Prompt emission of GRB 121217A from gamma-rays to the near-infrared

Abstract

The mechanism that causes the prompt-emission episode of gamma-ray bursts (GRBs) is still widely debated despite there being thousands of prompt detections. The favoured internal shock model relates this emission to synchrotron radiation. However, it does not always explain the spectral indices of the shape of the spectrum, which is often fit with empirical functions, such as the Band function. Multi-wavelength observations are therefore required to help investigate the possible underlying mechanisms that causes the prompt emission. We present GRB 121217A, for which we were able to observe its near-infrared (NIR) emission during a secondary prompt-emission episode with the Gamma-Ray burst Optical Near-infrared Detector (GROND) in combination with the Swift and Fermi satellites, which cover an energy range of 5 orders of magnitude (10 −3 keV to 100 keV). We determine a photometric redshift of z = 3.1 ± 0.1 with a line-of-sight with little or no extinction (A V ∼ 0 mag) utilising the optical/NIR SED. From the afterglow, we determine a bulk Lorentz factor of Γ 0 ∼ 250 and an emission radius of R < 10 18 cm. The prompt-emission broadband spectral energy distribution is well fit with a broken power law with β 1 = −0.3 ± 0.1 and β 2 = 0.6 ± 0.1 that has a break at E = 6.6 ± 0.9 keV, which can be interpreted as the maximum injection frequency. Self-absorption by the electron population below energies of E a < 6 keV suggest a magnetic field strength of B ∼ 10 5 G. However, all the best fit models underpredict the flux observed in the NIR wavelengths, which also only rebrightens by a factor of ∼2 during the second prompt emission episode, in stark contrast to the X-ray emission, which rebrightens by a factor of ∼100. This suggests an afterglow component is dominating the emission. We present GRB 121217A, one of the few GRBs that has multi-wavelength observations of the prompt-emission period and shows that it can be understood with a synchrotron radiation model. However, due to the complexity of the GRB's emission, other mechanisms that result in Band-like spectra cannot be ruled out.

The sharpness of gamma-ray burst prompt emission spectra

“…A possibly similar inference can be made on the related phenomena of prompt optical emission showing a similar temporal profile as the gamma-ray emission (e.g., Elliott et al 2014;Greiner et al 2014) or very early X-ray flares (e.g., Pe'er et al 2006; see also Hu et al 2014 for a recent large Swift sample study): if the prompt emission is not dominated by synchrotron emission, this is likely the case for this longer wavelength emission as well (see, e.g., Starling et al 2012;Peng et al 2014).…”

The Spectral Sharpness Angle of Gamma-ray Bursts

“…A possibly similar inference can be made on the related phenomena of prompt optical emission showing a similar temporal profile as the gamma-ray emission (Elliott et al 2014;Greiner et al 2014) or very early X-ray flares (e.g., Pe'er et al 2006; see also Hu et al 2014 for a recent large Swift sample study): if the prompt emission is not dominated by synchrotron emission, this is likely the case for this longer wavelength emission as well (see, e.g., Starling et al 2012;Peng et al 2014). Yu et al (2015b) demonstrated a new method to quantify the shape of the observed GRB spectra, providing a tool for distinguishing between various standard emission functions.…”

The optical/NIR afterglow of GRB 111209A: Complex yet not unprecedented

“…Tikhomirova & Stern (2005) have found multiple cases in the data of the Burst And Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory. Further individual examples are GRB 020410 (Nicastro et al 2004, T 90 ≈ 1500 s), the IPN GRB 080407 (Pal'shin et al 2012, with T 90 ≈ 2100 s), GRB 091024 (Gruber et al 2011, with T 90 ≈ 1020 s) which is also associated with an optical flash (Virgili et al 2013), the dark GRB 090417B (Holland et al 2010, T 90 > 2130 s), the "double burst" GRB 110709B (Zhang et al 2012, with a total duration of ≈ 1400 s) and the similar GRB 121217A (Siegel et al 2013;Elliott et al 2014, with a total duration of > 1070 s), the "Swift Birthday Burst" GRB 141121A (Golenetskii et al 2014;Cucchiara et al 2015, with a total duration of ≈ 1410 s), and, somewhat shorter, GRB 070616 , which lasted ≈ 600 s. Since GRB 111209A, three very similar events have been discovered. GRB 121027A features a highly variable X-ray light curve and extremely elevated emission at > 5000 s after the trigger (Serino et al 2012;Evans et al 2012, see Peng et al 2013and Hou et al 2014 for theoretical treatments), which, combined with the redshift z = 1.773, makes it one of the most luminous X-ray afterglows ever detected (L14, Starling et al, in preparation).…”