An X-ray-luminous, dwarf Seyfert companion of Markarian 273

We report the discovery of the brightest X-ray source hosted by a faint (M-B = -16) dwarf galaxy in the immediate vicinity of the ultraluminous IRAS merging galaxy Markarian 273. The dwarf galaxy, 1.3 away from Mrk 273, is at the tip of a faint northeast plume of Mrk 273. Its spectrum exhibits strong [O III], H alpha, and [N II] emission lines, which establish the redshift of the dwarf galaxy, z = 0.0376, the same as that of Mrk 273. The emission-line ratios are typical of Seyfert galaxies. The X-ray emission is consistent with a pointlike source coincident with the center of the dwarf galaxy. The intrinsic X-ray luminosity, 6.3 x 10(41) ergs s(-1), in the 0.1-2.4 keV energy range, is about 7 times larger than the B-band luminosity. The X-ray spectrum of the source can be fitted with a power law. All the evidence is consistent with the source being a Seyfert galaxy. Out of similar to 10 faint objects in the same field, only one is detected by ROSAT, and its ratio of soft X-ray to optical luminosity is as high as those for BL Lacertae objects and few active galactic nuclei (AGNs). If there is a population of such dwarf AGNs hidden as companions of major merger galaxies (such as Mrk 273), they may contribute to the luminosity function of AGNs and the cosmic X-ray background at the faint end.