Triggered by the earlier surprising detection of pulsed hard X-ray emission from 1E 1841-045, we investigated the time-averaged high-energy spectral characteristics of the established Anomalous X-ray Pulsars, 1RXS J1708 -4009, 4U 0142 + 61, 1E 2259 + 586 and 1E 1048.1 -5937, all with persistent X-ray emission. We report on the discovery of hard spectral tails for energies above 10 keV in the total and pulsed spectra of AXPs 1RXS J1708 -4009, 4U 0142 + 61 and 1E 2259 + 586 using RXTE PCA (2-60 keV) / HEXTE (15-250 keV) data and INTEGRAL IBIS ISGRI (20-300 keV) data. 1E 1048.1 -5937 appeared to be too weak to be detected with the presently available exposure. Furthermore, improved spectral information for 1E 1841-045 is presented. The pulsed and total spectra measured above 10 keV have power-law shapes and there is so far no significant evidence for spectral breaks or bends up to ∼ 150 keV. The pulsed spectra above 10 keV are exceptionally hard with indices measured for four AXPs approximately in the range -1.0 -1.0. The indices measured for these pulsed spectra below 10 keV were in the range 2.0 -4.3, indicating the very drastic spectral changes in a narrow energy interval around 10 keV, where clear minima are found in luminosity. The best fit power-law models to the total spectra between ∼ 10 and 150 keV are significantly softer, with indices measured for three AXPs, 1E 1841-045, 1RXS J1708 -4009 and 4U 0142 + 61 in the range 1.0 -1.4. For the latter AXPs the pulsed fractions are consistent with 100% around 100 keV, but are different at 10 keV: 4U 0142 + 61 ∼ 10%, 1E 1841-045 ∼25%, and 1RXS J1708 -4009 consistent with 100%. The luminosities of these total and pulsed spectral tails (10-150 keV) largely exceed the total available spin-down powers by factors ranging from ∼ 100 to ∼ 600. This shows that also these new hard-X-ray components cannot be powered by rotational energy loss. We reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data to search for signatures from our set of AXPs. No detections can be claimed, but the obtained upper-limits to soft gamma-ray emission in this MeV range indicates for 1RXS J1708 -4009, 4U 0142 + 61 and 1E 1841-045 that strong breaks or bends must occur somewhere between ∼ 150 keV and 750 keV. We discuss predictions from first attempts to model our hard X-ray / soft gamma-ray spectra in the context of the magnetar model. Our spectral results cannot yet discriminate between the different proposed scenarios. Particularly, more constraining information is required on the detailed shape of the spectra between ∼ 150 keV (our highest data points from INTEGRAL) and 750 keV, from where we report the COMPTEL upper limits.

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We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV -ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder -ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft -ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of $4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

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The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ∼10 in amplitude) flare in γ-rays towards the end of a 3-week pointing by CGRO, in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ∼8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with RXTE, ASCA, ROSAT and IUE and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of ≃3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ∼1 day. The optical-UV light curves, which are not well sampled during the high energy flare, exhibit more modest variations (factor of ∼2) and a lower degree of correlation. The flux at millimetric wavelengths was near an historical maximum during the γ-ray flare peak and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical Stanford, CA 94305