EXOSAT observations of the Perseus cluster

EXOSAT CMA and GSPC observations of the Perseus Cluster of galaxies are reported. The soft X-ray image data confirm the earlier Einstein Observatory results in terms of the density distribution radially outward from NGC1275. We have used the GSPC spectral data at higher energies to study the strength of the X-ray continuum and of the Fe line emission over the cluster. An asymmetry is found in the distribution of the hot gas around NGC1275, which is consistent with the E-W direction of the ellipticity discovered at soft X-rays by the Einstein Observatory.     

Coronal X-ray temperatures from Einstein and EXOSAT observations

Spectral analysis of coronal X-ray emission from stars observed with both the Einstein and EXOSAT Observatories is presented. Using computer codes developed by Raymond and Smith /1/ and Landini and Fossi /2/ to calculate the X-ray emission from optically thin plasma in collisional equilibrium we find that the derived coronal parameters depend only rather insensitively on the details of the calculated theoretical X-ray spectrum and demonstrate how both the Einstein Observatory IPC spectra and the EXOSAT LE filter ratios can be naturally and simultaneously explained by assuming an underlying continuous emission measure distribution as is the case in the solar corona.     

Measurements of X-ray scattering from interstellar dust

We have examined the surface brightness profiles of four of the brightest compact galactic X-ray sources observed with the Imaging Proportional Counter (IPC) aboard the Einstein Observatory for the existence of halos produced by the scattering of X-rays from interstellar dust. The sources are CYG X-3, 4U1658-48, GX13+1, and 4U1254-69. The halos are apparent when a comparison is made between each source's measured surface brightness profile and a model profile based upon a point response function (PRF) for each source. These model profiles depend upon knowledge of the source's spectrum, which is derived from the IPC itself and corroborated by the Einstein Monitor Proportional Counter and/or previous measurements.As conclusions rest entirely on a knowledge of the system (IPC and Mirror) PRF, we began this study with a comprehensive examination of the calibration data taken for the Observatory prior to launch. Point-source images for both the IPC and the HRI have been analyzed at various energies in order to gain a quantitative understanding of scattering by the mirror surface elements and the IPC's spatial resolution.All four sources show a significant excess above the response from a point-source. The attribution of the excess to X-rays scattered from interstellar dust is strengthened by the positive correlation of the magnitude of this excess with the observed column density of material along the line of sight. Furthermore, we have examined the surface brightness profiles of LMC X-1 and 3C273 and found them to have a greatly reduced excess above their model profiles. Because of their high galactic latitude and because only dust within our own galaxy will lead to the formation of a halo that extends beyond a few arcminutes, only a small effect is expected for these sources.     

EXOSAT observations of two gamma-ray burst sources

EXOSAT observations of the 19 Nov 1978 and 25 Mar 1979b gamma bursters are presented, for the 0.02–2.5 keV energy range. No source was detected in either case, leading to temperature upper limits for the neutron stars assumed to be present of 10⁵–10⁶ °K. Polar cap accretion rates are constrained to values of 10⁻¹²–10⁻¹⁸ M_⊙/y km². The EXOSAT non-detection of the source discovered by the Einstein satellite in the 19 Nov 1978 error box may be explained if the burster is more distant than about 1.5 kpc.     

Observational evidence for the binary model of X-ray bursters

Most, but not all, theoretical models of X-ray bursters require a binary system consisting of a mass donating star and a neutron star. The observational evidence in support of this model, however, is both indirect and meager. We have detected absorption dips in the X-ray spectrum of the Burster MXB 1916-05 with the IPC and the MPC on the Einstein Observatory which occur with a binary period of 2985 seconds. These dips are shown to be the result of a gas stream emanating from a companion star and hence this data represents the first direct evidence of the binary nature of X-ray bursters. Detailed models of the interaction of the gas stream with the accretion disk are presented. A 22nd mag. optical candidate for the system has been found.     

Evolution of the chromospheric and coronal activity of intermediate mass stars

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4–6 M₀) stars and the evolution of their structure are presented. A distance-limited (d ≤ 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars showing hybrid-chromospheric and coronal outer atmospheric structures. EXOSAT observations have been obtained for three hybrid stars, of which only α TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be ∼10⁶K. Observations of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.     

Search for millisecond rotational periods in some low-mass X-ray binaries observed by EXOSAT

We present the results of a search for millisecond rotational pulsations in the X-ray emission of five low-mass X-ray binaries: MXB1728-34, 4U1702-42, 4U1705-44, 4U1755-33 and GX17+2. The data were obtained by the EXOSAT Medium Energy Experiment Argon Counters (1–20 keV) with a time resolution of 7.8125 milliseconds. We searched for periods in the range from 16 msec to 8 sec. Upper limits of 1.5 – 5% were found at a 3 sigma level.     

Exosat observations of quiescent and flaring emission from active late-type stars

We present the highlights of several Guest Investigator programs carried out with the EXOSAT satellite. We discuss quiescent as well as flaring emission from normal F to M dwarfs, from dMe flare stars and from RS CVn binaries.     

Coronal models of late-type stars observed with EXOSAT

Differential emission measure distributions for transition region and corona able to reproduce X-ray and UV emission are investigated. The EXOSAT, EINSTEIN and IUE data for the solar type star K Cet are analyzed. A multitemperature differential emission measure distribution gives a satisfactory fit of the data.     

X-ray emission from the planetary nebula NGC 1360

The EXOSAT observatory has detected the nucleus of NGC 1360 in four photometric energy bands. The data rules out that the emission is from blackbody origin. Initial fits made with LTE model atmosphere spectra require the presence of highly ionized Oxygen and Neon in the stellar atmosphere.     

X-ray,UV, optical and radio observations of the visual binary 53 AQR

We present optical (ESO), ultraviolet (IUE), X-ray (EXOSAT) and radio (VLA) observations of the visual binary 53 Aquarii. We show that the two components of the system can be used to infer the properties of the Sun at a much earlier epoch.     

Exosat observations of the galactic bulge X-ray source GX17+2

We have measured the X-ray flux of the bright galactic bulge source GX17+2 in the energy range 1–20 keV using the EXOSAT ME experiment. During 8 hours of continuous observation an X-ray flare was observed (lasting ～1 hr) followed by an intensity increase. The data show intensity dips with a quasiperiod of ～1.4 hours and quasi-periodic oscillations on time scale of 200–500 sec, which are possibly connected with oscillations of an accretion disc. The spectrum can be fitted by two blackbody spectra with kT₁～1keV, and kT₂～2keV, respectively, and an iron line at 6.3 ± 0.3 keV having 130 eV equivalent width. While the low energy component is rather stable, the 2keV-component shows considerable intensity variations. We suggest that the latter component represents emission from the inner part of the accretion disc while the soft spectrum is blackbody emission from the surface of the neutron star.     

Optical and X-ray observations of 4U2129+47/V1727 CYG in a quiescent state

We observed the 5.2 h X-ray binary 4U2129+47 for more than one orbital cycle on 29 September and 4 October 1983 using the LE, ME and GSPC detectors of the ESA X-ray satellite EXOSAT. In neither detector an X-ray flux from the source could be detected. Quasi-simultaneous UBV observations using the 2.2 m telescop at the German-Spanish Astronomical Center, Calar Alto, failed to detect the large amplitude light curve reported in earlier observations but show the optical companion in a low intensity state. The large amplitude light curve has been interpreted as due to X-ray heating of the optical star by the X-ray source similar to the system Her X-1/HZ Her. Our optical observations indicate that the heating X-ray source has been shut off and nicely explain that EXOSAT failed to detect the source. 4U2129+47 seems to behave similar to Her X-1 in the optical quiescent state, seen for instance from 1949 to 1956.     

Exosat observations of the supernova remnant cas a

The young supernova remnant Cas A has been observed with the imaging proportional counter (PSD) of the EXOSAT observatory. High quality spatially resolved, spectral data allow for the first time the determination of the temperature structure of the remnant. Preliminary results on the distribution of temperature and emission measureover the remnant are presented.     

A simultaneous X-ray and radio observation of a flare from Algol

An X-ray flare was observed from Algol using the low and medium energy detectors on the European Space Agency's EXOSAT observatory. Spectra obtained during the flare are well fitted by thermal continua while an Fe XXV emission feature was also detected. The strength of this feature indicates a cosmic abundance for iron. The data indicate that the flare occurred in a loop of height approximately 0.25 of the K star radius & with a magnetic field >300 Gauss.     

Magnetic reconnection and energy release in a long-duration stellar flare

A dynamical model of magnetic reconnection in solar two-ribbon flares is applied to EXOSAT observations of a long-decay flare from the star EQ Peg. We show that the model is able of reproducing correctly the energy release rate and temporal evolution of the decay phase of the observed flare. We conclude that the flare was the stellar counterpart of solar two-ribbon flares and we derive the physical parameters of the emitting region.     

Einstein X-ray observations of cataclysmic variables

Observations with the imaging X-ray detectors on the Einstein Observatory have led to a large increase in the number of low luminosity X-ray sources known to be associated with cataclysmic variable stars (CVs). The high sensitivity of the Einstein instrumentation has permitted study of their short timescale variability and spectra. The data are adding significantly to our knowledge of the accretion process in cataclysmic variables and forcing some revision in our ideas concerning the origin of the optical variability in these stars.     

Is cygnus X-3 a low-mass X-ray binary?

We present examples of the quasi-periodic variations in the X-ray flux of Cyg X-3 which we have recently found during observations of this source with EXOSAT. Amplitudes and periods of the variations range from 5% to 20% of the total flux and from 50 to 1500 s, respectively. Our tentative interpretation of these quasi-periodicities, the occurrence of quasi-periodic phenomena in an accretion disk which partially occults the X-ray source, points towards an analogy of Cyg X-3 with certain ‘dipping’ low-mass X-ray binaries such as 4U 1822-37, as suggested /1/ previously. We point out, however, that there are also fundamental differences between Cyg X-3 and this type of low-mass X-ray binary.     

Nature of gamma-ray burst sources

Observational evidence suggests that gamma ray bursts have a local galactic origin involving neutron stars. In this light we make a critical review of physics of the thermonuclear runaway model placing emphasis on self-consistency. We further show that some of the proposed models can be observationally excluded in the light of existing data from the Einstein Observatory. The possibility of gamma bursts arising in low mass binaries is finally discussed in the light of evolutionary scenarios leading to low luminosity systems.     

Tunable interactions in ultracold Bose gases

We have created bright matter wave solitons by using a Feshbach resonance to tune the interactions in a Bose–Einstein condensate of ⁷Li. The solitons are made to propagate in a one-dimensional potential formed by a focused laser beam. We observed dispersive wave-like properties for repulsive interactions and soliton-like behavior for attractive interactions. Adjacent solitons are observed to interact repulsively.