Non-absorption dips in spectra of γ-ray bursts

Abstract

The spectra of more than 20 γ-ray bursts show dips in the 30–70 keV range1–3, that have been interpreted as cyclotron absorption lines1,2. The presence of these absorption lines, is one of the arguments in support of the belief that γ-ray bursts originate from neutron stars with very strong (1012–1013 G) magnetic fields1. This interpretation, however, poses serious problems4–6; for example, if the continuum is produced by optically-thin thermal cyclotron radiation it implies a magnetic field increasing with the distance from the surface of a neutron star5. Here we propose a different interpretation of the dips in the spectra of γ-ray bursts. We show that these dips could result from a superposition of two spectra: an optically thin thermal synchrotron spectrum with an (observed) turnover at about 55–110 keV and a softer one that is best fitted with a black body. We find black-body temperatures in the range of 7–12 keV. Due to poor statistics and the uncertain quality of the data7 our results should be considered only as tentative, our main objective is to point out that using a two-component interpretation for the dips in γ-ray bursts spectra could confirm the synchrotron mechanism.