Origins of the UV/X-ray Relation in Arakelian 120

Ra'ad David Mahmoud, Chris Done, Delphine Porquet, Andrew Lobban

Submitted on 3 July 2022


We explore the accretion geometry in Arakelian 120 using intensive UV and X-ray monitoring from Swift. The hard X-rays (110 keV) show large amplitude, fast (few-day) variability, so we expect reverberation from the disc to produce UV variability from the varying hard X-ray illumination. We model the spectral energy distribution including an outer standard disc (optical), an intermediate warm Comptonisation region (UV and soft X-ray) and a hot corona (hard X-rays). Unlike the lower Eddington fraction AGN (NGC 4151 and NGC 5548 at L/LEdd0.02 and 0.03 respectively), the SED of Akn 120 (L0.05LEdd) is dominated by the UV, restricting the impact of reverberating hard X-rays just from energetics. Illumination from a hard X-ray corona with height 10 Rg produces minimal UV variability. Increasing the coronal scale height to 100 Rg improves the match to the observed amplitude of UV variability as the disc subtends a larger solid angle, but results in too much fast variability to match the UV data. The soft X-rays (connected to the UV in the warm Comptonisation model) are more variable than the hard, but again contain too much fast variability to match the observed smoother variability seen in the UV. Results on lower Eddington fraction AGN have emphasised the contribution from reverberation from larger scales (the broad line region), but reverberation induces lags on similar timescales to the smoothing, producing a larger delay than is compatible with the data. We conclude that the majority of the UV variability is therefore intrinsic, connected to mass accretion rate fluctuations in the warm Comptonisation region.


Comment: Submitted to MNRAS

Subject: Astrophysics - High Energy Astrophysical Phenomena