IGR J17091-3624: Newly Formed Periodic Dips and Multiwavelength Activity During the 2025 Outburst

TL;DR

This study reports the first observation of quasi-periodic dipping behavior in IGR J17091-3624 during its 2025 outburst, revealing obscuration by ionized material likely linked to the binary orbit and suggesting a non-standard donor star.

Contribution

First detection of periodic X-ray dips in IGR J17091-3624, linking dips to ionized absorber modulated by the binary orbit, and proposing a non-standard stellar companion.

Findings

Dips recur every 2.83 days with increased spectral hardness.

Obscuration caused by ionized absorber with specific ionization and column density.

Radio spectrum differs from typical hard state 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 $log{\xi}$ ~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.