Re: Would an object traveling near c produce X-rays upon striking a gas?

The short answer is: IT DEPENDS. :-)

There are several mechanisms that can occur when an object impacts a gas at high velocity, and which ones occur largely depends on how much energy is involved and by what mechanisms the object is losing energy to the gas. Remember that "relativistic" particles can be generated ranging in energy from a modest medical accelerator in the million electron-volt (MeV) range all the way up to the city-spanning colliders like Fermilab's trillion electron-volts (TeV) to even higher energies seen in cosmic ray particles that we presume were born in the violent collisions of neutron stars or even black holes.

Since you used the word "object," I'll assume you meant a macroscopic thing rather than a subatomic particle. Lots of energy, but it will get distributed widely because you collide with many gas atoms at once. The primary energy transfer mechanism will be in the huge pressure wave you generate in the gas. You're obviously moving much faster than sound in the gas, so you create a shock front. This shock front will cause the gas to heat up. Any hot gas will radiate with the frequency of radiation depending on the temperature, ranging from the infrared up into ultraviolet. At very high temperatures, the collisions between atoms will knock electrons into higher energy states, and might even strip the electrons off. As the atoms de-excite, or recombine, some of that energy will be given off as x-rays, UV, and visible light. It should be noted that at these temperatures the atoms of your impact object will also be coming apart.

Up to this point, I've assumed you're transmitting up to a several thousand electron-volts (keV) per gas particle. Not hard to do if you've got your bullet going near light speed. At higher energies per particle (MeV range), you start to overcome the electric repulsion between atoms, and even between nuclei, and start having nuclear interactions. These will result in gamma rays being radiated, as well as multiple other particles as the nuclei break apart and/or fuse together. The gamma rays are at high enough energy to start spontaneously converting into electron-positron particle pairs under the right circumstances. At higher energies still (GeV and above) you might even break apart the protons and nuetrons in the nuclei and get the whole zoo of particles that we see in big accelerators and in cosmic rays hitting our atmosphere.

If the fast particle has an electric charge (and there will be plenty of fast charged particles once you start stripping electrons off atoms), there are a host of other radiation mechanisms such as bremstrahlung and Cherenkov radiation. Bremstrahlung just results from the deceleration of a charged particle, and can range from high-energy gamma rays all the way down to radio. Cherenkov occurs when a charged particle finds itself travelling faster than the speed of light in the local medium. (Remember - it's impossible to travel faster than the speed of light in a vacuum, but light travels more slowly in a gas or solid so a fast particle can out-race it then - until it Cherenkov radiates its extra energy.) Cherenkov light tends to be in the blue-to-UV range.

I hope this answers your question. Stay curious.