NICER observations of the X-ray transient MAXI J1807+132: flaring activity
ATel #13173; Stefano Rapisarda (Shanghai Astronomical Observatory), Peter Bult (CRESST/NASA/GSFC), Renee Ludlam (Cahill Center for Astrophysics, CalTech), Wenda Zhang (Astronomical Institute of the Czech Academy of Sciences), Wenfei Yu (Shanghai Astronomical Observatory)
on 8 Oct 2019; 18:26 UT
Credential Certification: Stefano Rapisarda (s.rapisarda86@gmail.com)
Subjects: X-ray, Transient
MAXI J1807+132 is an X-ray transient detected by MAXI/GSC on 2019 September 10 (ATel #13097). Since September 16, NICER has been observing the transient on a regular basis (ATel #13139). From this date until October 08, NICER collected ~22.6 ks of exposure. At the beginning of the NICER monitoring (September 16-17), the source showed a mean NICER count rate of ~250 c/s in the 0.2-10 keV energy band. After that, NICER lost visibility of the target for 4 days (September 18-22). When NICER started observing again (September 23), the source count rate had dropped to ~60 c/s and continued decreasing for the next three days. On September 26 the source had a count rate of ~15 c/s, possibly indicating a transition towards quiescence. This decreasing trend was interrupted on September 27, when the source count rate showed a peak (~46 c/s) mainly due to increased count rate in the 0.2-2 keV energy band. The following day (September 28), the source count rate returned to values similar to those before the peak (~12 c/s).
NICER observed the source again on October 03 and 06-07. On October 03 the source count rate was ~90 c/s in the 0.2-10 energy band. The rate increased to ~150 c/s on October 06 and slightly decreased the day after (~115 c/s). These last observations are characterized by a softer spectrum compared with those at maximum count rate (September 16-17). In particular, fitting the data with an absorbed power-law and a multi-temperature blackbody, we obtained a spectral index of Γ ~2.4 (Γ ~2 on September 16-17), black body temperature of ~0.14 keV, and neutral gas column density of ~1.7 1021 cm-2. As in our previous report, we note that our fits are part of an exploratory analysis, therefore the reported values of best-fit parameters represent only an indicative description of the spectral properties of the source.
MAXI J1807+132 showed several brightening episodes in the optical and X-ray band during the decay (~40 days) of its 2017 outburst (F. Jiménez-Ibarra et al. 2018, MNRAS, Volume 484, Issue 2, p.2078-2088). The two reported episodes of increased count rate observed by NICER (September 27 and October 03, 06-07) seem to be analogous to such flaring activity. However, due to the sparse sampling of our observations, we cannot exclude the presence of more brightening episodes.
At present, no new NICER observations of the target are scheduled.
NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
