Confirmation of dust scattering echo around MAXI J1421–613 by Swift observation

Abstract

MAXIJ1421–613 is an X-ray burster discovered by Monitor of All-sky X-ray Image (MAXI) on 9 January 2014 and is considered to be a low-mass X-ray binary. A previous study analyzing follow-up observation data obtained by Suzaku on 31 January to 3 February 2014 reported that an annular emission of ∼3′–9′ radius was found around the transient source. The most plausible origin of the annular emission is a dust scattering echo by the outburst of MAXIJ1421–613. In this paper, we confirm the annular emission by analyzing the data of the Swift follow-up observation which was conducted by the photon counting mode on 18 January 2014. In a radial profile, we found an annular emission at ∼2′.5–4′.5. Its spectrum was well explained by an absorbed power law, and the photon index was higher than that of MAXIJ1421–613 itself by ΔΓ∼2. The flux and radius of the annular emission observed by Swift are explained by dust scattering of the same outburst as is responsible for the annular emission observed by Suzaku. Assuming that the dust layer causing the annular emission found by Swift is located at the same position as the CO cloud in front of MAXIJ1421–613, the distance to the transient source was estimated to be ∼3 kpc, which is consistent with the value estimated by the previous study of Suzaku.