The Intrinsically X‐Ray Weak Quasar PHL 1811. I. X‐Ray Observations and Spectral Energy Distribution

2006/11/10 by Karen M. Leighly, Jules P. Halpern, J. P. Halpern +6
Physics and Astronomy · #Accretion (finance) #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Emission spectrum #Galaxies: Formation, Evolution, Phenomena #Galaxy #Gamma-ray bursts and supernovae #Light curve #Luminosity #Optics #Photometry (optics) #Physics #Power law #Quasar #Spectral energy distribution #Spectral line #Spectral slope #Stars #X-ray #astro-ph

paper · pdf · doi:10.1086/518017

published as Astrophys.J.663:103-117,2007 · 45 pages, 9 figures, accepted for publication in ApJ

arxiv created 2006/11/10 · openalex publication_date 2007/06/22 · arxiv updated 2011/02/11 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06

Abstract

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 suggests that the X-rays are not scattered emission. The XMM-Newton spectra are modeled in the 0.3-5 keV band by a steep power law with Γ = 2.3 ± 0.1, and the upper limit on intrinsic absorption is 8.7 × 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 α ox inferred from the Chandra and contemporaneous HST spectrum is -2.3 ± 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.

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