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An IXPE-Led Campaign on the Soft State of Cygnus X-1

Presentation #107.65 in the session Stellar/Compact Objects - Poster Session.

Published onMay 03, 2024
An IXPE-Led Campaign on the Soft State of Cygnus X-1

The persistently-active Cygnus X-1 is the prototypical black hole X-ray binary system. It is bright and very well-characterized, e.g., with precise mass, distance, and spin measurements, and exhibits long-lived hard/soft spectral states. In 2022, Cyg X-1 was the first black-hole X-ray binary studied by IXPE. It was in a hard state, and a surprisingly strong 2-8 keV polarization degree of ~4% was reported. After it underwent a state transition to the soft state in 2023, IXPE carried out a sequence of 5 observations of Cyg X-1’s soft state. Several of these observations were accompanied by other X-ray and multiwavelength monitoring to better characterize it’s broadband properties. We present the results of this soft-state campaign in which the polarization degree is found to be ~2%. The polarization angle is consistent with that found in the hard state and appears to be aligned to Cyg X-1’s AU-scale compact radio jet as well as its pc-scale radio lobes. This polarization cannot be ascribed to the thermal disk emission, and instead must be related to Comptonization and/or reflection. Compared to the hard-state observations, the soft state exhibits a lower polarization degree, but in all other respects the polarimetric behavior is strikingly consistent despite Cyg X-1 being vastly different spectrally between the two states. When scaling by the natural unit of the disk temperature, a consistent trendline in the polarization degree between soft and hard states is apparent.

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