Starburst activity in a ROSAT narrow emission-line galaxy

We present multiwaveband photometric and optical spectropolarimetric observ ations of the R = 15.9 narrow emission-line galaxy R117_A which lies on the edge of the error circle of the ROSAT X-ray source R117. The overall spect ral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far-infrared and radio emission is extended along the major axis of the ga laxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identificatio n of R117, and the observed X-ray flux is very close to what would be expec ted from a starburst of the observed far-infrared and radio fluxes. Althoug h an obscured high-redshift QSO cannot be entirely ruled out as contributin g some fraction of the X-ray flux, we find no candidates to K = 20.8 within the X-ray error box, and so conclude that R117_A is responsible for a larg e fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117_A have so far proven neg ative; deep spectropolarimetric observations show no signs of broad lines t o a limit of 1 per cent and, for the observed far-infrared and radio emissi on, we would expect 10 times greater X-ray flux if the overall emission wer e powered by an AGN. We therefore conclude that the X-ray emission from R11 7 is dominated by starburst emission from the galaxy R117_A.