Changing-look event in NGC 3516: continuum or obscuration variability?
  The Seyfert-1 galaxy NGC 3516 has undergone major spectral changes in recent
years. In 2017 we obtained Chandra, NuSTAR, and Swift observations during its
new low-flux state. Using these observations we model the spectral energy
distribution (SED) and the intrinsic X-ray absorption, and compare the results
with those from historical observations taken in 2006. We thereby investigate
the effects of the changing-look phenomenon on the accretion-powered radiation
and the ionized outflows. Compared to its normal high-flux state in 2006, the
intrinsic bolometric luminosity of NGC 3516 was lower by a factor of 4 to 8
during 2017. Our SED modeling shows a significant decline in the luminosity of
all the continuum components from the accretion disk and the X-ray source. As a
consequence, the reprocessed X-ray emission lines have also become fainter. The
Swift monitoring of NGC 3516 shows remarkable X-ray spectral variability on
short (weeks) and long (years) timescales. We investigate whether this
variability is driven by obscuration or the intrinsic continuum. We find that
the new low-flux spectrum of NGC 3516, and its variability, do not require any
new or variable obscuration, and instead can be explained by changes in the
ionizing SED that result in lowering of the ionization of the warm-absorber
outflows. This in turn induces enhanced X-ray absorption by the warm-absorber
outflows, mimicking the presence of new obscuring gas. Using the response of
the ionized regions to the SED changes, we place constraints on their density
and location.
