Spitzer reveals infrared optically thin synchrotron emission from the compact jet of the neutron star X-ray binary 4U 0614+091

Spitzer observations of the neutron star (ultracompact) X-ray binary (XRB) 4U 0614 + 091 with the Infrared Array Camera reveal emission of nonthermal origin in the range 3.5 - 8 mu m. The mid-infrared spectrum is well fit by a power law with spectral index of alpha = -0.57 +/- 0.04 (where the flux density is F-nu proportional to nu(alpha)). Given the ultracompact nature of the binary system, we exclude the possibility that either the companion star or the accretion disk can be the origin of the observed emission. These observations represent the first spectral evidence for a compact jet in a low-luminosity neutron star XRB and furthermore of the presence, already observed in two black hole (BH) XRBs, of a "break" in the synchrotron spectrum of such compact jets. We can derive a firm upper limit on the break frequency of the spectrum of nu(thin) = 3.7 x 10(13) Hz, which is lower than that observed thin in BH XRBs by at least a factor of 10. Assuming a high-energy cooling cutoff at similar to 1 keV, we estimate a total ( integrated up to X-rays) jet power to X-ray bolometric luminosity ratio of similar to 5%, much lower than that inferred in BHs.