Abstract We present optical identification results for four ROSAT PSPC soft X-ray companions of Mrk 231 based on the deep BATC 6660 Å -band image and the optical spectra obtained by the 60/90-cm Schmidt telescope and the 2.16-m telescope at the Xinglong Station, NAOC. Three optical counterparts are quasars with redshifts z > 1 and the remaining X-ray source is probably a background galaxy cluster. Therefore, none of these soft X-ray companions are physically connected with the central X-ray source Mrk 231. Incorporating the previous results of Arp 220 and Mrk 273 (Xia et al. 1998, 1999), we suggest that the apparent soft X-ray associations with ULIRGs are chance coincidence in most cases.
Keywords galaxies: individual (Mrk 231) --- X-rays: galaxies --- quasars
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