On March 20, 1969, we scanned the Southern sky for 8 hours in a manner similar to that during our October 15, 1967 observations. A quick look at the data shows clearly that the brightness of Cen XR-2 in the energy range above 20 keV has significantly decreased since our first observation. At this stage of the data analysis we can put an upper limit on the intensity of Cen XR-2 which is 1/3 the intensity observed by us on October 15, 1967. From further data analyses we will be able to determine the intensity of the source more precisely.

During the same flight we observed Sco X-l, Nor XR-2, and the sources M-l, M-2, M-3, and M-4. Results on these observations will become available by May 1970.

No text nor Summary was communicated by the Author.

Observation of the field strength of low frequency radio waves (164 kHz) transmitted during night hours from Tashkent and received at Ahmedabad show increased absorption around the sidereal times of the transit of the X ray stars Sco X-1 and Tau X-1. It is estimated that the ionization in the D region produced by these X-ray stars can explain the observed changes in field strength.

An optical astronomer enters this field only by courtesy of those X-ray astronomers who pay some attention to accurate positional measurements of X-ray sources. So my first and last words are to ask X-ray observers to give more time to establishing positions of X-ray sources. It appears that in fact most effort has been spent on spectral measurements of X-rays, and this has led just to the classification of sources according to either of two mechanisms for the production of the continuum. In one or two early instances the extrapolated X-ray spectrum has been useful for predicting the brightness of the optical counterpart to be found. A typical uncertainty of making optical identification is that of Vel XR-1 for which Gursky et al. (1968) have given a position with an error box of 3 square degrees. One candidate I can suggest for this is CU Vel, the only variable star of the 1958 General Catalogue of Variable Stars inside the error box. It is interesting because it is assigned to the U Gem class with a range of photographic magnitudes from 10.7 to 15.5. The stellar spectrum has not been observed.

Among the little less than 50 known X-ray sources, only 7 had been identified with optical objects at the end of 1968 (Gratton, 1968) and a couple or so have been added during the present Symposium; some identifications are still uncertain.

The telescope used for identification of X-ray sources at the Cerro Tololo Inter-American Observatory is the Curtis Schmidt Telescope. This telescope is located at Tololo by agreement with the University of Michigan. The field of the telescope is 5° × 5° and its correcting aspheric plate has negligible chromatic aberration over the range 3200 to 10000 Å. This latter feature makes it an ideal instrument for multicolor survey work.

On several occasions in the past month we attempted to observe the bright X-ray source in Scorpio using the OAO 8-inch photometers. These instruments isolate ultraviolet spectral bands by means of interference filters which have half-widths of about 300 Å. Digital and analog outputs both are available and integration times of 8 and 64 sec were used. The length of observing time was about 30 min per orbit. Sky background measurements about 30 arc min away from Sco X-1 source were interspersed with those of the X-ray source, enabling the large contribution of the background to be subtracted. These OAO observations are near the limit of our equipment and are therefore rather noisy. A sudden increase in the intensity of Sco X-1 less than 25% would be undetectable in the OAO photometers. During the observations described here, which were made on April 9, 11h14m UT to 11h43m UT, simultaneous observations were made by Dr. Jerome Kristian with the 200-inch telescope and multichannel photometer. Kristian observed a slow decrease in the flux from Sco X-1 during the first several minutes of simultaneous observations. We also observed such a decrease in intensity but did not see a 10% flare which the 200-inch saw at 5550 Å.

While the short-term fluctuations in the brightness and color of the source Sco X-1 have been found to be correlated in the sense that the system is bluest when brightest, such a result does not appear in the observations of Cyg X-2 or WX Cen described here.

Preliminary evidence is presented which indicates that the star WX Cen, associated with the X-ray source Centaurus X-2 belongs to the Scorpio-Centaurus association. It is pointed out that this result greatly strengthens the similar results for the source Scorpius X-1, and lends support to the idea of the existence of a class of X-ray stars formed by young, subluminous objects, probably supernova remnants, thus neutron stars.

I wish to report the preliminary results obtained by Hale Bradt, William Mayer and Saul Rappaport of MIT in a recent rocket flight devoted entirely to the observation of the X-ray emission from the Crab Nebula. The flight had been planned originally for April 3. Because of administrative difficulties, it was delayed until April 26.

This paper reports the detection of repetitive pulses of hard X-rays and gamma rays in data obtained in mid-1967 from the Crab Nebula. The pulses are believed to originate from NP0532 since the observed repetition period is what would be expected on the basis of an extrapolation of recent optical data obtained for NP0532 back to the time of the observation.

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

No text nor Summary was communicated by the Author.

The 10-m optical reflector at Mt. Hopkins, Ariz., was used to search for cosmic γ radiation from pulsars by detection of atmospheric Čerenkov light generated by energetic particle showers. In the energy region of 1011–1012 eV, no evidence of pulsed γ-ray emission was found from either NP0532 (Crab Nebula) or CP1133.

We examine the electromagnetic spectrum of the Crab Nebula pulsar NP 0532. The present observations are in rough agreement with the idea that the orbit of the radiating particles involves two different radii of curvature. One of the possible models requires electrons with a typical energy of the order of 100 MeV and a density ≈ 109 cm−3. A correlation length of about 10 cm could then give the coherence of the radio emission. Slow pulsars are unlikely to emit an appreciable amount of optical or X-ray radiation but a weak infrared emission is possible.

The purpose of this paper is to discuss the relationship between rotating neutron stars, pulsars, and cosmic X-ray sources. The latter may be divided into at least two classes: the sources with large angular diameters, such as the Crab Nebula, and those with small angular diameter, such as Sco X-1. I submit that a basic model, consisting of a rotating neutron star losing mass in the presence of a large magnetic field, can account for both types of X-ray source. The extended sources represent the case where the energy in the ‘neutron-star wind’ is greater than the magnetic energy. The streaming protons and electrons deposit their energy far out into the nebula in a shock transition region. The relativistic electrons responsible for the extended sources of radio, optical and X-ray emission are produced in the transfer of energy between the protons and electrons in the shock wave, and by magnetic pumping in hydromagnetic waves which are generated by fluctuations in the mass loss rate. The compact sources, such as Sco X-1, represent the other extreme where the magnetic energy dominates, so that no mass loss occurs. The particles are then accelerated and radiate in radiation belts around the neutron star, resulting in a source with a small angular diameter.

A large body of spectral information on X-ray sources has now become available, but the interpretation remains ambiguous. If temperature variations and finite optical depth effects are taken into account, almost any spectrum can be fitted to a model of thermal bremsstrahlung. If a suitable energy spectrum is adopted for the relativistic electrons, a wide variety of synchrotron spectra becomes possible. Although one or the other interpretation may seem artificial in some cases, it nevertheless should be pointed out that a strictly isothermal source would be a miracle and that power-law type energy spectra of the relativistic particles can apply over only a limited range of energies. More satisfactory progress can be made when the spectral data are augmented with structural information, when emission lines can be studied and polarization can be measured. Not only do the intensities of emission lines give much more detailed information on the temperature and density in a hot gas than can be derived from the continuum, but at sufficient resolution velocity fields can also be studied. Useful structural information probably can be obtained only if spatial resolution of 1 arc-min or better is achieved. But one has only to look at the situation in radio astronomy to see how essential this information is for the building of quantitative models.

This is a critical review of theories of known discrete X-ray sources. The Crab is omitted, having been dealt with in Woltjer's review. Two of the identified sources, Sco X-1 and Cyg X-2, seem to be of the same sort. A binary or gas-stream model like that of Prendergast and Burbidge, with dimension R ∼ 109 cm and density n ∼ 1016 cm−3, appears reconcilable with the observed features of these sources, though much detailed work remains to be done. Neither object is yet known to be binary. Theoretical work becomes more difficult if, as appears to be the case at least for Sco X-1, the objects are optically thick due to electron scattering; this may affect the optical and X-ray spectra.

The recent searches for iron lines in the X-ray spectrum of Sco X-1 are reviewed briefly. The calculations and the energy resolution are not yet good enough to make this a dependable test of models.

Several possibilities are offered for explaining the excess radio flux from Sco X-1.

Other theories of Sco X-1-type sources are discussed briefly. The theory of Manley and Olbert seems a little superfluous when the gas-stream theory is still in a strong position.

There are serious discrepancies between X-ray and optical estimates of the distance to Sco X-1. 21-cm measurements must also be considered. The situation is reviewed, and ways out of the difficulty are discussed.

Cen X-2 seems to be like Sco X-1, but several other unidentified sources have hard spectra like the Crab. It is tempting to speculate that most of the galactic sources are supernova remnants.

The extended γ-ray source in the galactic plane may be the extrapolated unresolved sum of galactic X-ray sources, as suggested by Ogelman. There are several other possibilities.

M87 is the only established extragalactic source. Radio, optical and X-ray observations are summarized and graphed. A power-law extrapolation to the X-ray band is far from mandatory; nevertheless the optical flux from the jet is known to be synchrotron radiation. The time-scale difficulties in the jet are described, and several theories of the survival of the optical electrons are reviewed.

Processes for producing X-rays other than thermal bremsstrahlung and synchrotron radiation are listed. These other processes are characterized by low efficiency, and are likely to be unimportant in discrete sources, though several have attracted attention with reference to the diffuse background.

Models of convection zones and corresponding mechanical energy fluxes were computed for 90 stellar photospheres with effective temperatures ranging from 2500 K to 41 600 K and acceleration of gravity values between 1 and 105. The most intense X-ray fluxes may be expected from stars with T ≈ 7200 K and logg ≈ 4. Detectable X-ray fluxes could be expected from Procyon, α Cen and β Cas.

In developing models for the point X-ray sources the general aim has been to find a common explanation for all such objects. However, important observational differences have already become apparent among the systems having optical identifications. A close examination of the evidence shows that those with the most certain optical identifications, Scorpio X-1 and Cygnus X-2, have very few features in common and, in fact, the model proposed here for Cyg X-2 can probably be excluded for Sco X-1.