Astrophys. J., 592, 710-720 (2003/August-1)
Lens or binary? Chandra observations of the wide-separation broad absorption line quasar pair UM 425.
ALDCROFT T.L. and GREEN P.J.
Abstract (from CDS):
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 3.8x1022 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.
Cosmology: Gravitational Lensing - Galaxies: Quasars: Absorption Lines - quasars: individual (UM Q2345, 425+007 (read: UM 425)) - X-Rays: General
Fig.8: UM NNNA (Nos 425F, 425G) added.
Bright galaxy NW from UM 425 = LEDA 1201057
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