Optical and X-ray discovery of the changing-look AGN IRAS23226-3843
  showing extremely broad and double-peaked Balmer profiles
  We detected a very strong X-ray decline in the galaxy IRAS23226-3843 within
the XMM-Newton slew survey in 2017. Subsequently, we carried out multi-band
follow-up studies to investigate this fading galaxy in more detail. We took
deep follow-up Swift, XMM-Newton, and NuSTAR observations in combination with
optical SALT spectra of IRAS23226-3843 in 2017. In addition, we reinspected
optical, UV, and X-ray data that were taken in the past. IRAS23226-3843
decreased in X-rays by a factor of more than 30 with respect to ROSAT and Swift
data taken 10 to 27 years before. The broadband XMM-Newton/NuSTAR spectrum is
power-law dominated, with a contribution from photoionized emission from cold
gas, likely the outer accretion disk or torus. The optical continuum decreased
by 60 percent and the Balmer line intensities decreased by 50 percent between
1999 and 2017. The optical Seyfert spectral type changed simultaneously with
the X-ray flux from a clear broad-line Seyfert 1 type in 1999 to a Seyfert 1.9
type in 2017. The Balmer line profiles in IRAS23226-3843 are extremely broad.
The profiles during the minimum state indicate that they originate in an
accretion disk. The unusual flat Balmer decrement Ha/Hb with a value of 2
indicates a very high hydrogen density of n_(H) > 10 exp(11) cm^(-3) at the
center of the accretion disk. IRAS23226-3843 shows unusually strong FeII blends
with respect to the broad line widths, in contrast to what is known from
Eigenvector 1 studies.
