W. Neil Johnson

The average X-ray/gamma-ray spectra of Seyfert galaxies from Ginga and OSSE and the origin of the cosmic X-ray background

Abstract : We have obtained the first average 2-500 keV spectra of Seyfert galaxies, using the data from Ginga and GRO OSSE. Our sample contains 3 classes of objects with markedly different spectra: radio-quiet Seyfert 1s and 2s, and radio-loud Seyfert 1s. The average radio-quiet Seyfert 1 spectrum is well- fitted by a power law continuum with the energy spectral index approximately equal 0.9, a Compton reflection component corresponding to a ~ 2x covering solid angle, and ionized absorption. There is a high-energy cutoff in the incident power law continuum: the e-folding energy is E(sub c) approximately equal 0.6 (sup +0.8, sub -0.3)MeV. The simplest model that describes this spectrum is Comptonization in a relativistic optically-thin thermal corona above the surface of an accretion disk. Radio-quiet Seyfert 2s show strong neutral absorption, and there is an indication that their X-ray power laws are intrinsically harder, although the Seyfert 1 spectrum with = 0.9 and strong reflection cannot be ruled out by the data. Finally, the radio-loud Seyfert spectrum has alpha approximately equal 0.7, moderate neutral absorption, E (sub c) = 0.4(sup +0.7, sub -0.2) MeV, and no or little Compton reflection. This is incompatible with the radio-quiet Seyfert 1 spectrum, and probably indicating that the X-rays are beamed away from the accretion disk in these objects. The average spectra of Seyferts integrated over redshift with a power law evolution can explain the hard X-ray spectrum of the cosmic background. The hump at ~ 30 keV in that spectrum is due to the dominant contribution of Seyfert 2s.

Observations of Seyfert Galaxies by OSSE and Parameters of Their X-Ray/Gamma-Ray Sources

We present a summary of spectra of Seyfert galaxies observed by the OSSE detector aboard Compton Gamma Ray Observatory. We obtain average spectra of Seyfert galaxies of types 1 and 2 and find they are well fitted by thermal Comptonization. We present detailed parameter ranges for the plasma temperature and the Compton parameter in the cases of spherical and slab geometries. We find both the average and individual OSSE spectra of Seyfert 2s are significantly harder than those of Seyfert 1s, which difference can be due to anisotropy of Compton reflection and/or Thomson-thick absorption.

X-ray and soft γ-ray spectra of broad-line radio galaxies

ABSTRA C T We study X-ray and soft g-ray spectral properties of nearby broad-line radio galaxies (BLRGs) using data from Ginga, ASCA, OSSE and EXOSAT. The X-ray spectra are well fitted by an intrinsic power-law continuum with an energy index of a , 0:7, moderately absorbed by a cold medium. In addition, the Ginga spectra show fluorescent Fe Ka lines with an average equivalent width of ,100 eV, and, in some cases, Compton reflection humps. However, the latter are significantly weaker than both those seen in radio-quiet Seyfert 1s and those expected if the Fe Ka lines were due to reflection. We find that this weakness of reflection cannot be explained by dilution by another continuum component, e.g. from a jet. Some ASCA and EXOSAT spectra show soft X-ray excesses below ,3 keV. When that component is taken into account, the Fe Ka lines in the ASCA data are found to be unresolved in most cases, and to have equivalent widths &200 eV, consistent with the Ginga data. Multiple observations of 3C 382 and 390.3 show the Fe Ka line approximately constant in flux but accompanied by strong continuum variations. This indicates the bulk of the line is formed by matter at a distance much larger than an accretion-disc scale, consistent with the ASCA line width measurements. The column density of the matter required to account for the observed line fluxes is NH * 10 23 cm π2 . Such a medium is in the line of sight of 3C 445, but it has to be out of it in other objects in which the observed NH are substantially lower. Thus a cold medium with that NH and covering a large solid angle is common in BLRGs, but in most objects it is out of the line of sight, consistent with the unified AGN model. The spectra of BLRGs break and become softer above ,100 keV, as shown by a simultaneous ASCA/OSSE observation of 3C 120 and by the OSSE spectra being on average much softer than the X-ray spectra. Finally, we find the X-ray and g-ray spectral properties of Cen A, a bright narrow-line radio galaxy ‐ a . 0:8, no or weak Compton reflection, NH * 10 23 cm π2 (which is consistent with its Fe Ka line flux), and a high-energy break at ,100 keV ‐ consistent with Cen A being intrinsically very similar to BLRGs studied here, again in agreement with the unified model.

The average X-ray/gamma-ray spectrum of radio-quiet Seyfert 1s

We have obtained the average 1--500 keV spectrum of radio-quiet Seyfert 1s using data from EXOSAT, Ginga, HEAO, 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 well-fitted by a power-law X-ray continuum with an energy spectral index of

X-ray and γ-ray spectra and variability of the black hole candidate GX 339—4

\alpha \simeq 0.9

Mobile imaging and Spectroscopic Threat Identification (MISTI): System overview

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 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

Spatial resolution and imaging of gamma rays with germanium strip detectors

\alpha \simeq 0.9

Seyferts and radio galaxies

) and the previous results of EXOSAT and HEAO (with

Gamma-ray instrument for polarimetry, spectroscopy, and imaging (GIPSI)

\alpha \simeq 0.7

Mobile imaging and spectroscopic threat identification (MISTI)

) 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 gamma-rays at energies of at least a few hundred keV, not at