IGR J17591-2342 second re-brightening observed with NICER

Following the reports (ATel #11957, Sanna et al. 2018, arXiv:1808.10195) that IGR J17591-2342 is an accreting millisecond pulsar in outburst, NICER has continued to monitor this source. After the initial outburst, multiple instruments observed a re-brightening that began on 2018 August 18 (MJD 58348; as reported in ATel #12004).

During that re-brightening, NICER observed the count rate to increase to about 3.8 mcrab (1-10 keV) around MJD 58362, then decline more quickly to about 2.3 mcrab by MJD 58367. Since then, the source has again re-brightened to its highest flux yet, climbing to 5.4 mcrab as of MJD 58376.8 (see Figure at link below for a 60-s binned light curve of the NICER data on this source).

The background contribution in this energy band is about 0.5 c/s. Note that in the 1-10 keV energy band, 1 mcrab corresponds to approximately 8 c/s in NICER. The pulsations remain strongly present after almost 60 days of outburst, with a fractional RMS amplitude of approximately 7% (1-10 keV), in the most recent data, so they have not disappeared as observed in some long outburst AMXP systems, such as SAX J1748.9-2021 (Altamirano et al. 2008, ApJL 674, 45) and HETE J1900.1-2455 (Galloway et al. 2007, ApJL 654, 73).

The 1.2-10 keV energy spectrum can be described by an absorbed power-law model with a soft blackbody component. No evidence of a spectral state change has been seen. We found a photon index varying between 0.9-2.1, positively correlated with flux, across the NICER observations. At the same time, blackbody temperatures ranging between 0.7-1 keV are seen.

The column density is 2-3E22 cm^-2, consistent with the values reported in Sanna et al. (2018, arXiv:1808.10195). Continued multiwavelength observations of this ongoing transient are encouraged. 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.