Lens or Binary? Chandra Observations of the Wide-Separation Broad Absorption Line Quasar Pair UM 425

Abstract

We have obtained a 110 ks Chandra ACIS-S exposure of UM 425, a pair of QSOs at z=1.47 separated by 6.5", which show remarkably similar emission and broad absorption line (BAL) profiles in the optical/UV. Our 5000 count X-ray spectrum of UM 425A (the brighter component) is well fitted with a power law (photon spectral index Γ=2.0) partially covered by a hydrogen column of 3.8×10²² 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 Γ for the much fainter image of UM 425B, 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 σ excess of galaxies in the UM 425 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. UM 425 may instead be a close binary pair of BALQSOs, which would boost arguments that interactions and mergers increase nuclear activity and outflows.