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Thermal radiation from a neutron star in SN 1987A

The supernova 1987A in the Large Magellanic Cloud has provided a new opportunity to study the evolution of a young neutron star right after its birth. A proto-neutron star first cools down by emitting neutrinos that diffuse out of the interior within a minutes. After the neutron star becomes transparent to neutrinos, the neutron star core with > 10 g c m 3 cools predominantly by Urea neutrino emission. However, the surface layers remain hot because it takes at least 100 years before the cooling waves from the central core reach the surface layers (Nomoto and Tsuruta 1981, 1986, 1987). From the hot surface, thermal X-rays are emitted. The detection limit for Xrays from SN 1987A by the Ginga satellite is 3 xlO erg s _ 1 (Makino 1987; Tanaka 1987). If the thermal Xrays are to be observed by Ginga, the surface temperature should continue to be as high as T8 > 8 xlO (.R/lOkm)-/ K until the ejecta becomes transparent. The exact value of the initial surface temperature depends on various factors during the violent stages of explosion, cooling stages of the proto-neutron star through diffusive neutrinos, and possible re-infalling of the ejected material. Therefore, until the surface layers become thermally relaxed Ts may satisfy the above condition. Figure 1 shows the cooling behavior of neutron stars during the first one year after the explosion. The total luminosity of the surface photon radiation £ph (left) and the surface temperature T8 (right), both to be observed at infinity, are plotted as a function of time, for three nuclear models PS (stiff), FP (intermediate), and BPS (soft). The temperature scale refers to the FP model. From Figure 1 it is clear that the surface radiation falls significantly below the Ginga detection limit within a few to ~ 20 days for all the models considered. Such a decrease is caused by the plasmon neutrino emission from the outer layers of p = 10 10 g cm 3 . The surface layers at the lower densities (p < 10 g cm 3 ) are so thin that the time scale of thermal conduction is shorter than the time scale of the plasmon neutrino cooling. Consequently, the surface cooling quickly follows the plasmon neutrino cooling in the layers just beneath the surface. This mechanism of surface cooling by the plasmon neutrino process clearly wipes out the initial conditions. We note that during these early stages the surface temperature is independent of the complicated thermal behavior in the central core.