IGR J17091-3624: Newly Formed Periodic Dips and Multiwavelength Activity During the 2025 Outburst
Zikun Lin, Yanan Wang, Shuyuan Wei, Yongkang Sun, Long Ji, Samaporn Tinyanont, Meng Sun, Song Wang, Diego Altamirano, Douglas J. K. Buisson, Wenxiong Li, Qian Chen, Jifeng Liu, Shuang-Nan Zhang, Wei Wang, Zhen Guo, Pathompong Butpan, Rungrit Anutarawiramkul
TL;DR
IGR J17091-3624, a Galactic BH LMXB candidate, underwent a 2025 hard-state outburst during which intermittent X-ray dips were detected by IXPE and contemporaneous missions. A multi-instrument timing and spectroscopy analysis reveals dips recurring with a period of $2.83 \pm 0.07$ days and a concurrent rise in spectral hardness, consistent with obscuration by an ionized absorber. Spectral modeling returns an ionization parameter $\log\xi$ in the range $1$--$3$ erg cm s$^{-1}$ and an equivalent hydrogen column density $N^{\rm zxipcf}_{\rm H}$ of $(1$--$30)\times10^{22}$ cm$^{-2}$, indicating absorber material in the outer accretion disk modulated by the binary orbit. If the period is confirmed, it implies the donor is likely a (partially) stripped giant rather than a main-sequence star. The optical band shows no clear periodicity, while the radio spectrum transitions from flat to steep, differing from the inverted spectrum often seen in the hard state of LMXBs.
Abstract
The black hole low-mass X-ray binary (LMXB) candidate IGR J17091-3624 experienced a hard-state-only outburst in 2025. In this paper, we show that IXPE detected a series of intermittent X-ray dips, spanning a total interval of ~1 day. Subsequent observations with NICER, EP, NuSTAR, and Swift reveal that these dips recur with a period of 2.83$\pm$0.07 days and are accompanied by an increase in spectral hardness. This is the first time such quasi-periodic dipping behavior has been observed in this target since discovery. Our spectral analysis shows that the dips can be explained by obscuration from an ionized absorber characterized by an ionization parameter of $logξ$ ~1-3 erg cm s$^{-1}$ and an equivalent hydrogen column density of $N^{\rm zxipcf}_{\rm H}$~(1-30)$\times10^{22}$ cm$^{-2}$. The periodic reappearance of the absorber is likely caused by obscuring material located in the outer accretion disk, modulated by the binary orbital period. If confirmed, this period would suggest that the donor star in IGR J17091-3624 has deviated from the standard main-sequence evolutionary path and is likely a (partially) stripped giant. In the optical band, no significant periodicity or correlation with the X-ray dips was detected, whereas the radio counterpart exhibited a flat to steep spectrum, in contrast to the inverted spectrum typically observed during the hard state of LMXBs.