Lens or Binary? Chandra Observations of the Wide Separation Broad Absorption Line Quasar Pair UM425

Details Lens or Binary? Chandra Observations of the Wide Separation Broad Absorption Line Quasar Pair UM425 Lens or Binary? Chandra Observations of the Wide Separation Broad Absorption Line Quasar Pair UM425

2003-04-22

arxiv.org/abs/astro-ph/0304401v1

Astrophys.J. 592 (2003) 710-720

Astronomy and Astrophysics

Astrophysics

13 pages, 9 figures, Accepted for publication in the Astrophysical Journal

Scientific paper

10.1086/375863

We have obtained a 110 ksec Chandra ACIS-S exposure of UM425, a pair of QSOs at z=1.47 separated by 6.5 arcsec, which show remarkably similar emission and broad absorption line (BAL) profiles in the optical/UV. Our 5000 count X-ray spectrum of UM425A (the brighter component) is well-fit with a power law (photon spectral index Gamma=2.0) partially covered by a hydrogen column of 3.8x10^22 cm^-2. The underlying power-law slope for this object and for other recent samples of BALQSOs is typical of radio-quiet quasars, lending credence to the hypothesis that BALs exist in every quasar. Assuming the same Gamma for the much fainter image of UM425B, we detect an obscuring column 5 times larger. We search for evidence of an appropriately large lensing mass in our Chandra image and find weak diffuse emission near the quasar pair, with an X-ray flux typical of a group of galaxies at redshift z ~ 0.6. From our analysis of archival HST WFPC2 and NICMOS images, we find no evidence for a luminous lensing galaxy, but note a 3-sigma excess of galaxies in the UM425 field with plausible magnitudes for a z=0.6 galaxy group. However, the associated X-ray emission does not imply sufficient mass to produce the observed image splitting. The lens scenario thus requires a dark (high M/L ratio) lens, or a fortuitous configuration of masses along the line of sight. UM425 may instead be a close binary pair of BALQSOs, which would boost arguments that interactions and mergers increase nuclear activity and outflows.

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