Swift observations identified the X-ray transient MAXI J1909+094 as 4U 1907+09
ATel #14920; H. Negoro (Nihon U.), J. A. Kennea (PSU) and P. A. Evans (U Leicester)
on 15 Sep 2021; 13:26 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)
Following the possible detection of new activity in XTE J1908+094 by MAXI (ATel #14912), NICER made an observation of XTE J1908+094 at 14:39 on DOY 246 (September 13) of 2021. NICER found no X-ray flux of XTE J1908+094. The visibility windows for this part of the sky for NICER at that time were too short for NICER to execute a raster scan, so Swift observations were requested. The 4 tiling observations with 500 sec each to cover the MAXI error region were performed from 20:56 UT on September 13. Only the supergiant HXMB 4U 1907+09 was detected in the FOVs, and XTE J1908+094 was below the detection limit. The XRT observation revealed significant variability in 4U 1907+09 which is typical in this source, and also consistent with that seen in the MAXI data. Thus, we conclude that the MAXI transient is 4U 1907+09.
4U 1907+09 is out of the MAXI error region but inside it if the systematic error is taken into account. In MAXI data analysis, each X-ray photon is assumed to come from the center of the FOV with +/- 1.5 degree in the scan direction, and the source position is determined
assuming that the source flux is constant during scans for multiple scans data. This time, SFXT-like activity and/or pulsations during some scans may cause an artificial large offset from the true source position, and result in the mis-identification.
The MAXI team thanks the NICER team, especially K. Gendreau, for the prompt scheduling and sharing the result.
