2025ApJ...986..173Z

Magnetars in quiescent states continue to emit hard X-rays with a power far exceeding the loss of rotational energy. It has recently been noted that this hard X-ray continuum may bear a direct signature of quantum electrodynamic effects in magnetic fields stronger than the Schwinger field (B_Q = 4.4 × 10¹³ G). Where the current flowing into the magnetosphere is driven by narrow structures in the solid crust, the e^± pair plasma supporting the current relaxes to a collisional and transrelativistic state. The decay of a pair into two photons produces a broad, bremsstrahlung-like spectrum of hard X-rays, similar to that observed and extending up to 0.5-1 MeV. The conversion of two gamma-rays to a pair is further enhanced by a factor ∼B/B_Q. Monte Carlo calculations of pair creation in a dipole magnetic field are presented. Nonlocal particle injection is found to be strong enough to suppress the high voltage that otherwise would accompany a weaker, global twist; the hard X-rays are mostly emitted away from the magnetic poles. Some of the pairs annihilate in an optically thin surface layer. The prototypical anomalous X-ray pulsar 1E 2259+586, which shows a hard X-ray continuum but relatively weak torque noise, slow spindown, and no radio emission, is a Rosetta Stone for understanding the magnetar circuit, consistent with the picture advanced here. For a 15-60 keV luminosity as low as 10³⁴ erg s^–1, the polar flux of subrelativistic pairs produces an optical depth of 3-30 to electron cyclotron scattering in the 1-10 keV band, reducing the net X-ray polarization.