PSR J0357+3205: the tail of the turtle

TL;DR

This study characterizes the X-ray properties of pulsar J0357+3205 and its tail, proposing thermal bremsstrahlung from shocked interstellar medium as the emission mechanism, and introduces a new class of nebulae called 'turtle's tail'.

Contribution

It presents the first detailed analysis of Morla's tail as a thermally-emitting nebula, proposing a novel emission mechanism and class of pulsar wind nebulae.

Findings

Pulsar's X-ray emission includes non-thermal and hot spot thermal components.

The tail's emission is best explained by thermal bremsstrahlung from shocked ISM.

Estimated pulsar distance is 300-900 pc with a velocity around 1900 km/s.

Abstract

Using a new XMM-Newton observation, we have characterized the X-ray properties of the middle-aged radio-quiet gamma-ray pulsar J0357+3205 (named Morla) and its tail. The X-ray emission from the pulsar is consistent with a magnetospheric non-thermal origin plus a thermal emission from a hot spot (or hot spots). The lack of a thermal component from the whole surface makes Morla the coldest neutron star in its age range. We found marginal evidence for a double-peaked modulation of the X-ray emission. The study of the 9'-long tail confirmed the lack of extended emission near the pulsar itself. The tail shows a very asymmetric brightness profile and its spectrum lacks any spatial variation. We found the nebular emission to be inconsistent with a classical bow-shock, ram-pressure dominated pulsar wind nebula. We propose thermal bremsstrahlung as an alternative mechanism for Morla's tail emission. In this scenario, the tail emission comes from the shocked interstellar medium (ISM) material heated up to X-ray temperatures. This can fully explain the peculiar features of the tail, assuming a hot, moderately dense interstellar medium around the pulsar. For a bremsstrahlung-emitting tail, we can estimate the pulsar distance to be between 300 and 900 pc. A pulsar velocity of about 1900 km/s is required - which would make Morla the pulsar with the largest velocity - and high inclination angles (>70 degrees) are preferred. We propose Morla's nebula as the first example of a new "turtle's tail" class of thermally-emitting nebulae associated to high velocity pulsars.