NICER has started to observe exotic variability pattern of IGR J17091-3624

ATel #15304; Jingyi Wang (MIT), Erin Kara (MIT), Diego Altamirano (Southampton), Jon Miller (Univ. Michigan), Yanan Wang (Southampton), James F. Steiner (SAO, CfA), Paul Draghis (Univ. Michigan), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), for the NICER team
on 30 Mar 2022; 17:26 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jingyi Wang (jingyiw@mit.edu)

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IGR J17091-3624 is a black hole X-ray binary (BHXB) that shows different exotic variability patterns (mostly high amplitude and highly structured), similar to the peculiar BHXB GRS 1915+105 (e.g., Altamirano et al., 2011; Court et al., 2017). After its last outburst in 2016, IGR J17091-3624 entered a new outburst in March 2022. From March 14 to 17, it brightened in the hard state (ATels #15282, 15283, #15284, #15286), and starting March 18, it transitioned into the intermediate state (ATels #15287, 15295, 15297, 15298).

Following NICER monitoring between 2022-03-14 and 03-19 (ATels #15282, #15287) and a Moon-avoidance gap in visibility, NICER observed the source again from 2022-03-22 to 03-28 on a daily cadence with a total exposure of 23.0 ks. During this 6-day period, the flux is variable from 400 to 900 counts/s (see Fig. 1). The 0.5-10 keV flux-energy spectra can be well approximated by a disk blackbody with Compton up-scattering into a continuum power-law (modeled by simplcut*diskbb; Steiner et al. 2017), absorbed by a Galactic column density (using Wilms et al. 2000 abundances) of (1.438+/-0.013)E22 cm$-2$. The iron K reflection signature is only weakly seen in the residuals, compared to a much stronger line detected earlier in the intermediate state on 03-19 (ATel #15287). The modeled absorbed flux ranges from (4.70+/-0.02)E-9 to (2.96+/-0.01)E-9 erg cm$-2$ s$-1$, corresponding to 156.7+/-0.7 and 98.7+/-0.3 mCrab. The photon index ranges from 3.19+/-0.06 to 2.014+/-0.014, and the disk temperature from 1.296+/-0.005 to 1.508+/-0.004 keV. We also note that the data on 03-28 can also be modeled well alone with only absorbed disk blackbody, with the disk temperature of 1.858+/-0.007 keV. The photon index and the disk temperature suggest that the source is still in the intermediate state.

The 0.3-10 keV power spectra have an integrated 1-10 Hz fractional rms decreasing from 6.2% to 3.4% over the most recent 6 days of observations. We also observe QPOs with the centroid QPO frequency ranging from 2.32+/-0.02 to 2.70+/-0.05 Hz in the data from 03-22 to 03-26. Starting from 03-27, that QPO component becomes much weaker, and the power spectra show an increase of variability at low frequencies (see Fig. 2). The 0.3-10 keV lightcurves with a time resolution of 2 seconds are shown in Fig. 3. In the lightcurve on 03-28, the variability pattern is similar to the μ class in Belloni et al., 2000, or Class V in Court et al., 2017.

NICER will continue monitoring, and we encourage further multi-wavelength observations. 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.

The March 14 to 28 NICER lightcurve; the power spectra; and the NICER lightcurves with a time resolution of 2 seconds.