Multi-Wavelength Monitoring and Reverberation Mapping of a Changing Look
  Event in the Seyfert Galaxy NGC 3516
  We present the results of photometric and spectroscopic monitoring campaigns
of the changing look AGN NGC 3516 carried out in 2018 to 2020 covering the
wavelength range from the X-ray to the optical. The facilities included the
telescopes of the CMO SAI MSU, the 2.3-m WIRO telescope, and the XRT and UVOT
of Swift. We found that NGC 3516 brightened to a high state and could be
classified as Sy1.5 during the late spring of 2020. We have measured time
delays in the responses of the Balmer and He II 4686 lines to continuum
variations. In the case of the best-characterized broad H-beta line, the delay
to continuum variability is about 17 days in the blue wing and is clearly
shorter, 9 days, in the red, which is suggestive of inflow. As the broad lines
strengthened, the blue side came to dominate the Balmer lines, resulting in
very asymmetric profiles with blueshifted peaks during this high state. During
the outburst the X-ray flux reached its maximum on 1 April 2020 and it was the
highest value ever observed for NGC 3516 by the Swift observatory. The X-ray
hard photon index became softer, about 1.8 in the maximum on 21 Apr 2020
compared to the mean about 0.7 during earlier epochs before 2020. We have found
that the UV and optical variations correlated well (with a small time delay of
1-2 days) with the X-ray until the beginning of April 2020, but later, until
the end of Jun. 2020, these variations were not correlated. We suggest that
this fact may be a consequence of partial obscuration by Compton-thick clouds
crossing the line of sight.
