[Paper Review] Polarized Blazar X-rays imply particle acceleration in shocks

Ioannis Liodakis, Alan P. Marscher+126 more|arXiv (Cornell University)|Sep 13, 2022

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This study presents the first polarization measurements of X-ray emission from the blazar Mrk 501 using the IXPE satellite, revealing intrinsic polarization degrees of 4±1% and 5±1% during two observations. The results imply that particle acceleration in relativistic shocks is responsible for the X-ray emission, providing direct evidence for shock acceleration in blazar jets.

ABSTRACT

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to $\sim 1$ TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization - the only range available until now - probe extended regions of the jet containing particles that left the acceleration site days to years earlier (Jorstad et al., 2005; Marin et al., 2018; Blinov et al., 2021), and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian~501 (Mrk~501). We measure an X-ray linear polarization degree $Π_X \sim10\%$, a factor of $\sim2$ higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration, and also implies that the plasma becomes increasingly turbulent with distance from the shock.

Motivation & Objective

• To measure the intrinsic X-ray polarization of the blazar Mrk 501 using the Imaging X-ray Polarimetry Explorer (IXPE).
• To investigate the physical origin of X-ray emission in blazars by analyzing polarization properties.
• To test whether shock acceleration mechanisms can explain the observed X-ray polarization and spectral curvature.
• To compare multiwavelength polarization and flux variability across radio to X-ray bands to constrain emission models.
• To determine whether the observed X-ray polarization is consistent with shock acceleration or alternative mechanisms like magnetic reconnection.

Proposed method

• Conducted simultaneous multiwavelength observations using IXPE for X-ray polarization, Swift/XRT for X-ray flux and spectrum, and NuSTAR for high-energy X-ray coverage.
• Measured total X-ray flux and spectral energy distribution using standard XSPEC fitting procedures with absorbed log-parabola and power-law models.
• Extracted background using blank-sky observations and source-specific circular regions (47'' for Swift, 49'' for NuSTAR).
• Calculated intrinsic polarization degree by correcting observed polarization for host galaxy contamination using archival optical data.
• Fitted the X-ray spectrum in the 0.3–79 keV range with an absorbed log-parabola model: $N(E) = (E/E_p)^{(-\alpha - \beta\log(E/E_p))}$, with $E_p = 5$ keV.
• Compared the fit quality of log-parabola and single power-law models to assess spectral curvature and determine the best-fit parameters.

Experimental results

Research questions

• RQ1 What is the intrinsic X-ray polarization degree of Mrk 501, and does it support shock acceleration mechanisms?
• RQ2 How do the X-ray polarization and spectral curvature observed by IXPE compare to predictions from shock acceleration models?
• RQ3 Is the observed X-ray polarization consistent with a dominant contribution from synchrotron emission in a shock-accelerated electron population?
• RQ4 How does the X-ray flux and polarization state of Mrk 501 compare to its long-term archival behavior in the X-ray and optical bands?
• RQ5 Can the observed polarization degree and angle variability across the electromagnetic spectrum be explained by a shock-dominated emission model?

Key findings

• The median intrinsic X-ray polarization degree was measured as $\Pi_{\rm intr} = 4 \pm 1\%$ during the 8–10 March observation.
• The intrinsic polarization degree increased to $\Pi_{\rm intr} = 5 \pm 1\%$ during the 26–28 March observation.
• The X-ray spectrum was best fit by an absorbed log-parabola model with $\alpha = 2.27 \pm 0.01$ and $\beta = 0.28 \pm 0.01$ during the 8–10 March observation, indicating significant spectral curvature.
• The single power-law model provided a significantly worse fit ($\chi^2/\mathrm{dof} = 2005/851$) compared to the log-parabola model ($\chi^2/\mathrm{dof} = 862/850$), confirming spectral curvature.
• The 2–8 keV X-ray flux was measured as $(10.0 \pm 0.5) \times 10^{-11}~{}\mathrm{erg/s/cm^2}$ during the first IXPE observation, with no significant variability detected.
• During the second observation, the X-ray flux increased to $(21.0 \pm 0.6) \times 10^{-11}~{}\mathrm{erg/s/cm^2}$, corresponding to a 17% rise in count rate over the observation period.