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K. J. JOHNSTON andJ. H. SPENCER E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, D.C. 20375

G. H. KAPLAN, W. J. KLEPCZYNSKI, andD. D. McCARTHY U.S. Naval Observatory, Washington, D.C. 20390

Abstract. A review of the accuracyof radio interferometric positions of extragalactic sources is given. A mean catalog of radio positions is presented. With the accuracycurrently attainable, radio observations can contribute significantlyto the determination of astronomical constants.

1. Introduction

In today's field of astrometry, one has to consider the entire electromagnetic spectrum. Radio astronomy has evolved quickly since the discovery of radio radiation from the galactic center in the early 1930's by Karl Jansky. Since this discovery, compact extragalactic radio sources have been found to exist whose positions can be measured precisely by radio interferometry. A catalog of the precise positions of these objects can establish a quasi inertial reference frame against which the dynamics of the earth, solar system and galaxy may be measured. The purpose of this paper is to evaluate the progress to date by radio astrometry in attaining this goal. There are a large number of surveys of radio sources which may be classified as radio catalogs. These surveys are at specific radio frequencies ranging from 80 MHz to 10 G H z and may cover the entire sky as seen from any one observatory. The high frequency (5 GHz) surveys of strong sources of Pauliny-Toth et al. (1978) and of Shimmins et al. (1975) contain almost all radio sources with fluxes greater than 0.6 Jy. Many of the sources in these catalogs have complex angular structure. The size of this angular structure ranges from >0"0001 to <60". Compact components of radio sources usually have flat spectra or have a slight increase in flux with increasing frequency. Extended components typically have steep spectra, with flux decreasing rapidly with increasing frequency. In the S (strong) surveys (Pauliny-Toth et al. 1978) there were 771 sources detected to a flux level of 0.6 Jy. Since these surveys covered a solid angle of 6.42 sr or roughly half the sky, we can expect a maximum of approximately 1500 sources of intensity/>0.6 Jy over the entire celestial sphere. Out of these 1500 sources, perhaps as few as 20% will have unresolved or pointlike components that will make them suitable as sources defining a radio reference frame. Thus the number of candidates for reference sources will be smaller than that now contained in the fundamental optical catalog (FK4) which contains 1535 fundamental stars. * Presented at the Symposium'Star Catalogues, Positional Astronomyand Celestial Mechanics', held in honor of Paul Herget at the U.S. Naval Observatory, Washington, November 30, 1978.

CelestialMechanics 22 (1980) 143-151. 0008-8714/80/0222-0143 $01.35. Copyright 9 1980 by D. Reidel Publishing Co., Dordrecht, Holland and Boston U.S.A.

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In addition to these general catalogs of continuum radio sources, there are specialized catalogs of radio sources associated with stars (Wendker 1978), pulsars (Taylor and Manchester 1975), and celestial masers (Dickenson 1976). To establish a radio reference frame, the radio sources should be extragalactic. These sources include radio galaxies, quasars, and BL Lacertae objects. This will be a distinct advantage over the FK4 in which the stellar p r o p e r motions are a m a j o r source of error. Galactic radio sources, while useful for relating a radio reference frame to an optical reference frame, are not useful for the establishment of a quasi inertial reference frame. These catalogs of stars, pulsars, and masers all consist of galactic objects.

2. Comparison of Catalogs There is a limited number of precise catalogs of extra-galactic radio sources. Available catalogs are those of Clark e t aL (1976), Elsmore and Ryle (1976), W a d e and Johnston (1977), and Fanslow (1978). Clark et aL (1976) and Wade and Johnston (1977) established their zero point in right ascension by adopting the position of 3C273B determined by H a z a r d et al. (1971) from lunar occultation timings. Elsmore and Ryle (1976) established their zero point relative to the FK4 position of fl Persei. Fanslow (1978) chose the position of N R A O 140 as the zero point. The position of N R A O 140 was established such that it minimized the difference between the optical and radio positions for those sources in the catalog having optical counterparts. Recently a new catalog has b e c o m e available, that of Spencer, Waltman, and Johnston (1979). This catalog also uses 3C273B to set the zero point of right ascension. Table I displays the average differences in position for TABLE I Weighted mean differences. Wade and Johnston (1977)- other catatogs Catalog

Sources in common

Aa

A8

Clark etaL (1976) Elsmore and Ryle (1976) Fanslow (1978) Spencer, Waltman and Johnston (1979)

17 17 10 10

-0.~0001:k 0.0006 -0.0060:~0.0015 -0.0032• 0.0002•

0'.'002+0.009 0.049• 0.012• 0.009:k0.016

sources in the above catalogs versus W a d e and Johnston (1977). The disagreement in right ascension is dominated by the selection of the zero point. We see that overall the agreement is quite good on the 0"01 level with the exception of the catalog of Elsmore and Ryle (1976). Elsmore and Ryle (1976) used an east-west interferometer whose accuracy in the determination of declination degrades near the

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equator. This can easily be seen from the relationship of the deviation of interf e r o m e t e r phase with error in source position: zl0 = - z l a (By cos Ss cos H s - B x cos Ss sin Hs)

- A S ( B x sin Ss cos Hs +By sin Ss sin Hs - B z cos S~)

(1)

where AO is expressed in turns (multiples of 2r radians), Bx, By, and Bz are the components of the baseline vector in wavelengths in a left handed topocentric coordinate frame, Ss is the source declination, and Hs is the hour angle of the source. Ac~ and AS are the error in source position in radians. Clark et al. (1976) used very long baseline interferometry employing a baseline that had a significant z component. Their catalog of source positions went as far south as the equator. Wade and Johnston (1977) extended the declination coverage to - 2 0 ~while Fanslow (1978) included sources as far south as - 4 0 ~ There has been a substantial evolution towards increasing accuracy in these catalogs. This is especially true for sources near the equator. Figure 1 displays the errors quoted in declination

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Er r o r in source declination as a function of declination. Note the errors in the earlier observations increase substantially near the equator.

for each source as a function of declination. One can see an increase in accuracy with the later catalogs of Wade and Johnston (1977) and Fanslow (1978) especially for sources near and below the equator.

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ET AL.

T h e r e a r e d i s c r e p a n c i e s in s o u r c e p o s i t i o n s b e t w e e n t h e s e c a t a l o g s which in s o m e cases m a y b e d u e to b a s e l i n e g e o m e t r y , s o u r c e s t r u c t u r e a n d t h e use of different astronomical constants. T h e l e n g t h of b a s e l i n e s used r a n g e f r o m - 4 k m to > 7 0 0 0 k m with o r i e n t a t i o n s r a n g i n g f r o m e a s t - w e s t to n o r t h - s o u t h . T h e s e differences in b a s e l i n e length a n d o r i e n t a t i o n c o r r e s p o n d to s p a t i a l sensitivities on t h e o r d e r of 30 s to 1 ms of arc. D i f f e r e n t f r e q u e n c i e s w e r e used. T h e s t r u c t u r e of r a d i o s o u r c e s varies as a f u n c t i o n of f r e q u e n c y . W h i l e a l m o s t all the s o u r c e s u s e d in a s t r o m e t r y a r e e x t r a g a l a c t i c t h e y d o s h o w s o m e s t r u c t u r e in the miUi arc s e c o n d level ( c o r r e s p o n d i n g to a few parsecs) as well as s o m e t i m e s o n t h e arc s e c o n d level (kiloparsecs). A g o o d e x a m p l e of this is given b y t h e s o u r c e 3C371 m a p p e d b y P e r l e y a n d J o h n s t o n (1979). A m a p of this s o u r c e at a f r e q u e n c y of 4 9 9 0 M H z is d i s p l a y e d in F i g u r e 2a, while the r a d i o r a d i a t i o n as a f u n c t i o n of f r e q u e n c y is d i s p l a y e d in F i g u r e 2b. This r a d i o s o u r c e

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Fig. 2a. Radio map of 3C371 at 4990 MHz made with the VLA. The contour intervals are 1% of the peak brightness to the 5% level with the 50% contour added to display the half power beamwidth. The coordinates A RA and zl DEC are the right ascension and declination offsets from the position of the compact radio source in seconds of arc. Fig. 2b. Radio spectrum of 3C371 in the frequency range 10-10 000 MHz. The solid line is the spectrum due to the dominant low frequency extended component while the dotted line is the spectrum of the compact source that dominates at high radio frequencies. consists of two m a j o r c o m p o n e n t s . A n i n t e n s e c o m p a c t c o m p o n e n t with a r e p o r t e d size of 0.1 m a s (Shaffer (1979)) w h o s e flux d o m i n a t e s at high r a d i o f r e q u e n c i e s a b o v e 5 G H z , a n d an e x t e n d e d c o m p o n e n t l o c a t e d - 3 " n o r t h - w e s t of t h e p o i n t - l i k e c o m p o n e n t w h o s e i n t e n s i t y d o m i n a t e s its r a d i o flux b e l o w 1 G H z . M e a s u r e m e n t s of this s o u r c e b y E l s m o r e and R y l e (1976) m a d e at 5 G H z with a r e s o l u t i o n of - 3 " a n d W a d e a n d J o h n s t o n (1977) m a d e at 11.1 cm with a r e s o l u t i o n of -0'.'6 h a v e a d i s c r e p a n c y of 0~.1 in right ascension. In this source a n d o t h e r s d i s c r e p a n c i e s in p o s i t i o n can b e r e s o l v e d b y m o r e c o m p l e t e s t u d y of t h e s p a t i a l a n d s p e c t r a l c h a r a c t e r i s t i c s of r a d i o sources.

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In addition to discrepancies in source positions arising from differences in observing techniques and source geometry, some other differences in position may arise from the use of different astronomical constants in the data reduction. Elsmore and Ryle (1976) reduced their data using accepted astronomical constants, while Wade and Johnston (1977) used the 'new' short period constants for nutation. It is not completely clear in all cases what values of astronomical constants have been used. Radio astrometric observations should use the accepted constants defined by the International Astronomical Union. This becomes especially important for observations obtained over a period of several years, such as the VLB observations of Clark et al. (1976) which were obtained from 18 separate sets of observations obtained between April 1972 and January 1975. Also radio catalogs should specify the mean epoch of the observations.

3. Fundamental Radio Catalog The catalogs can be used to form a weighted mean of their radio positions. We present here a catalog using the above-mentioned radio catalogs with the exception of Spencer et al. (1979). The values of source positions described by the various authors were offset in right ascension zero point by -0.~0046 for Elsmore and Ryle (1976) which was the average value for the common sources 0316 + 413, 0333 + 321, 0831 + 557, 1404 + 286, 1638 + 398, 1641 + 399, and 2200 + 420, and by -0.~0031 for Fanslow (1978) which was the average value for the common sources 0224 + 671, 0316+413, 0333+321, 0430+052, 0552+398, 0742+103, 0851+202, 0923+ 392, 1 2 2 6 + 0 2 3 , 1 2 5 3 - 0 5 5 , 1 6 3 8 + 3 9 8 , 1 6 4 1 + 3 9 9 , 1 7 3 0 - 1 3 0 , 1 7 4 1 - 0 3 8 , 2134 + 004, 2200 + 420, 2230 + 114, and 2251 + 158. No right ascension shift was applied to Wade and Johnston (1977) and Clark et al. (1976). This establishes the right ascension zero point through the position of 3C273B. The values listed were weighted by the errors quoted in the catalogs. In cases where the discrepancy in position exceeded the quoted accuracy by twice the quoted errors, the questionable positions were not included in the average. The catalog listed in Table II contains the I A U designation, alternate source name, 1950 right ascension and declination, a 6 cm flux density at time 1978.5, and the number of catalog positions that went into the average position. The errors quoted are the larger of the external or internal errors of the weighted average. Since there is a maximum of only four positions in each average, this is the best estimate of the errors. The source positions with their respective errors are similar to those reported by Elsmore (1978) who formed similar weighted mean positions from source positions reported in the literature. Figures 3 and 4 display the errors of the catalog as a function of right ascension and declination, respectively. Three sources have large errors in right ascension; namely, 1611 + 3 4 3 , 1807 + 698, and 2021 +614. Two of these sources have high declinations. If their errors are multiplied by cos 6, they are then considerably smaller. There seems to be little or no correlation of errors with right ascension, while in declination there appears to be some correlation with the errors in both coordinates. The error in

148

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TABLE II Catalog of mean radio source positions Source

Name

Ot(1950)

~(1950)

S

N

6 cm 1978.5 Jy

0224+671 0316+413 0333+321 0336-019 0355+508 0430+052 0552+398 0742+103 0831+557 0851+202 0923+392 1226+023 1253-055 1404+286 1611+343 1638+398 1641+399 1730-130 1741-038 1807+698 2021+614 2200+420 2230+114 2251+158

3C84 NRAO140 CTA26 NRAO150 3C120 DA193 4C55.16 OJ287 4C39.25 3C273B 3C279 OQ208 DA406 NRAO512 3C345 NRAO530 3C371 OW637 BLLac CTA102 3C454.3

02h24m41~.165 • 0~.001 03 16 29.567+0.002 03 33 22.406+0.001 03 36 58.954• 03 55 45.261• 04 30 31.603• 05 52 01.409• 07 42 48.466• 08 31 04.380• 08 51 57.253• 09 23 55.319• 12 26 33.247• 12 53 35.833• 14 04 45.616• 16 11 47.912• 16 38 48.174• 16 41 17.609m0.001 17 30 13.537• 17 41 20.620• 18 07 18.548• 20 21 13.299• 22 00 39.364• 22 30 07.811+0.002 22 51 29.523•

+67~ , 39':70• +41 19 51.90• +32 08 36.65--0.02 -01 56 16.86• +50 49 20.28• +05 14 59.58• +39 48 21.93• +10 18 32.63• +55 44 41.36• +20 17 58.37~:0.02 +39 15 23.57• +02 19 43.26• -05 31 08.01• +28 41 29.23• +34 20 19.83• +39 52 30.09• +39 54 10.82• -13 02 45.93• -03 48 49.02• +69 48 57.07::e0.01 +61 27 18,13• +42 02 08.58• +11 28 22.76• +15 52 54.31•

1.3 51.8 2.6 2.7 10.2 4.9 4.9 3.6 5.4 2.8 7.4 33.5 13.6 2.9 2.2 0.4 7.4 5.4 2.2 2.0 2.3 3.1 3.2 8.7

2 4 4 2 2 3 2 2 2 3 4 3 4 3 2 4 4 2 2 2 2 4 3 3

right a sc e n s i o n m a y a p p e a r to i n c r e a s e as a f u n c t i o n of d e c l i n a t i o n w h i l e t h e e r r o r in d e c l i n a t i o n d e c r e a s e s as a f u n c t i o n of declination. T h e cause of the effect in d e c l i n a t i o n has a l r e a d y b e e n discussed a n d is d u e to the large e r r o r s in s o u r c e position n e a r t h e e q u a t o r in t h e e a r l i e r catalogs. T h e c o r r e l a t i o n of right ascen si o n e r r o r with d e c l i n a t i o n is m a r g i n a l a n d is the result of the p o s i t i o n of O W 6 3 7 . R a d i o a s t r o m e t r y has b e e n t o u t e d as m e a s u r i n g large angles o v e r t h e sky with u n i f o r m accuracy. H o w e v e r , F i g u r e 4 d o e s n o t e x c l u d e a ' z o n a l ' d e c l i n a t i o n d e p e n d e n c e of accuracy. S o m e of this d e p e n d e n c e is d u e to the b a sel i n es an d the o b s e r v i n g t e c h n i q u e s u s e d in m e a s u r i n g t h e positions. I m p r o v e m e n t s in o b s e r v i n g t e c h n i q u e s has r e m o v e d m u c h of this effect as c o m p a r i s o n of Clark et al.'s (1976) e r r o r s with thos e of F a n s l o w (1978).

4. Astronomical Constants T h e e p o c h of o b s e r v a t i o n of t h e r e p o r t e d r a d i o catalogs is elusive. A s s t a t e d earlier, C l a r k et al. (1976) o b t a i n e d their o b s e r v a t i o n s f r o m 18 s e p a r a t e sets of o b s e r v a t i o n s

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Fig. 3. Errors in right ascension and declination of the sources in the mean catalog as a function of right ascension. The units of the errors in right ascension and declination are seconds of time and seconds of arc, respectively. The triangles are the error in right ascension reported for 3C271 and OW637 multiplied by cos 8. There appears to be no correlation.

distributed between April 1972 and January 1975. Elsmore and Ryle (1976) made their observations between October 1972 and March 1973 for 12 sources and between January and April 1974 for 43 sources. Wade and Johnston (1977) m a d e their m e a s u r e m e n t s in D e c e m b e r 1974, February 1975, and January 1976. Fanslow (1978) p e r f o r m e d his observations between 1971 and 1977 making - 1 8 sets of observations. The epoch of the mean catalog was established by the mean epoch of the catalogs which is 1973.8. The epochs of radio catalogs should be stated and the ~bservations reduced to a m e a n epoch. The mean source position accuracy is 0'.'02. As the new I A U (1976) constant of ~recession is estimated to be known to an accuracy of approximately 0.1 arcseconds ~er century or 0.001 arcseconds per year, second epoch observations taken now ;hould yield an independent determination of the precessional constant as pointed )ut by Elsmore (1976), from consideration of his observations. Since the radio ;ources are extragalactic, this value of precession will be independent of the errors 9ontributed by the stellar proper motions in the optical determination. This should ;naMe improved values for the constants of galactic rotation and solar motion from a eanalysis of the optical data. However, in order to solve for and eliminate the

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Fig. 4. Errors in right ascension and declination of the sources in the mean catalog as a function of declination. There is a decrease in accuracy in declination near the equator and in right ascension near the pole. c o n s t a n t s arising f r o m n u t a t i o n , t h e m e a s u r e m e n t s s h o u l d s p a n a t i m e r a n g e c o m p a r a b l e to t h e l o n g e s t n u t a t i o n p e r i o d of 18.6 years.

5. C o n c l u s i o n s I m p r o v e m e n t s m a y b e m a d e in the a c c u r a c y a c h i e v a b l e b y r a d i o t e c h n i q u e s w h i c h m a y a p p r o a c h t h e milli a r c s e c o n d level. H o w e v e r at this level v a r i a t i o n s in the s p a t i a l s t r u c t u r e of r a d i o sources d u e i n h e r e n t l y to the e n e r g y m e c h a n i s m s giving rise to t h e s e s o u r c e s m a y d i c t a t e t h e limit w h i c h can b e r e a c h e d . A t the level of a c c u r a c y of p r e s e n t o b s e r v a t i o n s , the i m p o r t a n c e of a l o n g series of r a d i o a s t r o m e t r i c o b s e r v a t i o n s c a n n o t b e o v e r e m p h a s i z e d . This will l e a d to i m p r o v e d a c c u r a c y in o u r k n o w l e d g e of t h e values of the a s t r o n o m i c a l c o n s t a n t s of p r e c e s s i o n , n u t a t i o n , a n d t h o s e of galactic r o t a t i o n a n d s o l a r m o t i o n as well as e s t a b l i s h e d a q u a s i inertial r e f e r e n c e f r a m e d e f i n e d b y e x t r a g a l a c t i c r a d i o sources.

References Clark, T. A., Hutton, L. K., Marandino, G. E., Counselman, III, C. C., Robertson, D. S., Shaprio, I. I., Wittels, J. J., Hinteregger, H. F., Knight, C. A., Rogers, A. E. E., Whitney, A. R., Niell, A. E., R6nn~ing, B. O., and Rydbeck, O. E. H.: 1976, Astron. J. 81, 599.

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Dickenson, D. F.: 1976, Ap. Y. Supp. 30, 259. Elsmore, B.: 1976, M.N.R.A.S. 177, 291. Elsmore, B. and Ryle, M.: 1976, M.N.R.A.S. 174, 111. Elsmore, B.: 1978, Proc. of IAU Colloquium #48 "Modern Astrometry". Fanslow, J.: 1978, private communication. Hazard, C., Sutton, J., Argue, A. N., Kenworthy, C. M., Morrison, L. V., and Murray, C. A.: 1971, Nat. Phys. Sci. 233, 89. Pauliny-Toth, I. I. K., Witzel, A., Preuss, E., Kiihr, H., Kellermann, K. I., Fomalont, E. B., and Davis, M. M.: 1978, Astron. 9". 83, 451. Perley, R. and Johnston, K. J.: 1979, Astron. J., 84, 1247. Shatter, D.: 1979, private communication. Shimmins, A. J., Bolton, J. G., and Wall, J. V.: 1975, Aust. J. Phys. Suppl. Set. 34, 63. Spencer, J. H., Waltman, E. B., and Johnston, K. J.: 1979, in preparation. Taylor, J. H. and Manchester, R. N.: 1975, Astron. J. 80, 794. Wade, C. M. and Johnston, K. J.: 1977, Astron. Y. 82, 791. Wendker, H. J.: 1978, Pub. of Hamburg Observatory, Vol. 10, No. 1.