Is it possible to reverse_proxy a sitemap hosted in S3?

I have setup a rake task to refresh the sitemap of an application, and it's hosted in S3, because we're using heroku. Can I use this to reverse proxy in this case? I tried but I get an error 

```
<Error>
  <Code>InvalidArgument</Code>
  <Message>Unsupported Authorization Type</Message>
  <ArgumentName>Authorization</ArgumentName>
```