30th Annual Report 2017-18

37 Multi-wavelengthCapability ofAstroSat : UV/X-rayVariability ofActiveGalactic Nuclei Withmultiple UVIT broadband filters and three gratings, and 0.3-100 keVX-ray band covered by the X-ray instruments,AstroSat is the de facto multi-wavelength astronomy satellite available today. However, multi-wavelength science requires careful cross-calibration between different instruments. After the calibration of individual instruments, AstroSat's capability for the study of multi-wavelength cosmic sources such as the active galactic nuclei (AGN) is being realised. AstroSat's UV and X-ray instruments cover a significant fraction of the primary continuum emission from radio-quiet AGN. Compared to Swift/UVOT, AstroSat/UVIT covers shorter UV wavelength down to the far UV (1300 Angstrom), and performs simultaneous UV observations in the near and far UV bands. Unlike Swift,AstroSat performs continuous observations, and therefore, is well suited for short-termvariability on hours time scales. AstroSat has observed a number of nearby bright AGN both to study UV/X-ray variability and spectral energy distributions. Gulab C. Dewangan and collaborators have been working to bring out the multi-wavelength capability of AstroSat for the study of AGN. They have demonstratedAstroSat's capability tomeasure time delay betweenUVandX-ray variations and tomeasure near UV to X-ray spectral energy distributions. As an example, the results from a four day continuous UVIT and SXT observation performed in July 2016 by GulabC. Dewangan, DipankarBhattacharya andA. R. Rao are shown in Figure 8. The UVITFUV image of NGC 4395 shows the bright active nucleus and the spiral arms of the galaxy. The FUV and NUV light curves of NGC4593 extracted from the nucleus, and the X-ray light curve extracted from the SXT observations are also shown in Figure 8. Clearly, the UV and X-ray emission from the AGN are highly variable and appear correlated. The cross-correlation between the SXT and UVIT FUV light curves of NGC 4593, and the distribution of lags measured based on the technique of flux randomization and random sample selection (FR/RSS) are also shown. This analysis has enabled measurement of soft X-ray-to-far UV lag of 0.475±0.086 days, thus demonstrating AstroSat's multi-wavelength capability tomeasure such lags. Figure 8: Results of 4-days longAstroSat observations of the active galaxy NGC 4593. Top left : UVIT/FUV image of NGC 4593 showing the bright active nucleus and the spiral arms. Top right: SXT and UVIT FUV/NUV light curves of the active nucleus in NGC 4593 showing strong variability. Bottom left: Cross-correlation function between the FUV and SXT light curves. Positive lag indicates lag of UV variations relative to that of X-rays. Bottomright: Distribution of UV lag based on FR/RSSmethod.