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We report on the first observation of an X-ray outburst of a Be/X-ray binarypulsar LS V +44 17/RX J0440.9+4431, and the discovery of an absorption dipstructure in the pulse profile. An outburst of this source was discovered byMAXI GSC in 2010 April. It was the first detection of the transient activity ofLS V +44 17 since the source was identified as a Be/X-ray binary in 1997. Fromthe data of the follow-up RXTE observation near the peak of the outburst, wefound a narrow dip structure in its pulse profile which was clearer in thelower energy bands. The pulse-phase-averaged energy spectra in the 3$-$100 keVband can be fitted with a continuum model containing a power-law function withan exponential cutoff and a blackbody component, which are modified at lowenergy by an absorption component. A weak iron K$\alpha$ emission line is alsodetected in the spectra. From the pulse-phase-resolved spectroscopy we foundthat the absorption column density at the dip phase was much higher than thosein the other phases. The dip was not seen in the subsequent RXTE observationsat lower flux levels. These results suggest that the dip in the pulse profileoriginates from the eclipse of the radiation from the neutron star by theaccretion column.

Outburst of LS V+44 17 Observed by MAXI and RXTE, and Discovery of a Dip Structure in the Pulse Profile