NICER confirms the new outburst of IGR J17091-3624

ATel #17038; Federico Vincentelli (University of Southampton), Sian Woahene-Demehin (University of Southampton), Gaurava K. Jaisawal (DTU Space), Diego Altamirano (University of Southampton), Douglas Buisson (Independent), Jeroen Homan (Independent), K. C. Gendreau, Z. Arzoumanian (NASA GSFC), E. C. Ferrara (Univ. of Maryland, CRESST II, NASA GSFC), James F. Steiner, Ole Koenig (CfA), A. Sanna (University of Cagliari),
on 13 Feb 2025; 22:53 UT
Credential Certification: Diego Altamirano (d.altamirano@soton.ac.uk)

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IGR J17091-3624 (IGRJ17091) is a black hole candidate that shows outbursts every 2-3 years. IGRJ17091 is well known for its exotic X-ray variability (Altamirano et al. 2011, ApJL, 742, L17; Court et al. 2017, MNRAS, 468, 4748; Wang et al., ApJ 963, 14, +2024). A new outburst has been recently detected by INTEGRAL (Atel #17034). Here we report on follow-up NICER pointed observations.

NICER observed IGRJ17091 between 2025-02-12 16:17:54 TT and 2025-02-13 15:32:03 TT for a total exposure of ~3.8 ks. The source showed strong variability and the average count rate in the 0.5-10 keV band rose slowly from ~72 cts/sec to ~78 cts/sec over the time span of ~1 day.

Power spectral analysis of the lightcurve revealed the presence of broadband noise with a strong QPO at ~0.2 Hz and an harmonic component at ~0.4 Hz. A marginal increase in frequency could be observed between the beginning and end of the observations. We computed the amplitude of these features through a Lorentzian fit, and obtained a fractional rms in the 0.5-10 keV band of 12±3 % and 7±3 % respectively.

The 0.5-10 keV NICER spectrum can be modeled with an absorbed power-law and a disk blackbody component. The best-fitted spectral parameters are: the absorption column density N_H = (1.33 ± 0.04) × 10²² cm^-2, inner disk temperature T_in = 0.42 ± 0.07 keV, diskbb normalization of 93 (-46, +106), and photon index of 1.47 ± 0.04 with a power-law normalization of (7.5 ± 0.5) × 10^-2. The total unabsorbed flux in the 0.5-10 keV range is (6.54 ± 0.04) × 10^-10 erg/s/cm², which is mainly dominated by the contribution from the non-thermal power-law component with a value of (6.1 ± 0.1) × 10^-10 erg/s/cm². Uncertainties on parameters are reported for 90% confidence range.

These results are consistent with typical parameters of black hole candidates in the hard state. Very similar spectral and timing properties were also reported by Wang et al. (2024) during the initial phases of the 2022 outburst. Given the long period of inactivity of the source after its last outburst in 2022, our observations indicate that IGR 1709 is in the first stages of a new outburst. Further multiwavelength observations are encouraged.