Discovery of nine quasars behind the Large Magellanic Cloud

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: adam.dobrzycki@eso.org
² Observatoire de Genève, 1290 Sauverny, Switzerland e-mail: laurent.eyer@obs.unige.ch
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA e-mail: kstanek@cfa.harvard.edu
⁴ Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA
⁵ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA e-mail: lmacri@noao.edu

Received: 23 May 2005
Accepted: 17 July 2005

Abstract

We present the discovery of nine quasars behind the Large Magellanic Cloud, with emission redshifts ranging from 0.07 to 2.09. Six of them were identified as part of the systematic variability-based search for QSOs in the objects from the OGLE-II database. Combination of variability-based selection of candidates with the candidates' colours appears to be a powerful technique for identifying quasars, potentially reaching ~50% efficiency. We report an apparent correlation between variability magnitude and variability timescale, which – if confirmed – could put even more constraints on QSO candidate selection. The remaining three quasars were identified via followup spectroscopy of optical counterparts to X-ray sources found serendipitously by the Chandra X-ray Observatory satellite. Even though the locations of the candidates were quite uniformly distributed over the LMC bar, the confirmed QSOs all appear near the bar's outskirts.