Extended X-ray emission in the vicinity of the microquasar LS 5039: pulsar wind nebula?

TL;DR

This study reports the detection of extended X-ray emission around LS 5039, suggesting it may be a pulsar wind nebula, with analysis favoring a synchrotron origin over dust scattering.

Contribution

First high-significance detection of extended X-ray emission around LS 5039, analyzing its spectrum and morphology to explore its origin as a pulsar wind nebula.

Findings

Extended emission detected up to 1' from LS 5039

Spectrum of extended emission is softer than the point source

Extended emission possibly a pulsar wind nebula

Abstract

LS 5039 is a high-mass binary with a period of 4 days, containing a compact object and an O star, one of the few high-mass binaries detected in gamma-rays. Our Chandra ACIS observation of LS 5039 provided a high-significance (~10sigma) detection of extended emission clearly visible for up to 1' from the point source. The spectrum of this emission can be described by an absorbed power-law model with photon index Gamma=1.9pm0.3, somewhat softer than the point source spectrum Gamma=1.44pm0.07, with the same absorption, N_H=(6.4pm0.6)e21 /cm2. The observed 0.5-8 keV flux of the extended emission is 8.8e-14 erg/s/cm2, or 5% of the point source flux; the latter is a factor of ~2 lower than the lowest flux detected so far. Fainter extended emission with comparable flux and a softer (Gamma~3) spectrum is detected at even greater radii (up to 2'). Two possible interpretations of the extended emission are a dust scattering halo and a synchrotron nebula powered by energetic particles escaping the binary. We discuss both of these scenarios and favor the nebula interpretation, although some dust contribution is possible. We have also found transient sources located within a narrow stripe south of LS 5039. We discuss the likelihood of these sources to be related to LS 5039.