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.     

Spectral observation of the composite supernova remnant G 29.7-0.3

The X-ray properties of the supernova remnant G 29.7-0.3 are discussed based on spectral data from the EXOSAT satellite. In the 2 to 10 keV range a featureless power-law spectrum is obtained, the best-fit parameters being: energy spectral index =-0.77, hydrogen column density on the line of sight N_H=2.3.10²² cm^–2. The incident X-ray flux from the source is (3.6±0.1) 10¹¹ erg cm^–2 s^–1 in the 2 to 10 keV range corresponding to an intrinsic luminosity of about 2. 10³⁶ erg s^–1 for a distance of 19 kpc. The source was not seen with the imaging instrument thus constraining the hydrogen column density to be N_H=(3.3 ±0.3) 10²² cm^–2 and the energy spectral index =1.0±0.15. This new observation is consistent with emission by a synchroton nebula presumably fed by an active pulsar. An upper limit of 1.5% for the pulsed fraction in the range of periods 32ms to 10⁴ s has been obtained.     

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 bright X-ray source GX9+1

We present the results of the spectral and timing analysis of an observation of GX9+1/4U1758-205 performed with the Medium Energy Experiment aboard EXOSAT. During our observation the source flux varied irregularly in time scales from minutes to hours. No periodic emission in the period range from 16 msec to 2000 sec was found with an upper limit of around 1% (3 ) for the pulsed fraction. The hardness ratio shows a correlated change with the flux intensity (Sco X-1 behaviour). The spectrum could be fitted by a double component model, a black body component (kT=1.16–1.26 keV) together with a thermal bremsstrahlung law (kT=13–15keV). The black-body temperature-black-body flux relation follows a Stefan Boltzmann law with R_BB=15.3 km^*D/10 kpc. No iron line was detected. The upper limit for the line equivalent width of a 6.7 keV iron emission line is 40 eV (1). The X-ray spectral behaviour of GX9+1 indicates, that this source belongs to the class of Low-Mass X-ray Binaries (LMXB).     

EXOSAT 2–10 KeV observations of M87 and the Virgo cluster

A series of observations of the Virgo cluster of galaxies with the Medium Energy experiment on board EXOSAT are presented. These take the form of two orthogonal scans meeting at M87. The observed count rates have been compared with those obtained with the IPC on board the EINSTEIN observatory and are found to be consistent with the the IPC surface brightness distribution. This fact together with spectral data obtained with Exosat indicates that the region around M87 is more or less isothermal (T 2–3 keV) out to > 80 arc min from the centre. Although a slight hardening is seen near the optical centroid of the cluster the results do not support a model which involves a hot ( 8 keV) intra-cluster medium. A mass of 5 × 10¹³ solar masses is derived for the inner 90 arc minutes of M87.     

Hard X-ray observations of the eclipsing binary EX Hydrae

We present light curves of EX Hydrae in the 1–10 keV range obtained with the medium energy experiment of EXOSAT. The 67-min modulation was observed with a peak-to-peak amplitude of 36 % in the 1–4 keV range and of 24 % in the 4–9 keV range. Newly discovered was a partial X-ray eclipse of 3 min full width which coincides with the optical eclipse and is seen at all energies. The results are interpreted in terms of an intermediate-polar (DQ Her star) model of EX Hydrae.     

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.     

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.     

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.     

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.     

X-ray spectral variability of 3C273

3C273 has been monitored by EXOSAT over the period December 1983 to June 1984 in 4 observations. In the December observation the flux was high and the spectrum showed a power law index of 1.5 changing to 0.9 at8 keV. In subsequent observations the flux dropped to40% of its original value and the hard tail disappeared. In the last observation the LE flux increased by a factor of 2 with no accompanying ME flux increase.     

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 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.     

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.     

Thermal radiation from a radio pulsar PSR 1055-52

Conclusion The EXOSAT observation of PSR 1055-52 supports strongly the idea of a thermally radiating neutron star, either still cooling from its initially high temperature or reheated by unpinning of vortex lines in its superfluid interior. The data do not confirm the Cheng & Helfand conclusions about a synchrotron nebula with a power law spectrum. The discrepancy in the data is, nevertheless, only a 3.5 effect, as no ME data could be used to get a firm upper limit of the flux above 1 keV. With regard to the outstanding theoretical interest in the structure of neutron stars we hope for further observations to clarify this question.     

EXOSAT and EINSTEIN High Resolution Images of the Small Magellanic Cloud

In order to obtain optical identifications and further information about the X-ray emission of sources discovered in the EINSTEIN IPC survey of the Small Magellanic Cloud (SMC), we have used the EXOSAT CMA and EINSTEIN HRI at selected positions. These observations have so far resulted in several identifications (including 4 stellar objects with m_v 14 to 21 and a Seyfert galaxy), and the discovery of two new X-ray sources. Medium energy X-rays (2–6 keV) have been detected from the brightest SNR in the SMC, 1E0102.2-7219. We present here an initial report of these results.     

The variable iron line in cygnus X-3

Cygnus X-3 was observed with the GSPC on board EXOSAT on several occasions, one observation lasting for 7 orbital cycles. The width W and centroid energy E of the iron emission feature near 6.7 keV show a smooth, correlated, sinusoidal-type modulation, the iron line being widest and E being lowest just before X-ray maximum. The line profile may show a low-energy wing, but apart from this does not deviate strongly from a symmetric, Gaussian-type shape. The continuum at higher energies than the line is not completely smooth, but shows bumps which remain stable in time. Two possible explanations are discussed for the correlated variation of E and W as a function of orbital phase.     

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.     

Spectra and time variability of GX 5-1

From a short observation of GX 5-1 with EXOSAT we have derived information on spectral and temporal behaviour in the energy range 1–20 keV. The source was found to be variable on time scales from 10 s to 1 h. Describing the spectrum one is forced to assume at least two spectral components. The best fit is reached using a spectrum composed of two blackbody functions with typical temperatures 1 keV and 2 keV, corresponding to apparent blackbody radii of 43 km and 11 km, respectively (for a distance of 10 kpc). With respect to the hot component there is evidence for variability in temperature as well as in apparent blackbody radius. No periodic variability has been found over the period range 0.25 s to 2000 s. There is no evidence for an iron emission line.     

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.