Spectro-polarimetry of GRB 180427A: evidence for distinct emission sites with varying polarisation
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| Autores principales: | , , , , , , , |
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| Formato: | Preprint |
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2025
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| _version_ | 1866909989608620032 |
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| author | Sonawane, Rushikesh Iyyani, Shabnam Gupta, Soumya Chattopadhyay, Tanmoy Bhattacharya, Dipankar Bhalerao, Varun. B. Vadawale, Santosh V. Dewangan, G. C. |
| author_facet | Sonawane, Rushikesh Iyyani, Shabnam Gupta, Soumya Chattopadhyay, Tanmoy Bhattacharya, Dipankar Bhalerao, Varun. B. Vadawale, Santosh V. Dewangan, G. C. |
| contents | The dynamics of the origin of gamma-ray emissions in gamma-ray bursts (GRBs) remains an enigma. Through a joint analysis of GRB 180427A, observed by the Fermi Gamma-ray Space Telescope and AstroSat's Cadmium Zinc Telluride Imager, we identify emissions from two distinct regions with varying polarisation properties. Time-resolved polarisation analysis reveals a synchronous evolution of the polarisation angle (PA) and fraction (PF) with two emission pulses, peaking with a delay of $ 5.09 \pm 0.29\, \mathrm{s}$. Spectral analysis indicates that the first pulse is characterised by a stronger blackbody component, while the second pulse exhibits a more prominent non-thermal spectrum (power law with an exponential cutoff). Using a bottom-to-top approach through simulations, we decouple the polarisation properties of the individual spectral components, revealing polarisation fractions of 25\% - 40\% for the blackbody spectrum and 30\% - 60\% for the non-thermal spectrum. At a redshift of $z \sim 0.22$, the blackbody emission originates from the jet photosphere at $\sim$ a few $10^{11}\, \mathrm{cm}$, whereas the non-thermal emission arises from an optically thin region at a few $10^{13}\, \mathrm{cm}$. The changing dominance of these emissions explains the observed PA shift of $60^\circ \pm 22^\circ$. The spectral cutoff at 1 MeV suggests pair opacity due to the jet's relatively lower bulk Lorentz factor ($Γ\sim$ a few tens). The high polarisation fraction and hard low energy spectral slopes ($α> -0.5$) imply a top-hat jet structure observed off-axis, near the jet's edge. This off-axis viewing introduces anisotropy in the observed radiation within the viewing cone ($1/Γ$), accounting for the observed polarisation. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2501_09492 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Spectro-polarimetry of GRB 180427A: evidence for distinct emission sites with varying polarisation Sonawane, Rushikesh Iyyani, Shabnam Gupta, Soumya Chattopadhyay, Tanmoy Bhattacharya, Dipankar Bhalerao, Varun. B. Vadawale, Santosh V. Dewangan, G. C. High Energy Astrophysical Phenomena The dynamics of the origin of gamma-ray emissions in gamma-ray bursts (GRBs) remains an enigma. Through a joint analysis of GRB 180427A, observed by the Fermi Gamma-ray Space Telescope and AstroSat's Cadmium Zinc Telluride Imager, we identify emissions from two distinct regions with varying polarisation properties. Time-resolved polarisation analysis reveals a synchronous evolution of the polarisation angle (PA) and fraction (PF) with two emission pulses, peaking with a delay of $ 5.09 \pm 0.29\, \mathrm{s}$. Spectral analysis indicates that the first pulse is characterised by a stronger blackbody component, while the second pulse exhibits a more prominent non-thermal spectrum (power law with an exponential cutoff). Using a bottom-to-top approach through simulations, we decouple the polarisation properties of the individual spectral components, revealing polarisation fractions of 25\% - 40\% for the blackbody spectrum and 30\% - 60\% for the non-thermal spectrum. At a redshift of $z \sim 0.22$, the blackbody emission originates from the jet photosphere at $\sim$ a few $10^{11}\, \mathrm{cm}$, whereas the non-thermal emission arises from an optically thin region at a few $10^{13}\, \mathrm{cm}$. The changing dominance of these emissions explains the observed PA shift of $60^\circ \pm 22^\circ$. The spectral cutoff at 1 MeV suggests pair opacity due to the jet's relatively lower bulk Lorentz factor ($Γ\sim$ a few tens). The high polarisation fraction and hard low energy spectral slopes ($α> -0.5$) imply a top-hat jet structure observed off-axis, near the jet's edge. This off-axis viewing introduces anisotropy in the observed radiation within the viewing cone ($1/Γ$), accounting for the observed polarisation. |
| title | Spectro-polarimetry of GRB 180427A: evidence for distinct emission sites with varying polarisation |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2501.09492 |