Swift monitoring of NGC 5548: X-ray reprocessing and short-term UV/optical variability

McHardy, I. M.; Cameron D.T.; Dwelly T.; Connolly, S; Lira, P; Emmanoulopoulos D.; Gelbord J.; Breedt, E; Arevalo, P; Uttley, P.

Abstract

Lags measured from correlated X-ray/UV/optical monitoring of AGN allow us to determine whether UV/optical variability is driven by reprocessing of X-rays or X-ray variability is driven by UV/optical seed photon variations. We present the results of the largest study to date of the relationship between the X-ray, UV and optical variability in an AGN with 554 observations, over a 750 d period, of the Seyfert 1 galaxy NGC 5548 with Swift. There is a good overall correlation between the X-ray and UV/optical bands, particularly on short time-scales (tens of days). The UV/optical bands lag the X-ray band with lags which are proportional to wavelength raised to the power 1.23 +/- 0.31. This power is very close to the power (4/3) expected if short time-scale UV/optical variability is driven by reprocessing of X-rays by a surrounding accretion disc. The observed lags, however, are longer than expected from a standard Shakura-Sunyaev accretion disc with X-ray heating, given the currently accepted black hole mass and accretion rate values, but can be explained with a slightly larger mass and accretion rate, and a generally hotter disc. Some long-term UV/optical variations are not paralleled exactly in the X-rays, suggesting an additional component to the UV/optical variability arising perhaps from accretion rate perturbations propagating inwards through the disc.

Más información

Título según WOS: Swift monitoring of NGC 5548: X-ray reprocessing and short-term UV/optical variability
Título según SCOPUS: Swift monitoring of NGC 5548: X-ray reprocessing and short-term UV/optical variability
Título de la Revista: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volumen: 444
Número: 2
Editorial: OXFORD UNIV PRESS
Fecha de publicación: 2014
Página de inicio: 1469
Página final: 1474
Idioma: English
URL: http://mnras.oxfordjournals.org/cgi/doi/10.1093/mnras/stu1636
DOI:

10.1093/mnras/stu1636

Notas: ISI, SCOPUS