Insights on the X-ray weak quasar phenomenon from XMM-Newton monitoring of PHL 1092

PHL 1092 is a z~0.4 high-luminosity counterpart of the class of Narrow-Line Seyfert 1 galaxies. In 2008, PHL 1092 was found to be in a remarkably low X-ray flux state during an XMM-Newton observation. Its 2 keV flux density had dropped by a factor of ~260 with respect to a previous observation performed 4.5 yr earlier. The UV flux remained almost constant, resulting in a significant steepening of the optical-to-X-ray slope alpha_ox from -1.57 to -2.51, making PHL 1092 one of the most extreme X-ray weak quasars with no observed broad absorption lines (BALs) in the UV. We have monitored the source since 2008 with three further XMM-Newton observations, producing a simultaneous UV and X-ray database spanning almost 10 yr in total in the activity of the source. Our monitoring program demonstrates that the alpha_ox variability in PHL 1092 is entirely driven by long-term X-ray flux changes. We apply a series of physically-motivated models with the goal of explaining the UV-to-X-ray spectral energy distribution (SED) and the extreme X-ray and alpha_ox variability. We consider three possible models: i) A “breathing corona” scenario in which the size of the X-ray emitting corona is correlated with the X-ray flux. In this case, the lowest X-ray flux states of PHL 1092 are associated with an almost complete collapse of the X-ray corona down to the marginal stable orbit; ii) An absorption scenario in which the X-ray flux variability is entirely due to intervening absorption. If so, PHL 1092 is a quasar with standard X-ray output for its optical luminosity, appearing as X-ray weak at times due to absorption; iii) A disc-reflection-dominated scenario in which the X-ray emitting corona is confined within a few gravitational radii from the black hole at all times. In this case, the intrinsic variability of PHL 1092 only needs to be a factor of ~10 rather than the observed factor of ~260.

💡 Research Summary

This paper presents a decade‑long monitoring campaign of the luminous narrow‑line Seyfert 1 galaxy/quasar PHL 1092 (z ≈ 0.4) using XMM‑Newton, with simultaneous UV data from the Optical Monitor and complementary optical spectroscopy. In 2003 the source displayed a typical optical‑to‑X‑ray spectral slope αₒₓ ≈ –1.57, but a 2008 XMM‑Newton observation revealed an extraordinary drop in the 2 keV flux by a factor of ~260 while the UV flux remained essentially unchanged. Consequently αₒₓ steepened to –2.51, placing PHL 1092 among the most extreme X‑ray‑weak quasars known, despite the absence of broad absorption lines (BALs) in its UV spectrum.

The authors obtained three further XMM‑Newton observations in 2009–2010, together with quasi‑simultaneous optical spectra from the Hobby‑Eberly Telescope and the William Herschel Telescope. The X‑ray flux recovered partially, reaching levels comparable to the 2000 observation by 2010, yet the source remained X‑ray weak (αₒₓ ≈ –1.97). By constructing the long‑term light curve (0.5–2 keV) and measuring the monochromatic 2 keV and 2500 Å fluxes, they demonstrate a tight linear correlation between Δαₒₓ and log f₂keV. This shows that the αₒₓ variability is driven entirely by X‑ray flux changes, with the UV continuum varying by less than 10 % and therefore not responsible for the large αₒₓ swings. The factor of X‑ray weakness, f_X‑weak = 10^(–Δαₒₓ/0.384), reaches ≈ 480 in the deepest low‑state.

To interpret these findings, three physically motivated scenarios are explored:

1. Breathing Corona – The size of the X‑ray emitting corona contracts as the source dims, eventually collapsing to the marginally stable orbit (R_ISCO). A reduction of the coronal radius from ~10 R_g to ~1.5 R_g would dramatically lower the Comptonisation efficiency, reproducing the observed 260‑fold flux drop while leaving the UV disc emission essentially unchanged.

2. Absorption – The X‑ray weakness could be due to a highly ionised, high‑column absorber that selectively attenuates the X‑ray band while leaving the UV untouched. However, the X‑ray spectra show no clear absorption edges, and the UV spectrum lacks BALs, implying that any absorber would need extreme ionisation (ξ ≫ 10³) and a special geometry. This makes the absorption explanation less plausible without additional evidence.

3. Disc‑Reflection‑Dominated – If the corona is always confined within a few gravitational radii, relativistic reflection from the inner accretion disc can dominate the observed spectrum. In this picture the intrinsic X‑ray variability need only be a factor of ~10; the remaining apparent factor of 260 arises from changes in the reflected fraction and light‑bending effects that modulate the observed continuum. The model predicts a strong, blurred Fe Kα line and a soft excess, which are hinted at but not definitively detected given the current signal‑to‑noise.

The paper also examines the UV/optical emission‑line properties using archival HST spectra and new ground‑based data. The high‑ionisation C IV line is unusually weak, broad, and strongly blueshifted, whereas low‑ionisation Mg II is narrow and centered at systemic velocity. This pattern mirrors that of other non‑BAL X‑ray‑weak NLS1s such as PHL 1811, 1H 0707‑495, and IRAS 13224‑3809, suggesting a common physical driver—perhaps a disc wind that filters the ionising continuum and a compact, variable corona.

In summary, the authors conclude that the extreme αₒₓ variability of PHL 1092 is a manifestation of dramatic changes in the X‑ray emitting region rather than variations in the accretion rate. Among the three scenarios, the “breathing corona” offers the simplest, self‑consistent explanation, while the reflection‑dominated model remains viable pending higher‑quality spectra. The absorption scenario appears unlikely given the lack of UV BALs and X‑ray absorption signatures. The study underscores the importance of long‑term, simultaneous UV–X‑ray monitoring for disentangling the mechanisms behind X‑ray weakness in quasars that lack obvious BAL features.