A diversity of instruments have been utilized or proposed for use in X-ray astronomy. This in part reflects the fact that the field is very new, that many competing techniques have been devised for any given type of observation and that there has been a progression to increasingly sophisticated instruments. But most of this diversity is real and simply reflects the very broad range of observations that can be performed and the different kinds of observational opportunities that are presented to the several experimental groups. Useful measurements are being performed over three decades of energy extending from 0.25 keV to 500 keV, positional measurements extend from degrees to arc seconds precision, attempts have been made to measure the polarization of the X-ray sources, time variations are being studied and spectra are being measured to resolutions of the order of ∼20%. The required instruments are being flown on sounding rockets, balloons and satellites. In addition, observation of the diffuse X-ray backgrounds requires even different instruments.

Prof. Friedman did not communicate any text or Summary of his invited discourse.

The discovery and measurement of cosmic X-ray sources has almost invariably been performed with proportional counters which have large window areas. In the energy range from 1 to 50 keV, proportional counters have advantages over other types of detectors; they provide energy resolution and they can be made relatively easily with very large window areas.

The object of this paper, which describes work which has been carried out in close collaboration between members of our two groups, is to present the background of the upper-air fluorescence technique as a viable ground-based tool for studies of cosmic X-rays. After a discussion of the characteristics, merits and limitations of the technique, we describe some preliminary experiments which have been carried out, both separately and also jointly, with our respective installations. These are sited at Mount Hopkins (the SAO field station at 2300 m altitude near Amado, Arizona) and at Sparsholt Firs (approx. 300 m altitude, on the Berkshire Downs, near Harwell).

It was proposed that the COS payload should include an experiment to study cosmic γ-rays and one for cosmic X-rays. From a scientific point of view the association of these experiments is clearly valuable, the more so in view of Ogelmann's suggestion that the γ-rays observed by Clark, Garmire and Kranshaar and by Hutchinson represent the high energy extension of the spectra of galactic X-ray sources. It may, however, be argued that the X-ray sources can independently be studied and that, in any case, the most important sources near the galactic plane will be well enough known by the launch date of the COS satellite. This argument may not be valid in view of the observed variability of X-ray sources, but in any case a strong case can be made for the proposed kind of experiment.

The following is a brief summary of some of the recent results obtained at M.I.T. which bear on the properties of various discrete X-ray sources, both galactic and extragalactic. Most of the work referred to has been described in publications which contain more complete descriptions.

Observations to determine the spectra and time variations of hard X-rays from cosmic sources have been made from balloons and from the OSO-III satellite. These data have been obtained using actively collimated scintillation counters with apertures between 6 and 24° FWHM, areas between 10 and 50 cm2 and which operate over the 10–300 keV range. The Crab Nebula has been observed on three occasions over a 22-month period between September 1965 and July 1967. The power law spectrum has a number index of 2.0 ± 0.1. No long-term changes were observed over the 30–100 keV range with a limit at 3%/yr. A balloon search with a 10 cm2 Ge(Li) detector for X-ray lines at 62.5 keV, 110 keV and 180 keV due to heavy element radioactive decays which would be produced in the initial Crab explosion based on the Cf254 hypothesis has resulted in upper limits at about 10−3 γ-rays cm2-sec. This is about a factor of 20 above the predicted levels. Simultaneous X-ray and optical observations of SCO XR-1 from OSO-III confirm that X-ray and optical flaring are indeed coincident phenomena, and that although the X-ray intensity increases about a factor of two during the flare, the equivalent temperature of the excess radiation is nearly the same as that of the quiescent object. Upper limits, 95% confidence, on the flux of M-87 at 40 keV have been obtained. These are inconsistent with the flux of 1.2 × 10−4 photons/cm2-sec-keV reported in the literature. CYG X-1 has been observed to have a power law of number index 2.0 ± 0.2. The OSO-III has observed a number of sources in the southern skies including NOR XR-2 and the variable source Centaurus XR-2.

Prof. McCracken did not communicate any text or Summary of his invited discourse.

Skylark 723, launched at 10.30 GMT on 12th June 1968 was an unstabilised rocket carrying two proportional counter detectors, each of 1385 cm2 effective area. Detector C1 covered the energy range 1.4 to 2.5 keV and detector C2 the range 2.0 to 18 keV. The detector outputs were analysed into 4 and 9 energy channels respectively. The field of view of each detector was 28° and 4° (FWHM) with the greater collimator extension mounted parallel to the longitudinal rocket axis for C2 and 6/10 of the C1 detector, the remaining 4/10 of the C1 collimator being canted at 40° to the major axis. During the flight, the rocket spun at a constant rate of 75° per sec whilst the spin axis slowly precessed about a flat cone, thereby surveying some 80% of the visible sky. On 25 consecutive scans the Vir XR-1 source, which by chance lay near the precession axis, crossed the field of view of both detectors. The count rate profiles for twenty scans in which the source passed within 21° of the centre of the field of view have been added together for each detector, resulting in the totals shown in Figure 1. Significant peaks are seen in the position of Vir XR-1 for both detectors and these have been fitted with the measured transmission profiles of the respective collimators assuming parallel incident radiation. The peaks are consistent with the observation of X-rays from a single point source.

No text nor Summary was communicated by the Authors.

Preliminary results of two rocket flights carrying X-ray payloads conducted from Thumba Equatorial Rocket Launching Station (TERLS), Trivandrum, India, on November 3, 1968, and November 7, 1968, respectively, are presented. The results indicate the first evidence for the existence of low energy X-ray flux in the energy range 2–20 keV from Cen-X2 source since the reported extinction in May, 1967. The energy spectrum and the absolute flux of X-rays from Cen-X2, Sco-X1 and Tau-X1 are presented and compared with other observations.

In this paper we report on our observations of hard X-rays from several X-ray sources in the energy range 20–120 keV. The results were obtained from the data collected during two balloon flights made from Hyderabad, India (latitude 17.6°N, longitude 78.5°E). The first flight was made on April 28, 1968, and the balloon reached a ceiling of about 5.3 g cm−2 residual atmosphere and floated from 0230 to 0800 hrs. IST (Indian Standard Time). The second balloon was launched on December 22, 1968 and floated at about 7.5 g cm−2 of residual air from 1000 to 1130 hrs. IST.

In this note we wish to report briefly the observation of sudden changes in the intensity of Sco X-1 by a factor of about 3 recorded in the energy interval 29.9–52.3 keV on December 22, 1968 between 04 h 27 m and 05 h 53 m UT. The observation was made with an X-ray telescope flown in a balloon from Hyderabad, India. The balloon was launched at 0200 hr UT and reached the ceiling of 7.5 g/cm2 of residual atmosphere at 0435 hr UT. The X-ray telescope consisted of a NaI(T1) crystal with an area of 97.3 cm2 and thickness 4 mm, surrounded by both active and passive collimators. The telescope was mounted on an oriented platform which was programmed to look in four specified directions successively, of azimuths, Φ=0°, 110°, 180° and 310° (Φ=0° being North and Φ=90°, West), spending about 4 min in each direction during a cycle of period of about 16 min. The axis of the telescope was inclined at an angle of 32° with respect to the zenith. A pair of crossed flux gate magnetometers provided information every 8.2 sec on the azimuth of the telescope. The pulse heights from the X-ray detector were sorted into several channels extending from 10 to 120 keV. An Am241 source came into the field of view of the telescope once in 15 min for about 30 sec to provide in-flight calibration of the detector. The meridian transit of Sco X-1 was at 0454 hr UT. Just before the balloon reached the ceiling Sco X-1 was in the field of view of the telescope for 3 min and 41 sec. After the balloon reached ceiling, Sco X-1 was in the field of view of the telescope on five occasions between 0443 and 0553 hr UT. During the last observation, however, the balloon had lost altitude by about 1 g/cm2. The excess counts due to Sco X-1 were obtained by subtracting the counting rates corresponding to the North direction which did not include any known X-ray sources. The observation on Sco X-1 in the 1st cycle was made while the balloon was still ascending and consequently the interposed grammage was changing from 10.5 to 9.7 g/cm2. However, for the energy range under consideration, the change in the background counting rate was not significant and there cannot be any doubt regarding the genuineness of the excess counts recorded.

In the past 2 years the group at American Science & Engineering, Inc., including Gursky, Gorenstein, Kellogg and me, has been engaged in a program of rocket launches aimed at the solution of several outstanding questions in X-ray astronomy. Of four rocket launches, one was almost totally unsuccessful due to a mechanical failure; one was only partially successful due to a failure of the ACS (Attitude Control System); and two performed as planned. Notwithstanding the delays induced by this run of bad luck, a considerable amount of data has been gathered and analyzed. The major results obtained in the flights on October 11, 1966, and on February 2, 1968, have been published. It is my intent today to report on preliminary results of a recent flight (December 5, 1968) and briefly summarize conclusions from previous flights, and comment on a few selected topics to which this experimental program may contribute.

This paper reports upon an observation of hard X-radiation from the direction of M87. Earlier (Haymes et al., 1968a), we reported the apparent detection of hard radiation from the general direction of Virgo; the experiment discussed here, conducted 16 months later, recorded approximately the same flux from a smaller region of sky that tends to isolate M87 as the most likely candidate for the source.

Cosmic X-rays were observed with three sets of proportional counters covering the energy range between 0.15 and 20 keV. The detector born on a spinning rocket scanned a celestial region in which the galactic latitude bII changed from 30° to −55° across the galactic plane in the Cygnus-Cassiopeia region. The spectrum of Cyg XR-2 thus obtained is represented by a thermal bremsstrahlung of temperature 3.4 keV modified by the interstellar absorption for the hydrogen column density of 3 × 1021 cm−2. The diffuse component showed an interstellar absorption effect, which was however found much weaker than one would expect if the diffuse component were due entirely to be of extragalactic origin. The spectrum obtained in the highest latitude region is represented approximately by a power law E−1.8 but shows a possible trough at about 1 keV.

We performed a couple of balloon experiments to measure the size and the location of Cyg-X-1 using the techniques of the modulation collimator [1]. The angular periods of the modulation collimator were 26′ and 10′ for the respective flights corresponding to the approximate angular resolutions of the size determination and location, 3′ and 1′ respectively. Preliminary results of the experiment with 3′ resolution are reported here.

An X-ray observation of the Cassiopeia Region by the ASE group from a sounding rocket on December 5, 1968, has resulted in the determination of locations for two sources that are precise to about 0.1 of a square degree. The positions of two well-known radio sources Cas A and SN 1572 (Tycho's Supernova), objects which are remnants of relatively recent galactic supernova, are consistent with these locations. Inasmuch as that region of the galaxy does not appear to contain nearly as large a concentration of objects as the galactic center, it is reasonable to make the identification between the X-ray sources and the supernova remnants on the basis of there being a small a priori probability of having an accidental coincidence within 0.1 square degrees. Cas A is almost certainly the same source as Cas XR-1 which the NRL group saw in an earlier survey [1]. During the December flight the Crab nebula was also observed for a short time interval.

A rocket-borne measurement carried out on March 3, 1969 has yielded a net (dead-time-corrected) exposure of 104 cm2 sec to the differential spectrum of X-radiation from Sco X-1. The data are fully consistent with radiation from an optically thin thermal source, even to the extent that K-emission from high ionization states of iron appears to be present. Such iron emission is consistent with cosmic abundance at the measured temperature.