LMC X-4, AO538-66 and surrounding X-ray sources observed with EXOSAT

Repeated observations of LMC X-4 with EXOSAT were carried out in 1983/84 in order to study its 30.5 day cycle and to cover the expected outbursts of the recurrent LMC transient A0538-66. The latter source was inactive during our campaign although a variable circumstellar envelope was still present around the optical counterpart.At least ten further X-ray sources are detected in the CMA field of view around LMC X-4 including the SNR N49 which is the possible site for the March 5, 1979 -ray burster and N63A which appears to be variable in X-ray luminosity. We furthermore discuss the strongest sources that were not present in a previous EINSTEIN survey of the LMC.     

The atmospheric composition,extinction and luminosity of the LBV R71

ESO 3.6m Caspec spectra of the LMC luminous blue variable (LBV) taken at minimum have been analysed using NLTE model atmospheres and line formation calculations to derive atmospheric parameters and chemical composition. Using the silicon ionization balance and the hydrogen Balmer lines we deriveT _eff =17250, log g=1.80 and a microturbulent velocity of 15–20 km/s. The analysis yields abundance ratios by number of approximately 0.43 for He/H, 0.03 for C/N and 0.14 for O/N, implying that enrichment of the atmosphere by processed material has taken place. We have re-evaluated the reddening of R71 using IUE low resolution data and published UBVRIJHKL photometry and derive a value for A _V of 0.63. We also construct an extinction curve using archive IUE data for mid-B LMC supergiants and show that the extinction is anomalous; the 2175A bump being almost absent and the far UV rise very pronounced. A comparison of our model flux in theV-band with the observed (dereddened)V magnitude and the D.M. of the LMC (18.45), implies that the bolometric magnitude or R71 is –9.9. This is significantly higher than the value of –9.0 usually adopted for R71 and suggests that this object may not in fact be a subluminous LBV.     

Abell 1367 and 1060 observed with EXOSAT

Summary Two clusters of galaxies have been observed with EXOSAT to map the distribution of cool gas. For A1367 we have detected the two brightest sources found by EINSTEIN. We confirm the identification of a X-ray source with a blue object near NGC3842. By comparing the EXOSAT and the EINSTEIN count rates we can state, either that the source is variable, or that the HI column density is low and the spectrum is either a power-law with a spectral index > 1.5 or thermal with a temperature below 6×10⁶ K. For A1060 we confirm the classification from optical data of NGC3311 as the dominant galaxy but we find no evidence for a central source as inferred from the EINSTEIN data. We derive an accretion rate of 10M_o/yr.     

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.     

An X-ray study of M51 (NGC 5194) and its companion (NGC 5195)

Observations of NGC 5194/95 with the Einstein HRI show a very strong nuclear X-ray source, surrounded by a diffuse flux, three point sources and the companion. The diffuse flux, which correlates well with the radio continuum, is likely to originate from the disk population with age 2·10⁹ yrs. The large luminosity from the nuclear source, together with optical and radio observations, shows that it belongs to the low luminosity active nuclei, thus extending this class to luminosities less than 10⁴⁰ erg/s.     

An optical outburst from the periodic recurrent X-ray transient A0538-66

We present optical spectroscopy and photometry and IUE spectroscopy of the counterpart of the LMC recurrent X-ray transient A0538-66 during an outburst at the end of December 1980 which was consistent with the 16.6 day X-ray period (Skinner, 1980). The optical spectra show steadily increasing Balmer and HeI emission (indicative of a shell phase) superposed on a B2 IV spectrum with a substantial brightness increase of 2^m and the sharp turn-on of HeII 686 at the peak. Significant radial velocity changes have been detected but they show no correlation with the 16.6 day period. IUE spectra during a subsequent outburst show very strong and broad (5000 km s^–1) emission from C IV 1550 and HeII 1640. This behaviour is compared with other galactic transients and shell/Be stars.     

New EXOSAT results from the intermediate polar V1223 SGR

We report new results obtained from the EXOSAT AO- 1 observation of the intermediate polar V1223 Sgr. The detection of a 12.4 minute period in the medium energy X-ray flux with an associated hardness ratio variation has been previously reported in Osborne et al. (1984a). Further work has revealed: a narrow dip at the phase zero in the folded medium energy light curve; 30% modulation in the low energy X-ray (3000 Lexan) flux; a count rate ratio from 3 filters which allow the presence of a bright low temperature blackbody component (kT = .05 –. 40 KeV); and a phase resolved ME spectrum which must have two or more components when the source is bright. New optical ephemerides show that the X-ray and optical pulses are in phase at an orbital phase of = 0.31.Affiliated to the Astrophysics Division, Space Science Dept., ESA     

Long-term cycles in cosmic X-ray sources

Most of what we know about galactic X-ray binaries comes from their time variation, particularly periodic variations corresponding to neutron star rotation, and binary motion. Longer cycles or quasi-cycles are much harder to observe because of the shortage of instrumentation suitable for long-term monitoring. Nonetheless, cycle with periods up to a few years have been seen in several galactic binaries.Cycles of 30–300 days have been confirmed for four high-mass systems, LMC X-4, Her X-1, SS433, and Cyg X-1, and are suspected in several others. These cycles are observed in both the X-ray and optical bands, and represent cyclic variations in both the inner and outer parts of the accretion disk. Some component of these systems is precessing, but we are not certain which. It could be a misaligned companion star; the outer rim of the accretion disk, driven by radiative feedback; or the neutron star.Several low-mass X-ray binaries have quasi-periodic cycles, with periods ranging from 1/2 to 2 years. The amplitude of modulation ranges between 50 and 100%, i.e., both persistent and transient objects fall into this class. This activity is reminiscent of the superoutburst cycles of the SU UMa cataclysmic variables, and may be caused by similar mass-transfer instabilities.Periodic outbursts in the Be/neutron star systems seem to result from variable mass transfer in a wide, eccentric orbit. The relationship between the orbital cycle and the flux outbursts, however, is not well understood, and even the equivalence of the outburst and binary cycles remains hypothetical for most objects. Most likely, the periodic outbursts result from enhanced mass transfer at periastron.Compared to other aspects of X-ray astronomy, long-term activity has been much less intensively studied by both observers and theoreticians. A simple all-sky monitor in permanent operation could provide for the X-ray sky the same kind of data base provided to optical observers by the Harvard plates.     

LMC X-4: 13.5 s pulsations and an X-ray flare observed by EXOSAT

EXOSAT observed LMC X-4 on November 17/19, 1983 for one 1.4 day binary period during the high state of the 30.5 day cycle. An eclipse with sharp ingress and slow egress was detected with an eclipse angle of 27.1±1.0 dgr. In the medium energy experiment the source showed a hard power law spectrum. Outside eclipse the source was remarkably constant and only one flare was detected on November 17 at 19 UT lasting for about 1 h. The energy spectrum of the source softens considerably during that time and shows an emission line of cold iron. 13.5 sec pulsations are strongly present during the flare and have also been detected during the quiescent period and during several 1 min flares in another EXOSAT LMC X-4 observation on November 22, 1983. A pulse delay time analysis results in the determination of the pulse period (13.5019±0.0002) s and of the semimajor axis of the orbit of the X-ray star (26.0±0.6) It-sec. These results, together with other available information on LMC X-4, allowed to improve the binary parameters. The mass of the neutron star is found to be 1.34 _±0.44 ^0.48 M_o (95% confidence errors).     

Galaxien vom Magellanschen Typ

In the first part of this paper the morphological structure of Magellanic type galaxies (Irr I) is investigated. The galaxies of Magellanic type present a basic pattern consisting of a disk, a bar, stellar arms, rudimentary or well developed, spiral filaments and condensations in the disk. With the help of this pattern a well-defined classification scheme is set up. The subgroup of Irr II-systems consists of normal galaxies which are more or less tidally disturbed. Bursts of star formation have a great influence on structure and colour of irregular galaxies. Using the ESO-B Atlas, 580 galaxies of Magellanic type (out of a sample of 3187 galaxies) were classified; 57 are new SB(s)m systems (prototype Large Magellanic Cloud). The sample shows dominant bar structures at the classification stages d-, dm-, and m. A striking feature is the asymmetric position of bar and disk. This asymmetry is a general characteristic of galaxies of types SBd-SBm IB. The asymmetry can be discribed by a relative displacement parameter \(\tilde \Lambda \) = 0.78 ±0.15, defined as the quotient of small and great distance of the bar center to the optical edge of the disk. The displacement cannot be explained by tidal interaction with neighbouring galaxies. In the second part of the paper the kinematics and dynamics of the Large Magellanic Cloud (LMC) as the nearest and best-known example of a galaxy of Magellanic type is investigated. The main structural features of the LMC are disk, bar, rudimentary and well developed stellar arms as well as spiral filaments (not necessarily connected with density waves); the γ-structure is a broken up ring structure. Embedded into these features are young, asymmetrically located spiral arm filaments. As an explanation for these structures stochastic start formation in an ordered chain reaction is proposed. The pattern of the spiral arm filaments is determined by the rotation curve. The morphological peculiarities of the LMC can also be detected in other galaxies of that type. The mean absolute displacement of the centers of bar and disk, determined from 18 galaxies, is Λ = 800 pc. The displacement between the bar center and the symmetry center of the rotation curve is of the same order. The presently known radial velocities of planetary nebulae, star clusters, Hi and Hii regions and stars belonging to the LMC have been collected in a catalogue as the basis of a discussion of the kinematics and dynamics of the LMC. Contrary to earlier work, we have used, for the first time, the radial velocities of objects of all subgroups together by a proper weighting scheme. Thus the basic kinematics and dynamics of the LMC has been deduced. The radial velocity field shows no central symmetry; it is characterized by large scale (2–3 kpc) disturbances. By comparison with the velocity field of other galaxies three main disturbances are identified: an oval distortion of the velocity field in the bar region, a radial velocity field around 30 Doradus, and disturbances connected with a warp or material above the disk in the southern quadrants. The results of a detailed numerical analysis of these three facts can be summed up as follows:

1. The rotation curve is determined over 10° diameter; it shows differential rotation, an asymmetric behavior in the south and a double structure in its Hi component. The rotation center is displaced by 0°.7 from the bar center. The orientation of the kinematic line of nodes and the systemic velocity vary as functions of the distance from the center. Therefore, it is possible to show definitely that large scale disturbances (warping, z-structure and streaming motions) are existent.
2. By variation of the kinematical parameters (systemic velocity, inclination, orientation of the line of nodes, rotation center) the dispersion of the measured radial velocities was minimized and the basic rotation curve determined. The rotation curves for the north and south side of the LMC are significantly different. The south side is either warped or there is material above the main plane. There seems to be a connection between this structure, the Panmagellanic Gas and the Magellanic Stream. The north side appears to be free of distorsion.
3. The residual velocity field (observed minus model) deduced from a basic rotation curve shows that the displacement between the rotation center and the bar center is not caused by local streaming motions. The rotation center must be the mass center. The bar shows a radial velocity field; in the 30 Doradus region inward and outward motions are found.

The mean velocity dispersion of population I objects is 10.5 km s^-1 of population II objects 16.0 km s^-1. Red and blue globular clusters show different kinematical behavior. By comparison of eight mass models, taking into consideration thickness effects and controlled by surface photometric data, the mass of the LMC is found to be (0.5 ± 0.1) × 10¹⁰ \(\mathfrak{M}_ \odot \) (assuming the inclination 33°, the systemic velocity 46.9 km s^-1, and the distance 56 kpc). Dynamically, the LMC can be described by a dominating disk potential with an additional bar potential as a disturbance. The mass of the bar is 0.6 × 10⁹ \(\mathfrak{M}_ \odot \) . The stable neutral point of such a configuration can be found in the residual velocity field. The absorption feature crossing the bar coincides with the maximum velocity gradient of the computed radial velocity field in the plane of the system.     

Long term observations of Cyg X-3 with Vela 5B

We present long-term (1969–1979) observations of Cygnus X-3, obtained by the Vela 5B satellite. The 3–12 keV light curve has 10 day time resolution. Cyg X-3 is a peculiar high-luminosity X-ray source, radiating from the radio region to hard gamma rays of more than 10¹⁶ eV. It has a 4.8 hour period, probably orbital, which is not resolved by our present analysis. Long term periodicities of 17, 20, and 33–34 days have been reported by several authors, and explained as the effects of apsidal motion, precession, or an eccentric orbit. We do not observe the 17 and 33–34 day variations, and set upper limits significantly lower than the reported amplitude of the 33–34 day variation. There is weak evidence for a 20 day flux variation. The light curve shows high and low states which alternate with a characteristic timescale of 1 year. There is no counterpart, at this time resolution, of the giant radio outburst of 1972 September.     

The Energy Spectra and Anisotropies of Cosmic Rays

The existing paradigm of the origin of Galactic cosmic rays places strong supernovae shocks as the acceleration site for this material. However, although the EGRET gamma-ray telescope has reported evidence for GeV gamma rays from some supernovae, it is still unclear if the signal is produced by locally intense cosmic rays. Although non-thermal X-ray emissions have been detected from supernova remnants and interpreted as synchrotron emission from locally intense electrons at energies up to 100 TeV, the inferred source energy spectral slopes seem much steeper than the electron source spectrum observed through direct measurements. It remains the case that simple energetics provide the most convincing argument that supernovae power the bulk of cosmic rays. Two characteristics which can be used to investigate this issue at high energy are the source energy spectra and the source composition derived from direct measurements.     

The structure of low-mass X-ray binaries

The present knowledge of the structure of low-mass X-ray binary systems is reviewed. We examine the orbital period distribution of these sources and discuss how the orbital periods are measured. There is substantial observational evidence that the accretion disks in low-mass X-ray binaries are thick and structured. In a number of highly inclined systems, the compact X-ray emitting star is hidden from direct view by the disk and X-radiation is observed from these only because photons are scattered into the line of sight by material above and below the disk plane. In such systems the X-ray emission can appear extended with respect to the companion star, which can lead to partial X-ray eclipses. There are substantial variations in the thickness of the disk rim with azimuth. These give rise to the phenomenon of irregular dips in the X-ray flux which recur with the orbital period, or to an overall binary modulation of the X-ray flux if the source is extended. The X-ray spectra of low-mass X-ray binaries can be used to probe the innermost emission regions surrounding the compact star. The spectra of the bright Sco X-1 variables can be fitted with two components which are provisionally identified as originating in the inner disk and the boundary layer between the disk and the neutron star respectively. The characteristic energy dependent flaring of the Sco X-1 sub-class may be a geometric effect triggered by an increase in the thickness of the inner disk or boundary layer. The X-ray spectra of the lower luminosity systems, including the bursters, are less complex, and in many cases can be represented by a single power law with, in some sources, a high energy cut-off. Iron line emission is a characteristic of most low-mass X-ray binaries, irrespective of luminosity.     

The Medium Energy Instrument on Exosat

The Medium Energy Instrument on EXOSAT, although conceived as the main instrument for occultations, has been made sufficiently versatile to provide a significant advance over previous large area proportional counters when used for individual source studies of timing and spectra. The energy range is 1.2 to 50 keV, with E/E of 0.2 at 6 keV, sufficient to detect iron lines. The effective area of 1800 cm² and narrow field of view (3/4° × 3/4°) make it suitable for the detailed study of sources down to the 0.3 mCrab confusion limit. The unique facility provided by EXOSAT, allowing uninterrupted observations of X-ray sources for periods of up to 80 hours, backed up by a high capacity data link and on-board processing, enables timing studies to be performed over the range from milliseconds to days. Sophisticated background discrimination techniques giving a rejection efficiency of99% will control the background count rate to a suitably low value in the environment of the 200,000 km orbit.     

X-ray timing and spectral observations of active galactic nuclei

Conclusions X-ray variability is seen in all types of AGN but large amplitude ( factor 2) outbursts on short timescales (days) occur rarely, perhaps once every 100 days. There is no strong dependence of variability on luminosity, but radio-powerful AGN, particularly BL Lacs and 0VV QS0s, do vary most. Sensitive detectors, such as the EXOSAT ME, have been able to detect variability of smaller amplitude (20%) and on shorter timescales (1 hour) than previous experiments, but this too is not common. There is very little evidence of spectral variability during changes in intensity and so it is very likely that such changes are total power variations and not artefacts of variable obscuration. The variability timescales imply that most Seyfert galaxies are emitting well below the Eddington limit. On efficiency considerations only two observations of X-ray variability, those of the QS01525+227 and the BL Lac H0322+022, require exotic black hole models, relativistic beaming, or a change in the assumed value of H₀. The most dramatic observation of variability so far reported, that of repeated variations on a timescale of 4000 seconds in NGC4051 is probably related to a hydrodynamical timescale in the accretion disc and encourages us to believe that, with future observations, our understanding of AGN may approach that of galactic X-ray sources.Many Seyferts do have a canonical =0.7 spectral index, but it is becoming increasingly clear that a wide variety of spectral indices exist, both in Seyfert galaxies and in other classes of AGN. Both thermal and non-thermal emission mechanisms are tenable explanations for most of these spectra as, in general, the very high energy observations which could distinguish between the two are not available.Timing observations rarely require relativistic beaming, however, the (low) observed X-ray fluxes of BL Lacs and 0VV QS0s generally do. reacceleration of particles on short timescales is necessary to explain the continuous infrared to X-ray spectra of BL Lacs.The status of soft excesses in the low energy spectra of Seyfert galaxies which have canonical medium energy spectra is not clear. A separate soft component has been detected in EXOSAT observations of NGC4151 but this need not be associated with the nuclear continuum source. No SSS or EXOSAT observations definitely require such excesses. EXOSAT is, in principle, very sensitive to soft excesses but the uncertainty in the Boron filter calibration and in the value of the galactic absorption at present limit precise determinations.The absorbing column in the direction of many AGN is, in many cases, entirely accountable for purely by absorption in our own galaxy. In cases where a substantial absorbing column is detected, variations in the column are occasionally seen but it is not yet clear whether these variations are due to bulk movements of obscuring material or increased photoionisation (warm absorbers). All observations of iron lines are consistent with fluorescence in a cold gas which probably surrounds the X-ray emitting region in a sphere or shell-type geometry, though (by Gauss' law) this need not necessarily lie immediately next to the central black hole.Detailed observations of the time-variability of the complete X-ray to radio spectrum offer the best hope of further progress in this complex but interesting field.     

The X-ray imaging telescopes on Exosat

The instrument configuration and performance characteristics of the X-ray imaging telescopes on EXOSAT are described. The instrument comprises two fully independent Wolter I imaging telescopes. Each telescope can be used in either of two principal modes: (i) an imaging mode with either a position sensitive proportional counter or a channel multiplier array plate in the focal plane, (ii) a spectrometer mode which features a 500 lines/mm and/or a 1000 lines/mm transmission grating as dispersive element.Preliminary results from the calibration of the fully integrated experiment indicate an ultimate angular resolution of 8.5 arc sec full width at half maximum or 17.5 arc sec half-power beam width. The ultimate wavelength resolution in the spectrometer mode ranges from 1Å for wavelengths below 50Å, to 5Å at wavelengths near 300Å.A method for estimating the telescope performance is given which reasonably accounts for the influence of the X-ray source spectrum and the degree of interstellar absorption on the counting statistics.A comparison between EXOSAT and the EINSTEIN telescope in terms of band width/resolution and minimum source detectability shows an enhanced potential for EXOSAT relative to EINSTEIN for sources with T 10⁷K and low column densities (< 4 × 10²⁰cm^–2) and a reduced potential for sources with hard, or heavily cut-off, spectra.     

The spectral and temporal variability of the X-ray transient 4U1630-47

We present the results of four observations made by the European Space Agency's EXOSAT Observatory of the X-ray transient 4U1630-47 during its 1984 outburst. We observed marked spectral changes as the source decayed from a maximum observed intensity of 8×10^–9 erg/cm²/sec (1.5–10 keV). The spectrum could be modelled by a soft thermal-like component with a high energy power-law tail. The relative contribution of the soft to hard component decreased as the total luminosity decreased. We compare these changes with those observed from the black hole candidate Cyg X-1 when it transitions from a high to a low state. In addition we report the discovery of short timescale intensity variations (down to 50 msec) with a characteristic timescale of 20 sec. We present a precise position for this unidentified source.on leave from Università di Roma Dipartimento di Fisica G. Marconi.     

EXOSAT PSD and CMA observations of the SNR PKS 1209-52

EXOSAT PSD images and spectra are presented of the supernova remnant (SNR) PKS 1209-52 (G296.5+9.7. Milne 23). This source was observed for 8.5 hours in June, 1983. PSD images constructed in different energy intervals reveal that the spatial structure of the SNR is energy dependent. Comparison of the PSD and CMA images with the latest radio map of PKS 1209–52 shows some interesting correlations, especially between the X-ray and radio Hot Spots. The PSD spectrum of the SNR is fitted with a Raymond and Smith line-emission model: the best fit temperature is found to be 1.7×l0⁶ K and the absorbing column is less than 2×10²¹ cm^–2.A compact X-ray source lies within the radio shell of PKS 1209–52, near the centre of the remnant. The PSD spectrum of this object is somewhat harder than that of the SNR, but does not require a significantly different absorbing column density. The possible association of the SNR and the compact object is briefly discussed.     

High energy X-ray spectrum of her X-1

Summary On May 8, 1980, we conducted a 90 minute observation on hard X-ray emission (15-200 keV) from Her X-1, using a large area ( 1500 cm²), low background balloon borne X-ray telescope. The energy resolution of the telescope was 17% FWHM at 60 keV. Her X-1 was at binary phase 0.0725 and 2.7 ± 0.5 days after turn on in the 35 day cycle.Average pulsation light curves were obtained by sorting data into 25 equal bins, according to pulse arrival time, modulo the 1.24 sec pulsation period. The width of the main pulse is energy dependent and in the 45–75 keV region about 30% smaller than in the range from 15 to 30 keV.The data have been analyzed by taking the Her X-1 pulse minus background spectrum, where the pulse count rate is defined in a pulse phase interval around the pulse maximum of the 1.24 sec period. The background spectrum was intermittently obtained by a chopping collimator system.A spectral feature is present in emission at an energy of 49.5 (+ 1.5, -3) keV and a FWHM of 18 (+ 6, -3) keV and in absorption at an energy of 29.5 (+ 1.7, -1.5) keV and a FWHM of 17.0 (+ 2.6, -2.8) keV. The intensity of this line feature in emission is (1.8 ± 0.4) photons/cm sec. The line excess in emission over the continuum (with kT = 6.75 (+ 0.2, -0.4) keV) is 7.     

The system AM Her = 4U 1814 + 50

The binary system AM Herculis = 4U 1814 + 50 gives the first well ascertained example of an X-ray emitting magnetic white dwarf. The orbital period (3.1^h) is apparent from X-ray to IR frequencies and in linear and circular polarization. Since the time of the identification of the X-ray source the system has been extensively studied. The observations (which range from 1 MeV to 20 m) are reviewed and compared with the present theory of X-ray emitting white dwarfs.