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tokens: We thus expect an homogeneous envelope to become transparent after a timewhich is frustratingly well above the duration of the Eddington luminosity phase. Here we assume κ ≈ 0.1 cm 2 g −1 , which is a reasonable value for ordinary supernova material. At lower X-ray energies ( x ≲ 3 keV), photoionization may delay the time at which the envelope becomes transparent by an additional factor of . Despite this, the youngest neutron star we have uncovered has an age of about 340 yr (Fesen et al. 2006 ). In a few instances, a surviving binary companion has been detected in postexplosion deep optical imaging of extragalactic supernova (Maund Smartt 2009 ; Folatelli et al. 2014 ). In the case of 1993J, for example, the brightness of the transient dimmed sufficiently after about a decade so that its spectrum showed the features of a massive star superimposed on the supernova (Maund et al. 2004 ). It is perhaps a stinging fact that despite the expected manifestations of neo-neutron stars, one of the main issues in the field has been that most Galactic supernova remnants have no detectable central source. Figure 11 shows the current detections and upper limits of thermal emission in nearby neutron stars with model predictions. Observational selection effects are clearly at play when uncovering young objects yet there is the possibility that a sizable fraction of massive star collapses might produce black holes rather than neutron stars, with the clearest example being W49B (Lopez et al. 2013 ) and the other four examples, from Kaplan et al. ( 2004 , 2006 ), all plotted in Figure 11 . Figure 11. Plot of luminosity as a function of the age of the remnant for nearby neutron stars together with the predictions from neutron star cooling models. The (green) dots are measurements and the (blue) squares are upper limits from detected neutron stars (data taken from Beznogov Yakovlev 2015 ) while the (red) dotted error bars are upper limits on the compact objects, black holes or neutron stars, expected to be presents in five core-collapse supernova remnants. These remnants are, in order of increasing age: G043.3–0.2 (a.k.a. W49B, marked by a diamond) from Lopez et al. ( 2013 ), and G127.1+0.5, G084.2+0.8, G074.0–8.5, and G065.3+5.7 from Kaplan et al. ( 2004 , 2006 ). Shaded areas show model predictions of Page et al. ( 2004 , 2009 ) for the minimal cooling of neutron stars that cover uncertainties on the chemical composition of the envelope and nucleon pairing at high densities. In contradistinction, the dashed-pentadotted curve, Z , exemplifies the effect of fast neutrino emission from the direct Urca process expected to occur in massive neutron stars (Boguta 1981 ; Lattimer et al. 1991 ) resulting in very cold stars (Page Applegate 1992 ). Also plotted are the three different models (A, C, D) shown in Figure 9 . 6.2. Neutron Star Mergers, Short Gamma-Ray Bursts, and LIGO Events The merger of binary neutron stars and the subsequent production of a beamed,