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.     

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).     

Coronal activity in F-, G-, and K-type stars

Summary Soft X-ray (0.3–3.5 keV) observations with the Imaging Proportional Counter (IPC) onboard Einstein Observatory are presented for a sample of some 20 cool stars of luminosity classes III–V. The results are compared with the Ca II H and K emission, which had served as a selection criterion.The specific X-ray flux F_X is an increasing function of the specific Ca II H and K line-core flux F_H+K. This correlation can be considerably improved by replacing F_H+K by the excess flux (F_H+K) above a certain lower limit which varies with B-V. This relation holds with little scatter over the two decades in F_X in our sample. The F_X-F_H+K relation shows no significant dependence on spectral type or luminosity class, it suits close binaries as well as single stars. However, the coronal X-ray temperature T_c strongly depends on the luminosity class: T_c 3 10⁶ K for dwarfs and 10⁷ K for giants.The results are interpreted in the framework of magnetic activity. The X-ray emission and the excess Ca II H and K flux are attributed to magnetic structure in the corona and chromosphere, the magnetic features emerging from the stellar convective envelope, where they are generated by dynamo action.     

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.     

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 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.     

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.     

On the optical variability of a be star,HDE 245770, — the optical counterpart of the transient X-ray pulsar A0535+26

Photoelectric WBVR observations of Be star HDE 245770=V 725 Tau, the optical counterpart of the transient X-ray pulsar A0535+26, having a pulse period of about 104 s, were conducted for more than 10 years. An irregular long-term optical variability of the star with amplitudes of the order of a few tenths of magnitude was found to be a usual phenomenon. In some cases rapid changes of the star's optical luminosity with a characteristic period of a few tens of minutes or a few hours, and an amplitude of several hundredths of magnitude in all the spectral bands used, which have practically coincided or correlated with the X-ray pulsar outbursts detected by X-ray satellites, were observed.Photoelectric recording of the optical flux from HDE 245770 were made in 1981–1982 with a time resolution of 1 second and 10 s, respectively, in theR spectral band (₀ 7000 Å) and in the narrowH -emission-line band (_1/2 75 Å) using a 48-cm reflector of High-Mountain Tien-Shan observatory of the Sternberg Astronomical Institute near Alma-Ata. An analysis of autocorrelation functions of the flux changes from object under study and a comparison with the star BD+26° 876 indicated the variability of luminosity of V 725 Tau in theR spectral band on a time scale of a few tens of second; this variability resembles shot noise with a characteristic time of stochastic bursts of about 15–20 s and their amplitudes of about a few tenths of a percent. InH -emission-line radiation autocorrelation functions and power spectra show quasiperiodic variability of luminosity of HDE 245770 with a characteristic period of about 100–150 s and an amplitude in the neighbour-hood of 0.5%. The latter result is not quite reliable because of not quite fine weather conditions during the observations; independent observations and check-up are required.     

Fragmentation in rotating isothermal protostellar clouds

Summary From the extensive set of numerical calculations briefly described above, it seems apparent that rotating, isothermal gas clouds are unstable to fragmentation under a wide range of conditions. (Caution: This result for isothermal clouds cannot be generalized to all clouds, as is shown, for example, by Boss's analysis [these proceedings] of the stability of collapsing, adiabatic clouds.) It is of importance to note, however, that no fragmentation is apparent during a cloud's initial dynamic collapse toward a disk structure; rather it is the rotationally flattened disk/ring configuration that undergoes fragmentation. This is a considerably different picture of fragmentation than has been presented, for example, by Hoyle (1953).The degree of instability and the mode (ring vs. blob) of fragmentation is sensitive to , but insensitive to . The initial amplitude of a perturbation does not appear to be crucial--fragmentation should occur eventually even for low amplitude initial NAPs.Finally, it is of some interest to know what the properties are of the fragments that break out of these isothermal clouds. Before outlining these properties we emphasize that in this set of calculations we have specifically excited the m = 2 (binary) non-axisymmetric mode; hence we have in some sense suppressed the development of other modes and we have promoted the development of equal mass components in the binary systems. In these evolutions, a typical fragment contained 15% of the initial cloud mass; had a specific angular momentum 25–30% that of the original cloud; had a ratio of spin angular momentum to orbital angular momentum 0.2; and itself had a ratio of thermal to gravitational energy _frag < 0.1. The formation of a binary system has therefore resulted in a conversion of some of the original cloud's spin angular momentum into orbital angular momentum, and has produced protostars with reduced specific angular momenta. It is also evident that each fragment is unstable to further collapse (having low ) under the isothermal assumptions imposed here.     

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     

Photometric effects of accretion disks in long-period eclipsing binaries

Summary A multi-year photometric program on long-period eclipsing binaries has begun to uncover some properties of accretion disks in these systems. Emission and transmission properties can sometimes be found from light curve features produced by partial eclipses of the disk by the cool star, and by partial occultations of the cool star by the disk. These disks do not have the classical alpha structure. They are optically thin normal to the orbital plane, but may be geometrically thicker than purely gravitationally-stratified disks. Disk gas may be contaminated by dust particles acquired from the outer layers of the cool loser. In some systems, high states, produced by elevated mass accretion by the hot star, occur, suggesting that the mass distribution in the disk is clumpy. However mass-transfer rates are found, they lie between 10^-7 and 10^-6 solar masses per year.While this binary sample is small at the moment, some of its properties are shared with other systems. The author has five-color observations of about a dozen additional systems, which may fill out this picture more fully.     

X-ray and optical variability at the hour timescale for 1E 0630+178 (GEMINGA) and its proposed optical couterpart

Einstein and EXOSAT data on the soft X-ray source IE 0630+178, the proposed counterpart of the -ray source GEMINGA, are analyzed for variability on the time scale of one to three hours. The EXOSAT September 1983 data, with an uninterrupted strech of over 10 hours offer the most interesting case. In parallel, a similar analysis is presented for the first time, for the optical data of the m_V21 proposed counterpart. About 30 CCD exposures, of 15 min. each, taken over two consecutive nights at the 3.6 m CFH telescope, yield evidence of variability, when compared to the data of similar nearby objects in the field.Visiting astronomer at the Canada-France-Hawaii Telescope, operated by the National Concil of Research, Canada, the Centre National de la Recherche Scientifique, France, and the University of Hawaii.     

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.     

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.     

LHG 83: Another outstanding X-ray binary in the LMC

The strongest X-ray point source, LHG 83, discovered in the EINSTEIN survey of the LMC and not being associated with a nearby coronal type stellar emitter or background AGN is identified with a faint blue variable object. Spectrophotometry reveals low mass X-ray binary characteristics at a mean velocity consistent with LMC membership. The He II 4686 emission exhibits a unique blue shifted component suggesting outflow velocities of several thousand km/s. Optical brightness changes by 0.3 mag in less than one hour are likely to be intrinsic to the source rather than induced by orbital motion. The low X-ray to optical flux ratio is probably due to the fact that the central X-ray source is blocked from direct view by the accretion disk.Based on observations obtained at ESO, La Silla, Chile     

Diffuse cosmic X- and gamma radiation: The isotropic component

SummaryA. Spectral features The ability of the various theories to explain the three main spectral features at 1/4 keV, 60 keV and 1 MeV is summarized in Tables II and III.Clearly, confirmation of the reality of these features, especially the soft X-ray and -ray excesses, is one of the key elements in enabling us to decide between the competing theoretical interpretations.B. Energy requirements None of the proposed interpretations are easily explained in terms of the available energy in cosmic rays (except perhaps the Seyfert galaxy proposal, and this runs into difficulties). It seems that one either has to regard normal galaxies at the present epoch as prolific sources of cosmic rays ( 10⁶⁰ erg/galaxy in protons), as is required by the Brecher-Morrison model, or to argue that at early stages in their evolution far more energy is available than at present. One ends up with much the same energy requirement in this approach.One could conceivably identify such an early phase with the radio galaxy or QSO phenomena: in any event, cosmological evolution plays a major role. Cosmology does ease the energy requirements, but only for the inefficient mechanisms, such as nonthermal bremsstrahlung or ° -production.It seems that one still needs the metagalactic cosmic ray flux to be 10^-2 of the galactic flux in the diffuse inverse Compton models, and 10^-2–10^-4 in the nonthermal bremsstrahlung models.Faced with problems of energetics, one is tempted to turn to the most energetic objects in the Universe, namely Seyfert nuclei and QSO's, to provide the basic energy source, whether directly or indirectly, for the diffuse X-ray background. A direct connection could be more readily investigated when X-ray observations are available of more extra-galactic sources.C. Angular variations Another approach, complementary to that of looking for remote discrete sources, is to seek angular fluctuations, or limits on such fluctuations in the diffuse X-ray background.The best results presently available are those from the X-ray experiment on board OSO 3. Schwartz (1970) reports a limit of I/Ifour percent on small-scale (10°) fluctuations over 10–100 keV over about one-quarter of the sky. If one assumes a astrophysics, namely the origin of cosmic rays, is intimately linked to the origin of the X-ray background.It may well be that no single mechanism suffices to account for the entire spectrum of isotropic X- and -radiation. Nature is sufficiently perverse for there to be a reasonable probability that several different processes are contributing, and considerable ingenuity will be required to ascertain which mechanism, if any, is assigned the dominant role in a given spectral region.This review is based on an invited paper presented at the joint meeting of the A. A. S. Division of High Energy Astrophysics, and the A. P. S. Division of Cosmic Physics, Washington, D. C., 28 April–1 May, 1970     

Ultra high-energy gamma-ray observations of Geminga

Results of the observations of Geminga (2CG 195 + 4) in the energy range E 10¹² eV, carried out in 1979, 1981, and 1983 with the Tien Shan high-altitude facility for recording the erenkov flashes of extensive air showers are reported. The mean flux density averaged over the whole protracted data is (5.7 ± 2.5) × 10^–11 quanta cm^–1 s^–1. The flux is variable with a period 59 s. The character of the period variation with time is hard to be reconciled with earlier findings by other authors. The importance of further simultaneous observations at various energies is indicated.     

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.     

High energy cosmic ray observations during August 1972

A series of spectacular cosmic ray events which included two relativistic solar particle enhancements and three major Forbush decreases were registered by ground-based cosmic ray monitoring stations beginning 4 August, 1972. These were associated with four major proton flare events on the Sun and with large interplanetary magnetic field disturbances and high velocity shock waves. This review attempts to discuss and interpret the high energy cosmic ray phenomena observed during this period in the light of the known behaviour of low energy particulate flux, interplanetary plasma and field observations and other associated solar and terrestrial effects recorded during this period.The first Forbush decrease event FD-1 occurred in the early hours of 4 August, exhibiting very strong north-south and east-west anisotropies. Immediately following the onset of FD-1, the first ground level solar particle enhancement occurred. This event, which had its onset almost 6 h after the flare event on 4 August, had a very steep rigidity spectrum. The major Forbush event of the series which had its onset at 2200 UT on 4 August, exhibited extremely interesting and complex behaviour, the prominent features of which are a precursory increase prior to the onset (PI-1), a large decrease (FD-2), the largest observed to date, followed immediately by an abrupt square wave like enhancement (PI-2). Interplanetary space during this entire period was highly disturbed by the presence of large low energy particulate fluxes and shock waves, at least one of which had a velocity exceeding 2000 km s^-1. Large north-south and east-west anisotropies existed throughout the event. Both FD-2 and PI-2 were characterized by almost the same rigidity spectrum, with a power law index of -1.2 ± 0.2, and a predominant anisotropy along the sunward direction. The square wave-like spike PI-2 during the recovery of FD-2 was associated with a similar abrupt change in low energy particle flux in space, as well as an abrupt decrease in the interplanetary magnetic field value from 50 to 10 .Based on the available particle, field and plasma observations, an unified model is presented to explain the Forbush event in terms of a transient modulating region associated with the passage of a narrow magnetic shock front. In this model, the reflection of particles from the approaching shock front account for the precursory increase PI-1. The main Forbush event is caused when the magnetic barrier at the shock front sweeps past the Earth. The square wave increase is due to the enhanced flux contained in the magnetic well just behind the shock front and bounded by magnetic discontinuities, which is explained as due to the transverse diffusion of particles into this region from the interplanetary space which have easy access to this region. In situ plasma, field and low energy particle observations are reviewed to support the model.Also Professor at Physical Research Laboratory, Ahmedabad 380009, India.     

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.