PhD defense : Ny Avo Rakotondrainibe

“Panchromatic analysis from the X-ray to the optical to characterize GRB and their hosts : SVOM/COLIBRÍ synergy”.

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Abstract:

Gamma-ray bursts (GRBs) are the most powerful energetic events in the Universe. Because of their extreme brightness and their simple spectral shape, they are powerful tools for studying their intermediate environments at all redshifts. This thesis focuses on understanding GRBs as probes of their host galaxies and especially studying dust extinction in the context of the French-Chinese Space Variable Objects Monitor (SVOM) mission and its French-Mexican ground follow-up telescope Catching Optical Light and Infrared BRIght transients (COLIBRÍ). COLIBRÍ is currently in operation with its visible camera (DDRAGO-blue and red) and will be equipped with its near-infrared camera (CAGIRE) in the summer of 2026.

One of COLIBRÍ’s scientific requirements is to provide a photometric redshift estimation in the minutes after a SVOM trigger. I first investigated COLIBRÍ’s capability to estimate the photometric redshift in the configuration without its infrared camera. I explored different observational strategies to optimize the balance between detection efficiency and estimation accuracy, based on simulations of GRB afterglows. At low redshifts (z < 3), it can only provide an upper limit estimation accurately for about ∼90% of the bursts. At higher redshifts (z ≥ 3), where the Lyman breaks enter COLIBRÍ’s wavelength coverage, the relative accuracy reaches ∼20%.

By fitting the X-ray-to-near-infrared GRB spectral energy distributions (SED), we can gain direct insights into the properties of the interstellar dust by studying the extinction curves. A simple model of extinction curve based on a power-law form with the addition of the characteristic bump at 2175 Å with a variable amplitude, was proposed as an alternative to the templates of extinction laws obtained for the Local Group, and the sophisticated parameterizations that are often used to fit spectroscopic observations of GRB afterglows. This model, intended for photometric study, was first tested on idealized afterglow simulations to assess its limitations and then validated on a sample selection of real extinguished GRBs by demonstrating that it can retrieve their extinction properties.

The variability of extinction and attenuation curves in GRB host galaxies was investigated in a previous study. I have extended it by adding a selection of new extinction curves of GRBs and compared them to the global attenuation curves measured for each host galaxy.

Keywords : gamma-ray burst, dust extinction curve, photometric redshift, X-ray-to-NIR spectral energy distribution modeling, galaxies

Jury members:

Antonio De Ugarte POSTIGO (LAM) – President

Susanna VERGANI Examinatrice (LUX, CNRS, Observatoire de Paris) – Examiner

David CORRE (Irfu, CEA Saclay) – Examiner

Patricia SCHADY (University of Bath) – Reviewer

Paolo D’Avanzo (INAF, Osservatorio Astronomico di Brera, Italy) – Reviewer

Véronique BUAT (LAM, Aix Marseille Université) – Thesis supervisor

Damien DORNIC (CPPM, CNRS, Marseille) – Co-thesis supervisor

Damien TURPIN (Irfu, CEA Saclay) – Invited member