A theory of photospheric emission from relativistic, collimated outflows

Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle dependent velocity profiles, Gamma = Gamma(r, theta, phi), where Gamma is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Gamma \approx Gamma_0/[(theta/theta_j)^p + 1], were theta_j is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, theta_v. We show that for narrow jets (theta_j Gamma_0 \lesssim few), the obtained low energy photon index is alpha \approx -1 (dN/dE \propto E^alpha), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at theta_v \approx theta_j. This result is surprisingly similar to the average low energy photon index seen in gamma-ray bursts. For wide jets (theta_j Gamma_0 \gtrsim few) observed at theta_v \ll theta_j, a multicolor blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.Comment: 15 pages, 11 figures; Minor changes in latest version, accepted for publication in MNRA

On the role of the magnetic field on jet emission in X-ray binaries

Radio and X-ray fluxes of accreting black holes in their hard state are known to correlate over several orders of magnitude. This correlation however shows a large scatter: black hole candidates with very similar X-ray luminosity, spectral energy distribution and variability, show rather different radio luminosities. This challenges theoretical models that aim at describing both the radio and the X-ray fluxes in terms of radiative emission from a relativistic jet. More generally, it opens important questions on how similar accretion flows can produce substantially different outflows. Here we present a possible explanation for this phenomenon, based on the strong dependency of the jet spectral energy distribution on the magnetic field strength, and on the idea that the strength of the jet magnetic field varies from source to source. Because of the effect of radiative losses, sources with stronger jet magnetic field values would have lower radio emission. We discuss the implications of this scenario, the main one being that the radio flux does not necessarily provide a direct measure of the jet power. We further discuss how a variable jet magnetic field, reaching a critical value, can qualitatively explain the observed spectral transition out of the hard state.Comment: 4 pages, 2 figures. Accepted for publication on ApJ Letter

Quantum lithography by coherent control of classical light pulses

The smallest spot in optical lithography and microscopy is generally limited by diffraction. Quantum lithography, which utilizes interference between groups of N entangled photons, was recently proposed to beat the diffraction limit by a factor N. Here we propose a simple method to obtain N photons interference with classical pulses that excite a narrow multiphoton transition, thus shifting the "quantum weight" from the electromagnetic field to the lithographic material. We show how a practical complete lithographic scheme can be developed and demonstrate the underlying principles experimentally by two-photon interference in atomic Rubidium, to obtain focal spots that beat the diffraction limit by a factor of 2.Comment: 6 pages, 4 figures, Submitted to Opt. Expres