Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO Programmes 63.H-0416 and 65.H-0643).

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Given that most radio pulsars are also isolated, a better name might be "thermally emitting'' neutron stars.

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... East

In the meantime, the proper motion has been measured accurately using HST (Walter 2001).

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... images

Photometry of star X using these images was presented by Neuhäuser (2001). He finds $B=25.1\pm0.4$ , consistent with (though substantially less accurate than) our result.

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... expected

We used $A_V=5.6(N_{\rm H}/10^{22} \,{\rm cm^{-2}})$ , as inferred by Predehl & Schmitt (1995) for distant, highly absorbed objects. In these, the line of sight samples all phases of the interstellar medium; hence, the relation may not be valid locally.

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... respectively

If they are giants, with $M_V\simeq0.8$ (Cox 2000), they would be at $\sim$ $20 \,{\rm kpc}$ and might be associated with the Sagittarius dwarf galaxy (Ibata et al. 1995), which is at $\sim$ $25 \,{\rm kpc}$ and $\sim$ $7\hbox{$^\circ$ }$ away from RX J1856.5-3754. Indeed, stars L and C have radial velocities of $+90\pm70 \,{\rm km} \,{\rm s^{-1}}$ , consistent with being associated. Star F, with a radial velocity of $-130\pm70 \,{\rm km} \,{\rm s^{-1}}$ , almost certainly is not associated.

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... processes

Assuming that the disk is optically thick and radiates as a black body. It is questionable whether these assumptions are justified for the very low accretion rate considered here, but given the speculative nature of the presence of an accretion disk, we felt a more detailed analysis was not warranted.

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