SED fitting

The SED fitting is performed using the X-ray Spectral-fitting program XSPEC v12.7.1 (Arnaud 1996). This tool is used in the individual SED fitting for the observations X-ray and is specially useful in the broad-band SED analysis as it can be extended to be use with any instrument at dif-ferent wavelengths and with user defined models. In the standard afterglow model the afterglow is described mainly by a synchrotron spectrum composed by four power-law segments join at three smooth breaks (Granot & Sari 2002). The SED analysis follow two steps:

First an analysis of the optical/NIR and X-ray data is performed in order to derived the spectral slopeβ and the dust and gas attenuation effects along the line of sight due to both the local envi-ronment and the host galaxy. The dust reddeningE(B−V)affects primarily the wavelength range from UV to NIR. The extinctionA_Vis related toE(B−V)asA_V=E(B−V)·R_V. In the case of the Galactic reddening A^Gal_v the values are based on previous measurements (Schlafly & Finkbeiner 2011) in several directions and a Milky Way extinction law withR_V =3.08 forA^Gal_v . For the host galaxy templates based on the Small and Large Magellanic Cloud are used (Pei 1992) and the val-ues for extinction A^host_v are derived from the observations. The gas absorption effects depends on the column gas along the line of sight of the source. The absorption factor is quantify based on the hydrogen column density and the photo-electric cross-section σ(E) (Balucinska-Church & Mc-Cammon 1992). The gas absorption effects due to the Galactic environmentN_H^Galare fixed while the effect due to the host galaxyN_H^hostare derived in the fit. Finally in terms of the input data, the time slice from the XRT repository are generally choose to overlap the optical/NIR measurements but due to statistical requirement on the counts per channel, the time interval expands over more than 1 decade usually. The time slices for the XRT SEDs are therefore renormalise in order to have the flux corresponding to the measured X-ray flux of the afterglow at the mid-time of the analysed epoch. The fit is generally performed separately for XRT and GROND data and then, a combined fit is performed (when possible) to obtain better constrains on the slope, on the gas and dust effects and, if existent, a measurement of the break between optical and X-rays.

The second step after the derivation of A^host_v , N_H^host andβ is the incorporation of submm and radio data to perform a broadband fit and measure all the three break frequencies. The only

con-3.3 Data handling and analysis straints introduced here are the A^host_v and N_H^hostderived in the previous step. The slope β of the GROND and XRT bands is not fixed but allowed to vary only within a 3σ uncertainty interval.

The smoothness of each break depends on the temporal slopes in the optical/NIR and the X-ray (Granot & Sari 2002). Here all the available multi-wavelength epochs are included and fitted si-multaneously. The simultaneous fit assures a unique spectral slopeβ, dust and gas effectA^host_v and N_H^hostdue to the host environment and a smooth transition between different spectral regimes. The break frequencies are left free to vary in all the cases. Although the data are expected to be de-scribed by a SED with three breaks, it is possible that fewer breaks are needed if the evolution of the afterglow is in a phase were one or more of the breaks are outside of the observational range used. Therefore the different fit profiles described in Eq. 3.1 are tested ( Eq. 3.1 ).

3.3 Data handling and analysis

Chapter 4

Microphysics and dynamics of the Gamma-Ray Burst 121024A ¹

The aim of the study is to constrain the physics of gamma-ray bursts (GRBs) by analysing of the multi-wavelength afterglow data set of GRB 121024A, covering the full range from radio to X-rays.

Using multi-epoch broad-band observations of the GRB 121024A afterglow, we measure the three characteristic break frequencies of the synchrotron spectrum. We use 6 epochs of combined XRT and GROND data to constrain the temporal slopes, the dust extinction, the X-ray absorption and the spectral slope with high accuracy. Two further epochs of combined data from XRT, GROND, APEX, CARMA and EVLA are used to set constraints on the break frequencies and therefore on the micro-physical and dynamical parameters. The XRT and GROND light curves show a simul-taneous and achromatic break at around 49 ks. As a result, the crossing of the synchrotron cooling break is not suitable as an explanation for the break in the light curve. Two plausible scenarios are analysed. The jet break model has been suggested by previous analysis of the observed linear and circular polarisation, although it requires a hard electron spectrum, a very low cooling break frequency, a non-spreading jet and an extreme prompt emission efficiency. The energy injection model avoids these issues but introduces otherwise problematic values for the microphysics and environment density. Broad-band spectral analysis on a larger sample of GRBs will contribute to previous studies with the aim of a better understanding of the wide range in the microphysical and environmental parameters within GRB shock fronts that have been observed so far, and thus will provide more grounds to favour certain model interpretations.

4.1 Observations and data reduction