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 observation revealed the pulsar's X-ray counterpart and a slightly asymmetric PWN suggesting possible jet structures. PSR J0855-4644 is a pulsar with one of the highest \dot{E}/d^{2} from which no GeV \gamma-ray pulsations have been detected. Methods: With a dedicated Chandra observation, we perform detailed spatial modelling to constrain the geometry of the PWN, in particular the pulsar's line of sight \zeta_{\rm PSR}. We also perform geometric radio and \gamma-ray light curve modelling to further constrain \zeta_{\rm PSR} and the magnetic obliquity \alpha. Results: The 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 \sim 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 (\Gamma=1.1 for a power-law model). Assuming the origin of the extended emission is from a double torus yields \zeta_{\rm PSR}=32.5\dg \pm 4.3\dg. Independent constraints from geometric light curve modelling yield \alpha\lesssim55\dg and \zeta\lesssim55\dg, and 10\dg\lesssim|\zeta-\alpha|\lesssim30\dg. A \chi^2 fit to the radio light curve yields a best fit at (\alpha,\zeta_{\rm PSR}) = (22\dg, 8\dg), with an alternative fit at (\alpha,\zeta_{\rm PSR}) = (9\dg, 25\dg) within 3\sigma. Such a geometry would explain, in the standard caustic pulsar model picture, the radio-loud and \gamma-ray-quiet behaviour of this high \dot{E}/d^{2} pulsar.