Significant Achievements in Space Astronomy 1958-1964

8/7/2019 Significant Achievements in Space Astronomy 1958-1964

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NASA SP-91

Space Astronomy1958-1964

Scientifir and Technical lnfwmrrtion Division 1 9 6 6NATIONAL AERONAUTICS AND SPACE ADMINISTRAnONWarhingta, D.C.


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FOR SALE BY THE SUPERINTENDENT O F DOCUMENTS. U . S . GOVERNMENT PRINT INGOFFICE, WASHINCTON. D.C. . 20402 - PRICE 4 5 CENTS


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Foreword

HIS VOLUME IS ONE OF A SERIES which summarizeThe progress made during the period 1958 through1964 in discipline areas covered by the Space Scienceand Applications Program of the United States. In thisway, the contribution made by the National Aeronauticsand Space Administration is highlighted against thebackground of overail progress in each discipline. Suc-ceeding issues will document the results from later years.

The initial issue of this series appears in 10 volumes(NASA Special Publications 91 to 100) which describe-the achievements in the following areas: Astronomy,Bioscience, Communications and Navigation, Geodesy,Ionospheres and Radio Physics, Meteorology, Partidesand Fields, Planetary Atmospheres, Planetology, andSolar Physics.

Although we do not here attempt to name those whohave contributed to our program during these first 6years, both in the experimental and theoretical researchand in the analysis, compilation, and reporting of results,nevertheless we wish to acknowledge all the contributionsto a very fruitful program in which this country maytake justifiable pride.

HOMER.NEWELLAssociate Adm inistrator for

Space Science and Applications, NASA

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PrefaEeTELLAR ASTRONOMY IS DEFINED aS hChding dlS galactic and extragalactic objects except those nor-mally associated with the solar system (the Sun is ex-cluded from this discipline) and is divided according tomethodology into gamma-ray and X-ray astronomy,

ultraviolet astronomy, infrared astronomy, and radio(millimeter and kilometer wave) astronomy. Study ofstars and galaxies at visual wavelengths would normallybe included in this category also;however, no work has- -been done in this area in space during the-period 1958-1964.In all the fields covered in the astronomy discipline,the &year period from 1958 to 1964 has been one ofdiscovery and development of the tools and techniquesof space astronomy in contrast to the extensive, detailedobserving programs of ground-based astronomy. Theuse of instruments carried above the Earth's atmospherehas enabled astronomers to study parts of the electro-magnetic spectrum which before had been inaccessibleto them. Significant consequences to astrophysical theoryhave already occurred from the new information, result-ing in a more accurate knowledge of the evolution ofthe universe.

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ContentsPIlge

INTRODUCTION ................................... 1.........-RAY AND GAMMA-RAY ASTRONOMY . d "IX-Ray Astronomy .................................. A+ 13Gamma-Ray Astronomy ............................. a2%

ULTRAVIOLET AND NFRARED ASTRONOMY ...... 3.rk /--1r /?- /

Frequencies ...................................... W ?

..............................Ultraviolet Astronomy H 2 c IInfrared Astronomy ................................ 4813LOW-FREQUENCY RADIO ASTRONOMY ............

Appearance of the Radio Sky at Low and HighRestrictions on Space Radio Observation .............. 5+ 5%Low-Frequency Measurements from Space ............. 5P '3Plans for the Future ................................ 64 b 2....................SUMMARY AND CONCLUSIONS c0bSX-Ray and Gamma-Ray Astronomy ................... 67 @Ultraviolet and Infrared Astronomy .................. 6@6bRadio Astronomy .................................. ~46\

REFERENCES .......................................

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Introduction

STRONOMY, THE OLDEST OF THE SCIENCES, probablyA has more to gain from the use of space technologythan any other science. A better understanding of theuniverse-its birth, history, and future-depends uponanswers to the fundamental questions of stellar evolutionand galactic structure. To answer these questions fully,a study must be made of information from all parts ofthe electromagnetic spectrum: gamma rays, X-rays, andultraviolet, visible, infrared, and radio radiation (fig. 1 .Electromagnetic radiation can be characterized by itswavelength, by its frequency, or by the quantum of en-ergy, hv, which is emitted or absorbed when radiation ofa given frequency interacts with matter. Thus a quan-tum of energy of 1 electron-volt is associated with awavelength of 1.2396X lo-' centimeters or a frequencyof 2.4184X lo" cycles per second.

Gamma rays and X-rays from celestial objects are com-pletely unobservable from the Earth's surface. The firstobservations of this part of the electromagnetic spectrumwere obtained from rocket flights in 1949 when X-raysemitted by the Sun were detected. Since then, steadilyimproving rocket methods have been used to map thesky and to search for other discrete sources of X-rays andgamma rays. As of 1964, there were 10 known galacticX-ray sources, which were far brighter than expected onthe basis of the intensity of the X-rays emitted by the Sun.

Gamma-ray astronomy has been less fruitful thanX-ray astronomy because the sources are weaker and

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. *I N T R O D U C T I O Ntherefore more difficult to detect. Instruments to detect 'very faint sources of gamma rays were developed, pri-marily by physicists, during the period from 1948 through1964. It was concluded from the results of initial experi-ments that a diffuse gamma-radiation background doesexist in the galaxy; however, no discrete sources havebeen discovered as yet.

The early-type supergiant stars, though less numerousthan older stars such as the Sun, are of great importancein cosmological studies because of their youth and asso-ciation with the hydrogen gas of the galaxy. Since mostof their radiation occurs at short wavelengths unobserv-able from the Earth's surface, it was quite natural thatthe first rocket work in stellar astronomy was on this partof the spectrum. These studies were at first limited tomapping the sky in the ultraviolet region from 1225 to1350 ii.

There have been extensive photometric studies of hotStars. The intensity of these stars in the ultraviolet wasmuch lower than had been predicted. Resolution of thisdiscrepancy has led to a revision of the stellar temperaturescale.

Balloon observations of celestial objects in the infraredconfirmed the few existing ground-based results. Thelimitations of available instruments restricted the initialefforts to bright objects such as planets. However, by1964 detector design had so improved that some of thebrightest red supergiant stars could be observed : theywere found to have water-vapor bands at 1.4 p and 1.9 p.

Absorption by oxygen and water vapor in the tropo-sphere sets a high-frequency limit on ground-based radioastronomy. At the low-frequency end, the cutoff is deter-mined by reflection, absorption, and scattering in theionosphere. Limitations on the size of antennas whichcould be carried on rockets and satellites have restricted

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SPACE ASTRONOMY

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SPACE A S T R O N O M Yradio observations to coarse resolution. As a result thy ,detected signals are actually composites of those from thegalaxy, extragalactic sources, planets, and the Sun. Theaverage cosmic-noise background shows a decrease at longwavelengths as predicted by astrophysicists.Measurement of radio flux at high frequencies is notpossible at present because of the lack of suitable instru-ments. The first reported low-frequency satellite radio-astronomy measurement was made in 1960 by a Canadiangroup, who used the Transit 2A satellite to measure thecosmic noise background at 3.8 million cycles per second.Two years later, a University of Michigan group used aJourneyman rocket with a 12.2-meter dipole antenna todetermine the mean cosmic background at 0.75, 1.225,and 2.0 Mc/sec. A large drop in intensity between 1.225and 2.0 Mc/sec was interpreted as resulting from absorp-tion by a local concentration of interstellar gas. Cosmicradio emission between 1.5 and 10 Mc/sec was measuredby the Alouette I satellite launched in 1962 (fig. 2 ) .At 2.3 Mc/sec, the brightest region of sky appears to becentered on the south galactic pole.

In ultraviolet astronomy, the era of satellite observa-tions is just beginning. A successful flight of the firstOrbiting Astronomical Observatory (figs. 3 and 4) isexpected to provide basic data necessary to plan futureobservations, as well as the first ultraviolet spectral energydistributions for a variety of normal and unusual starsand nebulae. Experiments are also being planned whichwill utilize the X-15 rocket plane to carry ultravioletcameras (fig. 5 ) . In X-ray astronomy, the major problemis to discover the nature of the X-ray stars. Within afew years we should be able to locate the brightest oneswith sufficient accuracy to permit their identification withphotographed objects- n important step in determin-

A

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I N T R O D U C T I O N

, Figure 2.-The United States-Canada satellite Alouette.

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SPACE A S T R O N O M Y

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INTRODUCTION

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. *SPACE A S T R O N O M Ying their nature. We should soon learn whether gamma-ray stars exist and, if so, what sort of objects they may be.

Current observations indicate that the brightness of theradio sky decreases at the longest observable wavelengths.An interpretation of this decrease must await at least theminimal resolution which the Radio Astronomy Explorer(figs. 6 and 7) will provide in 1967.The use of ionosphericfocusing to provide this resolution will be investigatedduring 1965 with a receiver on the Polar OrbitingGeophysical Observatory.

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- . INTRODUCTXONP

Figure 6.-Mdel of the Radio Astronomy Explorer.

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SPACE ASTRONOMY .

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X-RAY AND GAMMA-RAY ASTRONOl"


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' 4 A. G.W. ameronInstitute for Space StudiesG o h d prrcc Fligbt Center, ASA. .

X-Ray and Gumm a-Ray Astronomy

IntroductionHE PRESENCE of large amounts of neutral hydrogenT n space will prevent stellar ultraviolet astronomyobservations at wavelengths shorter than 912 A. This isthe ionization threshold for neutral hydrogen, and more

energetic photons will cause ionization of the neutralhydrogen in interstellar space and will be very rapidlyabsorbed. At still shortei; wavelengths, ionization thresh-olds for heavier atoms will be reached, and hence inter-stellar space will be very opaque to electromagnetic radia-tion until wavelengths considerably shorter than 100A arereached. According to calculations of Strom and Strom(ref. l ) , the interstellar medium becomes reasonablytransparent to distances of hundreds of parsecs only forwavelengths shorter than 20 A. The entire galaxy becomesrelatively transparent to X-rays in the wavelength regionnear 3 A.Detection and Identification of X-Ray ources

The earliest X-ray studies in space were carried outwith detectors scanning the Sun. A review of the solarX-ray results has been given by de Jager (ref. 2) . T h eearly measurements gave upper limits on the X-ray fluxesin space; Friedman (ref. 3) found an upper limit of lo-*ergs/cm'/sec/A for the influx of X-rays from beyond thesolar system.

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SPACE A S T R O N O M Y 8 .The first X-rays detected from sources outside the solar-.

system were found in an experiment of Giacconi, Gursky,Paolini, and Rossi (ref. 4).They flew a rocket, from theWhite Sands Missile Range, containing uncollimated,thin-window Geiger counters with some 60 cm2 of sensi-tive area. Discrimination against energetic particles wasachieved with an anticoincidence scintillator. The win-dows of the Geiger counters had thicknesses of 1.7 and7 milligrams per centimeter squared. Their transmission,together with that of the filling gas, gave a band of sensi-tivity for X-rays of wavelengths between 2 and 8 A. Thisrocket experiment detected a soft X-ray source from adirection near the galactic center. This very intense sourcewas later determined to be in the constellation Scorpio.There was also an indication of a second source in theneighborhood of Cygnus.

Further X-ray measurements were reported in 1963 byGursky, Giacconi, Paolini, and Rossi (ref. 5) . In theseflights the Geiger-counter windows were made of beryl-lium 0.002 inch thick. These counters were supplementedby sodium iodide and anthracene scintillation counterswhich were intended to measure the more energeticX-rays and any electrons which might be present. Theflights confirmed the presence of the source in Scorpio,reinforced the evidence for a source in Cygnus, and sug-gested the presence of the third source in the generaldirection of the Crab Nebula.Further evidence of celestial X-rays was given by Fisherand Meyerott (ref. 6 ) . Their analysis suggested that amultitude of X-ray sources is present in the sky, but thesesources are not statistically well established (refs. 7 and 8 ) .

Meanwhile, Bowyer, Byram, Chubb, and Friedman flewan instrumented rocket in April 1963 which carried propor-tional counters containing 65 cm2 of sensitive area havinga field of view of 10" at half-maximum sensitivity (refs.

,

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X-RAY AND GAMMA-RAY ASTRONOMY9 &nd 10). These counters had beryllium windows 0.005inch thick, and were sensitive to X-rays from 1.5 to 8 A.The source in Scorpio was confirmed and its positionlocated at RA 16h15m, ecl -15", the uncertainty in theposiriorr was stated to be abcct 2",m d he angular diame-ter was less than 5". This flight also located a source inthe direction of the Crab Nebula with a strength onlyone-eighth as great as that of the Scorpio source.

An extremely important advance was made by Bowyer,Byram, Chubb and Friedman (ref. 11) on July 7, 19H,when they launched a stabilized Aerobee rocket (fig. 8)guided to point Geiger counters with 114-cm2area at theMoon during the critical 5-minute phase in which theMoon was occulting the central portion of the CrabNebula (fig. 9 ) . There were two counters having Mylarwindows, coated with 60 A of Nichrome, one 0.001 inchthick and the other 0.00025 inch thick. The differencein counting rates between the counters was expected toindicate something about the spectral distribution of theX-rays in the low energy range. Both counters recordedessentially the same number of counts, which led Bowyeret al. to conclude that the X-rays from the Crab Nebulawere concentrated below 5 i.However, Friedman re-ported at the Symposium on Relativistic Astrophysics heldat Austin, Tex., in December 1964 that it had rained onthe day of the flight until nearly flight time and the mois-ture had apparently degraded the performance of thelower energy counter sufficiently to distort the results.Consequently, the data obtained from the July 1964 flightare open to question. This flight gave the very importantresult that the angular width of the X-ray source in theCrab Nebula was about 1 minute of arc. This indicateda diameter of about 1 light-year.

In August 1964, Giacconi et al. (ref. 12) flew anotherrocket carrying a number of Merent counters. This flight

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SPACE A S T R O N O M Y* -

Figure &-Launch of an Aerobee 150 rocket.

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X-RAY AND GAMMA-RAY ASTRONOMY-A .

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SP ACE A S T R O N O M Yindicated a source in Sagittarius, in addition to the well:determined Scorpio source.

In February 1965, Oda, Clark, Garmire, Wada, Giac-coni, Gursky, and Waters (ref. 13) reported some impor-tant data on the angular sizes of the X-ray sources inScorpio and Sagittarius. They used an ingenious X-raycollimation system consisting of two grids of parallel wires.These wires were separated by slightly less than one wirediameter. The two grids were mounted one behind theother 1.5 inches apart. As the collimator scans across apoint source, the shadow of the front set of wires will fallalternately on the back wires and on the intervals betweenthe back wires. Hence a point source gives a modulatedsignal, whereas an extended source gives a more nearlycontinuous signal.

Oda et al. concluded that the Scorpio source extendedin a direction approximately parallel to the galactic planedefinitely less than 30 minutes of arc and probably lessthan 8 minutes of arc. They were also able to concludethat the Scorpio source did not extend more than 1"perpendicular to the galactic plane. They further con-cluded that the Sagittarius source was either extendedin space or consisted of more than one point source. TheX-ray source is spread over a region more than 30 minutesof arc in diameter.

The number of known X-ray sources was considerablyextended as a result of two flights of Bowyer, Byram,Chubb, and Friedman (ref. 14), one on June 16, 1964,and the other on November 25, 1964. Geiger counterswere mounted facing outward through the skin of an un-guided Aerobee rocket. Aluminum-honeycomb collima-tors were used, limiting the field of view to 8.4" at half-maximum transmission. The rolling and precession ofthe rocket caused Geiger counters to scan a large portionof the sky. The counters were sensitive to X-rays in the18


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X-RAY AND GAMMA-RAY ASTRONOMY

Source

Tau XR-1' ......Sco XR-12 ........Sco XR-2 ..........ScoXR-3. .........Oph XR-l 3......Sgr XR-14 ........Sgr XR-25 ........Ser XR-1 ..........Cyg XR-1 ........Cyg XR-2 ........

iahge 1 to 15 K. The effective area for X-ray detectionwas 906 cm'.

These flights detected eight new X-ray sources. The-positions, designations, and flux intensities of these sourcesare listed in table 1, taken from bwyer- et 2. ref. 14).It must be emphasized that the shape of the X-ray spec-trum is extremely uncertain, as may be seen from thedifferences between the two methods of listing the flux intable 1. The positions of the sources are shown in the skymap of figure 10, taken from the same paper.Table 1.-X-ra y sources. Flux i s uncorrected for atmospbericabsorption. I t was measured witb a %-mil Mylar window.Co lum n A i s t b e frux (1W erg/cm'-sec) computed for a bluck-body at 2 x 10: OK, 1.5 to 8 A, and column B i s tha t for ablackbody at 5 x lo6 O K , 1.5 to 8 A .

{From ref. 141

RA05h31.5m16h15m17"8"17h23m17"32"17h55"18h10m18'45"1gh53"21'43"

Declination,deg22.0- 5.2- 6.4- 4.3

- 0.7- 9.2-17.1

5.334.638.8

FluxObserved,:ounts/cmz/sec

2.718.71.41.11.31.61.5

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5.52.92.32.73.33.01.57.31.7

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Within 1' of optical center of nebula.2 Previous measurement 12x10-8 erg/cmz-sec computed as for col. A.3 1.1" from $N1604.4 2.3" from galactic center.5 1.2" from M 17.The source Taurus XR-1 is associated with the Crab

Nebula; the lunar occultation experiment showed that itwas within 1 minute of arc of the center of the nebula.

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24 23 22 21 20 19 le 17 16 15 14 13 12 IIRIGHT ASCENSION

Figure IO.-Map of sky scanned by X-ray detectors in Aembeeflight June 16, 1964. Thin lines trace path of view vector acrosscelestial sphere on successive rolls. Shaded segments indicateportions of scan in which clearly detectable X-ray signals wereobserved above background. Circles are positions at which dis-crete sources have been identified.

The source Ophiuchus XR-1 corresponds to the positionof the Kepler supernova of 1604 to within 1.5", which iscomparable to the error of observation. However, Bowyeret al. (ref. 14) failed to find an X-ray source correspond-20


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- -X-RAY AND GAMMA-RAY A S T R O N O M Y

'ing to the position of the Tycho supernova. All the X-raysources lie rather close to the galactic plane and within90" of the galactic center. This suggests that the X-raysources are galactic rather than extragalactic, and theymay be associated in some way with the newer &k pp-lation of stars in the galaxy. Type I supernova remnantsare similarly associated with disk population stars.

Little information is yet available about the presenceof higher energy X-rays from these sources. In July 19M,Clark (ref. 15) flew a balloon carrying an X-ray detectorin the form of a scintillation counter with a sodium iodidecrystal of 97 cm2 in area and 1 mm thick. This detectorwas collimated to provide a field of view of 16" in onedirection and 55" in the other. Clark detected X-rays inthree energy channels between 15 and 60 keV.summaryThere have been many explanations of these X-raysources. Among the suggested source mechanisms we maymention are:( 1 Bremsstrahlung from a hot plasma(2 ) The inverse Compton effect in which energetic

(3) Synchrotron radiation produced by energetic

( 4 ) Thermal emission of a Planck spectrum fromThese mechanisms make different predictions about thespectral shape, angular width, and possible polarizationcharacteristics of the sources. More refined experimentswhich can be made as X-ray astronomy develops areneeded to determine which, if any, of these mechanismsis correct. It has, however, been possible to conclude thatintergalactic space is not filled with a very hot plasmasuch as has been suggested in some cosmological models(ref. 16).

electrons scatter starlight photonselectrons spiraling in a magnetic fieldthe surface of a hot, compact, neutron star.

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SPACE A S T R O N O M Y* *GAMMA-RAY ASTRONOMY

IntroductionGamma-ray astronomy is intrinsically more difficult than

X-ray astronomy, because of the low counting rates in-volved. The low counting rates require the use of largedetectors with large collimation angles. However, the ex-periments which have been carried out appear to indicatethat celestial gamma rays exist and may be detected inspace experiments.Difficulties Invalved in Gamma-Ray Detection

Gamma rays of energy near 1 MeV were detected in anexperiment on the Ranger I11 spacecraft by Arnold,Metzger, Anderson, and Van Dilla (ref. 17). Their de-tector was a 3-inch sodium iodide crystal surrounded bya plastic scintillator. One gamma-ray spectrometer wascarried on the end of a boom and one was placed nearthe spacecraft and the counting rates of the two werecompared. The two detectors subtended spacecraft solidangles differing by a factor of 20, but the gamma-rayintensity was greater by a factor of less than 2 near thespacecraft than remote from it, thus demonstrating thatthe majority of the gamma rays detected in the extendedposition came from space. Most of the gamma rays de-tected by Arnold et al. lay near 0.5 MeV, the lower energylimit of their detector. However, they detected no gamma-ray lines which might be attributed to positron annihila-tion or neutron capture by hydrogen.

Kraushaar and Clark (ref. 18) searched for primarycosmic gamma rays with an energy greater than 50 MeV.Their gamma-ray detector, together with a final rocketstage, constituted the satellite Explorer X I (figs. 11 and12). The detector consisted of a sandwich crystal scintil-lator with alternate slabs of cesium iodide and sodiumiodide. The solid angle of the detector was about 17O half-22


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1 . X-RAY AND GAMMA-RAY ASTRONOMY


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Figure 12.4utaway drawing of Explorer XI.

angle. There was a plastic anticoincidence detector sur-rounding the crystal.

These investigators detected a finite fluxof gamma raysin this high-energy region; however, they were uncertainof the background flux and decided to consider their re-24


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* *X-RAY AND GAMMA-RAY A S T R O N O M Y

sult only an upper limit to the high-energy gamma-rayA significantly higher flux of high-energy gamma rays

was obtained in high-altitude balloon flights by Duthie,Hafner, Kaplon, and Fazio (ref. i9 . Tneir pricipddetector element was a Cerenkov counter. There is atpresent no indication of the source of the discrepanciesbetween the data of Kraushaar and Clark and those ofDuthie et al.

* flux from space.

. .

s-ryThese high-energy gamma rays are believed to be pro-

duced by the decay of neutral pions produced in collisionsof cosmic rays with the interstellar medium. For thisreason an early attempt was made to measure them.Felton and Morrison (ref. 20) have suggested that thesehigh-energy gamma rays, aswell as he low-energy gammarays and the X-rays, may be explained by the inverseCompton effect. Because of the uncertainties in the ex-perimental measurements of high-energy gamma-rayfluxes, no positive conclusions can be drawn at present.However, it is possible to draw the negative conclusionthat the galaxy contains very little antimatter. This rulesout certain types of matter creation in a steady-stateuniverse.

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ULTRAVIOLET AND INFRARED ASTRONOMY



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N.G.Roman and J. K. GleimPhysics and A s t r o n o ~ y rogramsO&e of Space Science and Applicrdions, NASA

Induct ionHE 6 YEARS since the founding of NASA have beenTmarked by the development of rocket techniques formapping the sky in the ultraviolet. The only data thathad been obtained previously were from two rockets flown

by the Naval Research Laboratory. Results were limitedto the wavelength region from 1225 to 1350 K (refs. 21and 22). The first experiment, in 1955,was a crude surveywith a 20" field of view; the second flight used mechanicalcollimators to narrow the field of view to 3". Both indi-cated that detectable radiation was received from thedirection of hot, bright stars.

In the intervening period, most observations have beenmade from spinning rockets. This technique is inefficientfor studying individual sources. It also furnishes the datain a form which is difficult to interpret, thus most publi-cations have followed the flights by 2 years or more. Dur-ing the past year, a three-axis stellar pointing control hasbeen used for some flights, although no data from theseflights have been published to date. Such pointing con-trols will probably be used for most astronomy rocketflights in the future.

In 1958, the Naval Research Laboratory (NU) asthe only scientific group active in this field. Since then,not only has the Goddard Space Flight Center (GSFC)(NASA) been very active but the University of Wisconsin

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SPACE A S T R O N O M Y . *and Princeton University have flown rockets for ultrq- .violet astronomy. At least three groups are active in GreatBritain and several groups are developing on the Europeancon tinent .

c

Ultraviolet NebulositiesThe early flights, in spite of their poor angular resolu-

tion, indicated that emission in the wavelength range 1225to 1350K was being received from several extended areasof the sky. These areas included regions in Orion and inVela in which many hot stars and H I1 regions are locatedand from which strong ultraviolet signals were expected.However, they also included a puzzling strong nebulosityaround the high-latitude B star, a Virginis (Spica). Theindividual stars did not seem to stand out against thesenebulosities.

A partially successful rocket flown by Boggess, at GSFCin May 1960, seemed to confirm this result (refs. 23 and24 ) . Although the rocket rotated much too rapidly forgood photometric data or angular resolution to be ob-tained, the a Virginis source was clearly observed on 58consecutive scans covering an angle of 9". On the otherhand, a rocket-borne instrument with 1.5" angular resolu-tion flown by the NRL group late in 1959 (ref. 25)showed sources of about the same maximum intensity aswere observed earlier, but in the Orion region, showed noindication of angular extension.To recoqcile these results, Friedman suggested thattemperature effects had modified the short-wave limit ofthe calcium fluoride optics (which were known to be tem-perature sensitive) to such an extent that Lyman-a wasbeing received by the detector on the early flight. Thisled to the problem of explaining why Lyman-a formedsuch a bright halo around a star like Spica.The early observations are not understood, but these3 0


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* -ULTRAVlOLET AND lNFRARED A S T R O N O M Y

ixtended nebulosities have never been observed since.Chubb and Byram (ref. 26) suggest that the originalobservation may have been of a patchy, upper atmosphereairglow; however, the contributing molecule is not ob-vious. It probably suffices to say that the cr 'v'ii@~kregion was scanned with a detector identical to that usedon the 1957 flight by Byram, Chubb, and Friedman(NRL) in April 1963 (ref. 27). The star appeared asa point source and an upper limit of one-tenth the valuepreviously reported was placed on the intensity of thediffuse glow. Another telescope-photometer combinationsensitive to the region from 1050 to 1350 d also showedSpica as a point source and eliminated changes in filtertransmission as an explanation of the earlier results.Stellar Photometry

Observations of stars in the ultraviolet have been madeby two techniques: broadband photometry and low-resolution spectrophotometry. In the first technique, filterand detector cutoffs are used to isolate bands 100 to 200 din width; in the spectrophotometric experiments, a grat-ing is used to disperse the light and the extent of thespectrum is cleanly limited by a slit. The spectrophotome-ters flown to date have had bandwidths (or resolutions)of either 50 d or 100A.Most of the observations of stars in the ultraviolet havebeen photometric. Chubb and Byram published data forabout 50 stars observed near 1427 d ( 1350 to 1550 A ) andabout 80 stars observed near 1314 d ( 1290 to 1350 A ) on2 flights in 1960. Each rocket camed a set of 4-inch and6-inch paraboloidal mirrors which focused the radiationon a gas-gain ionization chamber with a view angle ofabout 2f i0 . The systems were calibrated before flightwith an estimated absolute uncertainty of less than a fac-tor of 2. Most of the stars fall below the brightness pre-

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Star

g 10.0Wz.I..dr:3.0c

Spectral Plot Startype no.

0 GIANTS AND BRIGHT GIANTS0 SUPERGIANTSA UNCLASSIFIED

45 E Cas66aG emAT Ceno Cenp Cen5~Dray Cenf Cen

IC

B3 IVpA l VB5 VB2 VeB3 VB7 VA0 I11B3 V

0.3 I I I I I I I I I I I I I08 09 80 81 6.2 83 84 85 86 87 88 89 A 0 A I ,SPECTRAL CLASS

151617181920

Figure 13.-A plot of the ratio of the ultraviolet brightness ofstars observed at 1427 A to their visible brightness at 5560 A asa function of spectral type. The numbers adjacent to each datapoint permit identification of the stars corresponding to indi-vidual data points. Also shown are curves describing theultraviolet-visible brightness ratios predicted by stellar atmos-pheric theory and by blackbody approximations to stellar spec-tral emission.

uLupa L u poLup~ L u p6Lupd L u ~

Plotno.

123456789101 1121314-32t2Cen B2 IV7 9 l U M a I A 2VB2 Vu1 CenL Lup

I

7 Lid2425 I zE:r

SpectraltypeB2 IVB2 I1B6 VB3 vB2 IVB3 IVB3 VB2 VBO IVB1 I11BO V08epBO I b


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ULTRAVIOLET AND l N F R A R E D ASTRONOMY, chcted either from blackbody curves or from stellar models

(figs. 13 and 14). However, these are a few stars whichappear much brighter than most.

Bogges published (m--m,,) colors for 13 well-observed, unreddened stars (ref. 28 ) . 1he magnitudeswere measured with either 60-mm refracting or 150-mmreflecting telescopes. The bandpasses of the 2200 di and2600 K filters were 210 A and 250 A, respectively, with theoverlapping region of the spectrum subtracted from theintensity measured with the 2200 K filter. The estimateduncertainties in the measured intensities for the multiplyobserved stars is less than 20 percent. In general, thecorrelation of these colors with spectral type is good; 10 ofthe 13 stars lie within 0.1 magnitude of the mean line.

Heddle has measured stellar radiation at wavelengthsnear 2000 K (ref. 2 9 ) . His data were obtained as part ofan experiment to measure the ultraviolet radiation ofcelestial objects in the Southern Hemispere. The experi-ment was flown on a Skylark vertical sounding rocketfired from Woomera, Australia. Identification of signalsreceived by 2 scanning photometers was made for 22 stars.The observed ultraviolet flux, when compared with thatpredicted by stellar model atmospheres, showed that themodels overestimated the ultraviolet radiation for earlyB stars by a factor of approximately 4.

-.

SpectrophotometryNarrowband photometry was introduced into rocket

astronomy by Stecher and Milligan, who flew narrowbandphotometers on an Aerobee 150A rocket to obtain stellarspectra (fig. 15) in the ultraviolet (ref. 30). Similar in-struments were subsequently used to study the reflectivityof Jupiter in the ultraviolet (fig. 16). Stecher and Milli-gan used four spectrometers (fig. 17), two instruments with

33


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SPACE A S T R O N O M Y . .I

STELLAR MODELS BY UNDERHILL

4 > +L i n C \BLACK BODY

+a +2e'o IZDO': 5.0n

OZ3 101 A+ DWARFS AND SUB GIANTSQ 2.0 0 GIANTS AND BRIGHT GIANTS0 SUPER GIANTS5 I AUNCLASSIFIEDy Lot- - .\ I I

"a!;d,Ld, io ;I i2 PECTRAL(3 @!4LASS5 d6 817Figure 14.-A plot of the ratio of the ultraviolet brightness of

stars observed at 1314 A to their visible brightness at 5560 A asa function of spectral type. The numbers adjacent to each datapoint permit identification of the stars responsible for the indi-vidual data points. Also shown are curves describing the ultra-violet-visible brightness ratios predicted by stellar atmospherictheory and by blackbody approximations to stellar spectralemission.

@ ! h O AI

-lotno.123456789101112

Star17cCas45E cas26 /3 Per45 E Per37~4Ori87r5 Ori10 9 Aur12 /3 Tau53 K Ori37 B Aur1 [ C M a2 p CMa

SpectraltypeB2 VB3 1V:pB8 VB0.5 VB2 IVB2 IVB3 VB7 I11cBO I1B9.5pvB3 VB1 I1

13 I 1 l p M o n14 15SMon13 ( CMa31 9 CMa66 h Gem A208 I OPUD

a Py;32 a Leo

25 I 8 P C e p

Spectralt w eB3 VeOe 5B2 VeB5 IaB7 VA1 VB1 VeB2 I1B7 VB8 I11B3 VB2 I11

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ULTRAVIOLET AND INFRARED A S T R O N O M Ya pair of 600-lines/mm gra$ngs for the range 1225 to3000 K and two near-ultraviolet instruments with a pairof 500-lines/mm gratings for the range 1700 to 4000 6.The four spectrometers were mounted in pairs 180" fromeach other in order to obtain the best sky coverage. aiiepair had 5 0 4 resolution, the other had 100-6 resolution.Signals from about 30 0-type and B-type stars were re-corded by the two long-wavelength spectrometers, but theshort-wavelength instruments were saturated throughoutthe flight. Every star observed (except a Carinae, spec-tral type FO I a ) was deficient in flux below 2400 A com-pared with that predicted by model atmosphere theory,and by a factor considerably in excess of photometricerrors or choice of models. This deficiency in flux couldbe an intrinsic property of the stars or caused by an ab-sorber in the interstellar medium. The stars have littleor no color excess, and so a mechanism that absorbs onlyat short wavelengths would have to be hypothesized. Nosuch mechanism in the interstellar medium appears suit-able, therefore, the observed ultraviolet absorption mustbe caused by some opacity in the stellar atmospheres.Ultraviolet Deficiency

The discrepancy between the observed flux and the fluxpredicted from model atmosphere theory for ultravioletwavelengths for early-type stars has been the subject ofseveral investigations.

One study by Morton takes into account the effect ofabsorption lines on the ultraviolet stellar radiation (ref.31). A total absorption of about one-third the total con-tinuum flux is probable in the ultraviolet spectrum ofB-stars. Morton finds that after correction for this line-blanketing effect, the discrepancy is reduced to a factorof 4 or even 2 at 1314 6, but remains a factor of 5 at 1427 6 .

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a LOO

0.4 0 .3 0.2 0.0

0.4 0.3 0.2 0.0

Figure I S.-FM-FM telemetry traces of ultravioletspectra of a Carinae, CY Leonis, and /3 CanisMajoris. The wavelength is given in microns.The resolution is 50 A, and the scanning rateis 5000 A/sec. The saturated signal to the rightof a Carinae is the southern airglow horizon.

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. .ULTRAVIOLET AND I N F R A R E D A S T R O N O M Y. .

6

I"EEc0

xb(u

37


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Figure 17.Echematic of spectrophotometers.However, if many weak lines contribute to the blanketing,these factors could be reduced even further.

Meinel pointed out that a mechanism exists which givesclose agreement between the observed and computed spec-tral distributions for a B-type star (ref.3 2 ) . This mecha-nism is a molecular-recombination process involving colli-sions between ground-state and excited-state hydrogenatoms. A similar process involving excited helium atomswas proposed for the ultraviolet deficiency effect in 0-typestars.

Another comparison between model atmospheres andobserved spectra of early-type stars was made by Avrettand Strom (ref. 33). Their nongray model atmosphereswere in strict radiative equilibrium, with effective tem-perature ranging from 10 000 to 20 000" K and surface3 8


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ULTRAVIOLET AND INFRARED A S T R O N O M Y'gravities of lo3 to lo'. For three models they added

, biended wings of the higher Balmer and Lyman hydrogenlinesand included an approximate correction for the ultra-violet line-blanketing effect. The effective temperaturesdenvea rrom m d e b fnr B-stars closely agree withthose determined observationally by Aller and Stecher fora Gruis (B5V), and hence they conclude that there isno longer a discrepancy in the ultraviolet between theoryand observation.

- - 1 r

Interstellar Extinction in the UltravioletThe first reliable estimate of interstellar extinction was

obtained from rocket-based photometry of six stars atwavelengths 2600 K and 2200 K by Boggess and Borgman(ref. 34). These stars have a narrow spectral range of09.5 to B 1. It appears that the extinction can be repre-sented by the usual reddening laws applicable to otherwavelength regions. A plot of extinction observations for allwavelengths, together with Van de Hulst's dielectric-grainextinction curve No. 15, shows close agreement down toabout 4000 A, but the two new observations at 2600 d and2200 K are not represented by this curve (fig. 18). A betterfit can probably be obtained with extinction models basedon composite particles.

INFRARED ASTRONOMYBecause the short-wavelength end of the spectrum is

only accessible from above the Earth's atmosphere, it wasonly natural that astronomers turned their attention tothat spectral region first when balloon and satellite astron-omy became possible. This situation s t i l l exists today,though to a lesser extent, since plans are well underwayto bring the infrared and visual regions of the spectrumunder investigation.

Before 1959 very little infrared astronomy of stellar or39


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SPACE A S T R O N O M Y. .

4

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. . ULTRAVIOLET A N D IN FR AR ED AnRONOMY'galactic objects had been attempted from above the-Ekth's atmosphere. Rocket astronomy was completelyinvolved with ultraviolet or X-ray wavelengths, and balloonastronomy was introduced in 1957 by the spectaculari;hctcgrq&s nF solar granulation by Stratoscope I (refs.35 to 37) . Not much work was done in the infrared fromthe Earth's surface, except for the extension of the funda-mental magnitude and color standards to longer wave-lengths. This s primarily because the data were difficultto obtain, even though the infrared transmission is quitehigh under the best conditions in certain restricted wave-length regions. The advent of balloons greatly facilitatedthe interpretation of ground-based observations. For ex-ample, Kuiper, Sinton, and Boyce discovered stellar water-vapor bands at 1.4 pl and 1.9 pl Mira Ceti, in spite oftelluric bands at 1.3 p and 1.8 p . In November 1963 thisdiscovery was confirmed by the spectra (fig. 19) taken bythe StratoscopeI1 balloon telescope.

Stratoscope I1 (figs. 20 and 21) is a balloon-borne36-inch telescope project of Princeton University directedby Martin Schwamhild (ref. 38). It is the successor toStratoscope I, a balloon-borne 12-inch telescope system,which, as mentioned previously, has given the highestresolution photographs of the Sun to date. The main pur-pose of Stratoscope I1 is to acquire high-resolution photo-graphs of celestial objects. Two successful preliminaryflights using infrared optics have been made; the first, inMarch 1963, gave valuable data on Mars, and the second,in November 1963, gave useful data in the range from1 to 3 p on Jupiter, the Moon, Sirius, and seven red giantstars. Two important results from this latter flight were:( 1 the infrared spectrum of Aldebaran is relatively freeof bands, though there is an intensity peak at 1.6p wherethe absorption coefficient of the negative hydrogen iongoes through a minimum; and ( 2 ) the strong water vapor

41


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SPACE A S T R O N O M Y . '. .

\3

a!H

I I I I II * rr) (u - 0Ia t I

42


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ULTRAVIOLET AND l N F R A R E D ASTROhlOMY* .

rj.*v).*e,Yu(J4mc,C00mJ2UU

e

. p; .. w ," .c -I *, i. i. :

43


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SPACE A S T R O N O M Ybands in Mira at 1.4 p and 1.9 p mentioned previously a k 'also strong in the spectra of Betelgeuse and R Leonis.Figure 22 shows the spectrum of a Orionis, obtained byStratoscope 11.

Figwe 21.Stratoscope 11 36-inch telescope before launch.44


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ULTRAVIOLET AND INFRARED ASTRONOMY. .

I . - .I C I I I 1 I I I

I 1 I 1 I I 1 1 I I

-0 9,

k- &...

.'8881i8..

45


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- .

LOW-FREQUENCY RADIO ASTRONOMY


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P. T. HaddockUniversity of M i c b i g eAstronomy Observatory

AND HIGH JXEQUENCIEST THE LOW-FREQUENCY END of the radio window,

(1 Sporadic bursts of emission from th e Sun and(2 ) A bright belt of radio emission in the galactic plane( 3 ) An overall glow from the entire sky(4)Scattered bright regions of emission from distant

radio galaxies(5 Small quasi-stellar objects of extreme radio bright-ness(6) A number of disk-like bright sources varying in

size and concentrated along the galactic plane,identified with remnants of the supernova( 7 ) Dark interstellar clouds of ionized gas which ab-sorb the general background emission; these alsovary in size and are concentrated in the galacticplane.

Figure 23 shows the radio spectra of a variety of thesecelestial objects, and illustrates how the radio picture ofthe sky changes when seen in bands of the radio spectrum.Figure 24 is a photograph in visible light of th e radiosource in Virgo A.

The appearance of the radio sky at high frequencies isgenerally dark and shows the following features:( 1) Strong emission from the Sun, Moon, Venus, and

A he prominent features in the sky are as follows:Jupiter

Jupiter49



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lo-'. ISPACE A S T R O N O M Y

. .

.

Figure 23.-Radio spectra of a variety of radiosources obtained from ground-based observationsthrough the radio window in the Earth's atmos-phere. The curves for solar and Jovian bursts areonly roughly indicative of these highly variablephenomena. The level of the spectrum for thecosmic background with respect to the otherspectra depends on the collecting area of theobserving antenna.50


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- .LOW-FREQUENCY RADIO A S T R O N O M Y. .

C."

51


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SPACE A S T R O N O M Y . .( 2 ) Occasional bursts of radiation from the Sun( 3 ) A steady narrow belt of emission along the galacticequator

(4)A few bright sources(5) Many faint sources scattered over the sky.In contrast, we expect to find from low-frequency ob-servations from large orbiting or lunar-based radio tele-scopes operating in the band around 1 Mc/sec that theradio sky will show the following:( 1 A very bright overall glow

( 2 ) A wide dark band distributed along the galacticequator, becoming very wide toward the galacticcenter( 3 ) A large number of bright sources of emission dis-tributed over the sky but because of strong absorp-tion by interstellar gas rather sparse in the Milky

(4)Frequent strong outbursts of radio waves fromJupiter(5) Occasional radio outbursts and noise storms fromthe Sun. The undisturbed Sun and most of theplanets will be inconspicuous.

Way

RESTRICTIONS ON SPACE RADIO OBSERVATIONBecause very large structures with the required dimen-

sional stability for resolution at low frequencies are notyet available, space radio observations are restricted tovery coarse resolution. To date, there have been no rocketor satellite observations with an angular resolution of lessthan a large fraction of the whole sky. Since the intensityof radio signals at low frequencies is generally strongerthan the receiver noise, accuracy is generally not limitedby receiver sensitivity for sufficiently large antennas, butby low angular resolution if there is adequate preflightcalibration and inflight monitoring of antenna impedance


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- . LOW-FREQUENCY RADIO A S T R O N O M Y. and receiver gain. Adequate angular resolution is required

to discriminate between the various sources of emissionin the same region of the sky in order to obtain usefuldata. With low angular resolution, the antenna outputis the composite of signais from the galaxy, eiiraga!acticsources, the planets, and the Sun. With the resolutionavailable at present, it is possible to study only a few im-portant astronomical problems : the average cosmic-noisebackground spectrum and the dynamic spectra of radiobursts from the Sun and planets.

*

LOW-FREQUENCY MEASUREMENTS FROM SPACEThe scientific potential of low-frequency radio measure-

ments from space was discussed over 6 years ago byGetmantsev, Ginzburg, and Shklovski, by Haddock, andby Lovell. Although many observations of the cosmicbackground noise have been made from the ground forthe past two decades, quantitative data for frequenciesbelow 10 Mc/sec are still available only from the measure-ments by Reber, by Ellis and his colleagues, and byParthasarathy, Lerfald, and Little (refs. 3947 . Theseobservations were made from Tasmania where conditionsfor radio propagation through the ionosphere areexceptional.

The first reported radio-astronomy measurement at lowfrequencies was by a Canadian group (ref. 48) who ob-tained a measurement of cosmic-noise background at 3.8Mc/sec from the Transit 2A ( 1960 Eta-1 ) satellite, whichwas launched in June 1960. These measurements werefirst discussed by Chapman and Molozzi (refs. 48 to 50)and later, independently, by F. G. Smith (ref. 51). Thecosmic brightness temperature deduced by Smith waslower by a factor of 6; this discrepancy was caused bydifferences in interpretation of the effect of the localionospheric medium on the receiver sensitivity. No infIight

53


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. *SPACE A S T R O N O M Yreceiver or antenna calibrations were made. The first .value reported is shown on figure 25.

The next reported measurements were by Walsh, Had-dock, and Schulte of the University of Michigan (refs.52 to 55). They used rocket-borne equipment (fig. 6)designed to measure the absolute value of mean cosmic

IOI-

K E Y :WALSH, HAODOCK, f SCHULTEC H A PM A NBENEDIKTOV,. I AL.E L L I SA L E X A N D E R e ST O N EH U G V E N I N e PAPAGlANNiSH A R T ZS M I T H ( P R E L I M I N A R Y 1

0.10. 1.0

F R E Q U E N C Y ( M e )10

Figure 25.-Measured values reported for the mean cosmic back-ground noise. All points represent measurements from rocketsor satellites, except those of Ellis. The solid curve is from theAriel I1 satellite and has not been corrected for local plasmaeffects. The broken curve represents relative values only, de-duced from uncalibrated sweep frequency measurements. Thehigh value at 0.7 Mc/sec is believed by Huguenin and Papa-giannis to be caused by locally generated noise, not cosmicnoise.

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- 1

LOW-FREQUENCY RADZO ASTRONOMY

Figure 26.-Payload of rocket for cosmic radio mise measure-ments.

background a t 0.75, 1.225, and 2.0 Mc/sec. The experi-ment included inflight calibration of the antenna andreceiver characteristics, required for the interpretationof the effects of the local ionosphere on system sensitivity.A 12.2-meter electric-dipole antenna was used. The pay-load was launched from Wallops Island on a four-stage

55


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, .SPACE A S T R O N O M Y

Journeyman rocket on September 22, 1962, shortly after 'midnight. The rocket and all components on the payload 'performed well and over 20 minutes of data were ob-tained. An altitude of 1691 kilometers was reached whichwas somewhat lower than originally planned.

The mean intensity of cosmic-noise background, aver-aged over a celestial hemisphere centered on new galacticlatitude of 144" and longitude of -19", was derived at1.225 and 2.0 Mc/sec. The system sensitivity was too lowto derive a useful value at 0.75 Mc/sec. The correspond-ing intensities obtained were 1O X lo-'" W/m'/cps/sr, and2.0 X lo-'' W/m'/cps/sr, respectively, with an estimateduncertainty of 40 percent in the absolute values but withhalf this uncertainty for the ratio of the two values. Thelarge drop of intensity between 2.0 Mc/sec and 1.225Mc/sec was rapid enough to require the assumption thatit was caused by free-free absorption by a local concen-tration of ionized interstellar gas. The magnitude of theabsorption can be accounted for if it is assumed (ref. 56)that the absorbing gas is concentrated within 200 parsecof the Earth and the mean electron density is 0.14/cm3.

Unexpected intense noise signals also detected are be-lieved to originate in the ionosphere (ref. 57) . A criterionfor the occurrence of this noise was obtained in terms ofthe propagation characteristics (the local gyro frequencyand plasma frequency) in the ionosphere at all three ob-serving frequencies (figs. 27 and 28). This region of localnoise occurs where the refractive index tends towardinfinity, in the absence of collisions. The effects of themagnetoionic medium on the radiation resistance werealso determined and measured quantitatively for the firsttime. Variations in the radiation resistance during thedisappearance of the extraordinary radio wave were con-sistent with theoretical expectations (refs. 58 and 59).

It was thus demonstrated that antenna behavior in the56


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. a LOW-FREQUENCY RAD10 A S T R O N O M Y. .

- >

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I(u 0

N- >

58

0

(u

tX

-

0


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LOW-FREQUENCY RADZO A S T R O N O M Y., imosphere is theoretically predictable and that corrections

to obtain the free-space cosmic noise intensity values arepossible except for those regions where the index of refrac-tion is infinite (ref. SO).As expected, there was no evidence of I~iiiiiiiiad~adksignals leaking through the ionosphere during this experi-ment. There was also no evidence of radio emission fromthe artificial radiation belt injected in the magnetosphereon July 9, 1962.

At the IAU Symposium No. 23 in Belgium duringAugust 1964, Huguenin and Papagiannis (ref. 61) re-ported results of measurements made during the summerof 1962 from a pickaback scientific payload on the satellite1962 cy@. The polar orbit had a perigee of 200 kilometersand an apogee of 370 kilometers. Because of the short life,only about 1 days worth of data was obtained on cosmic-noise levels at 4 and 7 Mc/sec, on antenna impedance,and on magnetic field. The background noise levels re-corded were a million times greater than expected, andare attributed to Earth-generated noise when the satellitewas below the maximum ionospheric electron density.The noise levels obtained in the nighttime phases over thesouth geomagnetic pole where the terrestrial interferencelevel was low and the critical frequency of the F, wasbelow 7 Mc/sec probably represent the cosmic back-ground. The value deduced was 1.5X lo OK ( k 2a),which is in general agreement with the previously reportedvalues. The descriptions of this work are not clear, anddetails of the calibration, instrumentation and data reduc-tion procedure have not been published.

Huguenin and Papagiannis (refs. 61 and 64) eportedmeasurements of the cosmic background noise averagedover the whole sky at frequencies of 0.7 and 2.2 Mc/sec.The instrumentation was carried by a Blue Scout Jr.( AP-3 ) high-altitude rocket probe which was launched

.

59


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I .SPACE A S T R O N O M Yfrom Cape Canaveral on July 30, 1963. It reached an.altitude of 11 100 kilometers. Although the antenna im-pedance probe did not wdrk, the high altitude insuredthat the measurements were made at essentially free-space conditions. There were unexpectedly intense burstsof radio emission which are attributed to terrestrial noisegenerated in the exosphere. There was no significantleakage of ground noise through the ionosphere, for astrong ground component of emission would have showna change of intensity by a factor of 20 over the range ofaltitudes of the experiment. The measured fluxes forthe average cosmic noise over the whole sky are:( 1 ) At 0.7 Mc/sec, S = 8 X W/m-z(cps)-', with

an estimated uncertainty of 2 to 3 dB.( 2 ) At 2.2 Mc/sec, S= .8X W/rn-'(cps)-',with an estimated uncertainty of 1.4 to 2 dB.

The sky brightness obtained at 2.2 Mc/sec is in generalagreement with previously reported values, but the bright-ness at 0.7 Mc/sec is about 15 dB higher than that ex-pected from the trend of the radio spectrum. The experi-menters state that the previously reported low-altitudeobservations, as well as these high-altitude observations atfrequencies below 2 Mc/sec, are both possibly correct.They suggest that the differences may be caused by strongradio emission in the Earth's exosphere and perhaps arisefrom harmonic gyroradiation from the Van Allen belt orthe artificial belt created by the high-altitude nuclearexplosion.

Hartz (refs. 62 and 63) has reported observations ofcosmic radio emission between 1.5 and 10 Mc/sec ob-tained from the Alouette I satellite (1962 pa 1 whichwas launched September 28, 1962. A nearly circular polarorbit at an altitude of 1000 kilometers and an inclinationof 80.5" was obtained. Five hours a day of sweep-frequency records covering the band from0.5 to 12 Mc/sec60


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. .SPACE A S T R O N O M Ystant at a level of 1.4X lo-'' W/m'-cps-sr from 2 Mc/seC .to about 1 Mc/sec but decreases to 0.7 X lo-" units at1.2 Mc/sec. The values at 1.2 and 2 Mc/sec are about30 percent below those obtained by the University ofMichigan group.Smith also reported that the antenna impedance effectsand the detection of locally generated noise in the Earth'sexosphere contfirm the findings of the Michigan group.Smith and Harvey also reported noise in region 4 wherethe magnetoionic parameters X and Y have values givenby l < X < ( l + Y ) and Y


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X-RAY AND GAMMA-RAY ASTRONOMYHE EARLIEST X-RAY TUDIES n space were carried

T ut with detectors scanning the Sun. I n 1962 thefirst X-rays from sources outside the solar system weredetected. They were found to come from the directionof Scorpio in the direction of the galactic center. A secondsource was found in the neighborhood of Cygnus. Animportant advance in X-ray work was made by NRLscientists in 1964 when they used the Moon as an occultingdisk to locate to within 1 minute of arc a source near thecenter of the Crab Nebula. By the end of 1964, 10 X-raysources had been detected and their positions located towithin a degree or two. All the sources lie rather close tothe galactic plane and within 90" of the galactic center.Hence they appear to be galactic rather than extragalacticobjects, and may be in some way associated with thenewer disk population of stars in the galaxy.

Gamma-ray astronomy is intrinsically more difficultthan X-ray astronomy, because of the low counting ratesinvolved. Experiments to date indicate that celestialgamma rays exist, though no discrete sources have yet beendetected. Gamma rays of energy near 1 MeV were de-tected by an experiment on Ranger 111. Explorer XIsearched for primary cosmic gamma rays with an energygreater than 50 MeV and established an upper limit onthe high-energy gamma-ray background flux of 3.7 to11X 10-'/ergs/cm*-sr-sec. In addition, upper limits for the

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v .

SPACE A S T R O N O M Ygamma-ray flux from several individual objects were alsbestimated.

Many of the problems of both gamma-ray and X-rayastronomy can be resolved by improvements in techniquessuch as larger and more sensitive detectors and longer ob-servation times. The immediate goals will be to map thegeneral background radiation more accurately, and todetect and identify discrete sources.

ULTRAVIOLET AND INFRARED ASTRONOMYAn early result from ultraviolet rocket photometry was

the suspected presence of ultraviolet nebulosity aroundthe high-latitude B-star, a Virginis (Spica). This wasobserved on two separate flights, and was tentatively ex-plained as a Lyman-a halo around Spica. Subsequentobservations, however, failed to confirm this and Spicahas since been observed as a point source rather than asan extended nebular one.

Both broadband photometry and low-resolution spectro-photometry have been used in making ultraviolet observa-tions of stars. Most of the stars fall below the brightnesspredicted from blackbody curves or stellar models. It isnot known at present whether this is an intrinsic propertyof the stars or is caused by an absorber in the interstellarmedium, but it probably results from the earlier failure totake adequate account of line-blanketing in computingmodel stellar atmospheres.

The advent of the first stabilized, accurately pointedsatellite specifically designed for astronomical observa-tions will permit ultraviolet investigations at a higher levelof accuracy and thus aid in resolving problems such asthose previously mentioned. Eventually, observations inthe visual region of the spectrum from large satellites willbe possible with a higher resolution than from ground-based telescopes.66


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. .

I 69

1. STROM,. E.; ND STROM, . M. : Interstellar Absorption Below100 A. Pub. Astronom. Soc . Pacific, vol. 73, 1961, p. 43.2. DE JAGER, C. : Solar Ultraviolet and X-Ray Radiation. Research

in Geophysics, H. Odishaw, ed., MIT Press, Cambridge,Mass., 1964.

3. FRIEDMAN,.: Rocket Observations of the Ionosphere. Proc.I.R.E., vol. 47, 1959, p. 278.

4. GIACCONI,.; GURSKY, .; PAOLINI,. R.; AN D ROSSI,B. B.:Evidence for X-Rays From Sources Outside the Solar System.Phys. Rev. Letters, vol. 9, 1962, p. 439.

5. GURSKY, .; GUCCONI, .; PAOLINI,. R.; AND, ROSSI,B. B.:Further Evidence for the Existence of Galactic X-Rays.Phys. Rev. Letters, vol. 11, 1963, p. 530.

6. FISHER, . C.; AND M E Y E R O ~ ,. J.: Stellar X-Ray Emission.Astrophys. J., vol. 139, 1964, p. 123.7. BOWYER,.: An Alternate Interpretation of the Paper Stellar

X-Ray Emission, by P. C. Fisher and A. J. Meyerott.Astrophys. J., vol. 140, 1964, p. 820.

8. FISHER, . C.; AND MEYEROIT, . J.: Reply to Letter of StuartBowyer. Astmphys, J., vol. 140, 1964, p. 821.

9. BOWYER,. ; Ta.X-Ray Astronomy. Space Research, vol. 4,10. Bo-, S.; BYRAM,. T.; CHUBB, . A.; AN D FRIEDMAN,.:

X-Ray Sources in the Galaxy. Nature, vol. 201, 1964, p. 1307.11. B~WYER,.; BYRAM,. T.;CHUBB, . A.; AND FRIEDMAN,.:

Lunar Occultation of X-Ray Emission From th e Crab Nebula.Science, vol. 146, 1964, p. 912.

12. GIACCONI,.; GURSKY,H.; WATERS,. R.; CLARK,G.; ANDROSSI,B. B.: Two Sources of Cosmic X-Rays in Scorpiusand Sagittarius. Nature, vol. 204, 1964, p. 981.

13. ODA,M. ;CLARK, .;GARMIRE, .;WADA,M.;GIACCONI,. ;GURSKY, .; AND WATERS,.: Angular Sizes of the X-Ray

1964, p. 966.


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SPACE A S T R O N O M YSources in Scorpio and Sagittarius. Nature, v01. 205, 1965,' ,p. 554.

14. BOWYER,. ; BYRAM,E. T. ; CHUBB, . A.; AND FRIEDMAN,.:Cosmic X-Ray Sources. Science, vol. 147, 1965, p. 394.

15. CLARK,G. W.: Balloon Obsewation of the X-Ray Spectrum ofthe Crab Nebula Above 15 keV. Phys. Rev. Letters, vol. 14,1965, p. 91.

16. GOULD, . J. ;AND BURBIDGE,. R. : X-Rays From the GalacticCenter, External Galaxies, and the Intergalactic Medium.Astrophys. J., vol. 138, 1963, p. 969.

17. ARNOLD, . R.; METZGER,. E.; ANDERSON, . C.; AND VANDILLA,M. A. : Gamma Rays in Space, Ranger 3. J. Geophys.Res., vol. 67, 1962, p. 4878.

18. KRAUSHAAR,. L.; AND CLARK,G. W.: Search for PrimaryCosmic Gamma Rays With the Satellite Explorer XI. Phys.Rev. Letters, vol. 8, 1962, p. 106.

19. DUTHIE, . G.; HAFNER,. M.; KAPLON,M. F.; AND FAZIO,G. G.: Gamma Rays at High Altitudes. Phys. Rev. Letters,vol. 10, 1963, p. 364.

20. FELTON,. E. ; AND MORRISON,. : Recoil Photons From Scat-tering of Starlight by Relativistic Electrons. Phys. Rev. Letters,vol. 10, 1963, p. 453.21. KUPPERIAN,. E.; AND MILLIGAN,. E.: Sources and StellarFluxes in the Far Ultraviolet. Astron. J., vol. 62, 1957, p. 22.

22. KUPPERIAN,. E.; BOGGESS,A.; AND MILLIGAN,. E.: Observa-tional Astrophysics From Rockets. I. Nebular Photometry at1300 A. Astrophys. J., vol. 128, 1958, p. 453.

23. BOGGESS,A.: Observational Results on Nebular and Inter-stellar Far Ulltraviolet Radiation. Mem. SOC.Roy. Sci. Li&ge,ser. V, vol. IV, 1961, p. 459.

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