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T. L. Aldcroft, P. J. Green (Harvard-Smithsonian Center for Astrophysics)
We have obtained a 110 ksec Chandra ACIS-S exposure of UM425, a pair of QSOs at z=1.47 which show remarkably similar emission and broad absorption line profiles in the optical/UV. Our X-ray spectrum of UM425A (the brighter component) has approximately 5000 counts, and is well fit with a power law (photon spectral index of 2.0) partially covered by a hydrogen column of 3.8\times 1022\,cm-2. These parameters and the optical to X-ray slope are consistent with the interpretation that UM425A is a normal QSO hidden behind a shroud of obscuring material. For the much fainter image of UM425B, we assume the same power law slope and find that the B component has a much larger obscuring column of nearly 20\times 1022\,cm-2. If UM425 is truly lensed, this difference has important implications for modeling the BAL outflow.
Our Chandra observation also reveals weak diffuse emission near the two QSO point sources, with an X-ray flux typical of a poor group of galaxies at redshift z ~0.5. This interpretation is supported by our analysis of archival HST WFPC2 and NICMOS data which shows a 3-\sigma excess of galaxies in the UM425 field. However, we find no evidence for a lensing galaxy, nor would the implied galaxy group have sufficient mass to produce the required separation of over 6\,arcsec. UM425 therefore remains an enigma, with conflicting evidence for both a lensed and a binary pair interpretation.
This work was supported by CXO grant GO2-3132X and NASA grant NAS8-39073.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.