Discovery of a Peculiar Pulsar in the Small Magellanic Cloud

Abstract

Abstract We report on a peculiar X-ray binary pulsar, IKT 1 $=$ RX J0047.3$-$7312, observed with XMM-Newton in 2000 October. The X-ray spectrum is described by a two-component spectrum. The hard component has a broken power-law with respective photon indices of 0.2 and 1.8, below and above the break energy at 5.8 keV. The soft component can be modeled by a blackbody of $kT = 0.6 \,\mathrm{keV}$. The X-ray flux shows a gradual decrease and periodic variations of about 4000 s. The averaged flux in 0.7–10.0 keV is $2.9 \times 10^{-12} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1}$, which is $\sim 10$-times brighter than that in a ROSAT observation in 1999 November. In addition to the 4000-s variation, we found coherent pulsations of $263 \pm 1 \,\mathrm{s}$. These discoveries strengthen the Be/X-ray binary scenario proposed by the ROSAT and ASCA observations on this source, and confirm that most of the hard sources in the Small Magellanic Cloud are X-ray binary pulsars. A peculiar property of this XBP is that the coherent pulsations are found only in the soft component, and the folded light curve shows a flat top shape with a sharp dip. We discuss the nature of this XBP focusing on the peculiar soft component.