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

TL;DR

This paper reports the first observed X-ray outburst of the Be/X-ray binary LS V+44 17, discovering a dip structure in its pulse profile caused by the eclipse of neutron star radiation by the accretion column.

Contribution

It presents the first detection of an outburst and a dip structure in LS V+44 17, with spectral analysis indicating the dip results from an eclipse by the accretion column.

Findings

First detection of an outburst in LS V+44 17 since 1997.

Discovery of a dip in the pulse profile linked to increased absorption.

Dip disappears at lower flux levels, indicating an eclipse phenomenon.

Abstract

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