Prolonged X-ray activity of IGR J17091-3624

ATel #17221; Sian A. Woahene-Demehin (University of Southampton), Federico M. Vincentelli (INAF-IAPS), Diego Altamirano (University of Southampton), Douglas Buisson (Independent), A. Sanna (University of Cagliari), James F. Steiner (SAO), K. C. Gendreau, Zaven Arzoumanian (NASA GSFC), Gaurava K. Jaisawal (DTU Space), Ole Koenig (CfA). G. Mastroserio (IUSS Pavia)
on 5 Jun 2025; 09:49 UT
Credential Certification: Diego Altamirano (d.altamirano@soton.ac.uk)

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IGR J17091-3624 (IGR J17091) is a transient stellar-mass black hole X-ray binary discovered in 2003. It is well known for its recurrent outbursts and sporadic highly structured X-ray variability similar to GRS 1915+105 (Altamirano et al. 2011, ApJL, 742, L17; Court et al. 2017, MNRAS, 468, 4748; Wang et al., ApJ 963, 14, 2024). The most recent outburst began in early February 2025 (ATel #17034, #17038).

Recent optical observations reported a decrease in the optical flux from a magnitude of r'=19.48 +/- 0.04 to a magnitude of r'=20.69 +/- 0.12 between 2025-04-1 and 2025-04-28, indicating that the source had entered into a quiescent phase (ATel #17166).

NICER has been monitoring IGR J17091 since the beginning of the outburst. Triggered by the optical results reported in Atel #17166, we analyzed the NICER observations of IGR J17091 taken between 2025-04-18 19:16:20 TT and 2025-05-17 23:37:00 TT with a total exposure of ~25 ks.

IGR J17091 was significantly detected in all our observations, revealing that although the optical flux is consistent with quiescence levels, IGR J17091 is declining but still in outburst in X-rays. The 0.5-10 keV light curve reveals substantial variability, with average count rates changing between 18 cts/sec and 12 cts/sec over a period of ~2 days. The most recent observations conducted between 2025-05-15 20:41:20 TT and 2025-05-17 23:37:00 TT reveal an average count rate between 3 and 7 cts/sec.

NICER will continue to monitor IGR J17091, and we encourage further multiwavelength observations of this source. NICER is an X-ray telescope operating on the International Space Station, devoted to time-resolved spectroscopy in the 0.2-12 keV band. The NICER mission and portions of the NICER science team activities are funded by NASA.