GRB 131014A: A LABORATORY FOR STUDYING THE THERMAL-LIKE AND NON-THERMAL EMISSIONS IN GAMMA-RAY BURSTS, AND THE NEW ${L}_{{\rm{i}}}^{\mathrm{nTh}}-{E}_{\mathrm{peak},{\rm{i}}}^{\mathrm{nTh},\mathrm{rest}}$ RELATION

Abstract

Over the past years, evidence has been accumulated on the existence of a thermal-like component during the prompt phase of Gamma Ray Bursts (GRBs). However, this component, often associated with the GRB jet's photosphere, is usually subdominant compared to a much stronger non-thermal one. The prompt emission of GRB 131014A-detected by the Fermi Gamma-ray Space Telescope (hereafter Fermi)-provides an unique opportunity to trace the history of this thermal-like component. Indeed, the thermal emission in GRB 131014A is much more intense than in other GRBs and a pure thermal episode is observed during the initial 0.16 s. The thermal-like component cools monotonically during the first second while the non-thermal emission kicks off. The intensity of the non-thermal component progressively increases until being energetically dominant at late time similarly to what is typically observed. This is a perfect scenario to disentangle the thermal component from the nonthermal one. The initial decaying and cooling phase of the thermal-like component is followed by a strong re-brightening and a re-heating episode; however, despite a much brighter second emission phase, the temperature of the thermal component does not reach its initial value. This re-brightening episode is followed by a global constant cooling until the end of the burst. We note that a shallower low-energy spectral slope than the typical index value +1, corresponding to a pure Planck function, better matches with the thermal-like spectral shape; a spectral index around +0.6 seems to be in better agreement with the data. The non-thermal component is adequately fitted with a Band function whose low and high energy power law indices are ∼-0.7 and <∼-3, respectively; this is also statistically globally equivalent to a cutoff power law with a ∼-0.7 index. This is in agreement with our previous results. Finally, a strong correlation is observed between the time-resolved energy flux, F nTh i , and the corresponding spectral peak energy, E nTh peak,i , of the non-thermal component with a slope similar to the one reported in our previous articles. Assuming a universal relation between the time-resolved luminosity of the non-thermal component, L nTh i , and its rest frame E nTh peak,i , E rest,nTh peak,i , that we derived from a limited sample of GRBs detected by Fermi, we estimate a redshift of ∼1.55 for GRB 131014A that is a typical value for long GRBs. These observational results are consistent with the models in which the non-thermal emission is produced well above the GRB jet photosphere but they may also be compatible with other scenarios (e.g., dissipative photosphere) that are not discussed in this article.

Cgro/Batse Data Support the New Paradigm for GRB Prompt Emission and the New – Relation

“…The C Th spectral shape is usually compatible with a Planck function -based on the quality of the Fermi data. However, a broader spectral shape more compatible with the GRB jet photospheric models was recently reported by Guiriec et al (2015b) who analyzed the spectra of GRB 131014A and showed that it exhibited an intense C Th component. Indeed, while a pure Planck function is well approximated with a cutoff PL (CPL) with an index a Th = +1, the thermal-like component of GRB 131014A is best described with a CPL with an index a » Th +0.6.…”

“…The recent discovery of a thermal-like component, C Th , together with a non-thermal one, C nTh , in the prompt emission of both long (Guiriec et al 2011a(Guiriec et al , 2015a(Guiriec et al , 2015bAxelsson et al 2012) and short (Guiriec et al 2013) GRBs observed with the Fermi Gamma-ray Space Telescope (hereafter, Fermi), challenges the well established Band function paradigm. The C Th spectral shape is usually compatible with a Planck function -based on the quality of the Fermi data.…”

“…The C nTh spectral shape is adequately described with a Band function but with spectral parameters usually very different from those resulting from fits to a Band function alone. For instance, a nTh is systematically lower than a Band and therefore more compatible with the synchrotron emission scenarios, b nTh is −3 and compatible with an exponential cutoff (i.e., the Band function can be replaced with a CPL with no change in the fit statistics), and E peak nTh is systematically shifted to higher values compared to E peak Band (Guiriec et al 2011a(Guiriec et al , 2013(Guiriec et al , 2015a(Guiriec et al , 2015b. In the case of short GRB 120323A, the α indices drop from positive values-in fits to a Band function alone-down to values low enough to be compatible with a pure fast-cooling synchrotron scenario (i.e., ∼−1.2)-in fits to C nTh + C Th (Guiriec et al 2013).…”

Gamma-Ray Burst Prompt Correlations

“…A further issue arises when one considers that ∝ Γ , with between 2 and 3 in different models [203,208,211]. An explanation could be that direct or Comptonized thermal radiation from the fireball photosphere [208,[210][211][212][213][214][215][216][217][218][219] can affect significantly the GRB prompt emission. This can be a good interpretation of the really energetic spectra presented for many events [220][221][222] and the flat shape in GRB average spectra.…”

“…The statistical or observational effects could not account for the large scatter of the power law index, and it was suggested to be an intrinsic feature, indicating that no relation common for all GRB pulses peak -peak would be expected. However, in the light of Fermi observations that revealed deviations from the Band function ( [181,[231][232][233][234]; see also [235]), it was proposed recently that the GRB spectra should be modeled not with the Band function itself (constituting a nonthermal component), but with additional blackbody (BB, thermal) and power law (PL, nonthermal) components [216,218,219,236]. The nonthermal component was well described within the context of synchrotron radiation from particles in the jet, while the thermal component was interpreted by the emission from the jet photosphere.…”