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

Swati Ravi; James F. Steiner; Megan Masterson; Lorenzo Marra; Philip Kaaret; Alexandra Veledina; Michal Dovciak; Adam Ingram; Fiamma Capitanio; Sudeb Ranjan Datta; Elise Egron; Javier A. Garcia; Ole Koenig; Honghui Liu; Herman L. Marshall; Guglielmo Mastroserio; Romana Mikusincova; Joey Neilsen; Edward J. R. Nathan; P. -O. Petrucci; Jakub Podgorny; Jiri Svoboda; Yuexin Zhang

arXiv · 2610.39384

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

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Abstract

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.

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Swati Ravi, James F. Steiner, Megan Masterson, Lorenzo Marra, Philip Kaaret, Alexandra Veledina, Michal Dovciak, Adam Ingram, Fiamma Capitanio, Sudeb Ranjan Datta, Elise Egron, Javier A. Garcia, Ole Koenig, Honghui Liu, Herman L. Marshall, Guglielmo Mastroserio, Romana Mikusincova, Joey Neilsen, Edward J. R. Nathan, P. -O. Petrucci, Jakub Podgorny, Jiri Svoboda, Yuexin Zhang. 2026-10-29. The Disk Awakens: IXPE's Sequel Observation of GS 1354-64 Suggests Changing-Look Behavior in the Steep-Power-Law State. https://arxiv.org/abs/2610.39384

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