Polarized blazar X-rays imply particle acceleration in shocks

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 around 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^1-3, 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 of around 10%, which is a factor of around 2 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.

Fig. 1. IXPE observations of Mrk 501.

a, IXPE image of Mrk 501 during the 8–10 March 2022 observation in the 2–8 keV band. The colour bar denotes the number of X-ray photons per pixel. b, Normalized Stokes Q/I and Stokes U/I parameters, where I is the total intensity, of both IXPE observations. The yellow and cyan shaded regions denote the uncertainty (68% confidence interval (CI)) in the polarization angle for the 8–10 March and 26–28 March observations, respectively. The dashed black line shows the jet direction and the magenta shaded area its uncertainty (68% CI). The dashed circles mark different levels of polarization degree, as labelled. Error bars denote the 68% CI.

Fig. 2. Multiwavelength and polarization archival observations of Mrk 501.

a–d, Optical brightness (R-band, a), observed optical Π in per cent (b), observed optical ψ in degrees (c) and X-ray flux in ×10⁻¹⁰ erg s⁻¹ cm⁻² (d). The black and red dashed lines indicate the level of the source during the 8–10 March and 26–28 March 2022 IXPE observations, respectively. The grey shaded area in c shows the direction of the jet axis. In all panels, the error bars denote the 68% CI.

Fig. 3. Multiwavelength polarization of Mrk 501.

a, Multiwavelength polarization degree of Mrk 501 from radio to X-rays. Black symbols are for the 8–10 March observation and red for the 26–28 March observation. The open symbols show the host-galaxy corrected-intrinsic optical polarization degree. b, Comparison between the observed logarithm of the X-ray and optical Π ratio and the expectations from single-zone model (red dashed line), two turbulent multizone jet models (dash-dotted blue and dotted magenta lines) and energy-stratified models (grey shaded area) for both IXPE observations (black for 8–10 March and red for 26–28 March). The solid error bars show the ratio uncertainty from the IXPE measurements; the dotted error bars show the full uncertainty including optical uncertainties. In both panels, the error bars denote the 68% CI.

Extended Data Fig. 1. Stokes Q/I and Stokes U/I parameters of our IXPE observations during 8–10 March 2022 (left) and 26–28 March 2022 (right).

The measurements are shown for the three detectors (DU1 [red x], DU2 [blue star], DU3 [magenta circle]) separately and combined (black triangle). In both panels error bars denote the 68% CI.