A new nearby PWN overlapping the Vela Jr SNR

PSR J0855-4644 is an energetic pulsar (Edot = 1.1x10^36 erg/s, P=65 ms) discovered near the South-East rim of the supernova remnant (SNR) RX J0852.0-4622 (aka Vela Jr) by the Parkes Multibeam Survey. The position of the pulsar is in spatial coincidence with an enhancement in X-rays and TeV gamma-rays, which could represent its pulsar wind nebula (PWN). We have revealed with an XMM-Newton observation the X-ray counterpart of the pulsar together with a surrounding extended emission thus confirming the suggestion of a PWN. The comparison of the absorption column density derived in X-rays from the pulsar with 12CO observations (tracing the dense gas) is used to derive an upper limit to the distance of the pulsar (d< 900 pc) and to discuss a possible association of the pulsar with the Vela Jr SNR. This new distance estimate implies that the pulsar is nearby and could therefore significantly contribute to the observed spectrum of cosmic-ray leptons (e-/e+).

💡 Research Summary

The paper presents a multi‑wavelength investigation of the young, energetic pulsar PSR J0855‑4644, which lies on the south‑east rim of the supernova remnant (SNR) RX J0852.0‑4622, commonly known as Vela Jr. The pulsar, discovered in the Parkes Multibeam Survey, has a spin period of 65 ms, a spin‑down power of Ė ≈ 1.1 × 10³⁶ erg s⁻¹, and a characteristic age of ~140 kyr. Earlier distance estimates based on radio dispersion measure placed it at ~4 kpc, but the complex environment (Gum Nebula, Vela and Vela Jr) makes that value uncertain.

To clarify the nature of the source and its possible association with Vela Jr, the authors obtained a dedicated XMM‑Newton observation (55 ks total, 40 ks MOS and 29 ks PN after flare screening). Images were produced in a soft band (0.5–0.8 keV) and a hard band (1.2–6 keV). The soft band is dominated by thermal emission from the large‑scale Vela SNR, while the hard band reveals a faint rim of Vela Jr and, for the first time, a point‑like X‑ray counterpart of PSR J0855‑4644 surrounded by diffuse non‑thermal emission.

A radial surface‑brightness profile centered on the pulsar was extracted and fitted with a two‑component model: the point‑spread function (PSF) of XMM‑Newton plus an extended Gaussian component. The Gaussian width is 5.7 ± 0.2 arcsec, and the diffuse emission is detected out to ~150 arcsec (≈0.1 pc at 1 kpc). No bow‑shock or cometary tail is seen, suggesting the pulsar is not moving at a very high transverse velocity.

Spectral analysis was performed on a 15 arcsec radius region around the pulsar. The background was derived from closed‑filter observations and normalized in the 10–12 keV band. The low‑energy part of the spectrum is modeled with an absorbed APEC component fixed to the thermal emission of the Vela SNR, while the high‑energy part is described by an absorbed power‑law. The best‑fit parameters are: N_H = (0.62⁺⁰·⁰⁷₋₀·₁₀) × 10²² cm⁻², photon index Γ = 1.30⁺⁰·⁰⁹₋₀·₀⁹, and an unabsorbed 2–10 keV flux of 3.0 × 10⁻¹³ erg cm⁻² s⁻¹.

To constrain the distance, the authors compared the X‑ray absorption column densities with the integrated ¹²CO (J=1‑0) intensity from the Dame et al. (2001) survey. The Vela Molecular Ridge (VMR) cloud C, at a distance of 700 ± 200 pc, dominates the CO emission along the line of sight. If the SNR and the pulsar were behind the VMR, a clear increase in N_H would be expected in regions of high CO intensity. However, the measured N_H values for the pulsar (0.62 × 10²² cm⁻²) and for the SNR rim in the same area (0.87 × 10²² cm⁻²) are not significantly higher than those in CO‑poor regions, and no correlation is observed in a plot of N_H versus CO intensity. This indicates that both the pulsar and the Vela Jr SNR lie in front of the VMR, establishing an upper distance limit of d < 900 pc. This limit is consistent with recent proper‑motion based estimates of the SNR distance (~750 pc) and with earlier suggestions of a ~750 pc distance for the pulsar.

Given the revised distance, the pulsar’s transverse velocity required to have traveled from the SNR centre to its current position in the SNR’s estimated age (~4 kyr) would be ~3000 km s⁻¹, far exceeding typical pulsar kick velocities and inconsistent with the lack of a bow‑shock morphology. Therefore, a physical association between PSR J0855‑4644 and Vela Jr is deemed unlikely.

The key outcomes of the study are: (1) the discovery of the X‑ray counterpart of PSR J0855‑4644 and its surrounding symmetric PWN with a radius of ~150 arcsec; (2) the detection of a faint non‑thermal rim of Vela Jr in the hard X‑ray band; (3) the establishment of an upper distance limit of 900 pc for both the pulsar and the SNR based on CO‑X‑ray absorption comparison; (4) the implication that PSR J0855‑4644 is one of the most energetic pulsars within 1 kpc, capable of contributing significantly to the local cosmic‑ray electron and positron spectrum. These results provide a new, nearby high‑energy particle source that must be accounted for in models of the Galactic cosmic‑ray lepton population.