H.E.S.S. detection of the PSR J0855-4644 nebula

TL;DR

The paper addresses the challenge of disentangling TeV emission in the Vela Junior region to identify a potential PWN around PSR J0855-4644. It employs a 3D forward-folding spectro-morphological analysis of 130 hours of H.E.S.S. data, complemented by a one-zone leptonic fit to X-ray data from XMM-Newton. It identifies a distinct extended TeV component near the pulsar with a hard spectrum and a significance of 12.2 sigma, and determines a magnetic field lower bound of 1.6 microgauss and an electron index around 1.88. The results place the PSR J0855-4644 PWN among the known TeV PWNe and demonstrate the power of 3D analysis to separate pulsar-driven emission from SNR shells.

Abstract

HESS J0852-463 is a TeV γ-ray source located in the Galactic plane. The region consists of a supernova remnant (SNR, RX J0852.0-4622) with a shell-like morphology, commonly referred to as Vela Junior, and a pulsar denoted PSR J0855-4644. Pulsars are among the most efficient leptonic accelerators in our Galaxy, making this region particularly interesting to study. We utilise the most recent data taken by the High Energy Stereoscopic System (H.E.S.S.), to investigate any γ-ray emission associated with the pulsar in this region, PSR J0855-4644. We applied a full forward folding method on the H.E.S.S. data. Utilising 3D modelling techniques, we evaluated the TeV γ-ray emission towards the various components of this complex system. The distinct energy-dependent morphology observed in our data motivates further investigation of this source. We resolved the emission in the Vela Junior region into various components, several of which correspond to the SNR itself. In particular, we find a new extended component which is coincident with the position of PSR J0855-4644. The spectrum follows a power-law distribution with a best-fit index of ΓE = 1.81 \pm 0.07stat which differs from the properties of the surrounding γ-ray emission of the Vela Junior SNR. A one-zone leptonic joint fit between the X-rays (from XMM-Newton) and γ-rays (from H.E.S.S.) leads to a lower limit on the magnetic field of 1.6μG and a spectral index of α = 1.88 \pm 0.01, in line with expectations of pulsar wind nebulae (PWNe). In this paper, we report the first detection of the PWN of PSR J0855-4644 at TeV energies with the H.E.S.S. experiment at a significance of 12.2σ. This is attributed to the advanced techniques of the 3D analysis. Based on the pulsar's characteristics, its PWN is consistent with the known TeV PWNe population in the Galaxy.

Figures (7)

• Figure 1: TeV $\gamma$-ray flux map (cm$^{-2}$ s$^{-1}$) of the Vela Junior region in three different energy bands. A smoothing radius of 0.1$^{\circ}$ is used. The Vela Junior SNR is indicated by the magenta circle, and PSR J0855$-$4644 is indicated by the green star. The significance contours of the TeV $\gamma$-ray emission at the 5, 10, and 20$\sigma$ levels are shown in white, with increasing line width for increasing significance.
• Figure 2: Significance maps of $\gamma$-ray above 0.7 TeV. Top: Significance map with Vela Junior indicated by the magenta circle and PSR J0855$-$4644 indicated by the green star. Bottom: Residual significance map with the best-fit models overlaid. The six model components are shown in different colours, together with their respective labels. The circles represent the $\sim68$% containment radii of the Gaussians.
• Figure 3: Spectral energy distribution for each H.E.S.S. component. The components from this work ('A' to 'F') are shown in red (dashed), green (dot-dashed), orange (dotted), purple (dashed), blue (solid), and maroon (dot-dashed), respectively. The flux points are derived from the respective models. These spectra are compared to the HESS J0852$-$463 result in solid black from hgps_2018.
• Figure 4: XMM-Newton flux image from the MOS+PN camera in the high-energy band from 2-8 keV. The map is smoothed with a Gaussian width of $4^{\prime\prime}$. The contour at $1.1\times10^{-7}$cm$^{-2}$s$^{-1}$ indicates the inner nebula of the PWN. The grey plus and circle indicate the position and size of component E in $\gamma$-rays, respectively. The diffuse emission to the north of the PWN is the rim of the Vela Junior remnant. The image is adapted from 2013_acero_xrays.
• Figure 5: Broadband spectral energy distribution of component E in a simple one-zone leptonic scenario. The IC emission is shown for the CMB (grey-dashed line) and FIR (dot-dashed line) components. The total IC emission is indicated by the orange line. The NIR cannot be seen on this plot as it is a subdominant component. The red and maroon lines represent the synchrotron emission and synchrotron emission with absorption, respectively. An X-ray absorption column density of $N_H=7.6\times10^{21}$cm$^{-2}$ is assumed for this model. The XMM-Newton X-ray flux points are shown by the dark-purple-filled squares, and the TeV $\gamma$-ray flux points from component E are indicated by the blue-filled triangles. Both sets of flux points were calculated from the one-zone single model.