D. Crampton

Discovery of a massive unseen star in LMC X-3

Spectroscopic observations of the optical counterpart of LMC X-3 show it to be a spectroscopic binary in the Large Magellanic Cloud with an orbital period of 1.70 days. The B3 main-sequence primary has a large radial velocity amplitude indicating a mass function of 2.3 solar masses. LMC X-3 is an extremely luminous and variable X-ray source, but no 1.7 day X-ray periodicity has been detected. The lack of optical and X-ray eclipses limits the inclination to less than 70 degrees and implies a mass for the unseen star of more than 9.0 solar masses, if the B star has a normal mass. The system now appears to be the strongest evidence for the existence of a stellar mass black hole and is the first extragalactic example.

The supergiant X-ray binary system 2S 0114 + 650

Spectroscopic observations show that LS I + 65 010 deg, the counterpart of the X-ray source 2S 0114 + 650, optically resembles the supergiant X-ray binary systems rather than the Be-type systems. The orbital period is 11.588 + or - 0.003 d with maximum positive velocity occurring at JD 2,444,4134.3. It is not possible to decide whether the orbit is circular or whether it has a small eccentricity of 0.16 + or - 0.007, both fit the observations equally well. The mass function is 0.006 solar masses, implying a mass of approximately 18 solar masses, for the B0.5 supergiant if the neutron star has a typical mass. It is argued that the source is 2.5 kpc from the sun, implying that the log of the X-ray luminosity is approximately 34.5 ergs/s, and that this system shares characteristics of both the supergiant X-ray binaries and the Be systems. The X-ray source appears to be fuelled by accretion of material from the stellar wind of the primary star, rather than by Roche lobe overflow. This accounts for its low X-ray luminosity compared to other supergiant X-ray binary systems. 26 references.

CAL 87 - An eclipsing black hole binary

Simultaneous spectroscopic and photometric observations have been made of the optical counterpart of the variable X-ray source CAL 87, an apparent low-mass X-ray binary in the Large Magellanic Cloud. The shape of the optical light curve, showing a deep minimum with very broad wings, suggests an eclipse of a very extended disk structure, which is the dominant light source. The only strong feature in the optical spectrum is a variable He II (4686 A) emission line. Radial velocities of the He II emission lines show a small-amplitude variation (K about 40 km/s) through the orbital period. The phasing of these velocities indicates that the emission arises on the degenerate star side of the center of mass, as is the case for many low-mass X-ray binaries. If this velocity reflects the motion of the compact star, then the mass ratio must be large (about 10), and there is a strong probability that the system may be an eclipsing black hole binary. 21 refs.

Optical Observations of the Black Hole Candidate GX 339−4 (V821 Arae)

GX 339-4 has long been known as a black hole candidate because of its rapid variability and high/low X-ray states, which are similar to those of Cyg X-1. Although GX 339-4 is assumed to be a binary, the orbital period has not yet been convincingly determined, and hence little is known about the nature of the component stars. In this study simultaneous photometric and spectroscopic observations have been made in an effort to address these problems. Although a definitive period has not been found, we present evidence that it lies near ~0.7 days. The I-band light curve has an amplitude of only ~0.2 mag, with flickering of >0.1 mag superposed, making it difficult to determine the orbital period from light variations alone. The emission-line velocity amplitudes are small, suggesting that GX 339-4 is seen at a low orbital inclination angle. Even though the period is still not determined, the relative phasing of the velocities and light minima are known, since the photometric and spectroscopic observing runs overlapped. Assuming the orbital period is near ~0.7 days, we can set limits on the mass ratio and stellar masses. For reasonable assumptions about the inclination angle, the mass ratio appears to lie between q ~ 3 and 15, where q = MX/Mdonor. If the donor star mass is greater than ~0.3 M⊙, the X-ray source is likely to be a black hole.

Optical counterparts of the Large Magellanic Cloud X-ray point sources

The results of a program of optical identification of the X-ray point sources in the direction of the Large Magellanic Cloud (LMC) are presented. Observational results are combined with accurate positions of about 3-arcsec obtained with the Einstein Observatory Imaging Proportional Counter (IPC). The sources include 13 foreground stars, three background active galaxies, and six LMC members which are known or suspected to be binaries. The 24 other sources in the Einstein LMC survey have only 30-arcsec positions and on average are less luminous and in more crowded regions of the LMC. On the basis of the new identifications, comparisons are made between the overall stellar X-ray population in the LMC and in the Galaxy. Although the mean X-ray luminosity of the LMC sources is significantly higher than that of the Galaxy, the total number of stellar X-ray systems per unit mass is similar in the LMC and the Galaxy.

Spectroscopy and kinematics of the low-mass X-ray binaries

The kinematics of low-mass X-ray binaries (LMXBs) were studied using moderate-resolution spectroscopic data in order to constrain the age and origin of LMXBs. Radial velocities were measured for some systems. Using known orbital periods, miniorbits were fitted in order to study the masses of the stars in these systems. Spectra of the observed LMXBs are presented; those which exhibit highly variable spectra are shown at several epochs. Analysis of the kinematics confirm that the LMXBs are among the oldest objects in the Galaxy. 42 references.

GX 339-4 - Black hole or neutron star X-ray binary

Optical spectra of the highly variable X-ray source GX 339-4 show an emission-line spectrum characteristic of an accretion disk. Velocity variations suggest the orbital period lies between 0.2 and 2 days. Periods in this range limit the mass of the compact star to less than 2.5 solar mass. The distance estimated from the interstellar lines and the color is about 4 kpc. At this distance the velocity expected from galactic rotation differs by over 100 km/s from the observed systemic velocity. 19 references.

Detection, identification, and observed properties of Large Magellanic Cloud supersoft X-ray sources

A small number of supersoft X-ray sources has been found in the direction of the Large Magellanic Cloud (LMC), although information has been published for only six of these. It has been suggested that these sources may represent a separate class of low-mass X-ray binaries. In an effort to learn more about the properties of these supersoft sources, new observational data are presented for four of them, including X-ray positions, optical identifications, optical spectroscopy and photometry, and the discovery of a previously unreported supersoft source. Three of the supersoft sources have been optically identified with LMC stars which show strong emission lines of H and He II, typical of accretion disks

Spectroscopic analysis of Scorpius X-1

Radial velocity and intensity measurements are presented for the strongest emission lines observed during four nights of continuous observation of Sco X-1. In general, the observations are consistent with a model in which the emission spectrum originates from an accretion disk surrounding the X-ray source. Some of the observations do not fit this simple model, but lack of phase overlap in the observations precludes further interpretation. A value of iapprox. =30degree and probable masses of M/sub x/approx. =1.3M/sub sun/ and M/sub 2/approx. =1M/sub sun/ are inferred from the observations. (AIP)

Optical observations of the X-ray nova EXO 0748-676 near maximum light

Photometric observations of the X-ray nova 0748-676 show that optical eclipses occur in phase with the X-ray eclipses. In addition, the light curve shows considerable modulation throughout the 3.8 hr orbital period. Spectra show emission lines of H and He II which vary in velocity, intensity, and profile throughout the orbital cycle. The phasing of the velocity variations suggests that the emission lines are formed in the vicinity of the compact star. However the lines are very weak, and accurate velocities cannot be determined from the data. Curiously, the emission lines have maximum flux near phase 0 (eclipse), indicating they cannot arise principally in the region which is eclipsed. 14 references.