay be the result of a projection effect, but it is compatible withdiffusion of electrons upstream of the shock with a mean freepath on the order of or less than 0.02 pc. A break in the circularoutline of the southeastern part of the remnant (referred to asthe “reverse” shell) is best explained by a density enhancementor cloud which has caused the shock to slow down considerablythus reducing the acceleration efficiency.

Acknowledgements. We are grateful to John Dickel for his thoroughreview of this paper, which guided us in clarifying the presentationof our results. The authors were supported by grants from the Natural


Sciences and Engineering Research Council of Canada (T.L.L & S.P.),and the Fonds FCAR of Quebec (S.P.). C.M.S. was a participant inthe Women in Engineering and Science program of NRC during partof this work. We could not have obtained the 22.25 MHz flux densitywithout the help of Dr. R.S. Roger. The DRAO Synthesis Telescopeis operated as a national facility by the National Research Council ofCanada.

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