IGR J17091−3624: Newly Formed Periodic Dips and Multiwavelength Activity during the 2025 Outburst

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, the Einstein Probe, NuSTAR, and Swift reveal that these dips recur with a period of 2.83 ± 0.07 days and are accompanied by an increase in spectral hardness. This is the first time such quasiperiodic 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>ξ</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> </mml:math> –3 erg cm s −1 and an equivalent hydrogen column density of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>N</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>zxipcf</mml:mi> </mml:msubsup> <mml:mo>∼</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:mspace width="0.1em"/> <mml:mtext>–</mml:mtext> <mml:mspace width="0.1em"/> <mml:mn>30</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>22</mml:mn> </mml:msup> </mml:math> 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.