HD3191, the high-mass X-ray binary that wasn't there

Abstract

The rapidly rotating B1 IV:nn star HD3191 lies within the error box of a flaring Fermi gamma-ray source. Although the counterpart of the Fermi source is likely an active galaxy, HD3191 has nevertheless been suggested to be a high-mass X-ray binary, possibly hosting a black hole companion. The star displays roughly sinusoidal photometric variations with amplitudes of about 12 mmag for two frequencies $\nu_1$ = 0.1235 d$^{-1}$ and $\nu_2$ = 1.6038 d$^{-1}$. Half of the former frequency ($\nu_1$/2) had previously been interpreted as the orbital frequency of a high-mass X-ray binary in which the B1 IV:nn primary undergoes ellipsoidal variations. We show that this scenario fails to account for the lack of significant radial velocity variations and for the overall properties of the star. Our spectroscopic observations instead unveil line profile variations, suggesting that the photometric and line profile variations arise from multi-mode pulsations rather than from orbital effects. Whilst we cannot rule out the possibility that HD3191 could be a nascent binary, consisting of a B1 IV:nn with a low-mass pre-main sequence companion, the most likely scenario is a single star displaying non-radial pulsations. The fast stellar rotation would thus be intrinsic to the B1 IV:nn star rather than being the result of a spin-up during a past mass-transfer episode.