Observation of X-ray sources outside the solar system

O B S E R V A T I O N O F X - R A Y S O U R C E S O U T S I D E T H E

S O L A R SYSTEM

RICCARDO GIACCONI and HERBERT GURSKY American Science and Engineering, Inc., Cambridge, Massachusetts

(Received November 30, 1964)

Introduction

X-ray astronomy is still in its infancy. So far only a few localized sources have been detected, and in most cases only crude measurements of location, angular dimensions, intensity, and spectra are available. Thus a complete review of the results and of the suggested models for the sources is at present not possible. A picture of the universe in X-rays is indeed unfolding, but it is a picture that changes with every new experiment, month by month. Furthermore, some of the observations are not easily recon- ciled with the present understanding of the prevailing conditions in Galactic and extragalactic space. With these difficulties recognized, the fact remains that experiments on X-rays of a few keV energy have revealed the existence of previously un- known celestial radiations. A progress report in this field seems, therefore, appropriate.

We have chosen to follow a chronological approach in the presentation of the experimental results in the belief that this approach will best outline the degree of uncertainty still existing and the advances already made. A brief review of the possible mechanisms for production of the observed radiation is given. No thorough discussion of the models is presented since there is little general acceptance for any of them, even as working hypotheses. Some recent advances in instrumentation which may permit the execution of crucial experiments are described.

Observational Facts

The Earth's atmosphere is practically opaque to electromagnetic radiation with wavelength shorter than 1800 •. Observations in this region of the spectrum only became possible with the advent of rocket- and satellite-borne equipment. Once the instru- ments are placed above the sensible atmosphere, limitations still exist as to the observable wavelength range. Absorption of electromagnetic radiation by interstellar gas is negligible for wavelengths greater than 912 A, although dust clouds obscure much of the Galactic plane. At 912 A, which corresponds to a photon energy of 13.5 eV, the energy required to ionize hydrogen atoms, the gas suddenly becomes opaque. A computation by STR6~ and STR6M (1961), based on CAMERON'S (1959a) estimate of the relative abundance of the elements is generally accepted as representa- tive of the absorption features in this region. In Figure 1 a plot deduced from their calculations is shown. In the direction of the Galactic plane, the atomic density is

Space Science Reviews 4 (1965) 151-175; �9 D. Reidel Publishing Company, Dordrecht- Holland

152 R . G I A C C O N I A N D H . G U R S K Y

roughly l atom per cm 3, and the absorption length can be read directly in centimeters rather than atoms per cm 2. One can see that absorption is negligible for interstellar distances (less than 102o cm or tens of parsecs) and becomes appreciable for distances of the order of 1000 parsec (3 x 1021 cm). It is possible that measurements in the

i 0 23 ,

Tcl " I022

O3

1021

Fig. 1.

io 2o I I I m ~

0 I0 20 30

WAVELENGTH (A )

I

I

4O I

5O

Absorption of X-rays in interstellar space. Absorption length is in units of number of atoms per cm "~

region of wavelength below 5/~ can be conducted on the Galactic core itself with little effect from interstellar absorption.

The Sun is the nearest star, and was discovered to be a source of X-rays in 1948 by BURNIGHT (1949), who used photographic plates covered with beryllium filters carried to an altitude of 96 km during the daytime. Subsequent experiments by a number of investigators with improved detection techniques yielded data on the emitted spectrum, its temporal variation and location of the sources on the Sun. A status report of the observations was given in the Symposium on the Exploration of Space, held in Washington, D.C., in April, 1959 (FRIEDMAN, 1959a). A useful discussion of solar data is given by MANDEL'gTAM and EFREMOV (1958). A more recent review is given by DE JAGER (1964).

The detectors which were used in solar experiments had small sensitive areas and were more appropriate to detect the high fluxes of X-rays from the Sun than to search for weaker extrasolar sources. A negative result obtained with detectors of this type yielded an upper limit quoted by FRIEDMAN (1959b) as 10- 8 ergs cm- 2 sec- ~/~- 1 on any influx from celestial sources in the soft X-ray range defined by his detectors.

X-rays from sources outside the solar system were first detected in June 1962 by a rocket launched from the White Sands Missile Range, White Sands, New Mexico. A picture of the recovered payload is shown in Figure 2. The experiment was conducted by Giacconi, Gursky, Paolini of ASE and Rossi of MIT (GIACCONI et al.,

OBSERVATION OF X-RAY SOURCES OUTSIDE THE SOLAR SYSTEM 153

1962) with uncollimated thin window Geiger counters totalling 60 cm2 of sensitive area. The energetic particle background was reduced by means of a well-type anti- coincidence scintillator in which the Geiger counters were placed. The counters were thus 100 to 1000 times more sensitive than those used previously. The counters had

Fig. 2. Experiment payload flown by the ASE-MIT group in June 1962 to detect cosmic X-rays.

mica windows 1.4 and 7 mg/cm2 thick. The transmission of the windows and the absorption of the filling gas resulted in a band of sensitivity for X-rays between 2 and 8 A. The experiment was designed to detect X-ray sources in the night sky with a flux at the Earth of the order of 0.1 to 1 photons cmF2 sec-l. Particular emphasis was placed on the detection of fluorescent X-rays produced on the lunar surface since this was then believed (Proceedings, 1960) to be the strongest night sky source.

The most remarkable feature of the data (shown in Figure 3) was the detection of a strongly anisotropic and very soft radiation. The authors presented arguments to show that the bulk of the observed radiation was not corpuscular, but electromagnetic in nature, and that it consisted of soft X-rays. They showed that the source for the radiation could not be of terrestrial origin or in the solar system. Based on the

154 I~. G I A C C O N I A N D H . G U R S K Y

results of this first flight, the authors concluded that the bulk of the measured X-ray

flux was due to an extrasolar source located between 16 and 17 h Right Ascension (RA) and at approximately - 4 0 ~ declination (the region of Scorpius). The presence

of a second source in the neighborhood of Cygnus was indicated. The spectrum of

radiation from the main source was found to show absorption characteristics con-

sistent with 3 ~ radiation. The flux from the main source was given as 5 photons cm - z sec- 1. Figure 4 shows the region of the sky explored in this first flight and the

location assigned to the source.

450 I I I I I I I I I I I Counter ~ ~ Moon ~ I Magnetic field vector --

1-- (f~ r.O mg/cm 2 M i c a ~ ~ o oO o Z 550

:DO Counter ~ 2 ~ . o o o o Co O0 0 -- 1.4 X o o

250 - rnglcm2Mico_~ o~ o o - - o

- oo ~ \ ~ : ILl 150 -- 0 0 0 0 0 0 0 2 ~ O0 ~ 0 --

0 ~176176176176176 ~ r,_ ~. �9 ^ o ~ -o "-o oooooo �9 ooooo Oo �9 OOo~OooOoOoOo~

Z 50 eoooOoOOoOo~ ~ o e ~ ~ O~OoNoOoOoOo~L I l I I I I I I I I I I

0 o 60 ~ 120 o 180 ~ 240 ~ 300 ~ 560 ~

N E S W N

Fig. 3. Azimuthal distributions of recorded counts from Geiger counters flown during June 1962.

N 6 0 ~

A 03 W UJ n~ (.9 UJ a

Z 0 6-

Z J

~J O

4 0 ~

2 0 *

0 ~

2 0 ~

4 0 *

S 6 0 ~

~ . ~ 3 0 ' ' i , , , , ~ ' 330'~ 11 ~ i . . /

tASS,OPE,A\ tYGNUS / ,OEA Z,N,T,O,,9,,T ,O0,l \ ,gJUNE gG2 /

TRAtE OF +

\ ' : ' " GALACTIC ~ " / . . . . \ 00,T0,\ /

\ / ~GALACTIC t E N T E R - ~ ' ~ O ' ~ - a S t O R y

~F~",r.-SOURGE POSITION HORIZON

, , , , , ~"~-------- '~"~, ~, "'~. , ,

2 4 22 20 18 t6 14 12 hrs

R I G H T A S C E N S I O N

Fig. 4. Map of celestial sphere showing location of source as determined from data of June 1962 flight. The numbers listed along "Trace of G. T. axis" correspond to the azimuth angles given in

Figure 3.

O B S E R V A T I O N OF X - R A Y S O U R C E S O U T S I D E T H E S O L A R S Y S T E M 1 5 5

In the two years following the initial discovery of cosmic X-ray sources described

above, further data have been reported as the result of eight rocket flights conducted

by three different groups of experimenters. In addition to the ASE-MIT group, these include BOWYER, BYRAM, CHUBB and FRIEDMAN of the U. S. Naval Research Laboratory, and FISHER and MEYEROTT of Lockheed.

In two additional rocket experiments in October 1962 and June 1963, the ASE-MIT group flew instrumentation almost identical to that used in the first flight except that the Geiger counters utilized 0.002 inch beryllium windows. Also NaI and anthra- cene scintillators were flown to measure harder X-rays and electrons. These two flights furnished additional evidence supporting the interpretation of the earlier data

(GuRsKY, 1963). The possible electron contamination was measured with the anthra- cene scintillator and found to be negligible. The existence of the main source in

Scorpius was clearly established, and its location given as approximately 17 h RA, - 2 2 ~ declination. A triangulation method was used, as shown in Figure 5. A better

9 0 ~

70 ~

Ixl 5 0 ~ Ld

3 0 ~

i 0 o Z 0 ~ 0 - i 0 o

Z - 3 0 ~ -J

- 5 0 ~ a

- 7 0 ~

_ 9 0 o [ t I i i I 0 9 8 7

t I r t I , r r ~ , r I r ~ I r j t I, 2 6 5 4 3 2 I 0 22, -/.2 21 20 19 18 [ 16 15 14 15

R I G H T A S C E N S I O N ( H O U R S )

Fig. 5. Map of celestial sphere showing region of sky explored during June 1962, October 1962 and June 1963 rocket flights. Also shown is location of source in Scorpius as given by triangulating lines

of peak intensity observed in June 1962 and June 1963 experiments.

determination of the flux yielded 25 photons cm-2 sec-1 at an effective wavelength

of 3 A. New evidence for the existence of a source in Cygnus (between 20 and 23 hour RA and + 10 ~ and + 50 ~ declination) was obtained. Evidence suggesting the existence

of a third source in the general direction of the sky containing the Crab Nebula was also obtained. In addition to the localized sources, a background of X-radiation from the night sky was observed in all three flights. The authors ascribed a celestial origin

to this background also and gave its intensity as about 6 photons cm-2 sec-1 ster-1 The data of the ASE-MIT group clearly established the existence of localized

extrasolar X-rays ources but gave only rough indications of their location, angular size and spectrum. The location of the source in Scorpius was given, for instance, as "close to the center of the Galaxy, but appearently not quite coincident with it". The question of whether or not this source was in fact coincident with the Galactic


center was of obvious importance for understanding the nature of the celestial X-ray

source. In April 1963 Bowyer, Byram, Chubb, and Friedman of N R L flew an instru-

mented rocket carrying proportional counters of 65 cm 2 area with a field of view 10 ~ wide at half maximum intensity (BowYER e t al . , 1964ab). The counters used 0.005 inch beryllium windows and were sensitive in the region of the spectrum from 1.5 to 8 lk, essentially the same chosen by the ASE-MIT group. The authors confirmed the

existence of the source in Scorpius and gave a location of 16 h 15 rain RA and - 1 5 ~ declination. The angular diameter of the source was stated to be less titan 5 degrees in diameter. Since, in their early flight, the center of the Galaxy was below the rocket horizon, the possible uncertainty in identification mentioned above could no longer exist. Figure 6 shows the region of the sky in which the source was located.

260 ~ 255 * 250 ~ 245 ~ 240 *

Fig. 6.

20 m 17 h 40 m 20 m 16h 40 m

Map showing passage of detector axis and location of X-ray source in Scorpius derived by BOWYER et al. (1964b) during April 1963 experiment.

During the same flight evidence was obtained for the existence of a localized source in the direction of the Crab Nebula approximately one-eight h as intense as the one in Scorpius. The signal from the Crab at the peak was given as 14 counts above a background of 9. The existence of an isotropic X-ray background from the night sky was also confirmed. The flux was given as 6 quanta cm -2 sec - t ster -1.

OBSERVATION OF X-RAY SOURCES OUTSIDE THE SOLAR SYSTEM

SECONDS OF TIME AFTER LAUNCH

Fig. 7. Counting rates observed in two Geiger counters during occultation of the Crab Nebula as obtained during rocket flight of July 1964 by BOWYER et al., (1964~).

Fig. 8. Position of Moon with respect to the Crab Nebula during the July 1964 rocket experiment are indicated by the dashed lines. The listed times correspond to the times after launch used in

Figure 7. From BOWYER et al. (1964~).

158 R. GIACCONI AND H. GURSKY

An important advance in the measurement of celestial X-ray sources was accomplished when in July, 1964, the NRL group flew a stabilized rocket carrying Geiger counters with 114 cm2 area to observe the lunar occultation of the Crab Nebula (BOWYER et al., 1964~). Two counters were flown with 0.001 and 0.00025 inch thick mylar windows coated with 60 A of Nichrome. The gas filling common to the counters was 89.5% Ne, 9.5% He and 1% isobutane at atmospheric pressure. The counting rate of both counters was identical and showed a gradual decrease during the occulation of a central portion of the nebula over a range of about 2 minutes of arc. The authors attributed the bulk of the data to X-rays in the wavelength range from 0 to 5 A. They concluded that the existence of an X-ray source in Crab was confirmed and that the angular width of the source was about 1 minute of arc. Figure 7 shows the time dependence of the counting rate during occultation, and Figure 8 shows the position of the Moon at difTerent times during the flight.

At the IAU Conference in Liege (IAU, 1964a) the NRL group discussed the observation during another rocket flight in the summer of 1964 of a source in the

3 O 1 ~ 1 ~ 1 i

DATA ACCUMULATION TIME

30

292.889 TO 306.440 SEC.

40 306.440 TO 319.9% SEC. FROM LAUNCH

4 0

g 2 0

334.685 TO 348.248 SEC. 4 o R o M A " "; " 20

348.248 TO 361.815 5% FROM LAUNCH

Fig. 10. Azimuthal distribution of counts recorded in a Geiger counter during the August

Fig. 9. Experiment payload flown by the 1964 rocket flight by the ASE-MTT group. ASE-MIT group in August 1964. The counts are grouped in six time intervals.


direction of the Galactic center and confirmed the existence of a source in the direction of Cygnus. No preprint or publication of the conference proceedings is yet available.

In August, 1964, the ASE-MIT group flew an instrumented rocket carrying a

number of different counters. A photograph of the payload is shown in Figure 9. The results obtained during approximately 83 seconds of flight with one of the counters

have been reported (GIACCONI et al., 1964a). The Argon-filled Geiger counter had a beryllium window 0.002 inches thick and an area of 70 cmz. The X-ray sensitivity of the detector extended from 1 to 9 A. A rectangular collimator was used with a

field of view of 15~ 21.5 ~ at half-maximum intensity. The data shown in Figure 10 and the map in Figure 11 demonstrate the existence of two sources, one in Scorpius,

I0~

z

z < -I0'

-20 ~

/ /

/ /

195 ] N / ' / ~

\ 205 IB5 e',, I

\ o $ERPENS ~ o 5 ~ " ~ l ~ / ~RPEN$

~P(, 12000 ~ / FigO bA / 1 z~l ~\Y~ / " \ ~ o t ~ , / . - " J "~ ?i

,~. \ ~ ~ 1 7 6 .,,~o:' ,y_ �9 \ I \

IBkr 17hr 16hr RIGHT ASCENSION

-30~ 285* 225* 19hr 15hr

Fig. 11. Traces of the detector axis are shown in the region of the celestial sphere containing the two sources. The circled numbers refer to correspondingly numbered distributions in Figure 10. The

angles given along the traces correspond to those in Figure 10.

and the other in Sagittarius, which may be coincident with Sgr A, the radio center o f the Galaxy. The angular widths of both sources were smaller than 5 ~ in the scan direction. The energy fluxes computed in the assumption of a 3/k effective wavelength were given as 1.0 x 10 -v erg cm -2 sec- 1 from the source in Scorpius, and 0.44 • 10 .7

ergs cm -2 sec-1 from the source in Sagittarius. Fisher and Meyerott of Lockheed have reported data from two rocket flights.

While the data from the N R L and ASE-MIT groups are in agreement, neither experiment confirms the conclusions reported by Fisher and Meyerott; namely the existence of a substantial number of specific regions of enhanced X-ray emission on the celestial sphere. I t would appear, however, that their experiments suffered from insufficient statistics (BoWYER et al., 1964d; FISHER and MEYEROTT, 1964b).

Sour

ce

Sco

rpiu

s X

(GIA

CC

ON

I et

al.,

19

62

)

Loc

atio

n

16 t

o

17 h

R

A

-- 4

0 ~

dec

lin

atio

n

TA

BL

E

I

SU

MM

AR

Y

OF

OB

SE

RV

AT

ION

S

Ang

ular

di

men

sion

In

tens

ity

~< 1

0 ~

5.0

qu

anta

cm

-~

sec

-~*

(2ef

~ =

3 A

)

Spec

tral

#~f

orm

atio

n

2 ef

fect

ive

= 3

F~

(GU

RS

KY

et

al.,

1963

)

(BoW

YE

R e

t al

., 1

96

4a)

~1

7

h R

A

-- 2

2 ~

dec

lin

atio

n

--

16

hl5

min

RA

--

15

~ d

ecli

nat

ion

~< 1

0 ~

20

.5 q

uan

ta

cm -

2 se

c -1

~<

5 ~

1.2

• 10

-7 e

rg c

m -

z se

c -1

(bla

ckb

od

y

at T

--

2

• 1

07

K ~

1.

4 •

10

s er

g c

m -

2 se

c-*

a,-

1

(fla

t sp

ectr

um

)

(GtA

cCO

NI

et a

L, 1

96

4a)

Cy

gn

us

X

(GIA

cCO

NI

et a

L, 1

96

2)

(GU

RsK

Y e

t al

., 19

63)

Tau

rus

X

(BoW

YE

R e

t al

., 1

96

4b

)

~<

5 ~

"Reg

ion

o

f C

assi

op

eia

A

and

C

yg

nu

s A

"

20

to

23

h R

A

10 ~

to

50

~ d

ecli

nat

ion

Co

inci

den

t w

ith

C

rab

to

_

+2

~

5 ~

1.0

• 10

-7 e

rgs

cm

2 se

c-1

(2el

f ~

3 A

)

0.6

ph

oto

ns

cm -

2 se

c 1

,,

O-e

~ ~

3 ~

)

~1

/10

S

corp

iusX

1.5

• 10

-s e

rg c

m -

2 s

ec -1

(bla

ckb

od

y

at T

=

2 •

107

K ~

2

.0

x 10

-9 e

rg c

m

2 se

c-1

Zk-

1

(fla

t sp

ectr

um

)

(Bow

YE

R e

t aL

, 1

96

4c)

C

ente

red

w

ith

in

1 ar

c m

inu

te

of

Cra

b

N

1 ar

c

min

ute

0

.45

co

un

ts

cm -

2 se

c -1

Sag

itta

riu

s X

(GIA

CC

ON

I et

al.,

19

64

a)

"Clo

se

to t

he

Gal

acti

c ce

nte

r, p

oss

ibly

co

inci

den

t w

ith

Sg

r A

" ~<

5 ~

0

.44

•

10 -7

erg

cm

~

sec

-1

(at

3 A

)

effe

ctiv

e =

3

1.5/

~,

2.5

/~

13

0 c

ou

nts

se

c -1

2

.5/~

,-8

•

40

0 c

ou

nts

se

c -1

0to

5A

Sour

ce

Loc

atio

n

Iso

tro

pic

B

ack

gro

un

d

Un

coll

imat

ed

Det

ecto

r (G

UR

SI(

Y e

t al

,, 1

96

4a)

(GU

RS

KY

et

al.,

1963

) U

nco

llim

ated

D

etec

tor

TA

BL

E I

(c

ontin

ued)

Ang

ular

di

men

sion

(BoW

YE

R e

t al

., 19

64b)

F

ield

of

Vie

w

10 ~

Hal

f A

ng

le

Co

ne

* C

orr

ecte

d

val

ue

= 2

8.0

qu

anta

cm

-~-

sec

-1 (

Gu

RsK

Y e

t al

., 1

96

3)

**

Co

rrec

ted

v

alu

e =

2.4

ph

oto

ns

cm -

2 s

ec=

1 (

Gu

RsK

Y e

t al

., 1

96

3)

t C

orr

ecte

d

val

ue

= 7

.0 q

uan

ta

cm -

~ s

ec -~

ste

r -a

(Gu

RsK

Y e

t al

., 1

96

3)

Inte

nsit

y

1.7

qu

anta

cm

-2

sec

-1 s

ter -

1 (2

elf

= 3

/~)

t

6 q

uan

ta

cm -

z se

c -~

ste

r -~

(2

o~r

= 3

A-)

6 q

uan

ta c

m

z se

c -~

ster

-~

Spec

tral

inf

orm

atio

n

2.5

A-8

A

0 7z

< x, 7~

>

,<

C

7~

0 �9

r~

,-]

t"

>

7~


Table I gives a summary of the experimental data hitherto obtained on the different X-ray sources. The sources are listed in order of their discovery, the data in order of publication. The sources are designated as Scorpius X, Cygnus X, Taurus X, and Sagittarius X. The data are presented without error assignment. This reflects

the view that the statistical data that could readily be quoted, do not reflect the much larger uncertainty due to systematic errors which could be present.

In order to clarify some of the apparent differences in the presentation of the

results, the following notes are pertinent. The ASE-MIT group has preferred to measure absorption coefficients of the radiation, thereby defining an effective wavelength. This value was then used to determine the value of the efficiency of the counters to be used in deriving X-ray intensities from counting rate. The N R L group has preferred to fit the data to thermal spectra, thereby determining an effective temperature. This temperature is used to derive the energy flux. When a thermal spectrum is fitted to the data of the ASE-MIT group, a temperature of 0.8___0.1 x 107 ~ is derived for Scorpius X (GIACCON! et al., 1964b). Also data obtained with a Na t scintillation crystal by the ASE-MIT group show a sharp cut-off at higher energy consistent with the quoted temperature and yielding an upper limit of 0.3_+ 0.08 counts cm-2 sec- ~ at wavelengths shorter than 1.8/~ (GIACCONI et al., 1964b). The two sets

of data from N R L and ASE-MIT are therefore in rough agreement with regard to intensity of the various sources. No change in intensity is apparent over the time period spanned by the experiments. Regarding the general background of soft radiation, it must be noted that it cannot yet be excluded that it originates in the Earth's

magnetosphere.

Comments on the Observations

I t may be noted that the discovery of a localized extrasolar X-ray source with flux intensity at the Earth of 10 .7 ergs cm -z was a totally unexpected event. This flux is

only 10 to 102 times less than the flux of the quiet Sun in the same wavelength interval (CHuBB et al., 1960). I f Scorpius X were a Sun-like star, it would have to be placed closer than about 10 AU to give the observed intensity. The detection of such an

object places a severe strain on a number of astrophysical theories and requires con-

siderable effort in interpretation. In the first experiment of the ASE-MIT group a large uncertainty existed in the

angular size of the source. The apparent vicinity of its location to the Galactic center

was considered significant. The observed radiation was believed to be due to phenomena which involved the Galaxy as a whole rather than to a localized source. Synchrotron radiation from cosmic electrons in the Galactic magnetic fields was suggested as a possible production mechanism (GIACCONI et al., 1962). Shortly after these data were published, a number of theories appeared which, while disagreeing with the proposed mechanism, still considered the Galaxy as a whole or the Galactic nucleus as the source of the observed radiation and endeavored to discuss conditions which would produce the measured flux.

The second experiment of the ASE-MIT group showed that the source was close


to the center of the Galaxy but apparently not quite coincident with it. The N R L

experiment definitely proved that the source in Scorpius was not the Galactic center and that its angular size was less than 5 ~ . The necessity of placing the source at Galactic distances was thus removed. Interpretations based on the assumption of a

nearby source could be advanced. The N R L group felt that their location assignment of - 16 11 15 m RA and - 15 ~

declination was sufficiently precise to state that "the Scorpius source does not coincide with any visible object". It was difficult to reconcile the absence of visible effects with the assumption of a nearby source. The authors, however, pointed out that the properties of this object were consistent with those predicted by Chiu for neutron

stars (Cmu, 1964). They also suggested that the source in the Crab Nebula was of the same nature. However, the lunar occultation measurement of N R L yielded an angular dimension of 1 minute of arc for Taurus X, which is not consistent with the

neutron star hypothesis. Measurements on Taurus X by the N R L group indicate a long wave-length

cutoff to the X-rays in the vicinity of 5 ~ that cannot be understood in terms of

interstellar absorption, while observations by the ASE-MIT group on the X-rays from Scorpius X indicate a rapid fall-off in intensity for wavelengths shorter than 2/~.

The recent discovery of Sagittarius X in close proximity to the Galactic center

and possibly coincident with Sgr A (the radio center of the Galaxy) suggests that one of the X-ray sources may be related to events taking place in the nucleus of our

Galaxy. However, it must be pointed out that it is not yet known whether the radiation from Sagittarius X is due to a diffuse source or to one or more localized sources.

The spatial distribution of the several observed sources tends to indicate a Galactic rather than an extragalactic origin. Taurus X is within our Galaxy and the remaining

sources fall within 25 ~ of the Galactic equator. The fact that Scorpius X is both the most intense source and the farthest f rom the Galactic equator can be taken as an indication of its relative proximity to the Sun. Extragalactic sources should be iso- tropically distributed on the celestial sphere. Such does not appear to be the case. For example, the data shown in Figure 10 of this paper represents a scan of approximately 1/8 of the celestial sphere in directions away from the Galactic equator and

not including Scorpius or Sagittarius. In that portion of the sky there is no indication of any source whose intensity might be as much as 1/20 that of Scorpius X.

Several different mechanisms may be at work to produce the observed celestial sources. Thus in order to distinguish between several possible origins, one must study in detail the angular dimensions, structure and spectra of each source. We will briefly review the mechanisms proposed up to now, and discuss some of the experi- mentally observable consequences.

Proposed Production Processes

Among the production processes which have been proposed are the following: (A) Bremsstrahlung Radiation; (B) Inverse Compton Effect with Photons of Starlight;


(C) Synchrotron Radiation; (D) Thermal Emission; (E) Characteristic X-ray Exci- tation. A more complete review of the possible production mechanisms is given by HAYAKAWA and MATSUOKA (1963).

A. B R E M S S T R A H L U N G R A D I A T I O N

Bremsstrahlung radiation occurs as a result of the deflection of electrons by atomic nuclei. The energy of the photon produced is a substantial fraction of the energy of the primary electrons. Thus electrons of only tens of keV are needed to produce the observed radiation. Bremsstrahlung can be produced as a result of collision of super- thermal electrons circulating in a "cold" plasma or by thermal electrons in a "hot" plasma. In the first case the energy lost in collisions with thermal electrons in the plasma and by excitation of atoms is several orders of magnitude greater than that lost in the production of X-ray bremsstrahlung. In the second case, that is, of plasma with approximately Maxwellian velocity distribution for both electrons and protons (corresponding to a temperature in the keV region), collisions between electrons do not result in any net energy loss. The second mechanism is therefore much more efficient for the production of X-ray photons in the sense that a much larger fraction of the energy dissipation results in X-rays.

A similar process was suggested by REIFFEL (1960) as a possible mechanism for production of a diffuse background of X-rays in interplanetary space from solar plasma. HOYLE (1963) calculated the electron-proton bremsstrahlung produced in the intergalactic radius of the "hot universe" model of the steady state theory, and found it consistent with the observed isotropic background. A more accurate calcu- lation by GOULD and BURBIDGE (1963) showed that the predicted flux would be two orders of magnitude higher than observed, and the authors concluded that the "hot universe" model was incompatible with the X-ray observations. They pointed out that the background is comparable to the expected contribution from all external Galaxies if they emit X-rays from their centers at the same rate as our own.

GOULD and BURBrDGE (1963) argued that electron-proton bremsstrahlung from a spectrum of fast electrons impinging on the cold plasma in the Galactic center would account for Scorpius X, then erroneously identified with the Galactic center. However, if one accepts this interpretation, the power dissipated in collisions in this region would be about 5 x 1043 ergs/sec, equal to the output of about 10 l~ Sun-like stars. On the other hand, Rossi (1964) pointed out that the hypothesis of bremsstrahlung by a "hot" plasma cloud in the Galactic center cannot be discounted.

HZILES (1964) discusses the heating of interstellar gas behind the shock wave from a supernova explosion and proposes a similar mechanism for the origin of

Scorpius X. Bremsstrahlung by a "hot" plasma can also supply the observed X-ray flux from

the Crab Nebula if the density Ne of electrons at, say, 10 keV energy is such that NeNp ~ 6 x 10 s cm - 6 (Np = proton density). A hot plasma cloud with Ne = Np ~ 400 cm- 3 would fulfil this requirement and would be consistent with the observations (RossI, 1964). This interpretation of Taurus X emission is discussed by BOWYER et al. (1964c)


with regard to the possible processes which may heat the gas to the required temperature. Radioactive energy production rates are considered to be adequate to supply the required power (WOLTJER, 1958).

B. I N V E R S E C O M P T O N E F F E C T

This effect is a process whereby a fast electron colliding with a photon of a few eV transfers part of its energy to the photon and shifts its wavelength toward the X-ray region. Electrons in the 20 to 30 MeV energy range colliding with starlight photons can produce X-rays of the observed energy. This process was proposed by FELXON and MORRISON (1963) to account for at least a portion of the observed flux.

CLARK and ODA (1963) pointed out that this process could account for X-ray production in Scorpius X if it were a nearby supernova remnant. However, the inverse Compton mechanism requires electrons of about 20 MeV, which should also produce bremsstrahlung 7-rays of similar energy in radiative collisions with protons. An experiment to detect ?,-rays of this energy from this source by OVERBECK, ODA, and CLARK (1964a) yielded a negative result which places an upper limit on the proton density N v in the region of the source of N v < 3 x 10-3 cm-3. This value appears unacceptable for a Galactic source.

C. S Y N C H R O T R O N R A D I A T I O N

Synchrotron radiation is produced by a fast electron as a consequence of its deflection in a magnetic field. For electrons of 2 x 1014 eV in a field of 3 x 10 - 6 gauss, the peak of synchrotron emission is 3 A.

Shortly after the first observation, Clark suggested that X-rays could be produced by this mechanism (CLARK, 1963). The apparent coincidence of regions of high radio emission in the center of the Galaxy and Cygnus with regions of high X-ray emission was considered significant. Subsequently, Clark discussed the relation between the intensities of cosmic 7-rays near 1014 eV and the X-ray synchrotron emission of cosmic electrons of comparable energy which arise together with the 7-rays in collisions between cosmic ray protons and interstellar matter. He concluded that the background X-ray intensity predicted from the upper limits on the 7-ray intensity set by air shower experiments was less than the background intensity apparently observed. GINZBURC and SYROVATSKII (1963) also discussed this mechanism and have shown that using current values of the Galactic magnetic field, matter density and cosmic ray flux, it was not possible to account for the observed X-ray fluxes. FRIEDLANDER (1963) also discussed this process.

ODA (1963) and SKLOVSKIJ (1964) have suggested that Scorpius X could be due to synchrotron radiation from electrons in a shell ejected by a Type II supernova. The suggestion is based on the fact that the position of the X-ray source in Scorpius appears to coincide with the position given (BROWN e t al . , 1960) for the center of the radio object known as "The Spur".

Both WOLTJER (1964) and SKLOVSKIJ (1964) have recently shown that assuming synchrotron production for visible light, radio and X-ray emission in the Crab,


the energy fluxes observed in the three spectral regions are consistent with a fairly smooth electron spectrum. WOI.TJER (1958), in addition, has shown that the flux at 2<200/~, as calculated from an extension of the synchrotron emission in the visible region, can account for the [OII] and [OIII] emission lines seen in the Crab. Using 10 .4 gauss as the magnetic field intensity in the Crab, the mean life of electrons of 4 x 1013 eV (required to produce 3 ,~ radiation) is only 20 years, so that a very efficient acceleration mechanism is required to maintain the necessary density of electrons. However, if one allows that the X-rays are produced in regions of higher magnetic field by lower energy particles, the requirements on the acceleration mechanism are less severe.

D, T H E R M A L EMISSION

Thermal emission has been proposed by BOWYER et al. (1964b) as the source of X-ray emission from Scorpius X and Crab X. They suggested that both sources are neutron stars, remnants of supernova explosions consistent with the X-ray emitting properties predicted by CHItJ (1964). A neutron star is the conjectured terminal phase of stellar evolution in which the interior has contracted to nuclear densities and consists of a degenerate neutron gas. A model by CAMERON (1959b) indicates a mass limit of about 1 solar mass and a radius of 10 km. Both Chiu and Cameron have argued that the surface temperature should be of the order of 107 ~ This temperature, in con- junction with the small diameter, could result in an object observable in X-rays at a distance of about 1000 parsecs but not observable in the visible or radio portions of the spectrum. ODA (1964a) pointed out that a neutron star assignment for Crab and Scorpius could be maintained in spite of the different characteristics in visible and radio emission. These differences would be accounted for by the different environment in which the parent supernova explosion occurred.

The recent experimental evidence on the Taurus X, which yields a finite angular width of about 1 minute of arc, is not consistent with the small size required for a neutron star. OVZRBECK (1964b) has studied the possibility that small angle scattering of soft X-rays by interstellar dust could increase the apparent angular diameter of a celestial X-ray source. He concludes that this effect cannot account for the observed angular diameter of Taurus X if one uses current values for the interstellar dust density and size. This does not exclude the possibility that Scorpius X or Sagittarius X is a neutron star or that neutron stars in general do exist and may account for some of the observed sources.

E. CHARACTERISTIC X - R A Y EXCITATION

Characteristic X-ray excitation is the process whereby K-series transitions following a K-shell ionization result in the emission of monochromatic X-radiation. This process occurs in regions where there is a substantial flux of high energy particles in collision with atoms other than hydrogen. It necessarily accompanies electron bremsstrahlung, in which case the characteristic radiation is superimposed on a continuous spectrum of X-rays or can result from proton excitation, in which case virtually only the charac-


teristic radiation results. It was shown by GOULD and BURBIDGE (1963) that such an effect could contribute observable intensities from the center of the Galaxy if high but not unacceptably high fluxes of energetic particles were present. HAYAKAWA and MATSUOKA (1964) also discussed the excitation of K X-rays by non-relativistic particles in interstellar space.

F. C O M M E N T S

It may be noted in the above discussion that more than one production mechanism is invoked to account for the same source. It is difficult to choose between the several processes on the basis of the X-ray observational data alone. Except for the Crab, known objects have not been uniquely identified with the X-ray sources. The measurement of the angular dimensions of the X-ray emission region in the Crab probably excludes a neutron star as the radiation source. The long wavelength cutoff reported by the NRL group is very difficult to explain. This cutoff does not appear in the spectra characteristic of synchrotron radiation or inverse Compton effect. Nor does it appear in the bremsstrahlung of a "hot" plasma cloud if the optical depth of the cloud is small compared with unity, as appears to be the case for the Crab. If the optical depth is large at 5 A, a cutoff would appear in the X-ray spectra at that wavelength independent of the production mechanism.

For the other X-ray sources the lack of supporting data obtained from radio or optical observations makes difficult any statement as to which production mechanism is at work.

Prospects of Observational X-Ray Astronomy

Planning and execution of celestial X-ray experiments has proceeded rapidly since 1962 when the existence of celestial X-ray sources was first announced. The results already obtained are extremely encouraging. We are convinced that observations in the X-ray region of the spectrum will ultimately have as profound an impact on our understanding of the universe as the observations in the radio region have had.

Clearly more experimental work is needed to survey the sky with greater com- pleteness and higher sensitivity. With sensitivities only a few orders of magnitude greater than presently available, ordinary stellar sources will be observed, and the detection of extragalactic X-ray sources will be possible.

More detailed measurements are needed to understand the nature of the observed sources. It is particularly urgent to measure their spectrum since it is intimately related to the production mechanism. Also, since interstellar absorption becomes appreciable at longer wavelength, spectrum measurements extended to the 10 to 50 A region can help in determining the distance of the sources. The detection of characteristic X-ray lines by means of high resolution spectrometers would be of great significance. Equally important is the determination of the angular dimensions and of the structure of the sources if extended. Accurate determination of the celestial location of the sources will permit a careful search for visible and radio emission.


These objectives can be achieved by the use of state-of-the-art techniques. The use of orbiting observatories will permit prolonged observations which will result in higher sensitivity and possibly in lifetime determinations. Also transient effects such as could be produced by flare stars could be observed. Recently developed detection techniques can be used to extend the measurements over the desired wavelength range. Improvements in X-ray optics offer the hope of constructing telescopes for soft X-rays with angular resolutions of 5 seconds of arc.

It appears useful to discuss some of these technical developments to indicate the direction that future experiments may take.

Experimental Techniques

C O N V E N T I O N A L D E T E C T O R S

The basic detector that has been used to date to measure cosmic X-rays has been the gas counter operated in either the proportional or Geiger region. Because of the low fluxes, a detector with large area, capable of withstanding the environment of a rocket flight, is mandatory.

The efficiency, E(2), of a gas counter for X-rays of wavelength 2 at normal incidence is given by the relation

E(2) = exp(-r ['1 - exp (#gXo) ]

where/~w and Xw are the mass absorption coefficient and the thickness of the window material, respectively, and/~g and X 0 are the corresponding quantities for the filling gas. The window determines the long wavelength response of a given counter as is expressed by the factor exp(-/~wX~). Table II lists some window materials that have

TABLE II LONG WAVELENGTH CUTOFF FOR THE TRANSMISSION OF SOFT X-RAYS

FOR SEVERAL MATERIALS

Material Thickness E (2) > 10%

Beryllium 0.002" = 9 mg/cm z 2 < 10

Beryllium 0.005" = 23 mg/cm z ,~ < 7

Mica* 0.0002" = 1.4 mg/cm z 2 < 10/~

Mylar 0.00025" : .9 mg/cm 2 )~ < 13 ,~

Mylar 0.001" = 3.6 mg/cm 2 ;t < 8

* In mica the K-edges of silicon and potassium give rise to narrow regions near 7 and 8/~ where the transmission falls below 10 % for the stated thickness.

been used in cosmic X-ray measurements. Also listed is the spectral cutoff of each of these materials, defined as the wavelength at which the transmission falls to 10%. At short wavelengths the limit for gas counters is imposed by absorption in the gas. In conventional counters the response is limited to 1 or 2 •. This short wavelength


limit can be extended almost at will by use of scintillation counters. Detection of

7-rays in the MeV energy region is conventional, and so is the detection of X-rays

of several tens of kilovolts of energy. The scintillation counters flown in the ASE- M I T experiments utilized a 3" CBS photomultiplier Type 7818 viewing the same

diameter NaI(T1) crystal. I t was found possible to operate at a pulse height threshold equivalent to 1.8/~ X-rays (GIACCONI et al., 1964b).

P H O T O E L E C T R I C D E T E C T O R

The extension of spectral measurements to longer wavelengths (2> 10 ~ ) than is possible with gas counters appears, however, to be more difficult than the extension to short wavelengths. With this in mind, the group at ASE has developed a detector that does not require a window and can therefore be utilized in any portion of the soft X-ray region. This counter can be constructed with large collection area.

The basis for the new detector was the discovery in 1960 by a group of Russian scientists of an anomalously large external photoelectron yield for several alkali halides, for incident soft X-rays (LuKIRSKIJ et al., 1960ab). Some of their early results

are listed in Table III . Recent investigations (SCHEMELEV et al., 1963abc) have revealed

TABLE HI QUANTUM EFFICIENCY OF VARIOUS MATERIALS FOR MONOCHROMATIC X-RAYS

Wavelength *

1.54A 2.28A 2.74A 3.35A 4.724 8.32]t 11.9h 13.3A 23.6/~44A 67/~

CaF~ 0.159 0.074 0.142

CsI 0.057 0.011 0.049 0.315 0.141 0.358 0.760 0.945

LiF 0.06 0.17

NaBr 0.013 0.046 0.035 0.036 0.061 0.033 0 .056 0.178

NaC1 0.135 0.195 0.135

NaF 0.032 0.125 0.26

SrFz 0 .014 0.058 0.036 0.066 0 .116 0.216 0.123 0.155 0.22 0.31 0.27

* Values for wavelength 13.3/~ and shorter are from LLrKIRSKIJ et al. (1960b); the other values are given in LUKIRSKU et al. (1960a).

that, in addition to the high quantum yield, the number of photoelectrons released per incident photon was greatly in excess of one. Despite the difficulty in understanding the results from a theoretical point of view, a practical device can be constructed for the efficient detection of soft X-rays that utilizes an alkali halide photocathode. The device is shown schematically in Figure 12. The ejected photoelectrons are focussed onto the first dynode of an electron multiplier structure that is operated to give a detectable pulse for each incident electron. Because the alkali halides have high work functions, the sensitivity to visible light is low, as is the thermionic noise

170 R, GIACCONI AND H. GURSKY from the photocathode. Although the device can be operated in vacuum without a window, in an application where ultraviolet light will be present, a window must be provided to limit the spectral response to X-rays. This need only be a few thousand angstroms of aluminium evaporated onto a thin organic film backing. Such a window will transmit efficiently X-rays well beyond the K-edge of carbon at 44 A. Since

CATHODE

INCIDENT X-RAYS

"~- -~ENTRANCE APERTURE

L ~ / / ~ - - ~ E LECTRON MULTIPLIER

ELECTROSTATIC FOCUSING ELEMENT ~//~/~,'~/- \? Fig. 12. Schematic drawing of photoelectric detector developed for detection of soft X-rays. Photo-

electrons ejected from an X-ray sensitive photocathode are detected in an electron multiplier.

the device is operated at hard vacuum both inside and out, the window is not required to support any pressure differential.

A detector of this type, flown during the fall of 1964 to measure cosmic X-rays, is shown in Figure 13. The device had a sensitive area of 40 cm 2, the photocathode was an evaporated layer of KC1 of 1 micron thickness, and a commercially available electron multiplier (EMR 541 A) was used to detect the photoelectrons. A 0.00025" thick aluminum window was used to exclude visible light and residual ions from the ambient. The data obtained with this detector are presently being analysed.

The net effficiency-area factor for the detector is comparable to that of gas counters used in cosmic X-ray experiments, but the device is freed from the restriction of a thick window needed to contain a counting gas at a substantial pressure. The lack of such a restriction permits construction of photoelectric detectors of substantially larger area than the one shown. The only limitation in size is the requirement that' electrons emitted at any point on the photocathode be focussed into the electron multiplier. The detector thus appears to offer important advantages for long wavelength measurements of cosmic X-rays over more conventional detectors.

OBSERVATION OF X-RAY OURCES OUTSIDE THE SOLAR SYSTEM

Fig. 13. Photoelectric detector developed by the ASE group and flown in the August 1964 launch. A honeycomb collimator in the detector aperture limits the field of view to 10" x 40" full angle. The "0"-ring sealed door is normally closed and opens only after the rocket has reached altitude.

HIGH ANGULAR RESOLUTION COLLIMATOR

The accuracy in the determination of the angular dimensions of a source of radiation as well as its location in the sky is directly a function of the angular resolution of the detector.

The problem of achieving high angular resolution with conventional mechanical collimators is a severe one. Aside from the problem of construction of a large area collimator with an angular resolution of arc minutes or less, the total number of counts accumulated by a given detector which scans across a source is directly proportional to its field of view. The experimenter is thus forced, in the quest for finer angular resolution, to even larger area detectors and more sophisticated experiments than are currently underway. ODA (1964b) has devised a collimator that incorporates


both a broad field of view and high angular resolution. The device whose principle of operation is illustrated in Figure 14, consists of two sets of parallel wires arranged so that parallel incident radiation is either transmitted or obscured depending on the angle of incidence of the radiation. If the collimator scans across a source of radiation of small angular dimensions (~ diD), the transmitted flux is modulated with a period related to the rate of change of the angle of incidence of the radiation on the collimator. The modulation will gradually disappear as the angular size of the source increases. Thus the degree of modulation observed can be used to determine the

I_ 13 _1

8(( a--- d D B ).-~-

Fig. 14. Schematic diagram demonstrating principle of the Oda collimator. The quantity d is the wire diameter, s is the separation between the wires, D is the separation between the wire planes, and 5 is the angular divergence of the incident radiation. The cross-hatched area shows the shadowing of

the wires which tends to disappear as 5 increases.

angular dimensions of a radiation source; or equivalently, one can determine the flux contained within an angle diD compared to the total flux from a source.

Collimators of this type have been used in recent ASE-MIT rocket experiments. Typical dimensions are: d=S=0.008", D=l .5" , total area= 5" x 6", yielding an angular resolution of 10 arc minutes. It appears feasible to construct an Oda collimator with an angular resolution of a fraction of an arc minute.

X-RAY FOCUSSING TELESCOPE

While the Oda collimator can yield data on the gross structure of a radiation source, its angular resolution in terms of the ability to resolve adjacent sources is very poor. Thus, small scale structure cannot be investigated with the Oda collimator. What is required is a device with high spatial resolution; namely, a telescope. A focussing X-ray telescope can in fact be constructed. The prime advantages of the instrument are the large area of collection, the high angular resolution and the improvement in signal-to-noise ratio. Its principle of operation was first suggested by GTACCONT and RossI (1960) and relies on the phenomena of total external reflection of X-rays at grazing incidence. In order to remove aberrations, two reflections of the incident radiation are required, the first from a paraboloid and the second from a hyperboloid. The ray path through the telescope is shown in Figure 15. A discussion of the theory of the focussing of X-rays by multiple reflections is given by WOLTER (1962) and


more recently by KIRKPATRICK (1963), and practical devices have been developed by

GIACCOM et al. (1964c). Such a telescope was used to obtain a solar X-ray photograph f rom a pointed

rocket in a joint experiment by G S F C and ASE ( I A U 1964b). The characteristics

o f the instrument are: focal length 25", projected collection area 2 cm z, angular

resolution 1 minute of arc, and efficiency (ratio of focussed to incident flux) 0.2%.

~PARABOLOID HYPERBOLOI D

J ~ _ _ m j . . - - ~ . ~ , ~ / - RADIATION

SINGLE REFLECTION FOCUS

Fig. 15. Ray path through focusing X-ray telescope for both single reflection and double reflection systems. For off axis radiation the single reflection system produces a circle in the focal plane, while

the double reflection system still produces a point focus.

Marked improvements in the reflection efficiency as well as in angular resolution

have recently been accomplished. Efficiencies of the order of 20% have been measured

by the group at ASE. Telescopes with angular resolution of 5 seconds o f arc appear

feasible. The large collection areas and fine angular resolution should permit the

execution o f significant measurements.

R e f e r e n c e s

BROWN, R. H., DAVIES, D., and HAZARD, C. : 1960, Observatory 80, 191. BOWYER, S. et al. : 1964a, Space Research 4, 966. BOWYER, S. et al. : 1964b, Nature 201, 1307. BOWYER, S. et al. : 1964c, Science 146, 912. BOWYER, S. et al.: 1964d, Astrophys. J. 140, 820. BURNIGHT, T. R.: 1949, Phys. Rev. 76, 165. CAMERON, A. G. W. : 1959a, Astrophys. J. 129, 676. CAMERON, A. G. W. : 1959b, Astrophys. J. 130, 884. CLARK, G. : 1963, Nuovo Cimento 30, 727. CLARK, G. and ODA, M.: 1963, 'X-Ray Production in Super Nova Remnants', Proceedings of the

International Conference on Cosmic Rays, Jaipur, India. CHIu, H.-Y.: 1964, Ann. Phys. (N.Y.) 26, 364. CtlUBB, T. A., FRIEDMAN, H., and KREPLIN, R. W.: J. Geophys. Res. 65, 1831.


FELTON, J. E. and MORRISON, P. " 1963, Phys. Rev. Letters 10, 453. FISHER, P. C. and MEYEROTT, A. J. : 1964a, Astrophys. J. 139, 123. FISHER, P. C. and MEYEROTT, A. J. : 1964b, Astrophys. J. 140, 821. FRIEDLANDER, i . : 1963, Submitted for publication in Nuovo Cimento. FRIEDMAN, H.: 1959a, J. Geophys. Res. 64, 1751. FRIEDMAN, H . : 1959b, Proe. IRE 47, 278. FRIEDMAN, H. : 1964, in Astronomical Observations from Space Vehicles, IAU Symposium No. 23,

Li6ge, Belgium. GIACCONI, R. and RossI, B. : 1960, d. Geophys. Res. 65, 773. G1ACCONI, R. et al. : 1962, Phys. Rev. Letters 9, 439. GIACCONI, R. et al. : 1964a, 'Observation of Two Sources of Cosmic X-Rays', submitted for publi-

cation in Nature. GIACCONI, R. et al. : 1964b, 'Measurements on Celestial X-Ray Sources', Space Research 5, to be

published. GIACCONI, R. et al. : 1964c, 'An Aplanatic Telescope for Soft X-Rays', submitted for publication in

J. Opt. Soc. Amer. GINZBURG, V. L. and SYROVATSKII, S. I. : 1963, Soviet Physics - JETP 45, 353. GOULD, R. J. and BURBIDGE, G. R.: 1963, Astrophys. d. 138, 969. GURSKY, H. et al. : 1963, Phys. Rev. Letters 11 , 530. HAYAKAWA, S. and MATSUOKA, M.: 1963, Progr. Theoret. Phys. (Kyoto) 29, 612. HAYAKAWA, S. and MATSUOKA, M. : 1964, 'Origin of Cosmic Rays', to be published in Progr. Theoret.

Phys. (Kyoto), Suppl.). HEILES, C. : 1964, 'Supernovae Shells and Galactic X-Rays', preprint. HOYLE, F. : 1963, Astrophys. J. 137, 993. JAGER, C. DE: 1964, 'Solar Ultraviolet and X-Ray Radiation', in Research in Geophysics, MIT Press,

Cambridge, Mass. KIRKPATR1CK, P. : 1963, 'Grazing-Incidence Telescopic Systems' in X-Ray Optics and X-Ray Micro-

Analysis (ed. by H. Patee et al.), Academic Press, New York, pp. 247-254. LINDSAY, J. C. : 'Astronomical Observations from Space Vehicles', IA U Syrup. No. 23, Li6ge, Belgium

(to be published). LUKIRSKIJ, A. P., RUM~, M. A., and SMIRNOV, L. A. : 1960, Opt. i Spekroskopija 9; 265. LUKIRSKIJ, A. P., RUMg, M. A., and SMIRNOV, M. A.: 1960b, Opt. i Spektroskop~a 9, 343. MANDEL'SHTAM, S. L. and EFREMOV, A. I.: 1958, 'Research on Shortwave Solar Ultraviolet Radi-

ation', in The Russian Literature o f Satellites, Part If, International Physical Index, Inc., New York.

ODA, M. : 1963, Proceedings of the International Conference on Cosmic Rays, Jaipur, India. ODA, M. : 1964a, Nature 202, 1321. ODA, M.: 1964b, d. Appl. Opt. 4, 143. OVERBECK, J. W." 1964, Thesis. Experimental Test o f the Inverse Compton Effect, MIT, Cambridge,

Mass. OVERBECK, J. W.: 1964b, 'Small Angle Scattering by Interstellar Grains as a Source of Angular

Broadening of Celestial X-Ray Sources', submitted to Nature. Proceedings o f the Conference on X-Ray Astronomy, Smithsonian Astrophysical Observatory, Cam-

bridge, Mass. 20 May 1960. REIEEEL, L. : 1960, Nature 185, 229. RossI, B. B. : 1964, Proceedings of the Solvay Conference on Physical Problems in the Structure and

Evolution of Galaxies, Brussels, 1964 (to be published). SCHEMELEV, V. N. and RUMSH, M. A. : 1963a, Soviet Physics - Solid State 5, 43. SCHEMELEV, V. N. and RUMSH, M. A., and DENISOV, E. P.." 1963b, Soviet Physics - SolM State 5, 827. SCHEMELEV, V. N. and RUMSH, M. A. : 1963c, Soviet Physics - Solid State 4, 2048. SKLOVSKIJ, I. : 1964, Astron. cirkuljar 304. STROM, S. E. and STROM, K. M.: 1961, Astron. Sac. Pacific 73, 43. WOr_TER, J. : 1952, Ann. Physik 10, 94. WOLTJER, L." 1958, Bull. Astron. Inst. The Netherlands 14, 39. WOLTJER, L." 1964, Astrophys. J. 140, 1309.


Note Added in Proof. At the second conference on Relativistic Astrophysics held at Austin, Texas during December, 1964 (Proceedings to be published) additional data was presented by GIACCONI of the ASE-MIT group and by FRXEDMAN of NRL based on rocket flights carried out in 1964.

GrACCONI reported that the region 20 ~ along the Galactic equator and containing the Galactic center (see Figure 11) appears to contain several point sources. The intensity from each of the sources is less than 1/10 the intensity from Scorpio. Giacconi gave the location of two of the sources and set an upper limit on their angular dimension of 30 minutes of arc. Also reported was a measurement of the angular size of Scorpio X by the ASE-MIT group using the Oda collimator which yielded an upper limit of 7 minutes of arc.

FRIEDMAN reported the observation of two sources in Cygnus and of several intensity peaks along the Galactic equator which were interpreted as six distinct sources. He also reported that another observation of the Crab with counters similar to those flown during the occultation experiment yielded evidence for a significant flux of X-rays above 5 ~. He added that there was a distinct possibility that the thinner mylar counter flown during the occultation experiment (see Figure 7, Counter A) was not sensitive at long wavelengths because of water vapor contamination. Thus their conclusion, at that time, regarding a cutoff in the spectrum of X-rays from the Crab was not valid.

At the same conference, CLARK of MIT reported having observed X-rays with energies between 15 and 60 keV from the Crab Nebula using a scintillation counter carried in a high altitude balloon. (See also Phys. Rev. Letters 14 (1965) 91.) The flux in this energy rangewas reported to be 0.02 photons cm -~ sec -1 or about 1 ~ of what is observed in the 2-8 ~ interval.

Documents

Observation of X-ray sources outside the solar system