P. Barr
An investigation of compact extragalactic X-ray sources
1980 (supervisor: P. W. Sanford)

The MSSL proportional counter spectrometer on the Ariel V satellite has been used to observe several active galaxies, including NGC 4151, 3C 390.3 and Markarian 501.

NGC 4151 is found to exhibit a strongly cut off spectrum, indicative of photoelectric absorption. The equivalent hydrogen column density is found to vary in a little over one year, indicating that the absorption is local to the source rather than in the intervening medium, and the material responsible is not uniformly distributed around the central source.

A feature in the spectrum at 7 keV is identified with absorption at the K-edge of iron, the abundance being approximately solar. No evidence is found for variability in the underlying continuum.

The spectrum of Markarian 501, on the other hand, is not strongly cut-off. Comparison with observations from IUE can support a synchrotron-self-Compton model with the X-rays produced in inverse Compton radiation from the UV component: self-Compton radiation from the low-frequency radio component is unlikely to be the dominant mechanism for X-ray production. In this scenario, the soft X-ray excess found by the HEAO-1 LED could be the high-energy tail of the UV synchrotron component.

3C 390.3 is observed to vary violently on a timescale of a few years; comparison of the Ariel V observations with previous Copernicus, OSO-7 and Uhuru data suggests that the source underwent an outburst in early 1973 and has subsequently decayed to a lower, quiescent state. The spectral index has apparently remained constant; this is hard to account for if the decay is caused by radiative losses of the emitting particles. Diffusion of the particles out of the emitting region, or (adiabatic) expansion would seem more likely.