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

TL;DR

This work presents the first X-ray detection of an extended halo around the middle-aged pulsar PSR B0656+14 using eROSITA PV data, identifying a non-thermal emission component with $\Gamma \approx 3.7$ extending out to about $0.2^\circ$. By modeling the halo as synchrotron radiation from electrons/positrons injected by the pulsar wind nebula and diffusing in the surrounding ISM, the authors constrain a magnetic-field strength of $4$–$10$ $\mu$G and find that a radial gradient $B(r) \propto r^{-1}$ helps reproduce the observed radial profile, while the gamma-ray halo observed by HAWC provides a cross-band consistency check. The analysis highlights a ~20% conversion efficiency of spin-down power into leptons, shows that a simple isotropic diffusion model cannot fully reconcile the X-ray and TeV spectra, and emphasizes the need for improved PSF calibration and deeper, multi-wavelength observations. Overall, the study demonstrates the viability of X-ray counterparts to VHE pulsar halos and motivates future missions like Athena to refine diffusion and magnetic-field structure near pulsars.

Abstract

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.

Figures (11)

• Figure 1: Fluxes of excess X-rays in 0.2 - 2.0 keV band of the 12 annuli, after correction of the inefficient detection of weak point sources beyond $16'$. The background model assumed is the physically motivated three APEC plus CXB model.
• Figure 2: X-ray spectra of the non-thermal component for the $4'-6'$ (black), $6'-8'$ (red), and $8'-10'$ (green) annuli. The background model is the $13'-16'$ data-driven background. The data are binned accordingly to enable that the signal-to-noise ratio (S/N) of each bin at least 3.
• Figure 3: Model predicted X-ray spectrum (left) and surface brightness profile (right) of the non-thermal component, compared with the eROSITA data. Dashed lines are for the constant magnetic field assumption, while solid lines correspond to the assumption of $B\propto r^{-1}$ in the inner pc region around the pulsar.
• Figure A1: Count map in 0.5 - 2.0 keV band of the one-degree diameter region around PSR B0656+14 observed by eROSITA with TM2-7. White ellipses mask identified point sources out. The solid green circles show the boundaries of the annuli divided for spectral analysis, from 4 arcmin to 28 arcmin with an increment of 2 arcmin. PSR B0656+14 is shown for illustration, but also excluded in spectroscopic analysis.
• Figure A2: X-ray spectrum of each annulus. The red points are the eROSITA observational data with the FWC data subtracted. The cyan, purple, and green lines represent the instrumental background, the sum of X-ray sky background, and the non-thermal excess emission. The black lines are the total model fitting results. Note that here the background model assumed is the physically motivated three APEC plus CXB model.