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Register a new userThe Intrinsically X-ray Weak Quasar PHL 1811. II. Optical and UV Spectra and Analysis
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This is the second of two papers reporting observations and analysis of the unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line quasar PHL 1811. The first paper reported that PHL 1811 is intrinsically X-ray weak, and presented a spectral energy distribution (SED). Here we present HST STIS optical and UV spectra, and ground-based optical spectra. The optical and UV line emission is very unusual. There is no evidence for forbidden or semiforbidden lines. The near-UV spectrum is dominated by very strong FeII and FeIII, and unusual low-ionization lines such as NaID and CaII H&K are observed. High-ionization lines are very weak; CIV has an equivalent width of 6.6A, a factor of ~5 smaller than measured from quasar composite spectra. An unusual feature near 1200A can be deblended in terms of Lyalpha, NV, SiII, and CIII* using the blueshifted CIV profile as a template. Photoionization modeling shows that the unusual line emission can be explained qualitatively by the unusually soft SED. Principally, a low gas temperature results in inefficient emission of collisionally excited lines, including the semiforbidden lines generally used as density diagnostics. The emission resembles that of high-density gas; in both cases this is a consequence of inefficient cooling. PHL 1811 is very unusual, but we note that quasar surveys are generally biased against finding similar objects.
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We report on a population of X-ray weak quasars with similar UV emission-line properties to those of the remarkable quasar PHL 1811. All radio-quiet PHL 1811 analogs are notably X-ray weak by a mean factor of ~13, with hints of heavy X-ray absorption. Correlations between the X-ray weakness and UV emission-line properties suggest that PHL 1811 analogs may have extreme wind-dominated broad emission-line regions (BELRs). We propose an AGN geometry that can potentially unify the PHL 1811 analogs and the general population of weak-line quasars.
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.
This paper was withdrawn due to a misidentification of the source.
We have compiled a catalog of optically-selected quasars with simultaneous observations in UV/optical and X-ray bands by the Swift Gamma Ray Burst Explorer. Objects in this catalog are identified by matching the Swift pointings with the Sloan Digital Sky Survey Data Release 5 quasar catalog. The final catalog contains 843 objects, among which 637 have both UVOT and XRT observations and 354 of which are detected by both instruments. The overall X-ray detection rate is ~60% which rises to ~85% among sources with at least 10 ks of XRT exposure time. We construct the time-averaged spectral energy distribution for each of the 354 quasars using UVOT photometric measurements and XRT spectra. From model fits to these SEDs, we find that the big blue bump contributes about 0.3 dex to the quasar luminosity. We re-visit the alpha_ox-L_uv relation by selecting a clean sample with only type 1 radio-quiet quasars; the dispersion of this relation is reduced by at least 15% compared to studies that use non-simultaneous UV/optical and X-ray data. We only found a weak correlation between L/L_Edd and alpha_uv. We do not find significant correlations between alpha_x and alpha_ox, alpha_ox and alpha_uv, and alpha_x and Log L(0.3-10 keV). The correlations between alpha_uv and alpha_x, alpha_ox and alpha_x, alpha_ox and alpha_uv, L/L_Edd and alpha_x, and L/L_Edd and alpha_ox are stronger amongst low-redshift quasars, indicating that these correlations are likely driven by the changes of SED shape with accretion state.
We present simple XSPEC models for fitting excess variance spectra of AGN. Using a simple Monte-Carlo approach, we simulate a range of spectra corresponding to physical parameters varying, then calculate the resulting variance spectra. Starting from a variable power-law, we build up a set of models corresponding to the different physical processes that can affect the final excess variance spectrum. We show that the complex excess variance spectrum of IRAS 13224-3809 can be well described by such an intrinsic variability model, where the power-law variability is damped by relativistic reflection and enhanced by an ultra fast outflow. The reflection flux is correlated with that of the power-law, but not perfectly. We argue that this correlation is stronger at high frequencies, where reverberation lags are detected, while excess variance spectra are typically dominated by low frequency variability.
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