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This document describes a proposal for a replica X-ray optics to be developed in Italy for NASA's X-ray spectroscopy mission AXAF-S. The program is based on state of the art technology for the production of replica X-ray optics. On the basis of the experience with the Jet-X mirror shells (to be flown on the Spectrum X-(gamma) satellite), a spatial resolution of 15 - 30 arcsec half power diameter (HPD) can be achieved for the AXAF-S optics. The characteristics of the proposed optical system are described and its performances evaluated by using the current baseline configuration for the array of X-ray calorimeters in the focal plane. The impact of the proposed replica X-ray optics is briefly outlined and a comparison with foil optics (1 - 3 arcmin HPD resolution) is carried out.

ROSAT PSPC observations of the blazars PKS 1034–293 and PKS 1335–127 are presented. In both cases the spectra are well fitted by a single absorbed power law. For PKS 1034–293 we obtain F1keV=0.23±0.02 μJy, Γ=1.23+0.54−0.50, nH=1.31.7−1.2 (1020 cm−2) (90% uncertainty). The column density is significantly smaller than the galactic value derived from Stark et al. 1992. For PKS 1335–127 we find FkeV=0.50±0.02 μJy, Γ=2.29+0.55−0.51, nH=6.60+3.00−2.23 (1020 cm−2): in this case the column density derived from the fit is in good agreement with the recent determination by Jackson et al. 1993.

The project of an Optical Monitor (OM) for X-ray satellites, in particular the JET-X (Joint European Telescope for X-ray astronomy) experiment (Wells et al., 1991), derives from the scientific need of having complete data coverage at various wavelengths, UV and optical, of the observed X-ray sources, because these data are essential for a deeper understanding of the various classes of objects. When studying variable sources and/or transient astronomical phenomena, one needs that the multifrequency observations be performed essentially at the same time, because it is the knowledge of the simultaneous optical and X-ray behaviour of a source which contributes substantially to the clarification of its nature. In principle optical observations simultaneous with X-ray ones can be performed from ground based telescopes. However the complexity of satisfying the constraints typical of the optical telescopes (weather conditions, source observability) and of the X-ray instrumentation (e.g. orbital constraints) lead inevitably to a substantial loss of observing time. Therefore the only practical way of having an optimal utilization of the time available for X-ray observations, together with the wealth of scientific potential of simultaneous UV-optical observations, is to have a small telescope to be part of the same space mission.

Daily monitoring of PKS 2155-304 with the IUE satellite throughout November 1991 has revealed dramatic, large-amplitude, rapid variations in the ultraviolet flux of this BL Lac object. Many smaller, rapid flares are superimposed on a general doubling of the intensity. During the five-day period when sampling was roughly continuous, the rapid flaring had an apparent quasi-periodic nature, with peaks repeating every ~0.7 days. The short- and long-wavelength ultraviolet light curves are well correlated with each other, and with the optical light curve deduced from the Fine Error Sensor (FES) on IUE. The formal lag is zero but the cross-correlation if asymmetric in the sense that the shorter wavelength emission leads the longer. The ultraviolet spectral shape varies a small but significant amount. The correlation between spectral shape and intensity is complicated; an increase in intensity is associated with spectral hardening, but lags behind the spectral change by ~1 day. The sign of the correlation is consistent with the nonthermal acceleration processes expected in relativistic plasmas, so that the present results are consistent with relativistic jet models, which can also account for quasi-periodic flaring. In contrast, currently proposed accretion disk models are stronly ruled out by the simultaneous optical and ultraviolet variability., Comment: 27 pages, plain TeX, STScI Preprint 714

The bright quasar Kaz 102, which lies in the vicinity of the North Ecliptic Pole, was monitored during the ROSAT All Sky Survey for 121.5 days from 1990 July 30 to 1991 January 25. In the course of the survey, optical photometry with various filters was peformed at several epochs, together with UV (IUE) and optical spectrophotometry. The spectral energy distribution in the 3 x 10(exp 14) -3 x 10(exp 17) Hz range is obtained simultaneously among the various frequencies to less than or = 1 day. No clear case of variability can be made in the X-rays, while in the optical and UV variability of 10%-20% is apparent. An analysis of IUE and Einstein archives indicates a doubling timescale of years for the UV and soft X-ray flux. The X-ray photon index, which in 1979 was rather flat (Gamma = 0.8(+0.6 -0.4), in 1990/1991 was found to be Gamma = 2.22 +/- 0.13, a typical value for radio-quiet quasars in this energy range. The overall energy distribution and the variability are discussed.

Simultaneous X-ray, ultraviolet, optical, infrared, and radio monitoring data were used to test and constrain models of continuum emission from the BL Lacertae object PKS 2155-304. Intensively sampled ultraviolet and soft X-ray light curves showed a clear temporal correlation with the X-rays leading the ultraviolet by 2-3 hr. This lag was found to be significantly different from zero after an exhaustive comparison of four different techniques for measuring temporal correlations. Variations in the ultraviolet trough optical wave bands were also strongly correlated, with no measurable lag down to limiting timescales of approximately less than 1-2 hr. This strong correlation extends to the near-infrared, but the less intensive sampling precludes measurement of any lag beyomnd an upper limit of approximately less than 1 day. These lags and limits of the order of hours are much shorter than most rapid observed single-band variations. Because of the very sparse radio sampling, it was not possible to measure quantitatively the correlation and lag with shorter wavelengths, but the data do suggest that the radio may lag the optical/ultraviolet by approximately 1 week, with longer delays and weaker variations to longer radio wavelengths. The epoch-folding Q(exp 2) statistic was used to test for periodicity, and no evidence for strict or quasi-periodicity was found in any of the light curves. Because they lead the lower frequencies, the soft X-rays (approximately less than 1 keV) cannot arise from synchrotron self-Compton scattering. These results also rule out the accretion disk model, which predicts a measurable lag between ultraviolet/optical wavelength bands and a correlation between hardness and brightness, neither of which were seen. They are consistent with the entire radio through X-ray continuum arising from direct synchrotron emission from a relativistic jet. However, the tapered jet model, in which the X-ray emission is produced closer in, has problems explaining the magnitude of the ultraviolet/X-ray lag, because the X-ray-emitting electrons have very short lifetimes (t(sub 1/2) much less than 1 s). The result that the lag is much smaller than the variability timescale suggests instead that the radiation may be produced in a flattened region such as a shock front.

Simultaneous observations of 3C 279 at radio, millimeter, near-infrared, optical, ultraviolet (with IUE) and X-ray (with ROSAT) wavelengths were obtained in 1992 December-1993 January, during a three week pointing at the source by the Compton Gamma Ray Observatory. The blazar was in a quiescent or 'low' state during this period. Comparing the multiwavelength energy distribution to that from 1991 June, when 3C 279 was in its brightest recorded gamma-ray state, we find the following: (1) 3C 279 faded dramatically at all frequencies above 10(exp 14) Hz, while the flux variations at low frequencies (radio to millimeter wavelengths) were minor. (2) The near-infrared-optical-ultraviolet spectral shape was softer (steeper) in the quiescent state, and the X-ray spectra also appear softer, although the spectral indix measured by ROSAT refer to a lower energy band than that measured earlier with Ginga. (3) The ratio of the gamma-ray luminosity to that across all other frequencies decreased from a value of approximately equal to 10 in the flaring state to a value approximately equal to 1 in the quiescent state. These findings imply that the production of gamma-rays is closely related to the optical-ultraviolet continuum, in agreement with models where gamma-rays are produced through inverse Compton (IC) scattering by relativistic electrons emitting the synchrotron continuum. The observed nonlinear relation between the synchrotron and IC requires both a change in the electron spectrum and an associated change in the seed photons.

The existence of two populations of BL Lac sources, characterized by different spectral properties and number densities, has been confirmed by the recently obtained complete samples of radio- and X-ray-selected sources. In the framework of relativistic jet models, the different properties of the two populations can be accounted for if the X-ray radiation is less beamed than the radio one. We propose that this can be due to a larger opening angle of the flow velocity in the inner, X-ray-emitting part of a jet of constant bulk Lorentz factor. With these assumptions we compute the expected luminosity functions (LFs) in the radio and X-ray bands for beamed objects deriving from the same parent population but observed at different angles

We present new EXOSAT spectral results for four X-ray-selected BL Lac objects (H0323+022, H0414+009, H1426+428, and H1722+119). The data were retrieved from the EXOSAT data base. A spectral analysis was performed on the overall low-energy plus medium-energy distributions, which were modeled with a single, absorbed power law. In one case (H1426+428), an indication of a spectral break at ∼5 keV is found. For H0323+022 and H0414+428, for which more than one observation is available, the spectral index is found to vary with intensity, in the sense of a hardening of the spectrum when the source brightens

PKS 0537-441, an object with properties intermediate between BL Lacertae objects and OVVs, was observed with ROSAT in 1991 April. The X-ray state F 1keV =0.79±0.05 μJy is brighter than those detected by the Einstein Observatory (1979-1980), and EXOSAT (1985) satellites, while the spectral slope α ph =2.1±0.4 is steeper. The hydrogen column density N H =(3.0±1.3)×10 20 cm −2 is consistent with that deduced from 21 cm observations. Optical spectrophotometry of 1991 February indicates a moderately high state of the source. The observations are discussed also in the light of the newly discovered gamma-ray emission

We discuss the evidence for relativistic bulk motion of the emitting plasma in the nuclei of ∼100 radio sources, which include BL Lacertae objects, radio quasars, and radio galaxies, with published VBLI measurements of the core angular dimension and radio flux. Comparing the predicted and observed high-frequency (X-ray) flux, in the framework of the synchrotron self-Compton model, we derive the beaming or Doppler factor for all sources. This is compared with other beaming indicators, such as the value of the expansion velocity (mostly superluminal and available for ∼40% of the objects) and the ratio of the core to the extended radio flux (available for all but two sources)

The physical conditions in the gamma-ray-emitting blazar 3C 279 are discussed. The requirement of transparency for gamma-rays, together with the observation of rapid variability, imply that the high-energy radiation is anisotropic. It is proposed that the gamma-rays are produced in a relativistic jet via the synchrotron self-Compton mechanism. The gamma-ray spectrum is the high-energy extension of the inverse Compton radiation responsible for the X-ray emission. It is softer than the X-ray spectrum, owing to upper cutoffs in the electron energy spectra along the jet. The same electrons are responsible for the low-frequency emission via synchrotron radiation. The expected correlation of variability at different frequencies is discussed. 38 refs.

High-resolution imaging of the brigth BL Lacertae object PKS 2155-304 reveals a symmetric nebulosity extending up to 15° from the nucleus and confirms the presence of a faint galaxy located ∼4″ away from the nucleus. It is shown that the redshift of z = 0.117, previously attributed to the BL Lac object, pertains in fact to the companion galaxy. The radial brightness profile of the surrounding nebulosity is consistent with that of an elliptical galaxy of effective radius of 4.5″ which, assuming an absolute magnitude typical of luminous ellipticals, puts the object at z∼0.1. The field of PKS 2155-304 appears to be rich in galaxies, one of which is at z = 0.116

A detailed power spectrum and cross-correlation analysis of the X-ray light curves of the BL Lac object PKS 2155-304, obtained with EXOSAT, is presented. The power spectra show a marked red noise-type variability, with the power increasing steeply toward low frequency. The variability is more pronounced in the 1-6 keV band than in the 0.1-2 keV band. It is shown that the presence of data gaps can flatten significantly the logarithmic slope of the power spectrum. A technique for bridging data gaps which reduces the power spectrum distortions is introduced. In this way an average power law slope of about -2.5 is obtained for the power spectrum from the 1-6 keV data.

The average amplitude of variability of BL Lac objects is larger at higher frequencies, and the spectra in the X-ray range show a hardening with increasing intensity. This is shown to be a natural consequence of the relativistic jet model proposed by Ghisellini G. et al. (1987, Astron. Astrophys., 146), where higher frequencies are produced nearer to the jet core. Time-dependent properties are computed. The evolution of the spectral shape with time and light curves at fixed frequencies are presented.

An analysis of five Exosat observations of Cyg X-2, taken around a full binary orbit is presented. The data were obtained using all instrumentation (1000 lines/mm Transmission Grating Spectrometer, Channel Multiplier Array + filters, Medium-Energy experiment, and Gas Scintillation Proportional Counter) simultaneously, and span the full energy range 0.5-20 keV. No clear evidence was found for a correlation of any of the source characteristics with orbital phase. During two of the observations, significant iron K emission at 6.7 keV was detected, the relative strength of which seems to correlate with total X-ray intensity during two sharp intensity dips. The previous detection of discrete emission features in the 12-19 A band with the Einstein grating is confirmed.

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.

IUE satellite data in the 1200-2000 and 1900-3200 A intervals of BL Lac objects are analyzed in terms of two discernible groups. A total of 25 BL Lac objects were observed, with differences between groups displayed in terms of the power slope of the energy of the UV emissions, i.e., slopes of 1 and 2. Comparisons of the spectra with those of quasars showed that quasars have a small spectral index in the 1000-6000 A band and no correlation exists between the spectral index and UV flux of the BL Lac objects. The comparisons underscore the lack of a thermal component for BL Lac objects. Steep spectral components in both BL Lac objects and highly polarized quasars emissions could both be due to synchrotron emission. Compton scattering of relativistic electrons off synchrotron photons could produce the X ray emissions. 44 references.

Spectral properties of X-ray selected and radio-selected BL Lacs are discussed. The results provide a direct indication that the X-ray emission is more isotropic than the radio emission. Recent results of multifrequency observations of the brightest X-ray selected BL Lac object, PKS 2155-304, are reviewed, showing how the fast X-ray variability is smeared out at low frequencies. A theoretical jet model is presented which can quantitatively account for the different spectral properties of X-ray selected and radio selected BL Lacs in terms of different line-of-sight angles to the jet.

3A0729+103 (= BG CMi) is an intermediate polar discovered through its X-ray emission (McHardy et al. 1981, 1984). The orbital period is 3.235 hours and the rotation period is 15.2 minutes, For ephemeris and references on the source we refer to McHardy et al. (1984). We report here on optical (4025 to 5090 A) and ultraviolet (1200 to 3200 A) spectroscopy obtained, respectively, on Dec 1, 1984 and April 21, 1985. Our data show clear modulation of spectral features with the orbital period.

The faint (1.8 micro Jy, 2-11 keV) X-ray QSO 4U0241+61 is considered in terms of its celestial position (galactic latitude = +2.4 plus or minus 30 sec), luminosity (M sub v = -25.8 plus or minus 1.8), redshift (z = 0.0438), and radio emission (0.2-0.5 Jy). Counting rates in two energy channels are presented, and found comparable to the Type 1 Seyfert 3C120. Spectrophotometric calibration to an approximately 8 A resolution was also achieved. Attention is given to the close relation between X-ray emitting QSOs and Seyferts.

The X-ray source H2155-304, identified with the radiosource PKS 2155 and with a 14 mag star-like object (Wade, Szkody and Cordova 1978; Schwartz et al. 1979; Agrawal and Riegler, 1979) is commonly believed to be a BL Lac object, on the basis of the blue featureless continuum, the presence of optical polarization and the violent variability, with mB varying from 12.8 to 14.2 on a time scale of months (Griffiths et al, 1979). Its X-ray emission has been observed to vary by a factor 2 on a time scale of 6 hours which is among the shortest found in extragalactic objects (Snyder et al, 1980). An X-ray flare of 2 sec duration, which would be hardly consistent with the BL Lac interpretation, has been reported by Agrawal and Riegler (1979).

The possibility of explaining the continuous emission of active galactic nuclei in the frame of a model of spherical accretion onto a massive black hole is discussed. Cool inhomogeneities ( T ⋍10 4 °K ) within the accretion flow could be responsible for the broad line emission if half of the accreting matter is in the dense phase. A crucial test of this hypothesis is the expected correlation between the ratio of the luminosity in lines to the total luminosity and the hardness of the continuous spectrum.

X-ray, UV and optical data are presented of the longest period AM Her object, E2003+225, from October 12, 1983, together with a new linear polarization ephemeris. The optical and X-ray data were obtained simultaneously and the UV observations were carried out on the same day. A 6-hr observation with the Exosat 500 line/mm objective grating restricts soft X-ray blackbody temperatures to the range 18-29 eV. The blackbody luminosity exceeds the hard X-ray luminosity by at least a factor of 4.5, but is of the same order as the optical/UV component. Soft (0.1-0.25 keV) and hard X-ray (1-6 keV) light curves covering almost two orbital periods are presented and discussed.

Quasi-simultaneous observations of H2215-086 (=FO Aqr), obtained with Exosat and IUE in 1983 and 1985 are reported. Secular variations in the X-ray flux and in the shape of the rotational light curve are observed. The modulation of the photoelectric absorption cutoff is studied by means of phase resolved X-ray spectra. A possible orbital modulation of the UV emission is also discussed.

V 861 Sco (HD 152667) is a well known single line binary(1,2).The proposed association with a variable X-ray source(3,4) has triggered renewed interest in the system. Recent optical, infrared and ultraviolet observations(5 - 12) have provided a better knowledge of its characteristics. A first estimate of the mass loss was given by Hutchings(13).Here we report on a series of photometric observations in the infrared (from 1.25 to 4.8 µ) taken at various orbital phases which improve and extend previous measurements by Tanzi et al.(12)The results give evidence of a phase modulated infrared excess which can be interpreted in terms of a non isotropic mass flow in the system or, alternately, of a contribution from a colder secondary component.

He report on a series of observations of Cyg X-2 obtained with EXOSAT in September 1983 at five phases in a single orbital cycle (P = 9.8 days, see Cowley et al. 1979). Here we present spectral data obtained with the Argon counters of the Medium. Energy experiment (HE) (see Turner et al. 1981), together with the search of Quasi Periodic Oscillations (QPOs) in high tine resolution data.

Analysis of the anisotropy of the γ-ray background observed by Clarket al. (1968) has lead to the suggestion that part of the anisotropy be due to a source at the Galactic Centre. The complex structure of radio and infrared emission of the region of Sgr A is considered. The γ-ray flux arising from Compton scattering and π0 decay is calculated. For a value of the magnetic field H≈10−4 Gauss, corresponding to equipartition, the γ-ray flux above 100 MeV at Earth from the Sgr A source is expected to lie between 10−6 and 10−5 gamma/cm2 sec. Such a source should be detectable in the near future.

A gamma-ray telescope with a 850 cm2 multiplate spark chamber as a detector, has been used in an experiment of six balloon flights to investigate the Crab Nebula Pulsar 0532. The triggering signal is provided by a plastic scintillator-directional Čerenkov counter system. The spark chamber events are photographed by a stereo camera. The time of arrival of each event is recorded in UTC time with one millisecond accuracy, to search for a possible pulsed gamma emission from NP0532 above 50 MeV.

WE have investigated the gamma-ray emission from the Crab Nebula in a series of balloon flights of a multiplate spark chamber triggered by a Cerenkov detector-scintillator coincidence system1. The previously quoted gamma-ray energy threshold for pulsed emission from the Nebula and NP 0532 (refs. 1, 2) is ∼50 MeV; a detailed investigation of the telescope efficiency as a function of energy has now shown that the efficiency extends down to 20 MeV.

The galactic contribution to the diffuse X-ray flux is calculated on the basis of working hypotheses as to the shape of the electron spectrum in interstellar space. The galactic contribution is found to be small if compared with the observed X-ray intensity; nevertheless it may cause an appreciable anisotropy which would give information on the true cosmic ray electron spectrum at low energies (below 10 MeV) in interstellar space.

In BL Lac objects the absence of broad emission lines may be due either to a lack of ionizing continuum or to a lack of cool gas (T ≲ 104K)1. UV observations of BL Lac objects are interesting in that they provide a measurement of the continuum, and they include the wavelength range of Lyα, which is in principle the most favourable line for detection. We report here several spectra of the BL Lac object PKS2155–304 in the 1,150–3,200 A band taken with the IUE when the object was in a bright phase. The UV flux connects smoothly with the optical and IR observations of the source in its brightest state and its extrapolation matches the soft X-ray flux, implying a change in spectral slope around 1015Hz.

THE transient X-ray sources A1524—62 and A0535+26 were briefly observed by the University College, London, proportional counter spectrometer, experiment C on the Ariel V satellite (P. W. Sanford and J. C. Ives, unpublished). The resulting spectra help to complete the spectral history of these two sources, making seven sources for which there is now extensive spectral information. A general scheme emerges from these in which the sources with hard spectra (n ≲ 2) show little spectral evolution and those with soft spectra (n >2) exhibit marked softening with time. We discuss here this subdivision in relation to other properties of the sources.

SS 433 appears in the list of radio stars possibly associated with SN remnants compiled by Ryle et al. (1978) together with Cir X-I. The similarity of the two sources also extends to their optical spectra as stressed by Clark and Murdin (1978). Another strongly variable radii star GT O236+610 was discovered in 1977 by Gregory and Taylor (1978). It was successively identified with an early type star with unusually broad Ha and HB emission (Gregory et al.j 1979). Within the class of "variable radio stars" this is the fourth in peak luminosity (2xlO 31 erg/s), the first being Cyg X-3, the second Cir X-I, and third SS 433, as noted by Gregory et al. (1979).

The results are presented of a survey regarding the X-ray source positions which fall within the error boxes of 10 unidentified gamma-ray sources observed with the aid of the COS-B satellite. In three cases, including CG 135-1, CG 312-1, and CG 327-0, an X-ray source was found within the gamma-ray error box. However, because of the large uncertainty regarding the gamma-ray source positions, the positional coincidence is not necessarily conclusive. It is, therefore, necessary to take into account additional information on the spectral or temporal characteristics of the X-ray sources. It is found that the X-ray source 4U 02416 plus 1 is a possible candidate as the X-ray-counterpart of CG 135 plus 1 in connection with both spectral hardness characteristics and positional coincidence.