NICER observations of rapid state transition of IGR J17091-3624
ATel #15287; Jingyi Wang (MIT), Erin Kara (MIT), Jon Miller (Univ. Michigan), Paul Draghis (Univ. Michigan), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Yanan Wang (Southampton), Diego Altamirano (Southampton), for the NICER team
on 19 Mar 2022; 19:05 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jingyi Wang (jingyiw@mit.edu)
Subjects: X-ray, Black Hole, Transient
IGR J17091-3624 is a black hole X-ray binary (BHXB) that has not been active since 2016, and is a very interesting source as it shows lots of similarities to the peculiar BHXB GRS 1915+105 (e.g., Altamirano et al., 2011). From the Swift/BAT lightcurves and the NICER data from 2022-03-14 to 03-15, the source was brightening in the hard state (ATel #15282). The brightening activity is also seen in hard X-rays with INTEGRAL and optical (ATels #15283, #15284). Radio observations obtained on 2022-03-17 were consistent with a compact radio jet, confirming that the source was in the hard state (ATel #15286).
NICER first observed the source on 2022-03-14 and 03-15 (ATel #15282), when the 0.5-10 keV NICER mean count rate was 142.7+/-0.2 counts/s. NICER observed this source again three days later on 2022-03-18 (MJD 59656), and within that 3.8 ks exposure, the count rate increased from 509.5+/-0.4 counts/s (09:00:23.175 â 15:15:40.341 UTC) to 617.8+/-0.8 counts/s (17:49:15.219 â 18:06:07.333 UTC). We preliminarily fit the flux-energy spectra of the newest observations with a model including a disk blackbody and a power-law, and an iron K reflection signature can be seen in the residuals. Therefore, we adopt a model including a disk blackbody and the relativistic reflection model reltransDCp (Ingram et al., 2019) that includes the illuminating coronal emission. The fitted column density, using Wilms et al. 2000 abundances, is (1.67+/-0.03)E22 cm. The modeled flux increases from (2.33+/-0.01)E-9 to (2.76+/-0.01)E-9 erg cm s, which correspond to 77.6+/-0.3 and 92.0+/-0.3 mCrab. The photon index Gamma is 2.15+/-0.13 and 2.03+/-0.010, which is higher than the typical photon index range of 1.4 to 1.8 in the hard state, and higher than the previous observation (reported as 1.46(1) in ATel #15282). The disk temperature is 1.12+/-0.07 keV in the latest epoch of data, consistent with the value reported in the 2016 outburst of this source in the intermediate state (Wang, Yanan, et al., 2018, MNRAS 478.4).
The 0.3-10 keV power spectra have an integrated 1-10 Hz fractional rms of 11.0% and 9.3%, which dropped slightly compared to the previous observation (13.4%). We also observe strong type-C QPOs and both sub- and second-harmonics, with the centroid QPO frequency increasing from 2.59+/-0.03 Hz to 3.46+/-0.05 Hz in the newest observations. As a comparison, we found the QPO frequency was 0.32+/-0.02 Hz in the previous observation. The QPO frequency and the spectral features, including the values of the photon index and the disk temperature, suggest that IGR J17091-3624 is currently making its hard-to-soft state transition, and is rapidly brightening.
NICER will continue monitoring; planned Target of Opportunity observations are on 03-22 to 03-25 due to a visibility gap in the next few days. 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.
