Unlocking the radio-gamma spectrum of the pulsar wind nebula around PSR J1124-5916 in SNR G292.0+1.8

TL;DR

This work tackles the challenge of separating magnetospheric and nebular gamma-ray emission in a young pulsar wind nebula embedded in a supernova remnant. Using time-resolved (phase-resolved) Fermi-LAT analysis, the authors isolate the unpulsed GeV component associated with the PWN around PSR J1124-5916 in SNR G292.0+1.8 and characterize its morphology and spectrum, complemented by a broadband SED that combines radio and X-ray data. The nebular emission is modeled with a single electron population featuring two breaks: an intrinsic injection break near tens of GeV and a cooling break near $\sim$25 TeV in a magnetic field of $\sim$15 $\mu$G, yielding a TeV-quiet yet GeV-bright PWN, reminiscent of 3C 58. These results illuminate particle acceleration and cooling in young PWNe and establish G292.0+1.8 as a valuable benchmark for high-energy PWN modelling and evolution.

Abstract

We present the first detection of GeV $γ$-ray emission potentially associated with the pulsar wind nebula (PWN) hosted by the young core-collapse supernova remnant G292.0+1.8, based on a detailed time-resolved analysis of \textit{Fermi}-LAT data. By isolating the unpulsed component from the dominant magnetospheric radiation of PSR~J1124$-$5916, we successfully disentangle a candidate nebular emission in the GeV range, characterise its morphology and extract its spectrum. This identification places G292.0+1.8 among the few systems in which the pulsar and PWN contributions have been spectrally resolved at high energies, offering new insight into their respective emission mechanisms. We characterise the $γ$-ray spectrum of the pulsar and model the broadband spectral energy distribution (SED) of the PWN using radio, X-ray, and GeV data. The emission is well described by a single electron population with two spectral breaks: one intrinsic to the injection spectrum and another produced by synchrotron cooling in a magnetic field of $\sim$15~$μ$G. Notably, the inferred magnetic field and the low TeV flux of the nebula resemble those of 3C~58, suggesting that similar low-field environments can arise in young PWNe. The high-energy portion of the SED is now tightly constrained by our GeV detection and existing TeV upper limits. Compared to our model, earlier predictions tend to underpredict the $γ$-ray flux, while others that succeed in reproducing the GeV component often overpredict the TeV emission. This mismatch underscores the challenges in modelling particle acceleration and radiation processes in young PWNe and establishes G292.0+1.8 as a valuable benchmark for testing and refining such models.

Figures (7)

• Figure 1: Left: Radio image of the SNR G292.0+1.8 from the Rapid ASKAP Continuum Survey at 887.5 MHz, showing the full extent of the remnant. The colour scale (square-root scale, in units of Jy beam$^{-1}$) is adjusted to enhance the visibility of the shell structure; as a result, the central PWN appears saturated. A zoom-in on the PWN is shown in the inset, where the square-root colour scale ranges from 0.015 to 0.4 Jy beam$^{-1}$ and is optimised to highlight the internal PWN morphology. The contours in the inset are drawn at 0.07, 0.13, 0.18, and 0.3 Jy beam$^{-1}$ and are shown for reference to delineate internal structures. The yellow cross marks the position of the PSR J1124$-$5916. The synthesised beam is $25^{\prime\prime} \times 25^{\prime\prime}$, and the rms noise level is 0.5 mJy beam$^{-1}$. Right: Updated radio continuum spectrum of the PWN associated with PSR J1124$-$5916 within SNR G292.0+1.8. Green diamond symbols correspond to the new flux density measurements obtained in this work from radio surveys, while orange symbols represent values compiled from the literature (see Table \ref{['tab:PWNradiofluxes']}). The solid line indicates the best fit to the weighted data, yielding a radio spectral index $\alpha=-0.012\pm0.010$. The grey and light blue shaded bands denote the 1$\sigma$ and 2$\sigma$ statistical uncertainties in the best-fit $\alpha$ value, respectively.
• Figure 2: Phase profile constructed using photons with energies above 50 MeV in a region of $0^{\circ}\!\!.2$ around PSR J1124$-$5916. The profile spans one full rotation, divided into 100 bins. The two dashed and solid vertical lines represent the off-pulse and on-pulse windows, respectively, as defined in the text and used in the phase resolved analysis.
• Figure 3: Fermi-LAT TS map of the region surrounding PSR J1124$-$5916 for energies above 1 GeV in Galactic coordinates during the OFF-pulse phase interval. All sources of the 4FGL-DR3 catalogue were included, except the pulsar source 4FGL J1124.7$-$5915 which was removed from the model. The colour bar indicates the TS value range. The white star marks the position of PSR J1124$-$5916. The black dash-dot circle represents the 1$\sigma$ error radius for the best-fit position obtained in the OFF-pulse phase interval analysis, while the black dashed circle indicates the 95% confidence level upper limit on the source's extension. Light blue contours show radio data from MOST highlighting the extent of the radio PWN, the green circle correspond to the X-ray PWN extension detected by Chandra hughes2003, and the red circle indicate the SNR G292.0+1.8 extension.
• Figure 4: Fermi-LAT $\gamma$-ray spectra of the pulsar PSR J1124$-$5916 (4FGL J1124.7$-$5915) for the pulsed P1 peak (black dots) and pulsed P2 peak (blues squares) phase intervals. The solid black and dashed blue curves represent the best-fit sub-exponentially cut-off power-law models for the P2 and P1 phase intervals, respectively, over the 50 MeV$-$300 GeV energy range. The model shape follows the one used for significantly curved pulsars in the 4FGL-DR3 catalogue. Red uncertainties account for both statistical and systematic contributions, with the latter primarily arising from the Galactic diffuse emission model and IRFs at low energy. The butterfly region denotes the 1$\sigma$ confidence interval of the best-fit spectral model.
• Figure 5: Zoomed-in views of the $>$1 GeV Fermi-LAT total dataset TS maps in a $2^{\circ} \times 2^{\circ}$ region centred on 4FGL J1124.7$-$5915. Left: Residual TS map after including all sources from the 4FGL-DR3 catalogue except the pulsar PSR J1124$-$5916 / 4FGL J1124.7$-$5915, and the new detected source. The map highligh the pulsar emission. Middle: Residual TS map obtained after modeling all 4FGL-DR3 sources, including 4FGL J1124.7$-$5915, with their best-fit parameters derived from the likelihood analysis. The map highlights the excess emission associated with a newly detected source associated with the PWN hosted in SNR G292.0+1.8. The black dash-dotted circle indicates the best-fit position of the new source, with the radius corresponding to the 68% confidence positional uncertainty. The 95% confidence upper limit on the source extension is represented by a larger black dash-dotted circle. The position of PSR J1124$-$5916 / 4FGL J1124.7$-$5915 is marked with a white star. Radio contours are overlaid in cyan, the green circle corresponds to the X-ray PWN extension detected by Chandra hughes2003, and the red circle indicates the SNR G292.0+1.8 extension. Right: Residual TS map for the complete source model, including all 4FGL-DR3 sources, PSR J1124$-$5916 /4FGL J1124.7$-$5915, and the new detected source, demonstrating that the emission is well accounted for.