Constraining the geometry of PSR J0855−4644: A nearby pulsar wind nebula with double torus/jet morphology

Abstract

Aims. PSR J0855−4644 is a fast-spinning, energetic pulsar discovered at radio wavelengths near the south-eastern rim of the supernova remnant RX J0852.0−4622. A follow-up XMM-Newton observation revealed the pulsar's X-ray counterpart and a slightly asymmetric pulsar wind nebula suggesting possible jet structures. Lying at a distance d ≤ 900 pc, PSR J0855−4644 is a pulsar with one of the highestĖ/d 2 from which no GeV γ-ray pulsations have been detected. With a dedicated Chandra observation we aim to further resolve the possible jet structures of the nebula and study the pulsar's geometry in order to understand the lack of γ-ray pulsations. Methods. We perform detailed spatial modelling to constrain the geometry of the pulsar wind nebula and in particular the pulsar's line of sight (observer angle) ζ PSR defined as the angle between the direction of the observer and the pulsar spin axis. We also perform geometric radio and γ-ray light curve modelling using a hollow-cone radio beam model together with two-pole caustic and outer gap models to further constrain ζ PSR and the magnetic obliquity α defined as the angle between the magnetic and spin axes of the pulsar. Results. The Chandra observation reveals that the compact XMM source, thought to be the X-ray pulsar, can be further resolved into a point source surrounded by an elongated axisymmetric nebula with a longitudinal extent of 10 . The pulsar flux represents only ∼ 1% of the XMM compact source and its spectrum is well described by a blackbody of temperature kT = 0.2 keV while the surrounding nebula has a much harder spectrum (Γ = 1.1 for a power-law model). Assuming the origin of the extended emission is from a double torus yields ζ PSR = 32.5 • ± 4.3 • . The detection of thermal X-rays from the pulsar may point to a low value of |ζ − α| if this emission originates from a heated polar cap. Independent constraints from geometric light curve modelling yield α 55 • and ζ 55 • , and 10 • |ζ − α| 30 • . A χ 2 fit to the radio light curve yields a best fit at (α, ζ PSR ) = (22 • , 8 • ), with an alternative fit at (α, ζ PSR ) = (9 • , 25 • ) within 3σ. The lack of non-thermal X-ray emission from the pulsar further supports low values for α and ζ under the assumption that X-rays and γ-rays are generated in the same region of the pulsar magnetosphere. Such a geometry would explain, in the standard caustic pulsar model picture, the radio-loud and γ-ray-quiet behaviour of this highĖ/d 2 pulsar.

Is PSR J0855$-$4644 responsible for the 1.4 TeV electron spectral bump hinted by DAMPE?

“…The contributed flux from PSR J0855−4644 to the observed TeV e ± s has been proposed for years (Acero et al 2013). The PWN lies projectionally near the south-eastern rim of the SNR Vela Junior (Maitra, Acero, & Venter 2017). As seen in radio, G266.97−1.00 is a bow shock PWN (Maitra et al 2018) with a faint trailing tail.…”

“…However, based on the measurements of the intervening hydrogen column density NH, Acero et al (2013) set an upper limit 900 pc to J0855−4644 considering the NH value towards the pulsar is less than that towards the rim of the SNR Vela Junior. In this paper, we assume PSR J0855−4644 is at a distance of 900 pc following Maitra, Acero, & Venter (2017). Due to its remarkably high spin-down luminosity at present (two orders of magnitude higher than those of other pulsars), PSR J0855−4644 is the most promising contributor (quantitative comparison can be found in §4).…”

“…Since the faint tail has blent into the background noise, the tail might be much longer than seen. Chandra X-ray observation reveals a compact part with double torus/jet morphology (∼ 10 ′′ , or ∼ 0.04 pc at a distance of 900 pc) in the very vicinity of the pulsar (Maitra, Acero, & Venter 2017), while XMM-Newton X-ray observation find a much larger diffuse outer nebula of ∼ 1 ′ (Acero et al 2013). The origin of jet-like features in bow shock PWNe have been detailedly studied in recent years; Olmi & Bucciantini (2019) show that the jets can be explained as the major tunnel of the escaping high energy electrons.…”

Discovery of a radio nebula around PSR J0855−4644

“…The morphology of the emission, with a bright partial ring-like structure in the south-east and two faint tail-like features in the northwest direction is reminiscent of a bow shock nebula. The direction of the westernmost tail is somewhat aligned along the spin axis of the pulsar derived in Maitra et al (2017) (offset by ∼ 10 • ). This is in further support of its bow shock origin.…”

“…Additionally, comparison of column densities provided an upper limit of 900 pc for the distance to the source. A dedicated Chandra observation revealed a further compact Xray nebula with an axisymmetric morphology resembling a double torus PWN, analogous to the Vela PWN which has an Ė similar to PSR J0855-4644 (Maitra et al 2017). However, this nearby energetic PWN has not been reported to be followed up in the radio wavelengths so far.…”