IXPE Detects X-ray Polarization from Swift J1727.8-1613 in the 2-8keV Energy Range

ATel #16242; Michal Dovciak, Ajay Ratheesh, Allyn Tennant, and Giorgio Matt on behalf of the IXPE team
on 13 Sep 2023; 10:43 UT
Credential Certification: Michal Dovciak (dovciak@asu.cas.cz)

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The Imaging X-ray Polarimetry Explorer (IXPE) has observed the newly discovered transient Swift J1727.8-1613 (ATEL #16205, #16206), probably a low-mass X-ray binary hosting a black hole candidate (ATEL #16207, #16208), during the period from 2023-09-07T20:00:16.184 to 2023-09-08T06:36:21.184 (UTC) with the total livetime of 19 ks (see also ATEL #16237). Our quick analysis with IXPEobssim v30.5.0 using unweighted PCUBE algorithm on the circular source region with 80 arcsec radius positioned at RA=261.9277deg and DEC=-16.2019deg (J2000) gave us a polarization degree of (4.71±0.27)% with polarization direction of (2.6±1.7)deg in the full energy band of 2-8keV. There is an indication that the polarization degree is rising with energy from 4% at 2 keV to almost 6% at 6-8keV, while the polarization angle is consistent with being constant (see figure https://owncloud.asu.cas.cz/index.php/s/EifzPG4fVGNyKi6). The polarization angle is aligned with the polarization direction measured in submillimeter wavelengths performed by SMA (ATEL #16230), with only a slight offset of about 6 degrees. (The X-ray polarization degree is twice larger than the one measured in the submm wavelengths.) The Imaging X-ray Polarimetry Explorer (IXPE) team is continuing its observational campaign on this new transient with X-ray timing, spectral, and polarimetric instruments (NICER, NuSTAR, INTEGRAL and IXPE) in a few days with 40ks IXPE exposure between 2023-09-16T17:15:16 and 2023-09-17T13:28:30 (UTC). Note that the start/end times have slightly changed compared to our previous announcement (ATEL #16237). Exact observing times are available at IXPE weekly plan at https://wwwastro.msfc.nasa.gov/for_scientists/weekly.html. Accompanying multiwavelength observations are encouraged since they promise to give new insights into the nature of this newly discovered transient.