No abstract

We study γ-ray observations of NGC 4151 by GRO/OSSE contemporaneous with X-ray observations by ROSAT and Ginga in 1991 June and with ASCA in 1993 May. The spectra are well modeled by thermal Comptonization and a dual neutral absorber. We also find, for the first time for NGC 4151, a Compton-reflection spectral component in the Ginga/OSSE data. When reflection is taken into account, the intrinsic X-ray energy spectral index is α ∼ 0.8 and the plasma temperature is ∼ 60 keV for both observations, conditions which imply an optical depth of ∼ 1. The X-ray spectral index is within the range, α ≃ 0.95 ± 0.15, observed from other Seyfert 1s. Also, the OSSE spectra of those and other observations of NGC 4151 are statistically undistinguishable from the average OSSE spectrum of radio-quiet Seyfert 1s. Thus, NGC 4151 observed in 1991 and 1993 has the intrinsic X-ray/γ-ray spectrum typical for Seyfert 1s, and the main property distinguishing it from other Seyfert 1s is a large absorbing column of ∼ 10 23 cm −2 . We find no evidence for a strong, broad and redshifted, Fe Kα line component in the ASCA spectrum of 1993 May. Also, the Compton-reflection component in the Ginga/OSSE spectrum is a few times too small to account for the strength of the broad/redshifted line reported elsewhere to be found in this and other ASCA spectra of NGC 4151. On the other hand, we confirm previous studies in that archival X-ray data do imply strong intrinsic X-ray variability and hardness of the intrinsic spectrum in low X-ray states. An observed softening of the intrinsic X-ray spectrum with the increasing flux implies variability in γ-rays weaker than in X-rays, which agrees with the 100 keV flux changing only within a factor of 2 in archival OSSE and GRANAT/SIGMA observations. The relative hardness of the intrinsic X-ray spectrum rules out the homogeneous hot corona/cold disk model for this source. Instead, the hot plasma has to subtend a small solid angle as seen from the source of UV radiation. If the hot plasma is purely thermal, it consists of electrons rather than e ± pairs. On the other hand, the plasma can be pair-dominated if a small fraction of the power is nonthermal.

We present simultaneous observations by EUVE, ASCA, and RXTE of the type 1 Seyfert galaxy NGC 5548. These data indicate that variations in the EUV emission (at 0 2 keV) appear to lead similar modulations in higher energy (1 keV) X-rays by 10-30 ks. This is contrary to popular models which attribute the correlated variability of the EUV, UV and optical emission in type 1 Seyferts to reprocessing of higher energy radiation. This behavior instead suggests that the variability of the optical through EUV emission is an important driver for the variability of the harder X-rays which are likely produced by thermal Comptonization. We also investigate the spectral characteristics of the fluorescent iron K« line and Compton reflection emission. In contrast to prior measurements of these spectral features, we find that the iron K« line has a relatively small equivalent width (W K« 100 eV) and that the reflection component is consistent with a covering factor which is significantly less than unity (ª 2 0 4-0.5). Notably, although the 2-10 keV X-ray flux varies by ¦ 25% and the derived reflection fraction appears to be constant throughout our observations, the flux in the Fe K« line is also constant. This behavior is difficult to reconcile in the context of standard Compton reflection models.

No abstract

drive the ultraviolet/optical variations. However, the medium energy X-ray NVA is 2-4 times that in the ultraviolet, and the single-epoch, absorption-corrected X-ray/γ-ray luminosity is only about 1/3 that of the ultraviolet/optical/infrared, suggesting that at most ∼1/3 of the total low-energy flux could be reprocessed high-energy emission.The strong wavelength dependence of the ultraviolet NVAs is consistent with an origin in an accretion disk, with the variable emission coming from the hotter inner regions and non-variable emission from the cooler outer regions. These data, when combined with the results of disk fits, indicate a boundary between these regions near a radius of order R ≈ 0.07 lt-day. No interband lag would be expected as reprocessing (and thus propagation between regions) need not occur, and the orbital time scale of ∼1 day is consistent with the observed variability time scale. However, such a model does not immediately explain the good correlation between ultraviolet and X-ray variations.

We have obtained the average 1-500 keV spectrum of radio-quiet Seyfert 1s using data from EXOSAT, Ginga, HEAO-1, and GRO OSSE. The spectral fit to the combined average EXOSAT and OSSE data is fully consistent with that for Ginga and OSSE, confirming results from an earlier Ginga/OSSE sample. The average spectrum is wellfitted by a power-law X-ray continuum with an energy spectral index of α ≃ 0.9 moderately absorbed by an ionized medium and with a Compton reflection component. A high-energy cutoff (or a break) in the the power-law component at a few hundred keV or more is required by the data. We also show that the corresponding average spectrum from HEAO-1 A1 and A4 is fully compatible with that obtained from EXOSAT, Ginga and OSSE. These results confirm that the apparent discrepancy between the results of Ginga (with α ≃ 0.9) and the previous results of EXOSAT and HEAO-1 (with α ≃ 0.7) is indeed due to ionized absorption and Compton reflection first taken into account for Ginga but not for the previous missions. Also, our results confirm that the Seyfert-1 spectra are on average cut off in γ-rays at energies of at least a few hundred keV, not at ∼ 40 keV (as suggested earlier by OSSE data alone). The average spectrum is compatible with emission from either an optically-thin relativistic thermal plasma in a disk corona, or with a nonthermal plasma with a power-law injection of relativistic electrons.

We re‐analyse the ASCAGinga X‐ray data from BY Cam, a slightly asynchronous magnetic accreting white dwarf. The spectra are strongly affected by complex absorption, which we model as a continuous (power‐law) distribution of covering fraction and column of neutral material. This absorption causes a smooth hardening of the spectrum below ∼ 3 keV, and is probably produced by material in the pre‐shock column which overlies the X‐ray emission region. The ASCA data show that the intrinsic emission from the shock is not consistent with a single‐temperature plasma. Significant iron L emission coexisting with iron K shell lines from H‐ and He‐like iron clearly shows that there is a wide range of temperatures present, as expected from a cooling shock structure. The Ginga data provide the best constraints on the maximum temperature emission in the shocked plasma, with kTmax = 21+18−4 keV. Cyclotron cooling should also be important; it suppresses the highest temperature bremsstrahlung components, so the X‐ray data provide only a lower limit on the mass of the white dwarf of M ≥ 0.5 M⊙. Reflection of the multitemperature bremsstrahlung emission from the white dwarf surface is also significantly detected.  We stress the importance of modelling all these effects in order to gain a physically self‐consistent picture of the X‐ray spectra from polars in general and BY Cam in particular.

No abstract