RX J185635-3754: What Is It?

It appears to be a neutron star. Fx/Fv, the ratio of the X-ray to visual flux, is about 75,000. At this V magnitude, a hot white dwarf (e.g., HZ 43) with the same color would have Fx/Fv about 1 and would be at a distance of 50 kpc.

observed spectral energy distributions.

The spectral energy distribution is remarkably close to thermal. The best fit X-ray black body (dotted line) underpredicts the F300W and F606W fluxes by factors of 2.4 and 3.7, respectively. A Greenstein & Hartke [4] model (solid line; dashed line is without interstellar extinction), with a sinusoidal flux distribution in latitude, fits the U and V points, but the F606W point is still high, by a factor of 1.5.

As an isolated, radio-quiet, non-pulsing neutron star, this object affords an opportunity to observe its surface. The thermal character of the radiation, and the lack of a hard X-ray tail, suggest minimal magnetospheric emission. If we can model the surface emission, we can determine the radius. Assuming a mass (all non-accreting neutron stars seem to have masses of 1.4 solar masses), we can then probe the interior equation of state.

The molecular cloud places a 130 pc upper limit to the distance. At this distance,

This is uncomfortably small, and may exclude many currently accepted equations of state [14].

Why is it detectable? Either it is still fairly young (less than a few Myr old), or it is accreting from the interstellar medium. If young, the proper motion should point to a nearby OB association. If any proper motion is detected, if cannot be accreting from the ISM.

4: Greenstein, G. & Hartke, G.J. 1983. ApJ, 271, 283.
14: Prakash, M. & Lattimer, J.M. 1997. in the Proceedings of the 18th Texas Symposium, in press.

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