New Hard X-Ray and Multiwavelength Study of the PeVatron Candidate PWN G0.9+0.1 in the Galactic Center Region

We present a new X-ray study and multiwavelength spectral energy distribution (SED) modeling of the young pulsar wind nebula (PWN) powered by the energetic pulsar PSR J1747-2809, inside the composite supernova remnant (SNR) G0.9+0.1, located in the Galactic Center region. The source is detected by NuSTAR up to 30 keV with evidence for the synchrotron burnoff effect in the changing spatial morphology with increasing energy. The broadband 2-30 keV spectrum of PWN G0.9+0.1 is modeled by a single power law with photon index $Γ=2.11\pm0.07$. We combined the new X-ray data with the multiwavelength observations in radio, GeV, and TeV gamma rays and modeled the SED, applying a one-zone and a multi-zone leptonic model. The comparison of the models is successful, as we obtained physically compatible results in the two cases. Through the one-zone model, we constrain the age of the system to $\sim2.2$ kyr, as well as reproduce the observed PWN and SNR radio sizes. In both the one-zone and multi-zone leptonic models, the electron injection spectrum is well-described by a single power law with spectral index $p \sim 2.6$ and a maximum electron energy of $\sim2$ PeV, suggesting the source could be a leptonic PeVatron candidate. We estimate the average magnetic field to be $B_{\rm PWN} \sim 20\ μ$G. We also report the serendipitous NuSTAR detection of renewed X-ray activity from the very faint X-ray transient XMMU J174716.1-281048 and characterize its spectrum.

💡 Research Summary

The authors present a comprehensive multi‑wavelength study of the pulsar wind nebula (PWN) G0.9+0.1, which resides in the composite supernova remnant (SNR) G0.9+0.1 near the Galactic Center. New NuSTAR observations, together with archival XMM‑Newton and Chandra data, provide a hard X‑ray view of the nebula from 2 to 30 keV. The NuSTAR images reveal a clear synchrotron burn‑off effect: the nebular size shrinks with increasing photon energy, indicating rapid cooling of the highest‑energy electrons. The integrated 2–30 keV spectrum is well described by a single power law with photon index Γ = 2.11 ± 0.07, consistent with previous soft‑X‑ray measurements.

High‑resolution Chandra imaging shows a torus‑jet morphology with a bright core of ~5″ × 8″, a torus of ~30″ radius, and a jet extending ~40″, all encompassed within a ~40″ radius region. XMM‑Newton, with its larger field of view, supplies complementary imaging of the surrounding environment.

The authors assemble a broadband spectral energy distribution (SED) that incorporates radio data (ATCA, VLA, MeerKAT), GeV measurements from Fermi‑LAT, and TeV observations from H.E.S.S., VERITAS, MAGIC, and HAWC. They model the SED using two independent approaches: (1) a one‑zone dynamical model based on Gelfand et al. (2009), which evolves the PWN radius, magnetic field, and particle population with time; and (2) a multi‑zone model following Kim & An (2020), which allows radial variations of magnetic field and particle spectra without explicit time evolution.

Both models converge on a physically consistent picture. The system age is constrained to ~2.2 kyr, the average nebular magnetic field to ~20 µG, and the injected electron spectrum to a single power law with index p ≈ 2.6. Crucially, the maximum electron energy is found to be ≈2 PeV, well above the 1 PeV threshold that defines a PeVatron. This high cutoff is robust because hard X‑ray synchrotron emission is not subject to the Klein‑Nishina suppression that limits TeV inverse‑Compton constraints. The inferred electron cutoff thus strengthens the case for G0.9+0.1 as a leptonic PeVatron candidate.

In addition to the primary target, the NuSTAR data serendipitously captured renewed activity from the very faint X‑ray transient XMMU J174716.1‑281048. Its spectrum is described by a power law with Γ ≈ 1.8, showing significant absorption and variability, indicating the source entered a new outburst phase.

Overall, the paper demonstrates that combining deep hard X‑ray observations with a full multi‑wavelength dataset enables precise constraints on particle acceleration in PWNe. The agreement between the one‑zone and multi‑zone models validates the derived parameters, and the detection of electrons up to ~2 PeV places G0.9+0.1 among the most promising Galactic PeVatron candidates.