Neutron stars and black holes that are part of a binary system sometimes emit strongly in the X-ray part of the spectrum. One of the problems is that the accretion disc (a disc of material destined to collide with the star or black hole) and the companion also emit radiation in the X-ray region. This means that any direct measurement must somehow exclude the contribution from these other sources. For black holes, this problem can be overcome by observing the radiation associated with jets of material being ejected from the star. This radiation is typically seen in the radio part of the spectrum, but it can (depending on the system) continue right into the visible. The companion and accretion disc do not emit much radiation in the radio spectrum and it has been observed that the intensity of the radio frequency radiation associated with the jet increases in proportion to the X-ray emission intensity. This gives astronomers a window into the world of X-ray binary black holes, provided that the theoretical models for the emissions of the jets is correct.
Now for the first time such jets have been observed in X-ray binary neutron stars in the infrared portion of the spectrum, which apart from showing that the jets are not as special as presumed also provides a great test for the jet models. These observations were made possible by using the incredibly sensitive infrared camera fitted to the Spitzer telescope, which is giving astronomers a fresh data set with which to test their understanding of various astrophysical phenomena. An important factor in the jet models is the amount of light emitted by the jets in different parts of the spectrum. At low frequencies (e.g., in the radio portion of the spectrum) the emission power increases as the frequency increases; in fact it increases as the square of the frequency. However, there comes a point where this cannot be sustained and the intensity of light must decreased with increasing frequency. Unfortunately, the frequency at which this decrease begins has always been inferred from radio and far infrared measurements and never directly observed because it seems to occurs in a very noisy part of the spectrum (e.g., visible light frequencies).
The fact that it exists is very good news since it means that our understanding of the basic driving principles is in good shape. The bad news is that this direct measurement is in very poor agreement with the inferred measurements gathered from X-ray binary black holes, which means that we don't yet really understand exactly what is going on in these jets. I think that is probably to be expected since the discovery that X-ray binary neutron stars possess such jets came as a bit of a surprise.