P. C. Schmidtke

Spectral variations of LMC X-3 observed with Ginga

The prime black hole candidate LMC X-3 was observed over three years with the Ginga satellite, and a characteristic spectral variation was found accompanying the periodic intensity variation of about 198 (or possibly about 99) days (Cowley et al., 1991). The energy spectrum of LMC X-3 consists of the soft, thermal component and the hard, power-law component, which are respectively dominant below and above about 9 keV. The soft component, which carries most of the X-ray intensity, shows a clear correlation between the intensity and the hardness, while the hard component varies independently of the soft component. It was found that the spectral variation of the soft component is well described by an optically thick accretion disk model with a remarkably constant innermost radius and variable mass accretion rate. The constancy of the innermost radius suggests it is related to the mass of the central object.

The 1990 Calán/Tololo supernova search

We have started a search for supernovae as a collaboration between the University of Chile and the Cerro Tololo Inter-American Observatory, with the aim of producing a moderately distant (0.01<z<0.10) sample of Type Ia and Type II supernovae suitable for cosmological studies. The project began in mid-1990 and continues to the present. This paper reports on the Calan/Tololo discoveries in the course of 1990, and on the spectroscopic and photometric observations gathered for these objects. All of these observations were obtained with CCDs, with the extensive collaboration of visiting astronomers. Great care was exercised in the reduction of the light curves in order to properly correct for the background light of the host galaxy of each supernova

MAGELLANIC CLOUD X-RAY SOURCES OBSERVED WITH ROSAT

This paper is the second in a series which presents ROSAT High-Resolution Imager (HRI) observations of previously known X-ray sources in the Magellanic Clouds. This paper includes new data on 31 X-ray sources discovered by Einstein Observatory as well as 16 serendipitous ROSAT sources. Optical photometry and spectroscopy were obtained to search for identifications and to determine the physical nature of these sources. Optical finding charts showing the ROSAT-HRI positions are given for 35 sources, and where appropriate candidates or identified counterparts are marked. Twenty-nine of these sources are optically identified with objects including X-ray binaries and supernova remnants in both the Small and Large Magellanic Clouds, foreground (Galactic) stars, and background active galactic nuclei. For previously known sources, there is evidence that more than half of them exhibit X-ray variability.

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.

Magellanic Cloud X-Ray Sources. III. Completion of a ROSAT Survey

This paper concludes a series of three papers presenting ROSAT High Resolution Imager (HRI) observations of unidentified Einstein and serendipitous ROSAT X-ray sources in the direction of the Magellanic Clouds. Accurate positions and fluxes have been measured for these sources. Optical photometry and spectroscopy were obtained to search for identifications in order to determine the physical nature of these sources. The present paper includes new data for 24 objects; identifications are given or confirmed for 30 sources. For six sources, optical finding charts showing the X-ray positions are provided. The results from this program are summarized, showing that the populations of luminous X-ray sources in the Magellanic Clouds are quite different from those in the Galaxy.

Determination of the optical counterpart of LMC X-1

The X-ray position of LMC X-1, derived from multiple ROSAT-HRI observations, confirms that the optical counterpart is a peculiar O7 III star, known as #32 (Cowley et al. 1978). This resolves a long standing uncertainty whether this star or the nearby supergiant R148 is associated with LMC X-1.

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.

LMC stellar X-ray sources observed with ROSAT. 1: X-ray data and search for optical counterparts

Observations of Einstein LMC X-Ray point sources have been made with ROSATs High Resolution Imager to obtain accurate positions from which to search for optical counterparts. This paper is the first in a series reporting results of the ROSAT observations and subsequent optical observations. It includes the X-ray positions and fluxes, information about variability, optical finding charts for each source, a list of identified counterparts, and information about candidates which have been spectroscopically observed in each of the fields. Sixteen point sources were measured at a a>3-sigma level, which 15 other sources were either extended or less significant detections. About 50% of the sources are serendipitous detections (not found in previous surveys). More than half of the X-ray sources are variable. Sixteen of the sources have been optically identified or confirmed: 6 with foreground cool stars, 4 with Seyfert galaxies, 2 with SNR in the LMC, and 4 with peculiar hot LMC stars. Presumably the latter are all binaries, although only one (CAL 83) has been previously studied in detail.

CAL 83 - a puzzling X-ray source in the Large Magellanic Cloud

Spectroscopic observations of the X-ray point source no. 83 in the Columbia Astrophysics Laboratory Einstein survey of the Large Magellanic Cloud have been accumulated over 4 yr. The optical spectrum shows no stellar absorption features, but only emission lines typical of an accretion disk. Radial velocity measurements reveal a small (K = 30 km/s) velocity variation modulated at 0.93 d, which appears to be the orbital period. No strong constraints can be put on the mass of the collapsed object, but its companion must be a low-mass, evolved star. Evidence for a precessing disk with a possible period of 69 days is presented. Exosat X-ray observations reveal no short-term (less than 6 hr) periodicities, although erratic, random variations were observed. The X-ray spectrum is very soft, reminiscent of some of the candidate black-hole sources. IUE ultraviolet spectra show only weak emission lines of N V and He II. The UV flux is variable with a mean effective temperature of about 19,000 K. Optical B, V photometry during November 1985 showed random variations of about 0.2 mag with a mean V = 17.3 mag. 30 references.

The Theta-z relation for HST bulges and disks out to z approximately equal 0.8

We present Hubble Space Telescope (HST) scale lengths and ground-based redshifts for 63 faint field galaxies down to I less than or approximately 21.5 mag from the Medium-Deep Survey. These have measured redshifts z less than or approximately 0.8 and half-light radii 0.1 sec less than or approximately r(sub e), r(sub s) less than or approximately 5 sec. We present the Theta-z relation for r(exp 1/4)-bulges and exponential disks separately for world models with q(sub 0) = 0.0-1.0. We show that selection against low surface brightness galaxies in the HST images and ground-based spectra is comparable to that in local surveys. We compare our HST disk-dominated galaxies to a magnitude-limited subsample of the European Southern Observatory (ESO)-Uppsala local spirals. Extrapolating the best-fit local exponential disk scale length (r(sub s) = 3.5 kpc for H(sub 0) = 75) out to z = 0.8, we find a rather symmetrical distribution of HST disks around this value. This implies that galaxy disks have been stable since z is approximately equal to 0.8. We compare our HST bulge-dominated galaxies with r(exp 1/4)-profiles to a magnitude-limited subsample of local Seven Samurai ellipticals. The latter have a local best fit r(sub e) = 5.7 kpc. Our HST bulge sample shows a similar distribution for z less than or approximately 0.8. Elliptical galaxy scale lengths have thus also remained rather constant since z less than or approximately 0.8. We set limits to their possible evolution.