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We present the results of spectral analysis of 42 simultaneous broad-band Ginga--OSSE and RXTE--OSSE observations of Cyg X-1 carried out in 1991 and 1996--1999. The hardest spectra in our sample in the energy range from 3 to \~1000 keV can be well described by thermal Comptonization model with reflection from the cold disc, while the rest of the spectra are more complex and require an additional component below 10 keV. We consider a number of physically realistic models to describe the shape of the E<10 keV excess. The additional soft component can result from thermal Comptonization by electrons with a low Compton parameter, or can be a part of a nonthermal, power-law like emission extending above 1 MeV. We study correlations between parameters obtained from the spectral fits with different models. We confirm a general correlation between the photon index $\Gamma$ and the amplitude of reflection R. We find that simple phenomenological models (like power-law plus Compton reflection) applied to the narrow band (3--20 keV) data overestimated the values of R and $\Gamma$, although the simple models did rank correctly the spectra according to R and $\Gamma$. The dynamic corona model provides a satisfactory description of the observed correlation, while the hot inner disc models have problems in reproducing it quantitatively. We do not find significant correlation between the electron temperature and other spectral parameters, while the optical depth of the hot medium seems to decrease when the spectrum becomes softer. It is also shown that spectral parameters are well correlated with the timing characteristics of the source., Comment: 17 pages, 11 figures, accepted to MNRAS. Changes to v2: added N_H notes, improved discussion on timing, new calibration used

A number of recent lines of evidence point towards the presence of hot, out-flowing plasma from the central regions of compact Galactic and extragalactic X-ray sources. Additionally, it has long been noted that many of these sources exhibit an ``excess'' continuum component, above 10 keV, usually attributed to Compton Reflection from a static medium. Motivated by these facts, as well as by recent observational constraints on the Compton reflection models - specifically apparently discrepant variability timescales for line and continuum components in some cases -- we consider possible effects of out-flowing plasma on the high-energy continuum spectra of accretion powered compact objects. We present a general formulation for photon downscattering diffusion which includes recoil and Comptonization effects due to divergence of the flow. We then develop an analytical theory for the spectral formation in such systems that allows us to derive formulae for the emergent spectrum. Finally we perform the analytical model fitting on several Galactic X-ray binaries. Objects which have been modeled with high-covering-fraction Compton reflectors, such as GS1353-64 are included in our analysis. In addition, Cyg X-3, is which is widely believed to be characterized by dense circumstellar winds with temperature of order 10^6 K, provides an interesting test case. Data from INTEGRAL and RXTE covering the 3-300 keV range are used in our analysis. We further consider the possibility that the widely noted distortion of the power-law continuum above 10 keV may in some cases be explained by these spectral softening effects., Comment: 16 pages, 5 figures, accepted for publication in the Astrophysical Journal and scheduled for 1 December 2004, vol 616 issue

We describe a method for the determination of black-hole masses based on information inferred from high-energy spectra. It is required that the spectral energy distribution consist of thermal and Comptonized components. One can then, in principle, infer the depth of the gravitational potential well for sources of known distance. The thermal component is inferred by the integration of a blackbody spectral form over the disk. We assume that the color temperature distribution in the disk has a specific shape given by the Shakura-Sunyaev (1973) disk model which goes to zero at the inner disk radius and at infinity and has a maximum at 4.2 Schwarzchild radii. In this formulation there is only one parameter, the so called color correction factor, relating the apparent temperature to effective temperature which characterizes the thermal emission component. We have made use of improved Galactic black hole binary dynamical mass determinations to derive, in effect, an empirical calibration of this factor. We then present our analysis of observational data for representative objects of several classes; Galactic black hole X-ray binaries, narrow line Seyfert galaxies (NLS1), and ``ultra-luminous'' extragalactic X-ray sources (ULX). We then apply our mass determination calculation and present our results. We argue that this approach can potentially fill a void in the current knowledge of NLS1 and ULX properties, and discuss how a deeper understanding of both classes has relevance to the broader issues of how cosmic black holes, beyond the stellar-mass realm, are formed and what is their overall mass distribution., Comment: 15 pages, 3 tables, 6 figures. Accepted for publication in ApJ, 1st of August

The theoretical predictions of the converging inflow, or Bulk-Motion Comptonization model are discussed and some predictions are compared to X- and gamma-ray observations of the high/soft state of Galactic black hole candidate XTE J1550+564. The 100 Hz QPO phenomenon tends to be detected in the high-state at times when the bolometric luminosity surges and the hard-power-law spectral component is dominant. Furthermore, the power in these features increases with energy. We offer interpretation of this phenomenon, as oscillations of the innermost part of the accretion disk, which in turn supplies the seed photons for the converging inflow where the hard power-law is formed through Bulk Motion Comptonization (BMC). We further argue that the noted lack of coherence between intensity variations of the high-soft-state low and high energy bands is a natural consequence of our model, and that a natural explanation for the observed hard and soft lag phenomenon is offered. In addition, we address some criticisms of the BMC model supporting our claims with observational results., Comment: 17 pages, 5 figures, accepted by the Astrophysical Journal

We offer a brief description of the bulk-motion Comptonization (BMC) model for accretion onto black holes, illustrated by its application to observational data for LMC X-1, and Nova Muscae 1991. We then extract some physical parameters of these systems from observables (within the context of the BMC model}, drawing from results on GRO J1655-40, for which we presented extensive analysis previously. We derive estimates of the mass, (16 +/- 1) solar masses and mass accretion rate in the disk in Eddington units around 2 for LMC X-1, and (24 +/- 1)d_{5.5} and the disk mass acretion rate around 3 for Nova Muscae 1991 [where d_{5.5} stands for the distance in 5.5 kpc units]. Differences between these estimates and previous estimates based on dynamical studies are discussed. It is further shown that the disk inner radius increases with the high-to-low state transition in Nova Muscae 1991. Specifically, our analysis suggests that the inner-disk radius increases to 17 Scwarzschild radii as the transition to the low-hard state occurs., Comment: 14 pages, 3 figures, The paper is accepted for publication in the Astrophysical Journal Letters

We present our analysis of the high-energy radiation from black hole (BH) transients, using archival data obtained primarily with RXTE observatory, and a comprehensive test of the bulk motion Comptonization (BMC) model for the high-soft state continuum. The emergent spectra of over 30 separate measurements of GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE J1755-32, and EXO~1846-031 X-ray sources are successfully fitted by the BMC model, which has been derived from basic physical principles in previous work. This in turn provides direct physical insight into the innermost observable regions where matter impinging upon the event horizon can effectively be directly viewed. The BMC model is characterized by three parameters: the disk color temperature, a geometric factor related to the illumination of the black hole (BH) site by the disk and a spectral index related to the efficiency of the bulk motion upscattering. For the case of GRO J1655-40, where there are distance and mass determinations, a self consistency check of the BMC model has been made. Using model parameters we present new, independent constraints on the black hole mass, mass accretion rate and the distance for the aforementioned sources. Notable is the relationship between the color temperature and flux, which for GRO J1655-40 is entirely distinct from a simple T^4 dependence, and consistent with the disk model we have invoked - standard Shakura-Sunyaev's disk. This allows us to impose an important estimation of the hardness parameter, the ratio of the color temperature to the effective temperature - we find T_h~2.6, higher than previous estimates used in the literature., Comment: 50 pages, 8 figures, accepted for publication in the Astrophysical Journal (scheduled for the May 20, 1999 issue)

We discuss the emergent spectra from accreting black holes, considering in particular the case where the accretion is characterized by relativistic bulk motion. We suggest that such accretion is likely to occur in a wide variety of black hole enviroments, where the strong gravitational field is expected to dominate the pressure forces, and this likely to lead to a characteristic high-energy spectriscopic signature; an extended power tail. It is in the high (soft) state that matter impinging upon the event horizon can be viewed directly, and intrinsic power-law seen. A test of the model is presented using observational data from the Compton Gamma Ray Observatory and the Rossi X-ray Timing Explorer, covering 2- 200 keV for recent galactic black hole X-ray nova outbursts., Comment: latex, 18 pages, 2 figures. Accepted by ApJLetters

Recent radio surveys have discovered a large number of low-luminosity core-dominated radio galaxies that are much more abundant than those at higher luminosities. These objects will be too faint in γ-rays to be detected individually by Fermi. Nevertheless, they may contribute significantly to the unresolved extragalactic γ-ray background. We consider here the possible contribution of these core-dominated radio galaxies to the diffuse extragalactic γ-ray background. Using published data available for all 45 of the radio galaxies listed as detected counterparts in the Fermi FL8Y source list update to the 3FGL catalog, we have searched for radio maps that can resolve the core flux from the total source flux. Using high-resolution radio maps we were able to obtain core fluxes for virtually every source. We then derived a relation between core radio flux and γ-ray flux that we extrapolated to sources with low radio luminosities that are known to be highly core-dominated. We then employed a very recent determination of the luminosity function for core-dominated radio galaxies in order to obtain the contribution of all possible γ-ray-emitting radio galaxies to the unresolved extragalactic γ-ray background. We find this contribution to be possibly non-negligible, 4%–18% of the unresolved γ-ray background observed using the Fermi-LAT telescope.

We analyze a 900 kilosecond stacked Chandra/HETG (High-Energy Transmission Grating) spectrum of NGC 3783 in the context of magnetically driven accretion-disk wind models in an effort to provide tight constraints on the global conditions of the underlying absorbers. Motivated by the earlier measurements of its absorption measure distribution (AMD) indicating X-ray-absorbing ionic columns that decrease slowly with decreasing ionization parameter, we employ 2-dimension (2-D) magnetohydrodynamic (MHD) disk wind models to describe the global outflow. We compute its photoionization structure along with the wind kinematic properties, allowing us to further calculate in a self-consistent fashion the shapes of the major X-ray absorption lines. With the wind radial density profile determined by the AMD, the profiles of the ensemble of the observed absorption features are determined by the two global parameters of the MHD wind; i.e., disk inclination theta (sub obs) and wind density normalization n (sub o). Considering the most significant absorption features in the approximately 1.8-20 angstrom range, we show that the MHD wind is best described by n(r) approximately equal to 6.9 times 10 (sup 11) (r/r (sub o)) (sup - 1.15) cubic centimeters and theta (sub obs). We argue that winds launched by X-ray heating or radiation pressure, or even MHD winds but with steeper radial density profiles, are strongly disfavored by data. Considering the properties of Fe K-band absorption features (i.e., Fe XXV and Fe XXVI), while typically prominent in the active galactic nucleus X-ray spectra, they appear to be weak in NGC 3783. For the specific parameters of our model obtained by fitting the AMD and the rest of the absorption features, these features are found to be weak, in agreement with observations.

We present a unified model for the structure and appearance of accretion powered sources across their entire luminosity range from galactic X-ray binaries (XRB) to luminous quasars, with emphasis on AG N and their phenomenology. Central to this model is the notion of MHD winds launched by the accretion disks that power these objects. These winds provide the matter that manifests as blueshifted absorption features in the UV and X-ray spectra of a large fraction of these sources; furthermore, their density distribution in the poloidal plane determines their "appearance" (i.e. the column and velocity structure of these absorption features and the obscuration of the continuum source) as a function of the observer inclination angle (a feature to which INTEGRAL has made significant contributions). This work focuses on just the broadest characteristics of these objects; nonetheless, it provides scaling laws that allow one to reproduce within this model the properties of objects extending in luminosity from luminous quasars to XRBs. Our general conclusion is that the AGN phenomenology can be accounted for in terms of three parameters: The wind maSS flux in units of the Eddington value, m(dot), the observers' inclination angle Theta and the logarithmic slope between the 0/UV and X-ray fluxes alpha(sub ox); however because of a correlation between alpha(sub ox) and UV luminosity the number of significant parameters is two. The AGN correlations implied by this model appear to extend to and consistent with the XRB phenomenology, suggesting the presence of a truly unified underlying structure for accretion powered sources.

The bright and highly variable X-ray and radio source known as Cygnus X-3 was among the first X-ray sources discovered, yet it remains in many ways an enigma. Its known to consist of a massive. Wolf-Rayet primary in an extremely tight orbit with a compact object. Yet one of the most basic of pa.ranietern the mass of the compact object - is not known. Nor is it even clear whether its is a neutron star or a black hole. In this Paper we present our analysis of the broad-band high-energy continua covering a substantial range in luminosity and spectral morphology. We apply these results to a recently identified scaling relationship which has been demonstrated to provide reliable estimates of the compact object mass in a number of accretion powered binaries. This analysis leads us to conclude that the compact object in Cygnus X-3 has a mass greater than 4.2 solar mass thus clearly indicative of a black hole and as such resolving a longstanding issue. The full range of uncertainty in our analysis and from using a. range of recently published distance estimates constrains the compact object mass to lie between 4.2 solar mass and 14.4 solar mass. Our favored estimate, based on a 9.0 kpc distance estimate is approx. l0 solar mass, with the. error margin of 3.2 solar masses. This result may thus pose challenges to shared-envelope evolutionary models of compact binaries. as well as establishing Cygnus X-3 as the first confirmed accretion-powered galactic gamma: ray source.

Simultaneous optical, UV, and X-ray spectroscopy of the low-redshift high-luminosity QSO H1821+643 is presented in order to examine the connection between the optical UV bump and the soft X-ray excess. The correlation of the short-term variability in the optical/UV and soft X-ray band is examined, and the applicability of the accretion disk models to the simultaneous continuum energy distribution is tested. By fitting the continuum energy distribution with the thin accretion disk model, it was found that bare disk models cannot easily account for the emission in both the UV and the steep excess in the soft X-ray band. The intrinsic X-ray spectrum of H1821+643 exhibited a typical power-law slope above about 0.3 keV, as detected by ROSAT and Ginga. The first detection of H1821+643 in the radio band is reported. This radio-quiet QSO was found to have a substantial radio flux at 20 cm and a typical steep spectral slope.

The Compton Observatory Science Workshop was held in Annapolis, Maryland on September 23-25, 1991. The primary purpose of the workshop was to provide a forum for the exchange of ideas and information among scientists with interests in various areas of high energy astrophysics, with emphasis on the scientific capabilities of the Compton Observatory. Early scientific results, as well as reports on in-flight instrument performance and calibrations are presented. Guest investigator data products, analysis techniques, and associated software were discussed. Scientific topics covered included active galaxies, cosmic gamma ray bursts, solar physics, pulsars, novae, supernovae, galactic binary sources, and diffuse galactic and extragalactic emission.

The first UV observations of the bright QSO H1821 + 643 are reported. With V = 14.2 mag and z = 0.297, H1821 + 643 is the second brightest object in the sky at z above 0.1. The IUE data are combined with new optical spectroscopy, and existing IR and X-ray data, to reveal a strong optical/UV 'big bump', which continues past the Lyman limit in the rest frame of the QSO. A possible turnover at the high-frequency side of the UV continuum constrains fits of a thin accretion disk model to a large black hole mass and high accretion rate, but a small disk size. The shape of the UV continuum was found to be variable, with a hardening of the spectrum when the source was brighter. Because of its location, only 3 deg from the ecliptic pole, H1821 + 643 will be an important object for simultaneous UV and soft X-ray monitoring to test for a common origin of the UV bump and soft X-ray excess.

Preliminary results of a multifrequency monitoring campaign for the bright, Seyfert 1 galactic nuclei Mkn335 are presented. Nearly uniform sampling at 3 day intervals is achieved quasi simultaneously at each wavelength band. Wavelength dependent variability is seen at the 20 to 30 percent level. Interpretation of variability in terms of geometrically thin, optically thick accretion disk models is discussed. The inferred blackhole masses and accretion rates are discussed. Possible correlation between continuum and emission line variations is discussed.

Preliminary results of new International Ultraviolet Explorer (IUE) observations of two major transient events, the April 1989 outburst of the Be star transient system A0535+26 and the remarkable May/June 1989 outburst of V404 Cygni, are discussed. The UV data for V404 Cygni, combined with contemporaneous published optical and x ray data are not satisfactorily represented by standard accretions disk models.

The International Ultraviolet Explorer (IUE) attitude control system originally included an inertial reference package containing six gyroscopes for three axis stabilization. The science instrument includes a prime and redundant Field Error Sensor (FES) camera for target acquisition and offset guiding. Since launch, four of the six gyroscopes have failed. The current attitude control system utilizes the remaining two gyros and a Fine Sun Sensor (FSS) for three axis stabilization. When the next gyro fails, a new attitude control system will be uplinked which will rely on the remaining gyro and the FSS for general three axis stabilization. In addition to the FSS, the FES cameras will be required to assist in maintaining fine attitude control during target acquisition. This has required thoroughly determining the characteristics of the FES cameras and the spectrograph aperture plate as well as devising new target acquisition procedures. The results of this work are presented.