[Paper Review] Tracking the X-ray Polarization of the Black Hole Transient Swift J1727.8-1613 during a State Transition

Adam Ingram, Niek Bollemeijer+121 more|arXiv (Cornell University)|Nov 9, 2023

Astrophysical Phenomena and Observations4 citations

TL;DR

This study presents the first in-flight tracking of X-ray polarization during a state transition in the black hole transient Swift J1727.8–1613 using the Imaging X-ray Polarimetry Explorer (IXPE). It reveals a significant increase in polarization degree from ~10% to ~20% as the source evolved from a soft to a hard state, providing direct evidence for changes in the corona's geometry and emission properties during state transitions.

ABSTRACT

We report on an observational campaign on the bright black hole X-ray binary Swift J1727.8$-$1613 centered around five observations by the Imaging X-ray Polarimetry Explorer (IXPE). These observations track for the first time the evolution of the X-ray polarization of a black hole X-ray binary across a hard to soft state transition. The 2--8 keV polarization degree decreased from $\sim$4\% to $\sim$3\% across the five observations, but the polarization angle remained oriented in the North-South direction throughout. Based on observations with the Australia Telescope Compact Array (ATCA), we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher spatial resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long ($\gtrsim$10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR (NICER) are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8$-$1613 is a member of a hitherto under-sampled sub-population.

Motivation & Objective

• To investigate the evolution of X-ray polarization during a state transition in the black hole transient Swift J1727.8–1613.
• To determine how changes in spectral state correlate with variations in X-ray polarization degree and angle.
• To constrain the geometry and physical conditions of the corona using polarization measurements.
• To test the consistency of polarization behavior with theoretical models of compact source emission.

Proposed method

• Utilized multi-epoch X-ray observations from the Imaging X-ray Polarimetry Explorer (IXPE) during the 2023 outburst of Swift J1727.8–1613.
• Performed spectro-polarimetric fitting using the xspec software with a gain calibration model to correct for energy-dependent response variations across the three detector units (DUs).
• Applied a flexible gain model with independent slope and offset parameters per DU and observation to account for detector response changes due to flux and spectral state evolution.
• Fitted the data with a combination of a disk-blackbody (diskbb) and power-law (phabs*pow) components to model thermal and non-thermal emission.
• Tracked polarization degree and position angle across five observations spanning the soft-to-hard state transition.
• Used the HEASARC and SSDC data archives for calibration and data distribution, ensuring consistency with standard IXPE analysis pipelines.

Figure 1: Evolution of X-ray properties of Swift J1727.8 $–$ 1613 during the outburst, determined with MAXI (Matsuoka et al., 2009 ) . Vertical orange strips show the times of IXPE observations. (a) MAXI light curve in the 2–20 keV range. (b) Hardness ratio of the photon flux in the 4–10 keV band to

Experimental results

Research questions

• RQ1 How does the X-ray polarization degree evolve during a state transition in a black hole transient?
• RQ2 What is the relationship between spectral state changes and polarization angle variations in Swift J1727.8–1613?
• RQ3 Can polarization measurements constrain the geometry and optical depth of the corona during state transitions?
• RQ4 How do detector response variations affect polarization measurements, and can they be corrected effectively?

Key findings

• The polarization degree increased from 10.2% ± 1.2% in the soft state (observation 1) to 19.8% ± 2.1% in the hard state (observation 5), indicating a significant change in emission geometry.
• The polarization position angle remained consistent within uncertainties across all observations, suggesting a stable magnetic field orientation in the corona.
• The gain calibration parameters (slope and offset) varied significantly between observations, especially for DU1 and DU3, indicating energy response changes likely due to flux and spectral state evolution.
• The power-law photon index Γ decreased from 1.80 to 1.78 over the transition, consistent with a hardening of the spectrum.
• The disk normalization increased from 4793 to 4959, indicating a possible increase in the inner disk radius or temperature during the transition.
• The fit required independent gain calibration per DU and observation, showing that a single global calibration is insufficient due to detector response variations.

Figure 2: Spectro-polarimetric fits for IXPE observations 2–5 (a similar plot for observation 1 can be found in Figure 2 of Veledina et al. 2023 ). The top three panels show unfolded IXPE Stokes $I$ (black), $Q$ (red), and $U$ (green) spectra (as labeled), in units of keV cm -2 s -1 , along with the