First Results of IXPE X-ray Polarization Measurements of Cyg X-1 in its Soft State
ATel #16084; Michal Dovciak, James F. Steiner, Henric Krawczynski, and Jiri Svoboda on behalf of the IXPE team
on 13 Jun 2023; 16:37 UT
Credential Certification: James F Steiner (jsteiner@cfa.harvard.edu)
Subjects: X-ray, Request for Observations, Black Hole
The Imaging X-ray Polarimetry Explorer (IXPE) team is currently performing an observational campaign on black hole X-ray binary Cyg X-1 with X-ray timing, spectral, and polarimetric instruments (NICER, NuSTAR, IXPE). Observations have been carried out on May 2 (IXPE), May 9 (IXPE), and May 24 (IXPE, NICER, and NuSTAR) of this year and trace Cyg X-1 in a highly-variable soft state. The IXPE, NICER, and NuSTAR observations show a soft state with a bright disk and a steep power law with photon index of ~2.5. The IXPE observations show a 2-8 keV linear polarization degree of 2.27% +/- 0.18% with the electric field vector being roughly parallel to the radio jet. These results are surprisingly consistent with the previous measurements in the hard state. The IXPE team therefore encourages multiwavelength spectral and polarimetric observations along the electromagnetic spectrum to accompany the ongoing observation campaign of Cyg X-1 with upcoming IXPE, NICER, and NuSTAR pointings on UTC 2023-06-13T23:05Â to 2023-06-14T16:10 and 2023-06-20T01:05 to 2023-06-20T18:09.
Accompanying multiwavelength observations promise to give new insights into the accretion disk and coronal plasma configuration in thermally-dominated states of accreting stellar mass black holes.
