X-ray variability of SDSS J000532.84+200717.4: from a normal state to an X-weak state

Abstract

We present a multi-epoch study of the extreme X-ray variability of the type~1 quasar SDSS~J000532.84+200717.4 using archival observations from \textit{XMM-Newton}, \textit{Swift}/XRT, \textit{EP-FXT}, and \textit{ROSAT}, together with new optical spectroscopy and multi-wavelength 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 ($Δα_{\rm ox} \lesssim -0.3$). 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 UV continuum varies only mildly and broadly tracks the X-ray evolution. Multi-epoch optical spectroscopy shows no significant long-term changes in either the continuum shape or the broad emission-line profiles. The \ion{Mg}{2} 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.

Figures (7)

• Figure 1: Representative XMM-Newton EPIC images of SDSS J0005+2007 at different flux states. The source is clearly detected in the high state (H5), while it becomes weak or undetectable in several low states (L2, L3, L5), demonstrating extreme long-term X-ray variability. The green circle in each image is the circular region of 54$\arcsec$ with a radius centered on the SDSS position of the source.
• Figure 2: The XMM-Newton EPIC-MOS light curve of SDSS J0005+2007 in the 0.2--10 keV band. The source exhibits extreme long-term variability over nearly two decades, with the count rate decreasing by more than an order of magnitude. The inset shows closely spaced observations in 2021, revealing significant short-term variability with flux changes of a factor of a few on day timescales. Downward arrows indicate $3\sigma$ upper limits.
• Figure 3: Unfolded XMM-Newton EPIC X-ray spectra of SDSS J0005+2007 in different flux states, shown together with the best-fit spectral models consisting of a Galactic-absorbed power law plus a blackbody component. For clarity, only the EPIC-MOS2 data are displayed. The lower panel shows the residuals.
• Figure 4: Multi-wavelength light curves between 1991--2025 of SDSS J0005+2007. (a) X-ray light curve in the 0.2--2 keV band compiled from XMM-Newton, ROSAT, Swift/XRT, and EP-FXT observations. For epochs with non-detections, 3$\sigma$ upper limits are shown. The vertical dot-dashed lines mark the epochs of the SDSS and P200 spectroscopic observations. (b) Ultraviolet light curve from Swift/UVOT in the UVW2 band. (c) Optical light curves from CRTS, ZTF, Pan-STARRS, and Gaia. (d) Mid-infrared light curves from the WISE/NEOWISE W1 and W2 bands, which were binned into 7-day intervals with the mean magnitude shown.
• Figure 5: Rest-frame optical spectra of SDSS J0005+2007 obtained at three epochs: 2014 October 17 from SDSS (orange line), 2024 September 09 from P200/DBSP (blue line), and 2025 December 14 from P200/NGPS (cyan line). All spectra have been corrected for Galactic extinction and shifted to the rest frame with $z=0.3814$. The fluxes of the P200 and NGPS spectra were recalibrated by scaling their narrow [O3] $\lambda$5007 emission line flux to match that of the SDSS spectrum.