Discovery of annular X-ray emission centered on MAXI J1421-613: Dust-scattering X-rays?

We report the discovery of an annular emission of $\sim$3'-9' radius around the center of a transient source, an X-ray burster MAXI J1421$-$613, in the Suzaku follow-up analysis. The spectrum of the annular emission shows no significant emission-line structure, and is well explained by an absorbed power-law model with a photon index of $\sim$4.2. These features exclude the possibility that the annular emission is a shell-like component of a supernova remnant. The spectral shape, the time history, and the X-ray flux of the annular emission agree with the scenario that the emission is due to a dust-scattering echo. The annular emission was made under a rare condition of the dust-scattering echo, where the central X-ray source, MAXI J1421$-$613, exhibited a short time outburst with three X-ray bursts and immediately re-entered a long quiescent period. The distribution of the hydrogen column density along the annular emission follows that of the CO intensity, which means that MAXI J1421$-$613 is located behind the CO cloud. The cloud position is estimated to be $\sim$2.6 kpc or $\sim$9 kpc. The Swift data which were taken 13 days before the Suzaku observation also show a hint of the annular emission. Unifying the above information, we will discuss the distance to MAXI J1421$-$613. Our result may encourage planning follow-up observations to search for structures around a transient object.