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CONTEXT: The masses previously obtained for the X-ray binary 2S0921-630inferred a compact object that was either a high-mass neutron star or low-massblack-hole, but used a previously published value for the rotational broadening(vsini) with large uncertainties. AIMS: We aim to determine an accurate massfor the compact object through an improved measurement of the secondary star'sprojected equatorial rotational velocity. METHODS: We have used UVES echellespectroscopy to determine the vsini of the secondary star (V395 Car) in thelow-mass X-ray binary 2S0921-630 by comparison to an artificially broadenedspectral-type template star. In addition, we have also measured vsini from asingle high signal-to-noise ratio absorption line profile calculated using themethod of Least-Squares Deconvolution (LSD). RESULTS: We determine vsini to liebetween 31.3+/-0.5km/s to 34.7+/-0.5km/s (assuming zero and continuum limbdarkening, respectively) in disagreement with revious results based onintermediate resolution spectroscopy obtained with the 3.6m NTT. Using ourrevised vsini value in combination with the secondary star's radial velocitygives a binary mass ratio of 0.281+/-0.034. Furthermore, assuming a binaryinclination angle of 75 degrees gives a compact object mass of 1.37+/-0.13Mo.CONCLUSIONS: We find that using relatively low-resolution spectroscopy canresult in systemic uncertainties in the measured vsini values obtained usingstandard methods. We suggest the use of LSD as a secondary, reliable check ofthe results as LSD allows one to directly discern the shape of the absorptionline profile. In the light of the new vsini measurement, we have revised downthe compact object's mass, such that it is now compatible with a canonicalneutron star mass.

The rotational broadening of V395 Car - implications on compact object's mass