Possible evidence for extended X-ray emission surrounding PSR B0656+14 with eROSITA

Extended very-high-energy $γ$-ray emission from middle-aged pulsars as revealed recently by several groundbased $γ$-ray experiments has strong implication on the transport of high-energy particles in the interstellar medium surrounding those pulsars. The $γ$-ray emission is widely believed to be produced by high-energy electrons and positrons accelerated by the pulsar wind nebulae when scattering off the interstellar radiation field via the inverse Compton process. Consequently, multiwavelength counterparts of the $γ$-ray halos are expected to be present, which have not yet been detected. In this work we report the possible detection of extended X-ray emission from a $\sim 0.2\degr$ radius region around PSR B0656+14 with eROSITA. In spite that there are uncertainties of the on-orbit point spread function of the pointing mode, the radial profile of PSR B0656+14 is found to be broader than that of a star at similar observational conditions, indicating that emission is possibly from the expected extended halo around the pulsar. The spectrum of the emission can be described by a power-law function with an index of $\sim3.7$. Its surface brightness declines with radius faster than the prediction of the particle diffusion and synchrotron radiation in a uniform magnetic field, suggesting the existence of a radial gradient of the magnetic field strength as $\sim r^{-1}$. The magnetic field strength in the X-ray emitting region is constrained to be $4-10~μ$G.

💡 Research Summary

This paper reports the first possible detection of an extended X‑ray halo surrounding the middle‑aged pulsar PSR B0656+14 (the Monogem pulsar) using data from the eROSITA telescope in pointing mode. The motivation stems from recent discoveries of very‑high‑energy (VHE) γ‑ray halos around similar pulsars, which are interpreted as inverse‑Compton (IC) emission from high‑energy electrons and positrons that have escaped the pulsar wind nebula (PWN) and are diffusing through the interstellar medium (ISM). If these particles also emit synchrotron radiation, a faint X‑ray counterpart should exist, but previous searches with XMM‑Newton, Chandra, and eROSITA all‑sky surveys have yielded only upper limits.

Observations and data reduction
The authors used a dedicated eROSITA pointing observation (ObsID 300000) performed on 14 Oct 2019, with an effective exposure of ~100 ks after filtering high‑background intervals. Because telescope module (TM) 1 failed and TM 5 and TM 7 suffered optical‑light leaks, only TM 2, 3, 4, and 6 were retained, providing good sensitivity down to 0.2 keV. An annular region from 4′ to 26′ around the pulsar was defined to avoid direct contamination from the bright point source. Within this annulus ~530 point sources were identified and excised. The annulus was divided into twelve concentric rings (2′ width each) and spectra were extracted for each ring, together with instrument background spectra derived from filter‑wheel‑closed data.

Spectral modeling
All twelve spectra were fitted simultaneously with a common background model but independent normalizations for any excess component. The background consists of (i) a fixed‑slope power‑law cosmic X‑ray background (CXB), (ii) an unabsorbed APEC component for the Local Hot Bubble, and (iii) two absorbed APEC components representing the Galactic halo / circum‑galactic medium (temperatures ≈ 0.2 keV and 0.7 keV). Absorption was modeled with tbabs using an HI column density in the range (1–2.8) × 10²⁰ cm⁻², consistent with previous measurements of the pulsar’s line‑of‑sight absorption.

The excess emission is well described by a power‑law with photon index Γ ≈ 4 (i.e., a very soft spectrum). Its surface brightness peaks in the 4′–10′ ring, drops to near zero between 10′ and 18′, and appears to rise again at larger radii. The authors argue that the rise is an artifact caused by decreasing detection efficiency for faint point sources beyond ~16′; after correcting for this effect the outer profile becomes flat.

PSF verification
A critical concern is whether the extended signal could be caused by the point‑spread function (PSF) of the bright pulsar. To test this, the authors observed a bright variable star (V* V702 CrA) at a larger off‑axis angle (4.5′) under similar conditions. The star’s radial profile is significantly more concentrated than that of PSR B0656+14 in the 0.2–2 keV band, and matches the on‑axis PSF model predictions. The estimated PSF leakage from the pulsar beyond 4′ is <10 % of the excess flux, well below the measured signal (≈ 1.4 % of the pulsar’s total flux). This suggests that the observed halo is not an instrumental artifact, although the authors acknowledge that a fully calibrated PSF for eROSITA pointing mode is still lacking.

Physical interpretation
Assuming the excess is genuine synchrotron emission from the same electron/positron population that produces the VHE γ‑ray halo, the authors construct a simple one‑zone diffusion model. Electrons are injected with a super‑exponential cutoff power‑law spectrum Q(E) ∝ E⁻α exp