The 450 Day X-Ray Monitoring of the Changing-look AGN 1ES 1927+654
C. Ricci, M. Loewenstein, E. Kara, R. Remillard, B. Trakhtenbrot, I. Arcavi, K. C. Gendreau, Z. Arzoumanian, A. C. Fabian, R. Li, L. C. Ho, C. L. MacLeod, E. Cackett, D. Altamirano, P. Gandhi, P. Kosec, D. Pasham, J. Steiner, C.-H. Chan
Abstract
Abstract 1ES 1927+654 is a nearby active galactic nucleus (AGN) that underwent a changing-look event in early 2018, developing prominent broad Balmer lines that were absent in previous observations. We have followed up this object in the X-rays with an ongoing campaign that started in 2018 May and that includes 265 NICER (for a total of 678 ks) and 14 Swift/XRT (26 ks) observations, as well as three simultaneous XMM-Newton/NuSTAR (158/169 ks) exposures. In the X-rays, 1ES 1927+654 shows a behavior unlike any previously known AGN. The source is extremely variable both in spectral shape and flux and does not show any correlation between X-ray and UV flux on timescales of hours or weeks/months. After the outburst, the power-law component almost completely disappeared, and the source showed an extremely soft continuum dominated by a blackbody component. The temperature of the blackbody increases with the luminosity, going from kT ∼ 80 eV (for a 0.3–2 keV luminosity of L 0.3−2 ∼ 10 41.5 erg s −1 ) to ∼200 eV (for L 0.3−2 ∼ 10 44 erg s −1 ). The spectra show evidence of ionized outflows and of a prominent feature at ∼1 keV, which can be reproduced by a broad emission line. The unique characteristics of 1ES 1927+654 in the X-ray band suggest that it belongs to a new type of changing-look AGN. Future X-ray surveys might detect several more objects with similar properties.