X-ray spectral shape variations in changing-look Seyfert galaxy SDSSJ155258+273728

Abstract: We analyze the X-ray, optical, and mid-infrared data of a "changing-look" Seyfert galaxy \sdssj15 at $z\simeq0.086$. Over a period of one decade (2009 - 2018), its broad H$\alpha$ line intensity increased by a factor of $\sim$4. Meanwhile, the X-ray emission in 2014 as observed by \chandra\ was about five times brighter than that in 2010 by {\it Suzaku}, and the corresponding emissions in V-band, mid-infrared W1 band brighten by $\sim$ 0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e. $${\rm N_{H}}\sim 10^{21} {\rm cm<sup>{-2}}$$, but the X-ray spectrum becomes harder in the 2014 \chandra\ bright state (i.e. photon index $\Gamma = 1.52<sup>{+0.06}_{-0.06}$) than that of the 2010 \suzaku\ low state ($\Gamma=2.03<sup>{+0.22}_{-0.21}$). With an Eddington ratio being lower than a few percent, the inner region of the accretion disk in \sdssj15\ is likely a hot accretion flow. We then compile from literature the X-ray data of "changing-look" AGNs, and find that they generally follow the well-established "V"-shaped correlation in AGNs, that is, above a critical turn-over luminosity the X-ray spectra soften with the increasing luminosity, and below that luminosity the trend is reversed in a way of "harder when brighter". This presents a 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.