RX J185635-3754: Introduction

Extrapolations of the pulsar birth rate, and estimates of the number of supernovae needed to enrich the galaxy, suggest that there are 10^8 to 10^9 neutron stars in our galaxy [1].

Fewer than 1000 neutron stars are known: most are radio pulsars [13] or are in X-ray binary systems. Most neutron stars are difficult to detect, because they are small (about 10 km radius) and old (typically about half the age of the Galaxy). Old neutron stars should be cool and spun down. Their magnetic fields should have decayed. At T=10,000K, the luminosity of a neutron star is about 10^25 erg/s, or twice the luminosity of the Earth (at 300K).

Nonetheless, theorists have held out hope that a sizeable number of isolated neutron stars might be detectable, either because they are not that old (less than a few million years), or because they are kept hot because of accretion from the interstellar medium [2,6,9,11]. Despite concerted efforts to find them, few candidates have appeared. Among these are:

Studies of old, radio-quiet neutron stars will allow us to study the surfaces of neutron stars, free of the complications from accretion and magnetospheric emissions.

1: Arnett, W.D., Schramm, D.N., & Truran, J.W. 1989. ApJ, 339, L25.
2: Blaes, O. & Madau, P. 1993. ApJ, 403, 690.
5: Haberl, F., Motch, C., Buckley, D.A.H., Zickgraf, F.-J., Pietsch, W. 1997. A&A, in press.
6: Helfand, D.J., Chanan, G.A., & Novick, R. 1980. Nature, 283, 337.
9: Narayan, R. & Ostriker, J.P. 1990. ApJ, 352, 222.
11: Ostriker, J.P., Rees, M.J., & Silk J. 1970. Ap. Lett., 6, 179.
13: Phinney, E.S. & Kulkarni, S.R. 1994. ARAA, 32, 591.
15: Stocke, J.T., Wang, Q.D., Perlman, E.S., Donahue, M.E., & Schachter, J. 1995. AJ, 108, 1199.
16: Walter, F.M., Wolk, S.J., & Neuhaeuser, R. 1996. Nature, 379, 233.

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