The emission mechanism of gamma-ray bursts: Identification via optical-IR slope measurements

Abstract

There is no consensus on the emission mechanism of γ-ray bursts (GRBs). A synchrotron model can produce γ-ray spectra with the empirical Band function form (Band et al., 1993), from a piece-wise two-power-law electron energy distribution (2EPLS). This synchrotron model predicts that for the same γ-ray spectrum, optical emission can be very different in fν log slope, and in flux relative to γ-rays, depending on model parameter values. This prediction is consistent with the huge range of optical/γ flux ratios observed. The model only allows a small set of fν log slopes in the optical—thereby allowing a clear path to verification or falsification. Measurements of prompt γ-ray burst emission in the optical thus far give no useful information about the spectral shape within the band, and therefore cannot be used to evaluate such predictions.We describe an experiment that responds to GRB position alerts with a fast-slewing telescope outfitted with three or more simultaneously recording, high-time resolution cameras, to measure the spectral shape of the prompt optical-IR (OIR) emission. Three channels measure two independent spectral slopes in the OIR region, the minimum information required to evaluate the model, assuming a single dominant component. We propose cross-correlation of γ and OIR light curves to verify that a given GRB is single-component dominated, or to model and quantify the contributions from other components. Previous CCD measurements have limited-time resolution due to read noise, limiting cross-correlation analysis. Electron-multiplied CCDS (EMCCDs) can be used to greatly reduce read noise, and allow exposure times of a few hundred ms. Our collaboration has begun a pathfinder experiment, the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO), with a 70 cm aperture telescope that can point anywhere above the local horizon in ≤8s, with three simultaneous optical channels. The NUTTelA-TAO is expected to measure the optical slopes of 3–8 GRB/yr, and should provide a clear verification/refutation of the 2EPLS model after a few single-component dominated, sufficiently bright GRBs are detected during prompt emission. A space-based platform would more easily extend the spectral coverage down to near-IR wavelengths, for greater precision in measuring spectral slopes, and increased chance of measuring the self-absorption frequency, which carries valuable information on physical conditions within the GRB jet. Additional science includes detection of dust evaporation due to the UV flash from the burst, which can be used to study dust around a single star at high redshift, independent of host galaxy dust.