Phd-Student (H/F) - Study of GRBs with Fermi and contribution to the multi-messenger astronomy.

Project description

The student will work within the EMA (Experiences and Modeling in Astroparticles) team at the Laboratoire Univers et Particules de Montpellier (CNRS/IN2P3 and Montpellier University) under the supervision of Frédéric PIRON and within the GRB Science Group of the Fermi/LAT international collaboration.

Analysis and interpretation of Fermi Gamma-Ray Bursts, implications for SVOM and CTA, and contribution to the multi-messenger astronomy.

Modelling the multi-wavelength emission of Gamma-Ray Bursts (GRB) is difficult owing to the complexity of its spectral and temporal evolution. In 10 years, the Fermi Gamma-ray Space Telescope has notably increased the number of these very energetic explosive events, with more than 150 GRBs jointly detected by the GBM and LAT instruments from keV to GeV energies. These observations revealed the existence of multiple spectral components whose interpretation remains incomplete in the framework of the widely-accepted scenario of an ultra-relativistic collimated outflow from a newly-formed stellar-mass black hole. The simplest models of electron synchrotron and inverse Compton radiations account with difficulty for the spectra observed in the first instants, whose interpretation calls for a better treatment of the jet (micro-)physical properties. The transition from this co-called prompt emission to the afterglow emission is also poorly understood. Whereas the GeV emission can be naturally explained by the jet's interaction with the surrounding medium at late times, new diagnostics are needed to pinpoint its origin through the entire duration of the burst, and to provide better constraints on the jet physical properties.

The approach that has been widely followed by the GRB community for more than 20 years consists of fitting GRB spectra with purely phenomenological functions that have little physical grounds (e.g., the well-known Band function). In a previous thesis work at the laboratory, the study of the prompt emission spectra of Fermi-detected GRBs showed a good agreement with the internal shock model. In particular, a generic spectral function was built from the model and applied with success to a large sample of Fermi GRBs. In this new thesis work, the student is expected to fully exploit the internal shock model and to explore its physical parameter space. This study will require an improved spectral reconstruction that is coupled to a full characterization of the temporal properties from keV to GeV energies (shape and width of the emission pulses, temporal lags between different energy bands, etc), in view of disentangling the contributions that have an internal or an external origin in the jet. In practice, the work will consist in developing a new and complex spectral analysis chain (e.g., based on a model truth bank) to achieve the data / model comparison. Moreover, a novel temporal analysis method will be designed and implemented in order to combine the relatively low photon counts recorded by the LAT at high energy with the large photon statistics available in the GBM. This method could rely on the maximum likelihood technique and make use of templates for the spectro-temporal distributions. It will also account for the light curve smoothing of the light curves at high energy that is caused by the gamma-gamma absorption in the jet. The sensitivity and the accuracy of the new methods will be evaluated and controled through extensive and dedicated Monte Carlo simulations before application to real Fermi data. Finally, the possibility of applying these methods outside the Fermi energy range will be explored, in view of contributing to the preparation of the GRB science with SVOM and CTA. The first GRB observations from these experiments should be used during the third (and last) year of the thesis work.

The student will work within the EMA (Experiences and Modeling in Astroparticles) at LUPM under the supervision of Frédéric PIRON and within the GRB Science Group of the Fermi/LAT international collaboration. In this context, he/she will carry ouf LAT Burst Advocate shifts to follow the real-time alerts generated by instruments monitoring the high-energy and/or gravitational transient sky (GBM, Swift, LIGO-Virgo, etc). This duty service will allow the student to get more familiar with the GBM and LAT instruments, and with their data analysis software. It will also allow him/her to get involved in the publication work for the most interesting bursts. In particular, the thesis work will start during the next observation campaign of the gravitational wave detectors. Though exploratory, the search with the LAT of short GRBs associated with the gravitational signal emitted from neutron star mergers could represent a complementary part of the thesis.

Job details

• duration: 36 months
• Start date of thesis: 1 October 2019