The Intrinsically X-ray Weak Quasar PHL 1811. I. X-ray Observations and   Spectral Energy Distribution
  This is the first 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, focusing on the X-ray properties and the spectral energy distribution. Two Chandra observations reveal a weak X-ray source with a steep spectrum. Variability by a factor of 4 between the two observations separated by 12 days suggest that the X-rays are not scattered emission. The XMM-Newton spectra are modelled in the 0.3--5 keV band by a steep power law with \Gamma = 2.3\pm 0.1, and the upper limit on intrinsic absorption is 8.7 x 10^{20} cm^{-2}. The spectral slopes are consistent with power law indices commonly observed in NLS1s, and it appears that we observe the central engine X-rays directly. Including two recent Swift ToO snapshots, a factor of ~5 variability was observed among the five X-ray observations reported here. In contrast, the UV photometry obtained by the XMM-Newton OM and Swift UVOT, and the HST spectrum reveal no significant UV variability. The \alpha_{ox} inferred from the Chandra and contemporaneous HST spectrum is -2.3 \pm 0.1, significantly steeper than observed from other quasars of the same optical luminosity. The steep, canonical X-ray spectra, lack of absorption, and significant X-ray variability lead us to conclude that PHL 1811 is intrinsically X-ray weak. We also discuss an accretion disk model, and the host galaxy of PHL 1811. 
Karen M. Leighly (1), Jules P. Halpern (2), Edward B. Jenkins (3),   Dirk Grupe (4), Jiehae Choi (1), Kimberly B. Prescott (...