Contains reports on three research projects.National Aeronautics and Space Administration (Grant NGL 22-009-016)National Science Foundation (Grant GP-8415)Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E

Barrett, Alan H.

Burke, Bernard F.

Ewing, M. S.

Price, R. M.

Schwartz, P. R.

DSpace@MIT

V. RADIO ASTRONOMYAcademic and Research StaffProf. A. H. Barrett Prof. R. M. Price D. C. PapaProf. B. F. Burke J. W. Barrett C. A. ZapataGraduate StudentsM. S. Ewing P. C. Myers J. W. WatersW. W. Gebhart G. D. Papadopoulos A. R. WhitneyL. P. A. Henckels P. W. Rosenkranz T. T. Wilheit, Jr.H. F. Hinteregger P. R. Schwartz T. L. WilsonC. A. Knight W. J. WilsonA. PULSAR FACILITYThe feeds for 16 dishes of the pulsar facility have been completed and put in place.All transmission lines from the feeds to the ground are installed.Four of the dishes in a North-South line have been phased to give a beam at thedeclination of the Crab Nebula for test purposes. Tests of the phasing errors andimpedance matching of the transmission lines are in progress.RFI tests at the site indicate that frequencies near 142, 146, and 148 MHz will bebest for avoiding interference. Modifications to the receiver to allow operation overthis frequency range are being undertaken.B. F. Burke, R. M. Price, M. S. EwingB. VARIATION OF SIX RADIO SOURCES AT FOUR ANDTWO CENTIMETERSDuring 1967 and 1968, an investigation of the variability of continuum radio sourcesat 4 cm and 2 cm wavelengths was conducted at the Haystack Microwave Facility ofLincoln Laboratory, M. I. T. The flux of 24 radio sources relative to 2 calibrationsources was measured at various times using observing techniques designed to eliminateall but the intrinsic secular variation of the objects. The well-known variable sources(Dent, l Kellerman and Pauliny-Toth2 ) were observed on a regular basis betweenJune 1967 and September 1968 at 2 cm, and more sporadically at 4 cm for the entireperiod.This work was supported principally by the National Aeronautics and Space Admin-istration (Grant NGL 22-009-016) and the National Science Foundation (Grant GP-8415);and in part by the Joint Services Electronics Programs (U. S. Army, U. S. Navy, andU. S. Air Force) under Contract DA 28-043-AMC-02536(E).QPR No. 94(V. RADIO ASTRONOMY)1. EquipmentThe Haystack antenna is a 37-m paraboloid with Cassegrainian optics on an altazi-muth mount, enclosed in a radome. The real antenna efficiency, including radomeeffects, is 45% at 4 cm and 35% at 2 cm, and the system has operated successfully atwavelengths between 18 cm and 0. 8 cm. Coordinate conversion, pointing, coordinateprecession, refraction, and systematic pointing corrections, and real-time data pro-cessing is provided by a Univac 490 computer. The rms antenna tracking error is0. 0030 and the real pointing errors are less than this.The radiometer and feed configurations that were used are summarized in Table V-l.The radiometers were Dicke-switched against a room temperature load with balancenoise injection or operated against an off-axis beam separated by 0. 250 from the mainbeam.Table V-1. Radiometer Configurations.Feed ConfigurationFront End Frequency T S  Bandwidth and Polarization1. Tunnel Diode 15. 5 2500 0 K 1. 0 GHz (1) Variable linear horn,load switched (3)2. Tunnel Diode 15. 5 900 0K 1. 0 GHz (1) or Variable linear horn,1. 2 GHz (2) beams switched (4)3. Tunnel Diode 8. O0 1200 0 K 1. 0 GHz (1) Variable linear clavin,load switched (3)4. Maser 7. 8 40 0 K 15 MHz Left circular horn,load switched(1) Two 500-MHz channels.(2) Single channel after June 1968.(3) June 1967 only.(4) After November 1967.2. Observing Techniques and Instrumental EffectsIn making absolute flux measurements, the program has been one of making a rela-tively large number of independent observation of calibration sources, whose flux iswell known, and of known or suspected variable sources. If all source sizes aremuch smaller than the antenna beam, this technique eliminates the need for knowledgeof the exact antenna gain and pattern. The major sources of error in this program arefluctuations of radiometer and antenna gain between observations of calibrators,pointing errors, and atmospheric losses.QPR No. 94(V. RADIO ASTRONOMY)Radiometer gain fluctuations may be overcome by frequent gas-tube calibrations. Ithas been the practice to calibrate every time there is a source change, since the masersystem gain is sensitive to antenna elevation angle, or every 20 minutes. In prac-tice, typical radiometer gain drifts have been less than 1% for the tunnel-diode ampli-fiers and less than 3% for the maser. Antenna gain and pattern variations have twoorigins: position- dependent gravitational deformation, and temperature -dependentthermal deformation of the antenna surface and backup structure. Careful study of thepositional dependence of the gain, an effect that reaches 15% with the Haystack antenna,by repeated observations of a number of point sources has resulted in the calibration ofthe elevation dependence for all angles above 10" and below 800, arid no azimuthal depen-dence has been found. The thermal effect was found to be much more complex, dependingupon the inside temperature of the radome and its time derivatives. Temperature controlof the radome seems to eliminate the effect, except during periods of very hot weatheror rapid temperature variation. By frequent observation of calibrators, at least every4 hours, and total avoidance of the worst periods during the summer and near sunriseand sunset, we hope that the thermal effect has been eliminated.Haystack pointing errors have been well calibrated and are automatically removedby the pointing system. Minor pointing and source position errors are eliminated bymanually inserting corrections derived from the use of the Discrete Source Scan (DSS)pointing routine (Allen 3 ). In the usual observation of a source there is a calibrationsequence, a DDS, and then a series of "on-offs" comprising a 90-sec source integra-tion, a 30-sec blanking period for antenna repositioning, and a 90-sec comparisonintegration at the same azimuth and elevation as the "on".The effects of variable atmospheric loss are countered by hard adherence to a ruleof no observations on days with more than 25% cloud cover and no observations below300 elevation. Also, a serious attempt is made to observe sources and calibrationsat the same elevation angles as close together in time as possible. Atmosphericattenuation corrections derived from observations of sea-level temperature, pressure,and water vapor taken every 4 hours are applied to the data in post processing (Allen3).3. ResultsThe flux of 6 known variable sources relative to 2 calibration sources, corrected foratmospheric loss and gain variations, is plotted in Figs. V-1 and V-2. The positions andidentifications of the 6 sources plus the 2 calibration sources, 3C274 and W75 (DR21),are given in Table V-2. Each data point represents the average of at least 4 sets ofobservations taken during a two-week period. Each set included measurements at 2orthogonal polarization angles or in the left circular polarization. The error bars arethe rms of the averaging.The sources represented here serve to illustrate the general characteristics of radioQPR No. 94Fig. V-1. Flux3C843C1203C2731966of three variable1967 1968sources relative to calibration sources.1515 -S105/-f25Fig. V-2. Flux of three3C454.32 cm4 cm1967 1968variable sources relative to calibration sources.QPR No. 94I- __ __ __2 cmS4 cmi /\\vll\ +"R~,Radio sources observed.Source Type R.A. 1950.0 Dec. 1950 Z (Red Shift) Flux7. 8 GHz 15. 5 GHzH M S D M S3C84 S. G. 3 16 29. 4 41 19 59.0 0.023C120 S. G. or N. G. 4 30 30.7 5 14 25.0 0. 033C273B Q.S.O. 12 26 33.3 2 19 42.0 0.163C279 Q. S. O. 12 53 35. 9 -5 31 14.0 0.543C345 Q. S. O. 16 41 17.6 39 54 10. 6 0. 603C454. 3 Q. S. O. 22 51 29. 5 15 53 11. 0 o. 863C274 12 28 18.4 12 39 57.6 42.3 28. 7W75 DR21 20 37 14. 0 42 9 0 19.5 20. 9S. G. = Seyfert galaxy.N. G. = N-type galaxy.Q. S. O. = Quasi-stellar object.Table V-2.(V. RADIO ASTRONOMY)variability: the relative dissimilarity of the types of objects involved, positive spectralindex in the variable region of the spectrum, small angular size, and the amplitude andtime scale of the variation. Furthermore, the observed double outburst of 3C120 seemsto bear a remarkable similarity to the observed behavior of 3C273, which experienceda spectacular outburst in middle and late 1966, and possibly to the observed variationof 3C345. The sources 3C84, 3C279, and 3C454.3 exhibited more gradual increasesto maxima in the period 1965-1968, and have not yet exhibited this multiple burst struc-ture. Thus this "ringing" may be related either to the amplitude or the time scale ofvariation of the source and, in any case, is strikingly reminiscent of the behavior of aperturbed resonant system with strong damping.P. R. Schwartz, A. H. BarrettReferences1. W. A. Dent, Science 148, 1458 (1965).2. K. I. Kellerman and I. I. K. Pauliny-Toth, Annual Reviews of Astronomy andAstrophysics, Vol. 6, 1968.3. R. J. Allen, Ph. D. Thesis, Department of Physics, M. I. T., January 1967.C. FURTHER OBSERVATIONS OF GALACTICWATER-VAPOR EMISSIONObservations of water vapor emission sources, begun in February and March, 1969,at the Lincoln Laboratory Haystack facility, '1,2 were continued in April and May usingthe 140-ft telescope of the National Radio Astronomy Observatory at Green Bank,West Virginia. Although the reduction of all of the data is not complete, several newresults concerning time variation, linear polarization, and spectral dispersion in thesesources can be reported.For the observations, the 140-ft telescope was equipped with a Dicke-switched mixerradiometer and linear horn feed, the IF signal was fed into the 400 N. R. A. O. channelautocorrelation receiver and the resulting power spectra were recorded on magnetictape, displayed real time on a storage oscilloscope, and post real time on a digital plot-ter. The system temperature was approximately 2400°K for single-sideband spectral lineobservations at 22 GHz. The aperture efficiency of the 140-ft telescope was found to bearound 10% on the meridian and strongly dependent upon local hour angle; consequently,the observations were usually limited to 3-4 h around source transit. The antenna beam-width was 1. 6 minutes of arc. For polarization measurements, the feed was orientedso that the E plane made an angle of 0, 450, 900 or 1350 to celestial North-South direc-tion; all other observations were made with the E plane North-South.The H20 Sources W30H, G133.7-1.2 (W3 Continuum), W490H, Orion A, andQPR No. 94(V. RADIO ASTRONOMY)VY Canis Majoris were observed on a daily basis with resolutions of 12.5 and 6.25 kHz;the sources W51OH, W75(S)OH, and W24(B2)OH were observed less frequently. Attemptswere made to measure the linear polarization of the first seven sources. Two of thesources observed, W30H and VY Canis Majoris, showed no evidence of variation duringthe month of observation, nor, for that matter, any significant change from earlierobservations in January and February 3 and February and March. 2 Both sources, how-ever, seemed to have small but significant linear polarization. For W51OH andW75(S)OH the analysis is not complete, but the shape of the W75(S)OH spectrum appearschanged from February. This source appears to be slightly linearly polarized.W51OH appears to have also changed since February, but this variation may be a resultof the higher frequency resolution used at N. R. A. O. The source G133.7+1.2 varied sig-nificantly during the month of April and is drastically changed from its appearance inFebruary; in particular, the strong features near 10 km/sec observed in February arecompletely absent in April. No significant linear polarization could be detected in thissource. In Fig. V-3, representative spectra for these 5 sources are shown.The strongest H20 source, W490H, has been shown to be highly variable in time andto possess an extremely high dispersion in radial velocity of features extending from-140 to +140 km/sec. 3 In particular, a strong line near 140 km/sec was observed todisappear over the space of three weeks in February-March.Z In Fig. V-4 time vari-ations in the main features observed during a one-month period, and rapid and complexchanges in line intensity and width are displayed. The high-velocity spectrum ofW490H is shown in Fig. V-5 with the 140 km/sec feature greatly weakened but present,and with even higher velocity features also present. Both the main and the low-velocity features of this source were found to be less than 5% linearly polarized. 2 ' 3The H20 source in Orion A has already been shown to be linearly polarized. 2 ' 3Time variations of two features of this source were found, including the very rapidappearance of a narrow, isolated, bright line that seems to be analogous to the 140 km/sec feature seen in W490H in February. It is interesting to note that these two variablefeatures are apparently unpolarized. Time variation and polarization of Orion A areshown in Fig. V-4.Although it is much too early to make strong generalizations about H2 0 emission,the data seem to indicate a tendency for variable features to be unpolarized and slightlynarrower than the steady, linearly polarized features, and for variability to be associ-ated with sources with a higher degree of velocity dispersion. If, as has been suggested,H20 emission is the result of some stimulated emission process in a circumstellarenvelope or near a star-formation region, the observed time variations may result fromthe effects of turbulence on an unsaturated maser process either through small changesin the density or length of the amplifying path or through changes in the orientation of ahighly beamed maser with respect to the observer. In any case, two approaches to theQPR No. 94040 40W510H 300 VY CANISMAJORIS200 -2 20o 10000-10 20 30 4(50 60 70 80 90 VR(km/s)VR(km/s)UlO100 - 100 -500 - W30H W75(S)0H G 133.7+1.250 50 -" 250 -0 0--60 -50 -40 0 10 -40 -30VR(km/s) VR(km/s)Fig. V-3. Water vapor emission from sources W510H, VY Canis Majoris, W30H,W75(S)OH and G133.7+1. 2, N. R. A. O. 140-ft telescope, April 1969.4/8/69THIS FEATURENOT PRESENTIN FEB 19694/16/691000 L750 -500 -250 - 4/26/69-10 0 10 20 30 40RADIAL VELOCITY (km/s)Fig. V-4. Time variation in the H 0 spectrum of W490H, N. R. A. O.140-ft telescope, April 1969.QPR No. 94 51400 LHAYSTACK, FEB 1969100 f-I I I i100 120 140 160VR(km/s)100 120 140 160 180 200RADIAL VELOCITY (km/s)Fig. V-5. High-velocity spectrum of W490H, in February 1969, at Haystack(small figure) and, in April 1969, N. R. A. O. 140-ft telescope.NRAO APRIL 196950-25z 0zz4/14/694/23/69500 L- ~E PLANE N-S--- E PLANE E-W250 [-4/25/69RADIAL VELOCITY (km/s)Fig. V-6. Time variation and polarization in the H20 spectrum of Orion A.QPR No. 94 53(V. RADIO ASTRONOMY)H 2 0 problem are indicated: (i) more careful studies of the relationship between varia-bility and polarization must be carried out, and (ii) the angular size, relative andabsolute positions of variable and nonvariable features must be determined, since thesequantities might be expected to be markedly different for saturated and unsaturatedemission.We would like to thank our co-workers in this experiment: Professor David H.Staelin and Mr. Joseph W. Waters, of M. I. T., Dr. David Buhl, Dr. Lewis Snyder,Dr. Edward T. Reifenstein, Dr. Robert Rubin, Dr. Barry Turner, Dr. Peter Mezger,of the National Radio Astronomy Observatory, and Mr. Edward Churchwell, of theUniversity of Indiana, for their efforts and advice, and also Mr. Dorsey L. Thacker,of N. R. A. O. , for engineering assistance.A. H. Barrett, P. R. SchwartzReferences1. A. H. Barrett, P. Rosenkranz, P. R. Schwartz, J. W. Waters, W. J. Wilson,and C. A. Zapata, Quarterly Progress Report No. 93, Research Laboratory ofElectronics, M.I.T., April 15, 1969, p. 29.2. M. L. Meeks, J. C. Carter, A. H. Barrett, P. R. Schwartz, J. W. Waters, andW. E. Brown III, "Observations of Interstellar Water Vapor" (to appear in Science).3. S. H. Knowles, C. H. Mayer, A. C. Cheung, D. M. Rank, and C. H. Townes,Science 163, 1055 (1969).QPR No. 94

Radio Astronomy

http://dspace.mit.edu/bitstream/1721.1/55881/1/RLE_QPR_094_V.pdf

Radio Astronomy

Abstract

Similar works

Full text

Available Versions

DSpace@MIT