PHL 1092 as a transient extreme X-ray weak quasar

We report a dramatic variability event in the X-ray history of the Narrow-Line quasar PHL 1092 (z=0.396). Our latest 2008 XMM-Newton observation reveals a flux drop of ~200 with respect to the previous observation performed about 4.5 years earlier, and a drop of ~135 with respect to its historical flux. Despite the huge X-ray variation, the UV flux remains constant producing a very significant steepening of the optical to X-ray slope alpha_ox from -1.56 to -2.44, making PHL 1092 one of the most extreme X-ray weak quasars. The similarity in the soft X-ray spectral shape between the present and previous observations, together with the persistent UV flux and the lack of any dramatic change in the optical spectrum suggest that an absorption event is not likely to be the origin of the observed variation. If absorption is ruled out, the sudden X-ray weakness of PHL 1092 must be produced by a transient significant weakening or disruption of the X-ray emitting corona.

💡 Research Summary

The paper presents a striking case of extreme X‑ray variability in the narrow‑line quasar PHL 1092 (z = 0.396). Using three XMM‑Newton observations (2000, 2003, and a deep 60 ks pointing in 2008), the authors show that the soft X‑ray flux (0.2–0.9 keV) dropped by a factor of ≈200 between 2003 and 2008, and by ≈135 relative to the long‑term historical average. In contrast, the simultaneous UV flux measured with the Optical Monitor (UVW2 filter, rest‑frame ≈1480 Å) remained essentially unchanged across all epochs (≈3.8–4.4 × 10⁻¹⁵ erg cm⁻² s⁻¹ Å⁻¹). This combination produces a dramatic steepening of the optical‑to‑X‑ray spectral index αₒₓ from –1.56 (2003) to –2.44 (2008), placing PHL 1092 among the most X‑ray‑weak non‑BAL quasars known (Δαₒₓ ≈ –0.96, i.e., an outlier at the ≲1 % level of the αₒₓ–L₂₅₀₀ relation).

Spectral analysis reveals that the soft‑band shape is unchanged: both the 2003 and 2008 spectra are well described by a steep power‑law (Γ≈4.5 in 2008, loosely constrained) and a disc blackbody component with kT≈100 eV when the higher‑flux 2003 data are fitted jointly. The neutral hydrogen column density is consistent with the Galactic value (NH≈3.6 × 10²⁰ cm⁻²) and does not increase in the low‑state spectrum; NH > 6 × 10²⁰ cm⁻² is ruled out. Consequently, intrinsic absorption cannot account for the flux drop. The hard‑band (2–10 keV) is background‑dominated in 2008, yielding only an upper limit on the flux.

UV monitoring with the OM shows no significant variability (χ²ν≈0.8), and quasi‑simultaneous optical spectra obtained with the Hobby‑Eberly Telescope and the William Herschel Telescope display the same strong Fe II emission, weak Hβ and