Contrasting X-ray/UV time-lags in Seyfert 1 galaxies NGC 4593 and NGC 7469 using AstroSat observations

We study accretion disc-corona connection in Seyfert 1 galaxies using simultaneous UV/X-ray observations of NGC 4593 (2016 July 14-18) and NGC 7469 (2017 October 15-19) performed with AstroSat. We use the X-ray (0.5-7.0 keV) data acquired with the Soft X-ray Telescope (SXT) and the UV (FUV: 130-180 nm, NUV: 200-300 nm) data obtained with the Ultra-Violet Imaging Telescope (UVIT). We also use the contemporaneous Swift observations of NGC 4593 and demonstrate AstroSat's capability for X-ray/UV correlation studies. We performed UV/X-ray cross-correlation analysis using the interpolated and the discrete cross-correlation functions and found similar results. In the case of NGC 4593, we found that the variations in the X-rays lead to those in the FUV and NUV bands by ~38 ks and ~44 ks, respectively. These UV lags favour the disc-reprocessing model; they are consistent with the previous results within uncertainties. In contrast, we found an opposite trend in NGC 7469 where the soft X-ray variations lag those in the FUV and NUV bands by ~41 ks and ~49 ks, respectively. The hard lags in NGC 7469 favour the thermal Comptonization model. Our results may provide direct observational evidence for the variable intrinsic UV emission from the accretion disk, which acts as the seed for thermal Comptonization in a hot corona in a lamp-post-like geometry. The non-detection of disk reverberation photons in NGC 7469, using AstroSat data, is most likely due to a high accretion rate resulting in a hot accretion disc and large intrinsic emission.