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The Radio Spectrum of Supernova Remnants

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The Radio Spectrum of Supernova Remnants THE RADIO SPECTRUM OF SUPERNOVA REMNANTS by D. E. Harris California Institute of Technology Radio Observatory Ovens Valley, California Abstract 960 Mc/s observations of thirteen galactic sources generally assumed to be supernova remnants, are coupled vith previous ob­ servations at other frequencies to derive spectral indices. Al­ though several values of spectral index are found in the neighbor­ hood of zero, arguments are presented that free-free transitions are not the primary cause of the radio emission. An interpretation of the relatively large range of spectral indices is suggested on the basis of an evolutionary sequence in vhich young, bright objects vith relatively steep spectra gradually evolve into old, faint objects vith flat or inverted spectra. I. Introduction The radio investigation of supernova remnants vas initiated in 1949 vhen Bolton and Stanley (1949) identified Taurus A vith the remains of the galactic supernova of 1054. Further radio vork on supernovae has been hampered by the inability of radio techniques to resolve individual features of the distant galaxies vhere most knovn supernovae have occurred. Therefore, radio observations have concentrated on the three knovn galactic supernovae, together vith an ever increasing number of extended galactic sources. The latter sources are non-thermal radio emitters vith optical features vhich often display a general expansion. Table 1 lists all such objects vith vhich ve are familiar. The present paper is based on part of the author's thesis and is a study of the first thirteen sources listed in Table 1. These thirteen sources, he~eafter assumed to be supernova remnants, vere observed at 960 Mc/s vith a 90' parabolic antenna at the Ovens Valley Radio Observatory. The receiver and the observing procedures have been described elsewhere (Harris and Roberts 1960; hereafter referred to as "CTA"). -2- Table 1 - Poaaible Supernova Reananta Object a 5 J_II Reterencea (1950) (1950) A) ObJect& atudied in the preaent paper. Caa A 3120±.150 23h21•11• +58°32' 111~7 -2':'1 Minkovaki (1959) Kepler'• SR 22± 3 17 27 40 -21 25 4.5 +6 . 9 Supernova obaerved Tycho '• SR 57± 3 00 22 46 +63 51 120.1 +l.IJ Supernova obaerved Puppia A l3U 12 08 21 20 -42 52 260.4 -3.3 Baade and Minkovaki (1954) II 1,4 211± 9 18 53 35 +01 15 34.6 -0.4 Lealie (1960) IC443 195± 2lo o6 14 16 +22 36 189.1 +3.1 Minkovaki (1959) Crab Rebula 1030± 45 05 31 30 +21 59 184.3 -6.2 Supernova obaerved 11»21 180± 60 20 .... 42 +50 32 89.0 +4.7 Minkovaki (1958) Cygnua Loop 265± 50 20 50 +30 30 74.1 -8.7 Minkoveki (1958) Vela X,Y,Z 1560 .. 08 32 48 -IJ5 37 263·9 -3 .2 Bolton (1960) 11»9 l60:t. 30 olo 57 30 +46 25 160.6 +2.8 Mintovati (1958) Sll,7 120± 60 05 36 +28 180.0 -1.7 Minkovaki (1958) CTAl 75± 18 00 02 35 +72 04 119.lo +9.8 Faint optical eaiaaion area B) Other radio aourcea ot aiailar type. RGC6888 20 10 55 +38 15 75.6 +2.3 Mintovati (1958) CTBl 56 .. 00 00 18 +62 11 117.3 +0.1 Vaa den Bergh (1960) Ci'Bl3 225 .. 04 21J +47 00 156.1, -1.2 Wilaon and Bolton (1960) 1128 240 .. 17 58 12 -23 22 6.6 -0.1 lleaterhout (1958) Galactic Spur 15 -20 3loO 33 Brown, Davie&, and Bazard (1960) Shtlovaky'e &election ot extended galactic object& (l960b) 1m3 02 16 +62 30 132.8 +1.6 BB7 03 23 30 +55 06 llo3 . 9 -1.1 Jm8 04 02 +IJ7 30 153.3 -3.1J Centaurua II 13 22 48 -59 2lo 307.3 +2.9 c) Optical object&. Mintovati'• aaall object&. (1958) 107 12 227.2 -1.0 Pe culiar double ring 522 01 27 28 ,-13+58 06 I 128.1 -4.1 07 26 15 +13 21 Van den Bergh'• tila111enta (1960) lro. 2 105 24 J+42 56 1166.2 +4.4 Lev denaity ring Ro. 6 18 02 -23 40 6 . 7 -1.1 S960 ia the flux denaity at 960 Me / a aeaaured in units ot lo-26 wa-2(c/•)-l. • CTB tlux denaity uaed (Wilaon and Bolton 1960) . - 3- The smaller supernova remnants have been studied previously by many workers and do not lend thems~lves to extensive investigation with the present equipment. Therefore, most of the observational effort has been spent on the larger objects. The Cygnus Loop is one of the strongest sources of this type at 960 Mc / s and has been examined previously in more detail than most other similar remnants. Furthermore, it is significantly resolved with beamwidths such as ours (098). For these reasons, the Cygnus Loop is presented in this paper as a prototype of the larger super­ nova remnants. In Section II, 960 Mc/s flux densities are used in conjunction with measurements of various observers at other frequencies to de­ termine spectra relative to the standard source, M87. Since the de­ rived spectra of many large supernova remnants are characteristic of HII regions, calculation~ based on optical observations are made to determine the extent of radio emission from free-free transitions. These calculations, given in Section III, indicate that most of the radio emission must be non-thermal. Section IV presents evidence which suggests that the spectral index of any remnant varies with time. This hypothesis is based on empirical correlations found for the 13 objects studied in this paper. When the spectral indices of the various remnants are plotted against the appropriate value of linear diameter or radio surface brightness, it appears that an initially steep spectrum may flatten with age. II. Observations and Spectra a) Intensity Measurements of the Cygnus Loop The present observations of the Cygnus Loop consist of about 30 drift curves of 40 minutes each taken at one-half degree intervals between +27°30' and +33030'. These intensity records, obtained be­ tween April and November 1959, were normalized by a drift curve taken each night at 5 = +30000' . Figure 6 (Appendix III) shows the resul­ ting Cygnus Loop contours of antenna temperature where a linear sub­ traction for the galactic background has been made. The galactic correction was performed on the average drift curves by noting the slope of the record of antenna temperature as the galaxy left the antenna beam before the source was encountered. Following Mathewson, Large and Haslam (1961), hereafter referred to as "MLH'', the Loop was divided into three parts: A, B, and c. (The boundaries are indicated by the dotted lines in Figure 6.) Each part was integrated with a planimeter, in order to derive a value of intensity for the individual sections as well as for the complete source. The intensity published earlier (CTA) was based on a prelimi­ nary contour diagram and the value given here supercedes that. The maximum intensity occurred on the 5 = +30°00' records (Source A) and the average of 7 observations gives a value of 0 .104 ± 0.008 (p.e.) for the peak intensity relative to M87. The flux density from the entire source is then found to be 0.88 ± 0.17 times that of M87 -4- [{265 ±50) lo-26wm-2(c/s)-l if SM87 = 300·lo-26wm-2(c/s)-~. The error of ± 20~ consists of 12~ from the integration process and 8~ ~rom the scatter in the observed peak intensity. b) Spectrum of the Cygnus Loop Observations made at other frequencies are presented in Table 2. For the purpose of deriving a spectral index, published flux densities have been normalized to the standard source (M87) whenever possible. The secondary calibrators Cas A and 3C 295 were used to relate Westerhout's 400 Mc / s measurement and the MLH values, respectively, to M87. The adopted spectra are SM87 = 42,ooo f-0.72 CTA Seas A = 757,000 f-0.8 Whitfield, 1957 948 f-0.5 8 295 = CTA where S is i n 1o-26w m-2(c/s)-l and f is in Mc / s. This normalizing procedure should not be viewed as a correction of published flux den­ sities; rather it is a method of deriving the spectrum of the Cygnus Loop relative to that chosen for M87 . The adopted flux densities are plotted in Figure 1 . The inter­ pretation in terms of spectral index must rest on a subjective choice o~ which data to weight most heavily. MLH have suggested that Westerhout's flux density of the Cygnus Loop at 1390 Mc/s may be low if the weaker sections of the source were not seen. The same comment may be made of the 915 Mc / s value of Eaton and Kraus. Costain's measurement at 38 Mc / s employed a corner reflector with a 4oo beam to half power poi nts in decli nation. As HB21 is only 20° away, and the galaxy is even closer, his intensity measurement may suffer from con­ fusion. If the spectrum follows a simple power law as assumed in Figure 1, the spectral index is XCL = -0.10 ± 0 . 15. The error quoted for this value vas estimated by fitting lines of various slopes to the data of Figure 1, and is a probabl e error . The relative spectral i ndex of each part of the Cygnus Loop may be obtained by comparing the contour diagrams at 408 and 960 Mc/s. Fortuitously, our 960 Mc / s beamvidth i s almost identical to that of the 250' telescope at 408 Mc/s.
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