Contains a report on a research project.National Science Foundation (Grant G-13904)National Aeronautics and Space Administration (Contract Nasr-101)National Radio Astronomy Observator

Weinreb, S.

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VIII. RADIO ASTRONOMYProf. A. H. Barrett J. W. Barrett J. R. CummingsProf. J. W. Graham V. J. Bates D. H. StaelinA. T. Anderson III J. C. Blinn III S. WeinrebV. K. ChungA. AN ATTEMPT TO MEASURE ZEEMAN SPLITTING OF THE GALACTIC21-cm HYDROGEN LINE1. IntroductionThe existence of a galactic magnetic field is presupposed by theories describing thepolarization of starlight, the emission of galactic radio noise, and the behavior of cos-mic rays. The magnitude of the field cannot be measured directly from these effects,although estimates of from 10-5 gauss to 10-6gauss are common. These facts are dis-cussed and referenced by Shklovsky, l and also in two papers, 3 that describe previousattempts to measure the field by measuring the Zeeman splitting of the 21-cm hydrogenline. The experiment was first suggested by Bolton and Wild. 4A brief description of the Zeeman effect on hydrogen line radiation may be given asfollows. The line-of-sight component of the galactic magnetic field causes a splitting ofthe radiation into left-hand and right-hand circularly polarized waves with a difference-6in frequency of 2. 8 cps per 10 - 6 gauss. The sharpest hydrogen lines found in absorp-tion of the strong discrete sources are ~10-20 kc wide, and thus the expected splitting of3-30 cps is quite difficult to measure. The procedure consists of alternately measuringthe hydrogen profile with feeds that are receptive to circular polarization of oppositesense and recording the difference profile. This difference profile, AT(f), is relatedto the observed (either polarization) profile, T(f), by the following relation which holdsif the frequency splitting, Af, is small compared with the width of the observed line:AT(f) = Af - T'(f) = 2.8 X 10 6 H - T'(f),where H is the magnetic field, and the prime denotes derivative with respect to fre-quency. The observed profile and the expected difference profile for the two observedsources, Cas A and Taurus A, are shown in Figs. VIII-1 and VIII-2.2. Experimental ProcedureObservations of Cas A and Taurus A were performed with the use of the 85' HowardTatel Radio Telescope at the National Radio Astronomy Observatory, Green Bank,West Virginia, in conjunction with a dual circular polarization feed built by Jasik Labo-ratories. Switching between polarizations was done at a 1/14 cps rate by using a*This work was supported in part by the National Science Foundation (Grant G-13904);and in part by N. A. S. A. Contract Nasr-101.CAS AOBSERVED PROFILE - CAS AZEEMAN PROFILEFOR 5 x 10- 6 GAUSS0 .260DATA ADATA A THEORETICAL RMSDEVIATION = 0.0 730THEORETICAL RMSDEVIATION = 0.053010 K C -20 KCDATA B0.20FREQUENCY -.Fig. VIII- 1. Results of attempt to measure Zeeman splitting of 21-cm galactichydrogen line observed in absorption of Cas A radio source.0.20-0.20F0.20TAURUS AOBSERVED PROFILE - TAURUS A600ZEEMAN PROFILETHEORETICAL RMSDEVIATION = 0.0440DATA ATHEORETICAL RMSDEVIATION = 0.080DATA B70 KC -60 KC -50 KC -Q KCCFREQUENCY\-30KCFig. VIII-2. Results of attempt to measure Zeeman splitting of 21-cm galactichydrogen line observed in absorption of Taurus A radio source.0.20-0.2 °0.20-0. 200.20(VIII. RADIO ASTRONOMY)mechanical coaxial switch. Double stub tuners were inserted between each arm of theswitch and the feed, and were adjusted so that the standing-wave ratio of each polariza-tion feed was less than 1.01 when measured through the switch.An electron-beam, traveling-wave, parametric amplifier was used for all observa-tions except for those of Taurus, Data B, for which a crystal mixer was used. Thesingle-channel noise temperature, including switch and stub losses, was 6000 for theparametric amplifier, and 10800 for the crystal mixer.Through conventional means a 50-kc portion of the band is filtered out and shifteddown to low frequencies at which its autocorrelation function is found by a digital cor-relator. 5 A computer Fourier transform of the autocorrelation function then gives thepower spectrum. This system is analogous to a multichannel radiometer having 7.5-kcwide filters spaced over the 50-kc band.The observational procedure consisted, first, of making a 10-minute observation ofthe absorption profile, with the use of the right-hand circular polarization feed. Typicalresults are the top curves in Figs. VIII-1 and VIII-2. A loss in sensitivity or a frequencyerror could be found in this manner. Next, after this run, a 5-hour run for measuringthe difference profile was made. The local oscillator was reset every half hour to cor-rect for Doppler shift resulting from the earth's motion. One or two 5-hour runs weremade on Cas A and Taurus A each day for approximately 35 days.3. ResultsA difficulty that arises in performing the Zeeman experiment is due to small dif-ferences in gain between the right-hand and left-hand circular polarization feeds. If thegain of one feed is 1 + a times the gain of the other feed, then the difference profile,AT(f), will contain a term a - T(f); the absorption profile will appear in the data. Sincethe absorption profile is known, it may be removed from the difference profile. However,care must be taken that the correction is not shifted in frequency, for this would causea false Zeeman effect. It is easily seen that if an unbalance signal, a - T(f), is correctedby subtraction of a profile, a - T(f+6f), then the result is the same as that which wouldbe found for a Zeeman splitting of value a - 6f.Approximately 20 per cent of the data that were taken showed values of a less than0. 003 (Data A); 40 per cent had a between 0. 003 and 0. 03 (Data B); and the rest hada greater than 0. 03 and were not used. The variations in a are probably due to temper-ature effects on the feed, stubs, and switch, and to tracking errors and flexure of thetelescope. The largest error in the local oscillators was less than 100 cps; thus evenwith a = 0. 03, the largest spurious Zeeman effect would be 3 cps, or ~10 - 6 gauss.The average of the Data A and Data B runs for Cas A and Taurus A are plottedin the figures. The Data A runs require no correction for feed unbalance, while theTable VIII-1. Zeeman results.Theoretical Actual Total Profile Observed Treatment of DataRMS RMS Noise Depth TimeFluctuation Fluctuationt Temperature TT (sec)Cas .0730 .099 0  10600 1800 1.2 x 105 No correction to dataData A 1.4 1. 9Cas . 0530 . 12 °  10600 1800 2. 1 X 105 Absorption profile ofData B 1.0 2.3 1.370 peak subtractedfrom raw dataTaurus .0440 .040 8500 600 2.2 X 105 Small slope correctionData A 2.2 2. O0 of 0. 120/10 kcTaurus .080 .050 13300 600 1.6 X 105 Absorption profile ofData B 4.0 2. 5 1.150 peak subtractedfrom raw dataAT = 2. 1 T/ B, where T = Antenna + Receiver Noise Temperature, B = 7. 5 X 103, and T is theObservation Time. (The quantity below AT is the magnetica peak signal = AT.)field in units of 10- 6 gauss which will givetAT' = rms value of data, with the mean taken over frequency.(VIII. RADIO ASTRONOMY)Data B runs have been corrected as indicated in Table VIII-1. Our conclusion is thatthe line-of-sight component of the magnetic field is less than 3 X 10 - 6 gauss in Cas A,-6and less than 5 X 10 gauss in Taurus A.This work was supported in part by the National Radio Astronomy Observatory.S. WeinrebReferences1. I. S. Shklovsky, Cosmic Radio Waves (Harvard University Press, Cambridge,Mass., 1960), pp. 182-190.2. J. A. Galt, C. H. Slater, and W. L. H. Shuter, An attempt to detect the galacticmagnetic field using Zeeman splitting of the hydrogen line, Mon. Not. Royal Astro.Soc. 120, 187-192 (1960).3. R. D. Davies, C. H. Slater, W. L. H. Shuter, and P. A. T. Wild, A new limitto the galactic magnetic field set by measurements of the Zeeman splitting of the hydro-gen line, Nature 187, 1088-1089 (September 24, 1960).4. J. G. Bolton and J. P. Wild, On the possibility of measuring interstellar mag-netic fields by 21-cm Zeeman splitting, Astrophys. J. 125, 296 (1957).5. S. Weinreb, Digital radiometer, Proc. IRE 49, 1099 (1961).

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