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In Zdziarski et al. (2007a), we have performed a comprehensive analysis of the superorbital modulation in the ultracompact X‐ray source 4U 1820–303, consisting of a white dwarf accreting onto a neutron star. Based on RXTE data, we measured the fractional amplitude of the source superorbital variability (with a ∼170‐d quasi‐period) in the folded and averaged light curves, and found it to be by a factor of ∼2. As proposed before, that variability can be explained by oscillations of the binary eccentricity. In that work, we presented detailed calculations of the eccentricity‐dependent flow through the inner Lagrangian point, and found a maximum of the eccentricity of ≃0.004 is sufficient to explain the observed fractional amplitude. We then studied hierarchical triple models yielding the required quasi‐periodic eccentricity oscillations through the Kozai process. We found the resulting theoretical light curves to match well the observed ones. We constrained the ratio of the semimajor axes of the outer and in...

Observations with Compton Gamma Ray Observatory have provided key insights to the high energy emission mechanisms in Active Galactic Nuclei (AGN) and clearly separate them into a class of Blazars, with apparent non-thermal jet emissions often peaking in the EGRET energy band, and into the Seyfert class with emissions more closely related to the accretion disk presumably energizing the nuclear black hole. OSSE measurements indicate that the high energy emission from Seyfert AGN has a high energy cutoff below ∼100–200 keV and no Seyferts have been detected by COMPTEL or EGRET. Here, we review the X-ray and γ-ray observations of Seyferts and Radio Galaxies and the broad band emission models that describe the observed spectral characteristics. A comparison of Seyfert 1, Seyfert 2, and radio galaxies can be made in the context of the AGN unification models.

We present X-ray/gamma-ray observations of Cyg X-1 in the soft state during 1996 May-June. We analyze ASCA, RXTE and OSSE data. The spectrum consists of soft X-ray blackbody emission of an optically thick accretion disk in the vicinity of a black hole and a power law with an energy index alpha around 1.2-1.5 extending to at least several hundred keV. In the spectra, we find the presence of strong Compton reflection, which probably comes from the disk., 5 pages, 3 figures, to appear in the Proceedings of 4th Compton Symposium, April 27-30, 1997, Williamsburg, Virginia, USA

Theoretical implications of the recent OSSE observations of two AGNs with spectral breaks in soft γ‐rays, NGC 4151 and IC 4329a, are discussed. Two major classes of models, with either thermal or nonthermal electrons, are discussed. The purely nonthermal e± pair model is ruled out for NGC 4151. On the other hand, a nonthermal e± pair model with inefficient acceleration fits the NGC 4151 spectrum very well. Still, the simplest interpretation of the data for both objects is in terms of a plasma cloud with thermal electrons that Compton upscatter soft photons from the UV bump. The thermal plasma can form an active corona above the surface of an accretion disk and/or be pair dominated in IC 4329a, but neither is possible in the case of NGC 4151. Superposition of the contributions from AGNs similar to these two objects can fit the spectrum of the cosmic X‐ray background.

The nonthermal e(+/-) pair model of the central engine of active galactic nuclei (AGNs) is discussed. The model assumes that nonthermal e(+/-) pairs are accelerated to highly relativistic energies in a compact region close to the central black hole and in the vicinity of some cold matter. The model has a small number of free parameters and explains a large body of AGN observations from EUV to soft gamma-rays. In particular, the model explains the existence of the UV bump, the soft X-rays excess, the canonical hard X-ray power law, the spectral hardening above about 10 keV, and some of the variability patterns in the soft and hard X-rays. In addition, the model explains the spectral steepening above about 50 keV seen in NGC 4151.