X-Ray Variability of SDSS J000532.84+200717.4: From a Normal State to an X-Ray Weak State

Abstract We present a multiepoch study of the extreme X-ray variability of the type 1 quasar SDSS J000532.84+200717.4 using archival observations from XMM-Newton, Swift/XRT, EP-FXT, and ROSAT, together with new optical spectroscopy and multiwavelength photometry. The 0.2–10 keV X-ray flux exhibits a transition from a high state to a subsequent low state, declining by more than an order of magnitude and placing the source in the X-ray–weak regime (Δ α ox ≲ −0.3). Significant variability on timescales of days to weeks persists within the low state. In contrast, the optical and mid-infrared emission remain stable over decade-long timescales, while the ultraviolet continuum varies only mildly and broadly tracks the X-ray evolution. Multiepoch optical spectroscopy shows no significant long-term changes in either the continuum shape or the broad emission-line profiles. The Mg ii emission is relatively weak compared with typical quasars, suggesting similarities to weak-line quasars. The pronounced wavelength-dependent variability indicates that the accretion disk remains largely intact while the X-ray emission undergoes dramatic changes. The spectral hardening in the low state and the viability of ionized partial-covering models are consistent with variable, largely dust-free absorbing gas, possibly associated with clumpy inner disk winds, although intrinsic coronal variations cannot be excluded. SDSS J0005+200717.4 therefore provides evidence that extreme X-ray weakness can arise as a transient phase in otherwise normal quasars.