Space telescopes track nearby quasar’s dramatic X-ray state transition

By analyzing the data from various space observatories, Chinese astronomers have inspected a nearby quasar designated SDSS J000532.84+200717.4. Results of the new study, published April 1 on the arXiv preprint server, shed more light on the X-ray variability of this object.

Quasars, or quasi-stellar objects (QSOs), are active galactic nuclei (AGN) in the centers of active galaxies, powered by supermassive black holes (SMBHs). They showcase very high bolometric luminosities (over 1 quattuordecillion erg/s), emitting electromagnetic radiation observable in radio, infrared, visible, ultraviolet, and X-ray wavelengths.

Nearby radio-quiet quasar worthy of being explored

SDSS J000532.84+200717.4, or SDSS J0005+2007, is a radio-quiet quasar at a redshift of just 0.38. The quasar has a super-solar metallicity and its supermassive black hole is estimated to have a mass at a level of 30 million solar masses.

Given that still very little is known about the properties and behavior of SDSS J0005+2007, a team of astronomers led by Xiao-Hui Yang of the Shenzhen Technology University in China decided to take a closer look at this quasar. For this purpose, they combed through the data from space telescopes such as XMM-Newton or Swift.

“We compiled multi-epoch X-ray data from several missions. In addition to the XMM-Newton observations, we utilized archival measurements from the Second Swift X-ray Telescope Point Source Catalog (2SXPS) and the ROSAT All-Sky Survey Faint Source Catalog (RASS-FSC). We also include a recent observation obtained with the Einstein Probe Follow-up X-ray Telescope (EPFXT),” the researchers write in the paper.

X-ray state transition

The study detected a dramatic X-ray state transition in SDSS J0005+2007. It was found that the quasar transitions from an X-ray–normal state to an X-ray–weak state. The collected data indicate that over a timescale of about five years, the 0.2–10 keV X-ray flux of SDSS J0005+2007 declines by more than an order of magnitude.

It turned out that during the high states, the X-ray spectrum of SDSS J0005+2007 is persistently soft, while the stacked low-state spectrum showcases substantial flux suppression together with indications of spectral hardening.

However, in contrast to the X-ray band, the ultraviolet continuum was found to vary only mildly, and the optical continuum, broad emission lines, and mid-infrared emission remain stable over decade-long timescales.

What do the new findings mean?

According to the paper, the observed stability of the optical continuum and broad-line region of SDSS J0005+2007 indicates that the accretion disk structure and photoionizing luminosity remain largely intact. When it comes to the dramatic suppression of the X-ray emission, the astronomers assume that it must originate in the compact coronal region or along our line of sight to it.

The researchers add that the spectral hardening observed in the X-ray low state suggests an absorption-driven scenario for SDSS J0005+2007 in which variable, largely dust-free gas located interior to or comparable to the broad-line region modulates the observed X-ray emission.

“Such behavior is similar to that observed in several X-ray–weak quasars and is consistent with clumpy inner disk winds or shielding-gas configurations,” the authors of the study conclude.

Further observations of SDSS J0005+2007 should be conducted in order to shed more light on the geometry and location of the absorbing material, and also to determine the recurrence timescale of the X-ray–weak phase.

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