IXPE detects a remarkably high X-ray Polarization Degree in a faint hard state of IGR J17091-3624
ATel #17093; M. Parra (Ehime University), M. Dovciak (Czech Academy of Sciences), M. Ewing (Newcastle University), G. Mastroserio (Universita degli Studi di Milano), M. Del Santo (INAF-IASF Palermo), J. Garcia (NASA Goddard), A. Veledina (University of Turku), A. Ingram (Newcastle University), G. Matt, S. Bianchi (Universita Roma Tre), P.-O. Petrucci (Universite Grenoble-Alpes), P. Gandhi (University of Southampton), M. Shidatsu (Ehime University), T. Russell (INAF-IASF Palermo) on behalf of a larger collaboration
on 20 Mar 2025; 16:48 UT
Credential Certification: Maxime Parra (maxime.parra@univ-grenoble-alpes.fr)
Subjects: X-ray, Black Hole, Transient
The Black Hole X-ray candidate IGR J17091-3624, first detected in 2003 (ATel #149) and well known for its recurrent outbursts and exotic variability patterns reminiscent of GRS 1915+105 (Altamirano et al. 2011, ApJL, 742, L17; Court et al. 2017, MNRAS, 468, 4748; Wang et al., ApJ 963, 14, +2024), was detected to undergo a new outburst in early February (ATels #17034, #17038, #17065). During the following month, high-cadence monitoring with Swift/XRT confirmed that the source remained in a purely hard spectral state, with a progressive decline in flux from 7e-10 erg/s/cm^2 (1-10keV band, unabsorbed) to ~3e-10 erg/s/cm^2 between February 14 and March 12.
The Imaging X-ray Polarimetry Explorer (IXPE) observed the source from 2025-03-07T07:17 to 2025-03-10T18:51, for a total of 163ks. Our analysis reveals a very significant (>5 sigma) polarization measurement, with a background subtracted Polarization Degree of PD = ( 9.3 +/- 1.8 )% and Polarization Angle of PA = ( 82 +/- 5 ) deg in the 2-8keV band (errors at a 1 sigma confidence level).
Simultaneous NuSTAR observations at the beginning and end of the IXPE exposure confirm hard state properties, with a spectrum dominated by a gamma~1.6 component, and a type-C QPO in the PDS. The flux of the source slightly decreased from 3.8e-10 to 3.3e-10 erg/s/cm^2 (1-10keV band, unabsorbed) over that period.
Follow-up observations and monitoring of the evolution of the source in the low-hard state are strongly encouraged.
We thank the IXPE, NuSTAR, NICER, and Swift operation teams for their efforts in scheduling the steps of this campaign.
