E-0336-248 - A NEW BL-LAC OBJECT FOUND BY AN OLD EINSTEIN

We obtained new ROSAT HRI and optical observations in the field of the Einstein X-ray source E 0336-248, which we use to identify it as a 19 th magnitude BL Lacertae object at z=0.251 with Lx=1X1045 erg s(-1). I t is also a 14 mJy radio source at 20 cm. An emission-line galaxy at z = 0.043 that was previously considered a Seyfert identification for E 0336-248 is shown instead to be an unrelated, nonactive H II region g alaxy that lies 78 '' from the X-ray source. The resolution of this hi storical case of mistaken identity illustrates that discoveries of non -AGN emission-line galaxies with high X-ray luminosity should be teste d carefully. The properties of E 0336-248 are similar to those of othe r X-ray selected BL Lacs, including its location in an apparent group or cluster of galaxies. Somewhat unusual is the weak contribution of n onstellar optical light relative to the starlight in the spectrum of i ts host galaxy, which raises once again the possibility that even high -luminosity BL Lac objects may be difficult to identify in X-ray selec ted samples. We discuss a possible manifestation of this problem that appeared in the recent literature. (C) 1997 American Astronomical Soci ety.