IGR J17591-2342 second re-brightening observed with NICER
ATel #12050; P. S. Ray (U. S. Naval Research Laboratory), K. C. Gendreau, P. M. Bult, Z. Arzoumanian, T. E. Strohmayer (NASA/GSFC), G. K. Jaisawal (DTU Space, Denmark), D. Chakrabarty (MIT), D. Altamirano (University of Southampton, UK), S. Guillot (IRAP-CNRS, France), C. Ferrigno, E. Bozzo (ISDC, University of Geneva, Switzerland), A. Sanna, L. Burderi (University of Cagliari, Italy), A. Papitto (Observatory of Rome, Italy), and T. Di Salvo (University of Palermo, Italy)
on 19 Sep 2018; 20:37 UT
Credential Certification: Paul S. Ray (paul.ray@nrl.navy.mil)
Subjects: X-ray, Request for Observations, Neutron Star, Transient
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.
NICER 60-s binned light curve
