2003/11/03 by K. Nandra, I. Georgantopoulos, M. Brotherton +1 · 2 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2004.07311.x
published as Mon.Not.Roy.Astron.Soc. 347 (2004) L41 · 5 pages, 3 figures. MNRAS, in press
arxiv created 2003/11/03 · openalex publication_date 2004/01/09 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We present follow-up observations of five hard X-ray sources from the ASCA 5–10 keV Search for the High Energy Extragalactic Population (SHEEP) survey, which has a limiting flux of ∼ 10−13 erg cm−2 s−1. Chandra data have been obtained to improve the X-ray positions from a few arcmin to <1 arcsec, which allows unambiguous optical identification. While the objects almost certainly house active galactic nuclei (AGNs) based on their X-ray luminosity, optical spectroscopy reveals a variety of properties. The identifications indicate that the SHEEP survey samples the same populations as deeper surveys which probe the origin of the X-ray background, but because the SHEEP sources are far brighter, they are more amenable to detailed follow-up work. We find a variety of classifications and properties, including a type II quasi-stellar object (QSO), a galaxy undergoing star formation and a broad-line AGN which has a very hard X-ray spectrum, indicating substantial absorption in the X-ray but none in the optical. Two objects have X-ray/optical flux ratios which, were they at an X-ray flux level typical of objects in Chandra deep surveys, would place them in the ‘optically faint’ category. They are both identified with broad line QSOs at z∼ 1. Clearly, this survey – which is relatively unbiased against obscured objects – is revealing a set of remarkable objects quite different to the familiar classes of AGNs found in previous optical and soft X-ray surveys.