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 approximate to Gamma(0)/[(theta/theta(j))(p) + 1], where 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) less than or similar to few), the obtained low-energy photon index is alpha approximate to -1 (dN/dE alpha 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) approximate to 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) greater than or similar to few) observed at theta(v) << theta(j), a multicolour blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.