The material expelled by core-collapse supernova ( SN ) explosions absorbs X-rays from the central regions .
We use SN models based on three-dimensional neutrino-driven explosions to estimate optical depths to the center of the explosion , compare different progenitor models , and investigate the effects of explosion asymmetries .
The optical depths below 2 keV for progenitors with a remaining hydrogen envelope are expected to be high during the first century after the explosion due to photoabsorption .
A typical optical depth is 100 t _ { 4 } ^ { -2 } E ^ { -2 } , where t _ { 4 } is the time since the explosion in units of 10 000 days ( { \sim } 27 years ) and E the energy in units of keV .
Compton scattering dominates above 50 keV , but the scattering depth is lower and reaches unity already at { \sim } 1000 days at 1 MeV .
The optical depths are approximately an order of magnitude lower for hydrogen-stripped progenitors .
The metallicity of the SN ejecta is much higher than in the interstellar medium , which enhances photoabsorption and makes absorption edges stronger .
These results are applicable to young SN remnants in general , but we explore the effects on observations of SN 1987A and the compact object in Cas A in detail .
For SN 1987A , the absorption is high and the X-ray upper limits of { \sim } 100 L _ { \sun } on a compact object are approximately an order of magnitude less constraining than previous estimates using other absorption models .
The details are presented in an accompanying paper \citep alp18 .
For the central compact object in Cas A , we find no significant effects of our more detailed absorption model on the inferred surface temperature .