Joint ROSAT-Compton GRO observations of the X-ray-bright Seyfert galaxy IC 4329A

We report a simultaneous ROSAT and GRO observation of the X-ray-bright Seyfert galaxy IC 4329A. For the GRO OSSE detector, we also present the sum of the data for this and earlier observations. The overall spectrum is very well described as a power law with an energy spectral index of ∼1 absorbed at low energies plus a strong Compton reflection component, typical for Seyfert 1 galaxies. The low energy absorption can be well described by a sum of a neutral column density of ∼3 × 10 21 cm -2 , most of which is associated with the edge-on galactic disk of IC 4329A, plus an edgelike feature at ∼700 eV; this feature implies either complex absorption (due to additional ionized material, or due to partial covering), or a soft excess. The data only weakly constrain the presence of a high-energy cutoff in the underlying power law; they are compatible with an exponential cutoff at any energy E c ≳ 100 keV. The relative steepness of the OSSE data, with the power-law energy index of 1.6 ± 0.2, can be accounted for entirely by the contribution of the high-energy tail of the reflection component when E c → ∞. (We find that the definite cutoff at an energy E c ∼ 130 keV suggested in the recently published analysis of the OSSE data for this object is due to a data reduction error.) Including nonsimultaneous Ginga observations with 2 keV fluxes matching well that of ROSAT gives us likely broadband X-ray/γ-ray spectra of the object from ∼0.1 keV up to several hundred keV. Joint spectral analysis of the data sets from the three instruments gives results similar to those from ROSAT/OSSE only, except that the cutoff energy is now constrained to be at 250 keV ≲ E c ≲ 1700 keV. The constraint on E c markedly distinguishes this object from NGC 4151, where E c was lower, ∼50 keV; this has implications for emission models for AGNs as well as for their contribution to the X-ray background. We also report the ROSAT spectrum of the companion object to the Seyfert galaxy, the elliptical galaxy IC 4329 located ∼3′ away. Its spectrum is well described by an optically thin thermal plasma with kT = 0.9 keV, with a 0.1-2 keV flux of 8 × 10 -13 ergs cm -2 s -1 , corresponding to a luminosity of ∼8 × 10 41 ergs s -1 .