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 Mrk 273 .
The dwarf galaxy , 1.3 \arcmin away from Mrk 273 , is at the tip of a faint northeast plume of Mrk 273 .
Its spectrum exhibits strong [ OIII ] , H \alpha , [ NII ] 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 point-like source coincident with the center of the dwarf galaxy .
The intrinsic X-ray luminosity , 6.3 \times 10 ^ { 41 } ergs s ^ { -1 } , in the 0.1–2.4 keV energy range , is about seven times larger than the B band luminosity .
The X-ray spectrum of the source can be fit with a power-law .
All the evidence is consistent with the source being a Seyfert galaxy .
It is mysterious why out of \sim 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 Lac 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 .