With magnitudes of B = 14.4 and R = 14.1, PHL 1811 is the second brightest quasar known with z > 0.1 after 3C 273. Optically it is classified as a Narrow-line Seyfert 1 galaxy (NLS1). Objects of this class are generally strong soft X-ray emitters, but a BeppoSAX observation of PHL 1811 showed that it is anomalously X-ray weak. The inferred α_ox was 1.9–2.1, much steeper than the nominal value of 1.6 for quasars of this optical luminosity, and comparable to the X-ray weakest quasars. Follow-up Chandra observations reveal a variable, unabsorbed X-ray spectrum and confirm that it is intrinsically X-ray weak.

HST STIS spectra of PHL 1811 reveal a very blue continuum with little evidence for absorption or scattering intrinsic to the quasar. High-ionization lines are very weak; C IV has an equivalent width of only ~ 5Å. Neither forbidden nor semiforbidden emission lines are detected. Fe II is the dominant line emission in the UV. High metallicity is implied by the large Fe II to Mg II ratio and relatively strong N V. Low-ionization emission lines of Al III, Na I D, and Ca II H & K are present, implying high optical depth.

We demonstrate that the emission-line properties of PHL 1811 are a direct consequence of the UV-peaked continuum and weak X-ray emission. We propose that these properties are a consequence of high accretion rate, which powers the UV emission from an optically thick accretion disk, while suppressing the formation of a hot corona. This is an extreme case of the same mechanism which is thought to be responsible for luminous NLS1s. Based on the similarity between PHL 1811 and the lineless SDSS quasars, we propose that the lineless quasars discovered in the SDSS are the high-z counterparts of local high-luminosity NLS1s.