We report the discovery of X-ray polarization from the X-ray-bright filament.G0.13-0.11 in the Galactic center (GC) region. This filament features a bright,hard X-ray source that is most plausibly a pulsar wind nebula (PWN) and anextended and structured diffuse component. Combining the polarization signalfrom IXPE with the imaging/spectroscopic data from Chandra, we find that X-rayemission of G0.13-0.11 is highly polarized PD=$57(\pm18)$% in the 3-6 keV band,while the polarization angle is PA=$21^\circ(\pm9^\circ)$. This high degree ofpolarization proves the synchrotron origin of the X-ray emission fromG0.13-0.11. In turn, the measured polarization angle implies that the X-rayemission is polarized approximately perpendicular to a sequence of nonthermalradio filaments that may be part of the GC Radio Arc. The magnetic field on theorder of $100\,{\rm\mu G}$ appears to be preferentially ordered along thefilaments. The above field strength is the fiducial value that makes our modelself-consistent, while the other conclusions are largely model independent.
We report the X-ray polarization properties of the high-synchrotron-peaked(HSP) blazar PKS 2155$-$304 based on observations with the Imaging X-rayPolarimetry Explorer (IXPE). We observed the source between Oct 27 and Nov 7,2023. We also conducted an extensive contemporaneous multiwavelength (MW)campaign. We find that during the first half ($T_1$) of the IXPE pointing, thesource exhibited the highest X-ray polarization degree detected for an HSPblazar thus far, (30.7$\pm$2.0)%, which dropped to (15.3$\pm$2.1)% during thesecond half ($T_2$). The X-ray polarization angle remained stable during theIXPE pointing at 129.4$^\circ$$\pm$1.8$^\circ$ and125.4$^\circ$$\pm$3.9$^\circ$ during $T_1$ and $T_2$, respectively. Meanwhile,the optical polarization degree remained stable during the IXPE pointing, withaverage host-galaxy-corrected values of (4.3$\pm$0.7)% and (3.8$\pm$0.9)%during the $T_1$ and $T_2$, respectively. During the IXPE pointing, the opticalpolarization angle changed achromatically from $\sim$140$^\circ$ to$\sim$90$^\circ$ and back to $\sim$130$^\circ$. Despite several attempts, weonly detected (99.7% conf.) the radio polarization once (during $T_2$, at 225.5GHz): with degree (1.7$\pm$0.4)% and angle 112.5$^\circ$$\pm$5.5$^\circ$. Thedirection of the broad pc-scale jet is rather ambiguous and has been found topoint to the east and south at different epochs; however, on larger scales (>1.5 pc) the jet points toward the southeast ($\sim$135$^\circ$), similar to allof the MW polarization angles. Moreover, the X-ray to optical polarizationdegree ratios of $\sim$7 and $\sim$4 during $T_1$ and $T_2$, respectively, aresimilar to previous IXPE results for several HSP blazars. These findings,combined with the lack of correlation of temporal variability between the MWpolarization properties, agree with an energy-stratified shock-accelerationscenario in HSP blazars.
Weakly magnetized neutron stars in X-ray binaries show complex phenomenologywith several spectral components that can be associated with the accretiondisk, boundary and/or spreading layer, a corona, and a wind. Spectroscopicinformation alone is, however, not enough to disentangle these components.Additional information about the nature of the spectral components and inparticular the geometry of the emission region can be provided by X-raypolarimetry. One of the objects of the class, a bright, persistent, and ratherpeculiar galactic Type I X-ray burster was observed with the Imaging X-rayPolarimetry Explorer (IXPE) and the X-ray Multi-Mirror Mission Newton(XMM-Newton). Using the XMM-Newton data we estimated the current state of thesource as well as detected strong absorption lines associated with theaccretion disk wind. IXPE data showed the source to be significantly polarizedin the 2-8 keV energy band with the overall polarization degree (PD) of 1.4% ata polarization angle (PA) of -2 degrees (errors at 68% confidence level).During the two-day long observation, we detected rotation of the PA by about 70degrees with the corresponding changes in the PD from 2% to non-detectable andthen up to 5%. These variations in polarization properties are not accompaniedby visible changes in spectroscopic characteristics. The energy-resolvedpolarimetric analysis showed a significant change in polarization, from beingstrongly dependent on energy at the beginning of the observation to beingalmost constant with energy in the later parts of the observation. As apossible interpretation, we suggest the presence of a constant component ofpolarization, strong wind scattering, or different polarization of the two mainspectral components with individually peculiar behavior. The rotation of the PAsuggests a 30-degree misalignment of the neutron star spin from the orbitalaxis.
Discovery of pulsations from a number of ULXs proved that accretion ontoneutron stars can produce luminosities exceeding the Eddington limit by severalorders of magnitude. The conditions necessary to achieve such high luminositiesas well as the exact geometry of the accretion flow in the neutron starvicinity are, however, a matter of debate. The pulse phase-resolvedpolarization measurements that became possible with the launch of the ImagingX-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometryand its orientation relative to the orbital plane. They provide an avenue totest different theoretical models of ULX pulsars. In this paper we present theresults of three IXPE observations of the first Galactic ULX pulsar SwiftJ0243.6+6124 during its 2023 outburst. We find strong variations in thepolarization characteristics with the pulsar phase. The average polarizationdegree increases from about 5% to 15% as the flux dropped by a factor of threein the course of the outburst. The polarization angle (PA) as a function of thepulsar phase shows two peaks in the first two observations, but changes to acharacteristic sawtooth pattern in the remaining data set. This is notconsistent with a simple rotating vector model. Assuming the existence of anadditional constant polarized component, we were able to fit the threeobservations with a common rotating vector model and obtain constraints on thepulsar geometry. In particular, we find the pulsar angular momentum inclinationwith respect to the line of sight of 15-40 deg, the magnetic obliquity of 60-80deg, and the pulsar spin position angle of -50 deg, which significantly differsfrom the constant component PA of about 10 deg. Combining these X-raymeasurements with the optical PA, we find evidence for at least a 30 degmisalignment between the pulsar angular momentum and the binary orbital axis.