Revisiting the continuum reverberation lags in the AGN PKS 0558−504

Funding: DHGB acknowledges CONACYT support #319800 and of the researchers programme for Mexico. Research by AJB is supported by National Science Foundation grant AST-1907290. JVHS acknowledges funds from a Science and Technology Facilities Council grant ST/R000824/1 research fellowship. YRL acknowledges the financial support from the National Natural Science Foundation of China through grant no. 11922304 and from the Youth Innovation Promotion Association CAS.We present a revised analysis of the photometric reverberation mapping campaign of the narrow-line Seyfert 1 galaxy PKS 0558 − 504 carried out with the Swift Observatory during 2008–2010. Previously, Gliozzi et al. (2013) found using the Discrete Correlation Function (DCF) method that the short-wavelength continuum variations lagged behind variations at longer wavelengths, the opposite of the trend expected for thermal reprocessing of X-rays by the accretion disc, and they interpreted their results as evidence against the reprocessing model. We carried out new DCF measurements that demonstrate that the inverted lag-wavelength relationship found by Gliozzi et al. resulted from their having interchanged the order of the driving and responding light curves when measuring the lags. To determine the inter-band lags and uncertainties more accurately, we carried out new measurements with four independent methods. These give consistent results showing time delays increasing as a function of wavelength, as expected for the disc reprocessing scenario. The slope of the re-analysed delay spectrum appears to be roughly compatible with the predicted τ ∝ λ4/3 relationship for reprocessing by an optically thick and geometrically thin accretion disc, although the data points exhibit a large scatter about the fitted power-law trend.Publisher PDFPeer reviewe