X-Ray Spectral Shape Variation in Changing-look Seyfert Galaxy SDSS J155258+273728

We analyze the X-ray, optical, and mid-infrared data of a “changing-look” Seyfert galaxy SDSS J155258+273728 at z  ≃ 0.086. Over a period of one decade (2009–2018), its broad H α line intensity increased by a factor of ∼4. Meanwhile, the X-ray emission in 2014 as observed by Chandra was about five times brighter than that in 2010 by Suzaku , and the corresponding emissions in the V -band, mid-infrared W 1 band brighten by ∼0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e., N H ∼ 10 21 cm − 2 , but the X-ray spectrum becomes harder in the 2014 Chandra bright state (i.e., photon index Γ = 1.52 − 0.06 + 0.06 ) than that of the 2010 Suzaku low state ( Γ = 2.03 − 0.21 + 0.22 ). With an Eddington ratio being lower than a few percent, the inner region of the accretion disk in SDSS J155258+273728 is likely a hot accretion flow. We then compile from literature the X-ray data of “changing-look” active galactic nuclei (AGNs), and find that they generally follow the well-established “ V ”-shaped correlation in AGNs, that is, above a critical turnover luminosity the X-ray spectra soften with the increasing luminosity, and below that luminosity the trend is reversed in the way of “harder when brighter.” This presents direct evidence that CL-AGNs have distinctive changes in not only the optical spectral type, but also the X-ray spectral shape. The similarity in the X-ray spectral evolution between CL-AGNs and black hole X-ray binaries indicates that the observed CL-AGNs phenomena may relate to the state transition in accretion physics.