We present spectroscopy and H \alpha imaging of a faint nebula surrounding the X-ray bright , nearby neutron star RX~J1856.5 $ - $ 3754 .
The nebula shows no strong lines other than the Hydrogen Balmer lines and has a cometary-like morphology , with the apex being approximately 1″ ahead of the neutron star , and the tail extending up to at least 25″ behind it .
We find that the current observations can be satisfactorily accounted for by two different models .
In the first , the nebula is similar to “ Balmer-dominated ” cometary nebulae seen around several radio pulsars , and is due to a bow shock in the ambient gas arising from the supersonic motion of a neutron star with a relativistic wind .
In this case , the emission arises from shocked ambient gas ; we find that the observations require an ambient neutral Hydrogen number density n _ { H ^ { 0 } } \simeq 0.8 { cm ^ { -3 } } and a rotational energy loss \dot { E } \simeq 6 \times 10 ^ { 31 } { erg } { s ^ { -1 } } .
In the second model , the nebula is an ionisation nebula , but of a type not observed before ( though expected to exist ) , in which the ionisation and heating are very rapid compared to recombination and cooling .
Because of the hard ionising photons , the plasma is heated up to \sim 70000 { K } and the emission is dominated by collisional excitation .
The cometary morphology arises arises naturally as a consequence of the lack of emission from the plasma near and behind the neutron star ( which is ionised completely ) and of thermal expansion .
We confirm this using a detailed hydrodynamical simulation .
We find that to reproduce the observations for this case , the neutral Hydrogen number density should be n _ { H ^ { 0 } } \simeq 3 { cm ^ { -3 } } and the extreme ultraviolet flux of the neutron star should be slightly in excess , by a factor \sim 1.7 , over what is expected from a black-body fit to the optical and X-ray fluxes of the source .
For this case , the rotational energy loss is less than 2 \times 10 ^ { 32 } { erg } { s ^ { -1 } } .
Independent of the model , we find that RX~J1856.5 $ - $ 3754 is not kept hot by accretion .
If it is young and cooling , the lack of pulsations at X-ray wavelengths is puzzling .
Using phenomenological arguments , we suggest that RX~J1856.5 $ - $ 3754 may have a relatively weak , few 10 ^ { 11 } { G } , magnetic field .
If so , it would be ironic that the two brightest nearby neutron stars , RX~J1856.5 $ - $ 3754 and RX~J0720.4 $ - $ 3125 , may well represent the extreme ends of the neutron star magnetic field distribution , one a weak field neutron star and another a magnetar .