HST and Chandra Observations of Quasar PHL 1811

Details HST and Chandra Observations of Quasar PHL 1811 HST and Chandra Observations of Quasar PHL 1811

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aas...20111410l&link_type=abstract

American Astronomical Society, 201st AAS Meeting, #114.10; Bulletin of the American Astronomical Society, Vol. 34, p.1288

Astronomy and Astrophysics

Astronomy

1

Scientific paper

PHL 1811 is a nearby, luminous (z=0.192; MV=-25.9) quasar. With magnitudes of B = 13.9 and R = 13.9, it 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), a class generally known to be bright in soft X-rays. Thus, it was surprising that PHL 1811 was not detected in the ROSAT All Sky Survey. A follow-up BeppoSAX observation detected the quasar, but revealed it to be 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. To investigate the cause of the X-ray deficiency, coordinated HST UV spectra and Chandra observations were obtained in December 2001. Two Chandra pointings, 9.4 and 9.8 ks in length and separated by 12 days, netted 84 and 338 photons respectively. The X-ray spectra, fitted jointly by a power law with Galactic absorption, yield a photon index of 2.09+/- 0.14. The flux varied by a factor of 4 between the two observations. The lack of intrinsic absorption and the strong variability are interpreted as evidence that we observe the central engine directly and unobscured. The HST STIS spectra, taken two days before the first Chandra observation, reveal a very blue continuum with little evidence for absorption or scattering intrinsic to the quasar. The inferred α ox for the two Chandra observations are 2.13 and 2.36, respectively. We conclude from these observations that PHL 1811 is intrinsically X-ray weak. The UV and optical emission-line spectra of PHL 1811 are remarkable. Neither forbidden nor semiforbidden emission lines are detected. Fe 2 is the dominant line emission in the UV. High metallicity is implied by the large Fe 2 to Mg 2 ratio and relatively strong N 5. Low-ionization emission lines of Al 3, Na I D, and Ca II H&K are present, implying high optical depth. High-ionization lines are very weak; C 4 has an equivalent width of only ~ 5 Å/. The spectrum bears marked resemblance to ``line-less'' high-redshift quasars discovered in the SDSS.

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