Swift observations of the Be/X-ray transient system 1A 1118−615

Abstract

We report results of Swift observations for the high-mass Be/X-ray binary system 1A 1118−615, during an outburst stage in 2009 January and at a flaring stage in 2009 March. Using the epoch-folding method, we successfully detected a pulsed period of 407.69(2) s in the outburst of January and of 407.26(1) s after the flare detection in March. We find that the spectral detection for the source during outburst can be described by a blackbody model with a high temperature (kT ∼ 1–3 keV) and a small radius (R ∼ 1 km), indicating that the emission results from the polar cap of the neutron star. On the other hand, the spectra obtained after the outburst can further be described by adding an additional component with a lower temperature (kT ∼ 0.1–0.2 keV) and a larger emission radius (R ∼ 10–500 km), which indicates the emission from around the inner region of an accretion disc. We find that the thermal emission from the hotspot of the accreting neutron star dominates the radiation in outburst; the existence of both this X-ray contribution and the additional soft component suggests that the polar cap and the accretion disc emission might co-exist after the outburst. Because the two-blackbody signature at the flaring stage is a unique feature of 1A 1118−615, our spectral results may provide a new insight to interpret the X-ray emission for the accreting neutron star. The time separation between the three main outbursts of this system is ∼17 yr and it might be related to the orbital period. We derive and discuss the associated physical properties by assuming the elongated orbit for this specific Be/X-ray transient.