The Disk Awakens: IXPE's Sequel Observation of GS 1354-64 Suggests Changing-Look Behavior in the Steep-Power-Law State

@misc{ravi2026disk,
  title = {{The Disk Awakens: IXPE's Sequel Observation of GS 1354-64 Suggests Changing-Look Behavior in the Steep-Power-Law State}},
  author = {Swati Ravi and James F. Steiner and Megan Masterson and Lorenzo Marra and Philip Kaaret and Alexandra Veledina and Michal Dovciak and Adam Ingram and Fiamma Capitanio and Sudeb Ranjan Datta and Elise Egron and Javier A. Garcia and Ole Koenig and Honghui Liu and Herman L. Marshall and Guglielmo Mastroserio and Romana Mikusincova and Joey Neilsen and Edward J. R. Nathan and P. -O. Petrucci and Jakub Podgorny and Jiri Svoboda and Yuexin Zhang},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.39384},
  eprint = {2610.39384},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.39384},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - The Disk Awakens: IXPE's Sequel Observation of GS 1354-64 Suggests Changing-Look Behavior in the Steep-Power-Law State
AU  - Swati Ravi
AU  - James F. Steiner
AU  - Megan Masterson
AU  - Lorenzo Marra
AU  - Philip Kaaret
AU  - Alexandra Veledina
AU  - Michal Dovciak
AU  - Adam Ingram
AU  - Fiamma Capitanio
AU  - Sudeb Ranjan Datta
AU  - Elise Egron
AU  - Javier A. Garcia
AU  - Ole Koenig
AU  - Honghui Liu
AU  - Herman L. Marshall
AU  - Guglielmo Mastroserio
AU  - Romana Mikusincova
AU  - Joey Neilsen
AU  - Edward J. R. Nathan
AU  - P. -O. Petrucci
AU  - Jakub Podgorny
AU  - Jiri Svoboda
AU  - Yuexin Zhang
PY  - 2026
DA  - 2026/10/29
PB  - arXiv
DO  - 10.48550/arxiv.2610.39384
UR  - https://arxiv.org/abs/2610.39384
AB  - After decades of only failed transitions, the black hole X-ray binary GS 1354-64 has undergone a complete outburst again. We present its first X-ray polarimetric investigation in the fading steep-power-law (SPL) state, following an earlier intermediate-state IXPE campaign during the same outburst. The 2-8 keV emission has a polarization degree (PD) of 2.3 +/- 0.2% and polarization angle (PA) of 5 +/- 3 degrees. Compared with the intermediate state, where PD=3.9 +0.3/-0.2% and PA=-2 +/-2 degrees, the broadband PD is lower while the PA remains consistent. The intermediate-state type-C quasi-periodic oscillation state has also disappeared, marking a major reconfiguration of the accretion flow. Assuming a distance of 7 kpc, the observed the intensity drop from 2.5% to 1.6% of the Eddington luminosity over 10.2 days is punctuated by several broad hour-to-day-scale troughs, tracing two distinct tracks in hardness-intensity space. Simultaneous spectroscopy shows the troughs' spectra contains a cooler disk with smaller apparent normalization, while no change in the Compton component's photon index or normalization is required between tracks. The model-independent analysis identifies a steeper increase in PD with energy outside the troughs, yielding G_E=1.5 +/- 0.2%/keV, compared with G_E=0.7 +0.3/-0.5%/keV during the troughs. Their duration and spectropolarimetric morphology distinguish the troughs from traditional short absorption dips seen in X-ray binaries, providing a possible stellar-mass analog of the recurrent accretion changes that drive "changing-look" behavior in supermassive black hole systems.
ER  -