Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow

Abstract

Gamma-ray bursts (GRBs), which are bright flashes of gamma rays from extragalactic sources followed by fading afterglow emission, are associated with stellar core collapse events. We report the detection of very-high-energy (VHE) gamma rays from the afterglow of GRB 190829A, between 4 and 56 hours after the trigger, using the High Energy Stereoscopic System (H.E.S.S.). The low luminosity and redshift of GRB 190829A reduce both internal and external absorption, allowing determination of its intrinsic energy spectrum. Between energies of 0.18 and 3.3 tera–electron volts, this spectrum is described by a power law with photon index of 2.07 ± 0.09, similar to the x-ray spectrum. The x-ray and VHE gamma-ray light curves also show similar decay profiles. These similar characteristics in the x-ray and gamma-ray bands challenge GRB afterglow emission scenarios.

Science 372, 1081-1085 (2021)
ArXiv pre-print

Auxiliary informations

Main paper

Figure 1: H.E.S.S. VHE spectra of GRB 190829A on the first and second nights.
Left plot, i.e. first night of H.E.S.S. observations
Data points: ECSV format (incl. additional information; can be used as input for astropy/gammapy/etc.)
E [TeV]     dNdE [1/(cm^2 s TeV)]     dNdE_err_pos [1/(cm^2 s TeV)]     dNdE_err_neg [1/(cm^2 s TeV)]     residuals     res_err_pos     res_err_neg    
0.216 9.074e-11 2.731e-11 2.389e-11 -0.435 0.149 0.170
0.320 5.165e-11 9.990e-12 8.954e-12 -0.126 0.152 0.169
0.473 3.151e-11 4.371e-12 4.016e-12 0.450 0.185 0.201
0.699 8.998e-12 1.690e-12 1.500e-12 0.126 0.188 0.211
1.033 3.563e-12 7.707e-13 6.801e-13 0.212 0.231 0.262
1.528 8.216e-13 2.938e-13 2.430e-13 -0.241 0.225 0.272
2.258 2.485e-13 1.385e-13 1.065e-13 -0.375 0.268 0.348
3.338 1.248e-13 7.421e-14 5.324e-14 -0.148 0.364 0.507
4.935 4.298e-14 (U.L.) – – – – –

Right plot, i.e. second night of H.E.S.S. observations
Data points: ECSV format (incl. additional information; can be used as input for astropy/gammapy/etc.)
E [TeV]     dNdE [1/(cm^2 s TeV)]     dNdE_err_pos [1/(cm^2 s TeV)]     dNdE_err_neg [1/(cm^2 s TeV)]     residuals     res_err_pos     res_err_neg    
0.236 1.692e-11 (U.L.) – – – – –
0.415 4.122e-12 2.422e-12 2.122e-12 -0.202 0.411 0.469
0.731 1.697e-12 6.879e-13 5.904e-13 0.255 0.437 0.509
1.286 3.675e-13 2.101e-13 1.706e-13 0.038 0.482 0.594
2.262 1.979e-13 (U.L.) – – – – –
Figure 2: Extragalactic background light absorption for three GRBs detected at VHE
Package containing the tabulated EBL values and a Jupyter notebook to create the figure can be found the following archive; Fig2_EBL.zip
Figure 3: Logarithmic X-ray and gamma-ray Multi-wavelength energy flux light-curves of the GRB 190829A afterglow.
The details of the H.E.S.S. light-curve are given in the paper in Table S2, those of Swift-XRT in Table S4.

The following package contains the data files and a python script to reproduce the figure: Fig3_lightcurve.tar.gz
Interval     Time after T0 [s]     Energy flux [10^-11 erg / (cm s)]     stat. uncert. [10^-11 erg / (cm s)]    sys. uncert. [10^-11 erg / (cm s)]   
Night 1, cluster 1   1.56e4 – 1.92e4 4.06 0.65 0.90
Night 1, cluster 2   1.92e4 – 2.44e4 3.57 0.42 0.77
Night 1, cluster 3   2.44e4 – 2.85e4 2.66 0.39 0.60
Night 2 9.79e4 – 1.15e5 0.64 0.14 0.14
Night 3 1.84e5 – 2.01e5 0.27 0.13 0.07
The code used for the MCMC fitting of the datasets used in the paper is available in the following archive: Modelling_GRB190829A.tar.gz

For a detailed description, usage instructions, etc. we point towards the README.md file contained in the package. It also contains all necessary experimental data (including the butterflies in Figure 4) and a Juyter notebook reproducing the figures of the paper. The output of the MCMC chains used for the figures in the paper can be found here.

The MCMC code is also available from Github.

Supplementary Material

Figure S1: Significance skymap of the H.E.S.S. observations
The following package contains the FITs file of the maps and a python script to reproduce the figure: FigS1_skymaps.tar.gz

Swift-XRT results

This archive, XRT_data_files.tar.gz, contains the files to recreate the time-sliced XRT analyses in this paper. The results are provided in Tables S3 and S4, and plotted in Figs. 3, 4, S4, and S5. The files “timeslices_tableS3.txt” and “timeslices_tableS4.txt” correspond to the time intervals described in Tables S3 and S4, respectively, and were uploaded to https://www.swift.ac.uk/xrt_spectra/addspec.php?targ=00922968 to build the time-sliced spectra. Note that the BAT trigger time (which is what the XRT uses) is 51.5 seconds after the GBM one, and the times listed in Tables S3 and S4 are calculated from the GBM trigger time (T0 in the paper).

Collaboration Acknowledgement

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the German Research Foundation (DFG), the Helmholtz Association, the Alexander von Humboldt Foundation, the French Ministry of Higher Education, Research and Innovation, the Centre National de la Recherche Scientifique (CNRS/IN2P3 and CNRS/INSU), the Commissariat à l’Énergie atomique et aux Énergies alternatives (CEA), the U.K. Science and Technology Facilities Council (STFC), the Knut and Alice Wallenberg Foundation, the National Science Centre, Poland grant no. 2016/22/M/ST9/00382, the South African Department of Science and Technology and National Research Foundation, the University of Namibia, the National Commission on Research, Science & Technology of Namibia (NCRST), the Austrian Federal Ministry of Education, Science and Research and the Austrian Science Fund (FWF), the Australian Research Council (ARC), the Japan Society for the Promotion of Science and by the University of Amsterdam. We appreciate the excellent work of the technical support staff in Berlin, Zeuthen, Heidelberg, Palaiseau, Paris, Saclay, Tübingen and in Namibia in the construction and operation of the equipment. This work benefited from services provided by the H.E.S.S. Virtual Organisation, supported by the national resource providers of the EGI Federation.

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