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

TL;DR

This study combines NuSTAR hard X-ray observations with archival XMM-Newton, Chandra, and multiwavelength data to model the PWN G0.9+0.1. Using both a dynamical one-zone model and a multi-zone model, the authors consistently derive an injection spectrum with $p\sim2.6$ and a maximum particle energy of $E_{ ext{max}}\sim2$ PeV, implying a leptonic PeVatron. The estimated PWN magnetic field is $B_{ ext{PWN}}\sim20\ \mu$G and the system age is $t_{ ext{age}}\sim2.2$ kyr, with the PWN not yet interacting with the SNR reverse shock. The joint XMM-Newton+NuSTAR analysis reveals a hard X-ray spectrum ($\Gamma\approx2.1$) extending up to tens of keV, and the results place PWN G0.9+0.1 among the few confirmed or strong PeVatron candidates, providing crucial insight into particle acceleration in extreme Galactic environments.

Abstract

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.

Figures (10)

• Figure 1: Exposure-corrected Chandra ACIS-S1 image of PWN G0.9+0.1 in the energy range 2--7 keV. We adopted an image bin size of one, corresponding to a pixel size of $0.492"$, and smoothed using a Gaussian kernel with radius $r=3$ and width $\sigma = 1.5$ pixels ($r=1.48"$, $\sigma=0.74"$). Solid arrows point to the torus (arc) and jet substructures of the PWN. A dashed arrow highlights the offset between the position of CXOU J1747422.8--280915, marked with a cross, and the bright PWN core. CXOU J1747422.8--280915 is most likely the X-ray counterpart to the pulsar.
• Figure 2: (a) Combined EPIC-MOS and EPIC-pn image of PWN G0.9+0.1 and its surrounding region for the 2000 observation. (b) Combined XMM EPIC-MOS image of PWN G0.9+0.1 and its surrounding region for the 2003 observation. In both images, the energy range is set to 2--10 keV, and we use a Gaussian smoothing kernel with radius $r=3$ and $\sigma = 1.5$ pixels. We marked with crosses the positions of VFXT XMMU J174716.1--281048 and V* BN Sgr, and with a triangle the position of the point source 4XMM J174717.7--281026.
• Figure 3: Left panel: MeerKAT 1.28 GHz image MeerKAT_2022 of the SNR shell associated with PWN G0.9+0.1. Right panel: RGB image of PWN G0.9+0.1, including observations from MeerKAT at 1.28 GHz MeerKAT_2022, XMM--Newton at 2--10 keV (green, this work), and NuSTAR at 10--30 keV (blue, this work). We also show the 95% and 99% confidence contours levels of XMM (solid) and NuSTAR (dashed). The best-fit position and 95% confidence level upper limit on the source extension as observed by HESS HESS_2005 are shown as the plus marker and dotted circle. The position of the VFXT XMMU J174716.1--281048 is marked with a black cross.
• Figure 4: FPMA (a) and FPMB (b) images of the NuSTAR FoV in the 3--79 keV energy range, highlighting the stray light pattern affecting the observation. (c) Merged (FPMA and FPMB), exposure- and vignetting-corrected NuSTAR image of PWN G0.9+0.1 in the 3--10 keV energy range, smoothed with a Gaussian kernel with $r=3$ and $\sigma = 1.5$ pixels. The solid circle represents the radial extension of PWN G0.9+0.1 estimated with Sherpa. The cross marks the position of VFXT XMMU J174716.1--28104. (d) Same as for (c), but in the 10--30 keV range. (e) Normalized (to one) radial profiles of PWN G0.9+0.1 in the 3--10 keV (orange) and 10--30 keV (green) energy ranges, compared to the PSF of NuSTAR (blue solid curve). The shaded areas represent the $1\sigma$ uncertainty bands.
• Figure 5: Joint fit of both XMM--Newton observations together with the new NuSTAR data of PWN G0.9+0.1 assuming an absorbed power law model in the energy range 2--30 keV (upper panel), along with the model residuals (bottom panel) expressed as (data - model)/error. The joint fit parameters are reported in the last row of Tab. \ref{['tab:PL_fit']}.