Chandra Observation of the Broad Absorption Line QSO Pair UM425

Details Chandra Observation of the Broad Absorption Line QSO Pair UM425 Chandra Observation of the Broad Absorption Line QSO Pair UM425

Mar 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003head....7.2113a&link_type=abstract

American Astronomical Society, HEAD meeting #7, #21.13; Bulletin of the American Astronomical Society, Vol. 35, p.637

Astronomy and Astrophysics

Astronomy

Scientific paper

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 3.8x 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 20x 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-σ 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.

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