Abstract
The spectral width and sharpness of unfolded, observed gamma-ray burst (GRB) spectra have been presented as a new tool to infer physical properties about GRB emission via spectral fitting of empirical models. Following the tradition of the “line-of-death”, the spectral width has been used to rule out synchrotron emission in a majority of GRBs. This claim is investigated via reexamination of previously reported width measures. Then, a sample of peak-flux GRB spectra are fit with an idealized, physical synchrotron model. It is found that many spectra can be adequately fit by this model even when the width measures would reject it. Thus, the results advocate for fitting a physical model to be the sole tool for testing that model. Finally, a smoothly-broken power law is fit to these spectra allowing for the spectral curvature to vary during the fitting process in order to understand why the previous width measures poorly predict the spectra. It is found that the failing of previous width measures is due to a combination of inferring physical parameters from unfolded spectra as well as the presence of multiple widths in the data beyond what the Band function can model.
Highlights
Catalogs of gamma-ray burst (GRB) observations contain spectra fit to the canonical Band function (Band et al 1993) which consists of two power laws that are exponentially connected (Greiner et al 1995; Briggs et al 1999; Goldstein et al 2012; Gruber et al 2014; Yu et al 2016)
Many of the spectra could be adequately fit with this model in regardless of the value of the spectral width derived from the Band function
These measures are derived from an empirical models that can poorly represent synchrotron emission
Summary
Catalogs of gamma-ray burst (GRB) observations contain spectra fit to the canonical Band function (Band et al 1993) which consists of two power laws that are exponentially connected (Greiner et al 1995; Briggs et al 1999; Goldstein et al 2012; Gruber et al 2014; Yu et al 2016). Empirical approaches to characterizing GRB spectra have focused on the Band fitted low-energy power law slope, α, from which conclusions are drawn about the physical process producing the observed emission (Crider et al 1997; Preece et al 1998) These studies find that a fraction, ∼1/3, of GRB spectra cannot be explained by the simplest so-called slow-cooled synchrotron emission models and disfavor the more preferred (on account of radiative efficiency) fast-cooled synchrotron models (Sari et al 1998; Beniamini & Piran 2013). Without much effort or the need for computationally expensive physical models, the community can quickly categorize thousands of observations and provide tests for theoretical predictions from which models can be rejected For this reason, many studies have begun adopting the width as a tool to advocate for photospheric emission (Ahlgren et al 2015; Iyyani et al 2015, 2016; Vurm & Beloborodov 2016; Bharali et al 2017). A method for measuring the width of the spectra directly in the data by fitting a sample of GRB peak-flux spectra is employed (Sect. 4)
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