An investigation into the nature of the 4.4 hr periodic source 4U1755-33

We present the results of a continuous 18 hour observation of 4U1755-33 made with the European Space Agency's EXOSAT Observatory. Four 50 min dips in X-ray intensity were observed equally spaced with a period of 4.4 hrs, confirming the periodicity first suggested in White et al. (1984). The dips are spectrally independent. We examine the properties of 4U1755-33 and conclude that the source is most probably point-like and that the metallicity of the absorbing material is at least 600 times less than cosmic values.     

Coordinated EXOSAT and optical observations of the X-ray burster 4U1735-44

We present the first results of an EXOSAT observation of the low-mass X-ray burster 4U1735-44. The ME data show low-amplitude variations in the persistent flux including two 5% dips separated by 4 hours. The structure of the single observed burst is briefly described. Five hours of simultaneous B-band photometry were obtained at SAAO with 12 minute time resolution; a strong anti-correlation is shown to exist between the X-ray and optical flux, with a high level of significance. A model for this behaviour is suggested, based on reprocessing of the X-ray flux in a corona or stellar wind.     

The big dipper: 4U1624-49

We present a 6.3 hour observation of 4U1624-49 with the EXOSAT Medium Energy experiment. The X-ray light curve is dominated by a series of sharp dips in which the observed flux falls to 25% of the steady level on timescales of seconds. These dips are accompanied by strong variations in the spectral hardness consistent with large changes in the absorbing column density. No evidence is found for any dip periodicity, in contrast to the other four sources in which dip activity has been reported. We discuss the implications of these observations for models of low mass X-ray binaries.     

EXOSAT observations of the contact binary VW Cephei

EXOSAT observations of the contact binary VW Cephei on 19th March 1984 are presented. The L1-telescope with CMA+thick Lexan filter was used. The observations cover one orbital revolution showing an asymmetrical X-ray light curve. This can be modelled by an active neck, connecting the two stars, and with enhanced coronal regions on the primary star. Nearly simultaneous IUE observations are also presented. The observations form a part of the program to observe contact binaries with EXOSAT.     

Temporal intensity fluctuations of Her X-1 around 100 sec time scales

Her X-1 has been observed with EXOSAT for one 35^d cycle between March 1 and April 5, 1984 at about 4 day intervals. During three observations, absorption dips were encountered showing fluctuations with time scales in the several hundred seconds range. The data are interpreted with a model where random superposition of absorbing blobs or inhomogeneous structures within the accretion disk, in the line of sight, cause these intensity variations. The calculations give a measure of the extent of the disk both in vertical and radial directions.     

Coordinated optical-EXOSAT-tenma observations of a burst from 2S1636-536

During a coordinated observations of 2S1636-536 with EXOSAT, Tenma and the 1.4 metre Danish telescope at La Silla, a single burst was detected at all three observatories. The burst was bright with a peak flux of 6 × 10^-8 ergs/cm ² /s and rapid expansion of the blackbody radius. EXOSAT and Tenma agree closely in the values of blackbody radius, temperature and flux which are very similar to the three bursts recently reported by Tenma which appear to reach the Eddington flux limit at their peak. Preliminary analysis of the X-ray and optical timing data reveal that this burst is unusual in having a sharp leading edge in both X-ray and optical data and a very short X-ray-optical delay compared with those previously observed. If confirmed, this result puts significant restraints on the location of the optical reprocessing site.     

X-ray,optical and UV observations of the AM Her system E2003+225

The AM Her type object E2003+225 was observed with EXOSAT, IUE and ground-based telescopes on 1983 Oct. 12. The brightness of the ultrasoft X-ray component allowed the Objective Grating Spectrometer (OGS) to be used, which gave a model-independent determination of the temperature of the blackbody spectrum. The star was observed again on 1984 July 24 by IUE with simultaneous optical spectrophotometry. The high resolution of this observation revealed complex line profiles, and a systematic velocity much smaller than previously reported. The composite energy distribution is presented.     

Detection of iron lines in the persistent emission of some X-ray burst sources observed by EXOSAT

We have observed the X-ray burst sources 4U1728-33 and 4U1813-14 with the ME detectors aboard EXOSAT, and present here results of a spectral analysis of their persistent emissions. For both sources the data can be well fitted by a double blackbody continuum and a Gaussian emission line. The two spectral components can be interpreted in terms of a blackbody radiation from the neutron star, and emission from the inner part of an accretion disc. The line feature is consistent with the 6.7 keV iron emission line expected in the presence of a hot extended object (X-ray corona) around the neutron star.     

The soft X-ray spectrum of NGC 4151

EXOSAT observations of the Seyfert galaxy NGC 4151 over the period July 1983 to April 1984 have revealed a decreasing flux in the 2 –10 keV band. In accord with previous measurements a power law spectrum attenuated with a simple column of cold gas does not provide a satisfactory spectral fit below 3 keV, where a lower relative opacity is required. Inclusion of additional low energy data from the EXOSAT telescopes allows the modelling of the absorbing column to be improved and reveals a second, separate, soft X-ray spectral component.     

EXOSAT observations of the X-ray pulsar 4U1145-619

EXOSAT observations of the X-ray pulsar 4U1145-619 during June and July 1983 and July 1984 confirm that this source shows a regular 187 day outburst cycle in X-rays. The results from pulse timing and X-ray spectroscopy are discussed in terms of a model for 4U1145-619 involving an eccentric binary system in which there is variable accretion from the Be primary star onto a companion neutron star.     

EXOSAT observations of the BL Lac object Mkn 421

The BL Lac object Mkn 421 was observed by EXOSAT four times over a period of six days in February 1984. Significant X-ray variability was apparent on a timescale of less than a day, but with no accompanying spectral change. The source exhibited a very soft power law X-ray spectrum with an extremely low intrinsic column density (N_H10²⁰ cm^–2). There was no evidence for an additional hard component attributable to synchrotron self-Compton emission. The observations when combined with other published data imply that significant changes occur in the form of the broad-band UV/X-ray continuum of this source.     

First detection of an X-ray burst and a one hour intensity dip in 4U1323-62

We report the results of a 1.4 10⁴s observation of the region of 4U 1323-62 with the EXOSAT ME. The source has a flux of 7–8 10^-11 erg/cm²s (2–10 keV) and a power-law spectrum with 1.1 < < 1.8. During our observation, the source showed a symmetric 60% dip in its X-ray flux of R~1 hr. The spectrum hardens during the dip. Inside the dip we observed an X-ray burst with a 2–10 keV peak flux of 7 10^-10 erg/cm²s. The burst spectrum is black-body, and shows evidence of cooling during the burst decay. The discovery of a burst from 4U 1323-62 settles the classification of the source; the observation of a dip suggests that we may be able to measure its orbital period in the near future.     

EXOSAT observations of active stellar coronae in the Cygnus Loop,T Tau & NGC 2264 regions

A highly variable point X-ray source, first seen by the Einstein IPC, has been positioned with the EXOSAT CMA and identified with a bright (V = 8.5) K0 star. Although in the direction of the southern half of the Cygnus Loop, this star is almost certainly a foreground object and typical of other active cool stars that are related to RS CVn systems.An EXOSAT program to study T Tauri stars failed to detect T Tau itself. However, a strong X-ray source was observed 15 from T Tau, which in its turn had not been seen by Einstein. This new source has been identified with a hitherto unstudied 13 mag star which is likely to be a dMe flare star.The young star cluster NGC 2264 was observed with the EXOSAT CMA in an attempt to identify the sources found during an Einstein IPC study of S Mon. Apart from S Mon itself, only UV-bright objects were seen, but several of these are considered likely counterparts of the Einstein sources.     

Optical observations of serendipitous EXOSAT sources in the Coma Cluster

A 7 hour observation of the central part of the Coma Cluster of galaxies has been performed with the EXOSAT LE telescopes and CMA detectors. Five serendipitous sources are detected within the inner 35 arcmin radius of the field. Optical spectroscopy demonstrates that at least three of these are background AGN not associated with the cluster. At the sensitivity level of the EXOSAT exposure, we would have expected to see only 0.01 background sources based on the Einstein Medium Sensitivity Survey. The EXOSAT and Einstein results may be reconciled if these AGN have a much softer average X-ray spectrum than previously assumed.     

High and medium resolution spectroscopy of the X-ray transient 4U1543-47

Preliminary results of an EXOSAT observation of the transient X-ray source 4U1543-47 are presented. The source was observed in August 1983, during a high state, following a Tenma alert that the source was again active. Results from the GSPC and the LE 1000 l/mm grating are presented. The spectrum is complex, but in the 2–10 keV energy range can be well described by a Comptonised thermal distribution. Extrapolating the same model into the lower energy band of the grating requires an absorption column density equivalent to 2 × 10²¹ H cm². A marked under-abundance of Oxygen and overabundance of Nitrogen, along with a strong, unidentified line feature at 9.8 Å, are necessary to model the grating data.     

The discovery of a 25 min regular modulation in the X-ray flux from 2S0142+61

A 13 hr observation of 2S0142+61 on 1984 August 27 by EXOSAT shows the X-ray flux of 2S0142+61 to be modulated with a period of 1456+/-6 s. The 1–10 keV spectrum is two component with a 0.7 keV thermal and 0.0 energy index power law, with 30% of the total luminosity in the thermal component. The spectrum is absorbed by 1 × 10²² H cm^-2. Only the hard component is pulsed with a 3 to 10 keV peak to mean amplitude of 35%. Below 2 keV the modulation is less than a few percent. The total 1–10 keV luminosity is 3.5 × 1032 erg s^-1 for a distance of 100 pc. Possible optical counterparts are discussed.     

X-ray variability of the AM Her star CW1103+254

The AM Her star CW1103+254 was observed with the low-energy and the medium-energy instruments of EXOSAT on 3 occasions for a total of 10.5 orbital cycles. Both, soft and hard X-rays from CW1103+254 were detected. We report preliminary results on the time variability of the X-ray emission on time scales from 1 minute to 1 month.     

Spectral variability of scorpius X-1, as observed with EXOSAT

EXOSAT measurements of Sco X-1 have been carried out with the 500 l/mm and 1000 l/mm grating spectrometers and simultaneously with the gas scintillation proportional counter (GSPC) on two occasions in August 1983. The first observing period, part of the instrument performance verification phase, on August 7 and 8, 1983, lasted for about twenty two hours and showed the source flaring during the first six hours. During the second observing period, August 29 and 30, 1983, the source was again active during part of the time. The lightcurves of the spectrometers and the GSPC show some correlation during the flaring activity. The grating spectra show clear emission features between 12 and 20 Å, which vary with time. When fitting the observed spectra to a thermal bremsstrahlungs model (exponential with gaunt factor) together with absorption by neutral matter, a reduced abundance of oxygen in the absorbing matter with respect to Solar is required.     

SS Cygni in outburst and quiescence

EXOSAT has been used to carry out a series of observations of the dwarf nova SS Cygni covering both quiescent and outburst states which provide a detailed picture of its behaviour at X-ray wavelengths. These results provide strong support for a new coronal model for the X-ray emission from non-magnetic white dwarf systems.