In addition to what everyone else has been saying, I thought I'd add a couple of pennies. I recently found myself in this situation, not because I was a new hire, but because someone else was hired and needed to be trained.
I sit in an odd position where everything I do generates more work for other people. So a certain amount of work needs to be done before I can start, and once I finish someone else has to do more work. Part of this is just because we're too siloed, and even though I could do much of this work I don't have access to some of the needed resources.
So try to keep in mind who you could unintentionally be inconveniencing by just trying to do your job. By all means, try to stay productive, but don't shoot yourself in the foot if you could be pushing someone who is overworked to do even more work to keep you productive.
If you can't find other work, look for clues that suggest some upcoming initiatives that you might be able to develop useful knowledge on. Also, think back to your interview. Were there any questions you felt you could have answered better? Take advantage of the extra time and learn what you can about those topics. Chances are, that knowledge will be highly relevant in the near future.
Also, take advantage of the "down time" to try to build relationships with the rest of the team if you can without interfering with their work. This will stand you in good stead, as your ability to build consensus will probably help you more than any technical skill you bring to the table. For example, if a teammate is struggling with his workload and owes assets to several different people, who do you think he'll work hardest to get the assets to? Probably not the new guy who has never even said "hello."