Swift Observations of the Be/X-ray Transient System 1A 1118-615

TL;DR

This study presents Swift observations of the Be/X-ray binary 1A 1118-615 during outburst and flaring stages, revealing spectral components from the neutron star's polar cap and accretion disk, and suggesting a possible orbital period of about 17 years.

Contribution

The paper provides detailed spectral analysis during different activity stages, identifying co-existing emission components and proposing a long orbital period for the system.

Findings

Detection of a 407.69 sec pulsation during outburst

Spectral modeling shows hot polar cap emission and soft disk emission

Identification of a unique two-blackbody spectral signature at flaring stage

Abstract

We report results of Swift observations for the high mass Be/X-ray binary system 1A 1118-615, during an outburst stage in January, 2009 and at a flaring stage in March, 2009. Using the epoch-folding method, we successfully detected a pulsed period of 407.69(2) sec in the outburst of January and of 407.26(1) sec 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 disk. We find that the thermal emission from the hot spot of the accreting neutron star dominates the radiation in outburst; the existence of both this X-ray contribution and the additional soft component suggest that the polar cap and the accretion disk 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 years 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.