2–10 KeV spectra of Crab-like SNR's as observed by EXOSAT

The EXOSAT ME observations of 3C58 and G21.5-0.9 are presented. The data for these objects is considered in conjunction with the data available from EXOSAT for the Crab-like SNR's Crab and Vela X. The objects have single power law spectra with a range of spectral indices of 0.6 to 1.5, 3C58 and Vela X being similar to the Crab whilst G21.5-0.9 and the source associated with the Vela pulsar have significantly flatter spectra. The derived column density for G21.5-0.9 is consistent with a distance of 5 kpc. The X-ray luminosities and overall electromagnetic spectra of these objects are investigated as age indicators and compared to current model predictions.     

The discovery of low-level iron line emission from Cyg X-1

Summary We present the results of seven observations of the X-ray spectrum of Cyg X-1, made with the GSPC on board the EXOSAT observatory. We report the discovery of an iron emission line in its spectrum. The likely origin of this feature is discussed.     

EXOSAT results on the X-ray burster 2S1254-690

EXOSAT observations of 2S1254-690 in August 1984 have confirmed the presence of periodic dips in the X-ray intensity. These dips occurred with a period of 3.9 hours and last for about 50 minutes. During individual dips the observed X-ray flux oscillated several times between the quiescent level and an undetectable level on time scales varying from a few seconds to approximately 5 minutes. In addition, a second type one burst has been observed, with characteristics which are very similar to those observed earlier. In this paper we report the results of a 20 hour EXOSAT observation made in 1984 August 6th to 7th, and discuss some properties of the dips and of the burst.affiliated to the Astrophysics Division, Space Science Department, ESA     

EXOSAT observations of late-type stars: The application of coronal loop models

We apply solar-type coronal loop models to X-ray and UV observations of late-type stars. We derive from EXOSAT and IUE observations constraints on the temperature, pressure and size of the emitting structures.     

X-ray emission from cataclysmic variables

EXOSAT results on cataclysmic variables are reviewed. The long continuous X-ray observations afforded by this observatory, coupled with the sensitivity of its instruments to medium energy and very low energy X-rays, have enabled the rotational and orbital X-ray light curves of these stars to be measured in unprecedented detail. Examples are given of data on synchronously and asynchronously rotating magnetic systems, and on disc accreting stars. The impact of the new observations on our understanding of cataclysmic variables is discussed.     

EXOSAT spectra of some bright QSO's

This paper reports on EXOSAT observations of four bright QSO's and active galaxies. The QSO PHL 1657 has a simple spectrum that can be fit with power law plus some low energy absorption. A soft excess is observed for both MR 2251 and E 1352.24+1830. Finally, PKS 2126-158 was detected and the results are consistent with a 0.7 energy index.     

EXOSAT observations of M dwarfs

We give a progress report of our EXOSAT observations of active M dwarfs. The possibilities of filter spectroscopy of coronal X-ray sources using the available CMA filters are discussed, and we confirm that M dwarfs are rather hot coronal sources with X-ray temperatures in excess of 10⁷ K, a result previously obtained with the Einstein Observatory.     

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.     

X-ray variability of the bright BL Lac object PKS 2155-304

We have observed the BL Lac object PKS 2155-304 with the EXOSAT observatory and with IUE on 1983 October 31 and November 29. At the same time, optical photometry has been obtained. Variability has been observed both on a time scale of hours during the October 31 observation and on the time scale of one month. During the November 29 observation the source was 2 times brighter than in the previous observation, but no spectral modification is found between the two observations. The source has been also detected in the ME instrument and an energy spectrum has been fitted with a power law. A marginal detection in the ME Xenon counters suggests a flattening of the spectrum in the high energy band. These X-ray data extending over three decades in energy are compared with the simultaneous UV and optical observations.     

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.     

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.     

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.     

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.     

2A0526-328: The white dwarf rotation period revealed

We report results from EXOSAT observations of the intermediate polar system 2A0526-328 (TV Col). The hard X-ray emission (2–8 keV) is modulated with a period of 1943 s, interpreted as the white-dwarf rotation period. Soft and hard X-ray emission show intensity minima, in phase with the orbital period of 0.2286 days; analysis of the hard X-ray spectra shows that these minima are caused by an extra low-energy absorption corresponding to a H column density of 4 × 10²² cm^-2.     

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.     

H0323+022: Classification as a BL Lac and EXOSAT coordinated observations

The object H0323+022 (Doxsey et al. 1983) has been shown to be a BL Lac object by virtue of a diversity of observational characteristics at radio, optical, and x-ray wavelengths, in agreement with the conclusion of Margon and Jacoby (1984). Multi-frequency coordinated observations of this highly variable object with EXOSAT in September 1984 found it to be in a faint quiescent state (1/3; Jy at 5 keV and V=16.55). Preliminary results from the latter observations are presented.     

Variability of BL Lac objects at X-ray and other frequencies

EXOSAT observations reveal that daily X-ray variability appears to be a common property of high luminosity BL Lac objects. Results are reported for PKS0735+178, OM280(=1147+245), OQ530(=1418+546), OV-236 (=1921-293) and OJ287(=0851+202). In OJ287, for which we have the most extensive coverage, the amplitude of variability at X-ray frequencies appears to be a factor of a few larger than at longer wavelengths.     

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.     

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.