1 Introduction

In recent years, the improved imaging capabilities and increased sensitivity of ROSAT, Chandra and XMM-Newton have allowed us to effectively study discrete sources in nearby galaxies beyond the Local Group. Particularly intriguing is the discovery of off-nuclear X-ray sources with luminosities well above the Eddington limit for a typical neutron star, $\sim$10³⁸ erg s^-1, and up to  $2\times 10^{40}$ erg s^-1 (e.g., Read et al. 1997; Colbert & Mushotzky 1999; Roberts & Warwick 2000; Makishima et al. 2000; Fabbiano et al. 2001). These sources are typically called ultraluminous X-ray sources (ULXs). Despite much effort, little is presently known about these sources. The identification of their optical counterparts is often problematical when the sources are superposed against regions of high surface brightness in the host galaxy. To date, only two ULXs appear to have a clear optical identification (Roberts et al. 2001; Wu et al. 2002), while for others it has been possible to study only the nearby environment (Pakull & Mirioni 2002; Wang 2002).

We have started a search for ULXs in a sample of nearby galaxies (Foschini et al. 2002). This paper concerns follow-up observations of the ULX in NGC 4698. We discuss the radio and optical counterparts and give a redshift estimate. We identify the source with a background source, most likely a BL Lac object at z = 0.43.

Figure 1: R-band image from VLT-U3/FORS1, superimposed in contours the smoothed image from the XMM-Newton EPIC-MOS2 data in the 0.5-10 keV energy band (white over NGC 4698, black in the remaining field). North is up and East to the left. The D25 ellipse $(3\farcm8)$ is shown for comparison.