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arXiv 2025年

作者： Peng, J.Q. Zhang, S. Chen, Y.P. Kong, L.D. Wang, P.J. Zhang, S.N. Shui, Q.C. Ji, L. Zhang, G.B. Yan, Z. Tao, L. Qu, J.L. Ge, M.Y. Yu, Z.L. Li, J. Chang, Z. Li, Z.S. Zhang, P. Xiao, Y.X. Zhao, S.J. Key Laboratory of Particle Astrophysics Institute of High Energy Physics Chinese Academy of Sciences Beijing100049 China University of Chinese Academy of Sciences Chinese Academy of Sciences Beijing100049 China Institute für Astronomie und Astrophysik Kepler Center for Astro and Particle Physics Eberhard Karls Universität Sand 1 TübingenD-72076 Germany School of Physics and Astronomy Sun Yat-Sen University Zhuhai519082 China Yunnan Observatories Chinese Academy of Sciences Kunming650216 China Key Laboratory for the Structure and Evolution Celestial Objects Chinese Academy of Sciences Kunming650216 China Center for Astronomical Mega-Science Chinese Academy of Sciences Beijing100012 China CAS Key Laboratory for Research in Galaxies and Cosmology Department of Astronomy University of Science and Technology of China Hefei230026 China School of Astronomy and Space Science University of Science and Technology of China Hefei230026 China Key Laboratory of Stars and Interstellar Medium Xiangtan University Hunan Xiangtan411105 China College of Science China Three Gorges University Yichang443002 China Center for Astronomy and Space Sciences China Three Gorges University Yichang443002 China

Context. Thermonuclear bursts, also known as type-I X-ray bursts, result from unstable nuclear burning of H/He accreted to the surface of neutron stars, lasting from tens of seconds to hundreds of seconds. Thermonuclear bursts have an important impact on accretion environments around the neutron stars, such as their disks and corona, and are therefore a subject of extensive research. Thermonuclear bursts can be used as probes to gain a deeper understanding of the properties of their disks and corona. Aims. By analyzing the data from Insight-HXMT and NICER, we can determine the evolution of the significance of the hard shortage in 4U 1636–536 with its spectral state, as well as the evolution of the fraction of deficit with energy. Additionally, we investigate the possible geometry and evolution of the corona in 4U 1636–536 by combining our findings with the results of spectral analysis. Methods. Extracting the light curves from the Insight-HXMT LE, ME, and HE data and subtracting their pre-burst emission allows us to estimate the significance of the hard shortages during the bursts. By fitting the spectra, the correlation between the persistent spectral parameters and the significance of the hard shortages can be established. The bursts are then grouped according to the spectral state in which they occurred, and the significance of the hard shortages is estimated. These in turn help to investigate the evolution of the fraction of deficit with energy. Results. We find that during the soft state, the significance of possible hard X-ray shortage in bursts is almost zero. However, in the hard state, some bursts exhibit significant shortages (>3 σ), while others do not. We attempt to establish a correlation between the significance of the hard X-ray shortage and the spectral parameters, but the data quality and the limited number of bursts prevent us from finding a strong correlation. For bursts with insignificant shortages in the soft state, their fraction of the deficit

关键词： Stars

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Back to business: SLX 1746–331 after 13 years of silence

arXiv

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arXiv 2025年

作者： Peng, Jing-Qiang Zhang, Shu Wang, Peng-Ju Zhang, Shuang-Nan Kong, Ling-Da Chen, Yu-Peng Shui, Qing-Cang Ji, Long Qu, Jin-Lu Tao, Lian Ge, Ming-Yu Ma, Rui-Can Chang, Zhi Li, Jian Li, Zhao-Sheng Yu, Zhuo-Li Yan, Zhe Zhang, Peng Xiao, Yun-Xiang Zhao, Shu-Jie Key Laboratory of Particle Astrophysics Institute of High Energy Physics Chinese Academy of Sciences Beijing100049 China University of Chinese Academy of Sciences Chinese Academy of Sciences Beijing100049 China Institute für Astronomie und Astrophysik Kepler Center for Astro and Particle Physics Eberhard Karls Universität Sand 1 TübingenD-72076 Germany School of Physics and Astronomy Sun Yat-Sen University Zhuhai519082 China CAS Key Laboratory for Research in Galaxies and Cosmology Department of Astronomy University of Science and Technology of China Hefei230026 China School of Astronomy and Space Science University of Science and Technology of China Hefei230026 China Key Laboratory of Stars and Interstellar Medium Xiangtan University Hunan Xiangtan411105 China Yunnan Observatories Chinese Academy of Sciences Kunming650216 China Key Laboratory for the Structure and Evolution Celestial Objects Chinese Academy of Sciences Kunming650216 China Center for Astronomical Mega-Science Chinese Academy of Sciences Beijing100012 China College of Science China Three Gorges University Yichang443002 China Center for Astronomy and Space Sciences China Three Gorges University Yichang443002 China

The black hole candidate system SLX 1746–331 was back to business in 2023, after a long silence of roughly 13 years. An outburst was observed thoroughly by Insight-HXMT and NICER. The outburst is characterized by spectral dominance of the soft state, where the joint Insight-HXMT and NICER spectral analysis shows the temperature dependence of the disk flux follows Tin3.98, and thus suggests that the inner disk reaches to ISCO during almost the entire outburst. By assuming 0.3 LEdd for the peak flux and an inclination angle of zero degrees, the lower limit of the compact object hosted in this system is estimated as 3.28±2.14M☉. We also look into the relation of the disk temperature and disk flux for a sample of black hole systems, and by taking the disk temperature derived in the outburst of SLX 1746–331, such a relation results in a mass estimation of 5.2 ± 4.5M☉. Finally, the spin of the compact object is constrained to larger than 0.8 with a spectral model of kerrbb. Copyright © 2025, The Authors. All rights reserved.

关键词： Black holes

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NICER, NuSTAR and Insight-HXMT views to the newly discovered black hole X-ray binary Swift J1727.8–1613

arXiv

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arXiv 2025年

作者： Peng, Jing-Qiang Zhang, Shu Shui, Qing-Cang Zhang, Shuang-Nan Kong, Ling-Da Chen, Yu-Peng Wang, Peng-Ju Ji, Long Qu, Jin-Lu Tao, Lian Ge, Ming-Yu Chang, Zhi Li, Jian Li, Zhao-Sheng Yu, Zhuo-Li Yan, Zhe Key Laboratory of Particle Astrophysics Institute of High Energy Physics Chinese Academy of Sciences Beijing100049 China University of Chinese Academy of Sciences Chinese Academy of Sciences Beijing100049 China Institute für Astronomie und Astrophysik Kepler Center for Astro and Particle Physics Eberhard Karls Universität Sand 1 TübingenD-72076 Germany School of Physics and Astronomy Sun Yat-Sen University Zhuhai519082 China CAS Key Laboratory for Research in Galaxies and Cosmology Department of Astronomy University of Science and Technology of China Hefei230026 China School of Astronomy and Space Science University of Science and Technology of China Hefei230026 China Key Laboratory of Stars and Interstellar Medium Xiangtan University Hunan Xiangtan411105 China Yunnan Observatories Chinese Academy of Sciences Kunming650216 China Key Laboratory for the Structure and Evolution Celestial Objects Chinese Academy of Sciences Kunming650216 China Center for Astronomical Mega-Science Chinese Academy of Sciences Beijing100012 China

Swift J1727.8–1613 is a black hole X-ray binary newly discovered in 2023. We perform spectral analysis with simultaneous Insight-HXMT, NICER and NuSTAR observations when the source was approaching to the hard intermediate state. Such a joint view reveals an additional hard component apart from the normally observed hard component with reflection in the spectrum, to be distinguished from the usual black hole X-ray binary systems. By including this extra component in the spectrum, we have measured a high spin of 0.98+0.02−0.07 and an inclination of around 40+1.2−0.8 degrees, which is consistent with NICER results reported before. However, we find that the additional spectral component can not be exclusively determined due to the model degeneracy. Accordingly, a possible jet/corona configuration is adjusted to account for the spectral fitting with different model trials. The extra component may originate either from a relativistic jet or a jet base/corona underneath a slow jet. Copyright © 2025, The Authors. All rights reserved.

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NICER, NuSTAR and Insight-HXMT views to black hole X-ray binary SLX 1746–331

arXiv

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arXiv 2025年

We study the spectral and temporal properties of the black hole X-ray transient binary SLX 1746–331 during the 2023 outburst with NICER, NuSTAR, and Insight-HXMT observations. Through the joint fitting of the spectra from NICER, NuSTAR, and Insight-HXMT, the spin and inclination angles are measured for the first time as 0.85 ± 0.03 and 53 ± 0.5°, respectively. Accordingly, the mass of the compact star is updated from the previous lower limit of 3.3±2.1M☉ to 5.5 ± 3.6M☉, which is consistent with 5.2 ± 4.5M☉ measured with an empirical mass-luminosity correlation of BH samples. With more NICER observations covering the later decay of the outburst, we confirm that the entire outburst was dominated by the disk emissions, and the thermal spectrum follows F ∝ Tin3.974±0.003, till a luminosity of over than two magnitudes lower than the maximum of the outburst. Copyright © 2025, The Authors. All rights reserved.

关键词： Black holes

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Position reconstruction and surface background model for the PandaX-4T detector

arXiv

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arXiv 2025年

作者： Qian, Zhicheng Gu, Linhui Cheng, Chen Bo, Zihao Chen, Wei Chen, Xun Chen, Yunhua Cheng, Zhaokan Cui, Xiangyi Fan, Yingjie Fang, Deqing Gao, Zhixing Geng, Lisheng Giboni, Karl Guo, Xunan Guo, Xuyuan Guo, Zichao Han, Chencheng Han, Ke He, Changda He, Jinrong Huang, Di Huang, Houqi Huang, Junting Hou, Ruquan Hou, Yu Ji, Xiangdong Ji, Xiangpan Ju, Yonglin Li, Chenxiang Li, Jiafu Li, Mingchuan Li, Shuaijie Li, Tao Li, Zhiyuan Lin, Qing Liu, Jianglai Lu, Congcong Lu, Xiaoying Luo, Lingyin Luo, Yunyang Ma, Wenbo Ma, Yugang Mao, Yajun Meng, Yue Ning, Xuyang Pang, Binyu Qi, Ningchun Ren, Xiangxiang Shan, Dong Shang, Xiaofeng Shao, Xiyuan Shen, Guofang Shen, Manbin Sun, Wenliang Tao, Yi Wang, Anqing Wang, Guanbo Wang, Hao Wang, Jiamin Wang, Lei Wang, Meng Wang, Qiuhong Wang, Shaobo Wang, Siguang Wang, Wei Wang, Xiuli Wang, Xu Wang, Zhou Wei, Yuehuan Wu, Weihao Wu, Yuan Xiao, Mengjiao Xiao, Xiang Xiong, Kaizhi Xu, Yifan Yao, Shunyu Yan, Binbin Yan, Xiyu Yang, Yong Ye, Peihua Yu, Chunxu Yuan, Ying Yuan, Zhe Yun, Youhui Zeng, Xinning Zhang, Minzhen Zhang, Peng Zhang, Shibo Zhang, Shu Zhang, Tao Zhang, Wei Zhang, Yang Zhang, Yingxin Zhang, Yuanyuan Zhao, Li Zhou, Jifang Zhou, Jiaxu Zhou, Jiayi Zhou, Ning Zhou, Xiaopeng Zhou, Yubo Zhou, Zhizhen Shanghai Key Laboratory for Particle Physics and Cosmology Shanghai200240 China School of Physics Beihang University Beijing102206 China New Cornerstone Science Laboratory Tsung-Dao Lee Institute Shanghai Jiao Tong University Shanghai201210 China Shanghai Jiao Tong University Sichuan Research Institute Chengdu610213 China Jinping Deep Underground Frontier Science and Dark Matter Key Laboratory of Sichuan Province China Yalong River Hydropower Development Company Ltd. 288 Shuanglin Road Chengdu610051 China Sino-French Institute of Nuclear Engineering and Technology Sun Yat-Sen University Zhuhai519082 China Department of Physics Yantai University Yantai264005 China Institute of Modern Physics Fudan University Shanghai200433 China Huanwu Collaborative Center for Research and Education Beihang University Beijing100191 China International Research Center for Nuclei and Particles in the Cosmos "& Beijing Key Laboratory of Advanced Nuclear Materials and Physics Beihang University Beijing100191 China Institute of Modern Physics Chinese Academy of Sciences Huizhou516000 China SJTU Paris Elite Institute of Technology Shanghai Jiao Tong University Shanghai200240 China School of Mechanical Engineering Shanghai Jiao Tong University Shanghai200240 China Department of Physics University of Maryland College ParkMD20742 United States School of Physics Nankai University Tianjin300071 China School of Physics Sun Yat-Sen University Guangzhou510275 China State Key Laboratory of Particle Detection and Electronics University of Science and Technology of China Hefei230026 China Department of Modern Physics University of Science and Technology of China Hefei230026 China Research Center for Particle Science and Technology Institute of Frontier and Interdisciplinary Science Shandong University Qingdao266237 China Key Laboratory of Particle Physics and Particle Irradiation of Ministry of Education Shandong University Qingdao266237 China School of Physics Peking University Beiji

We report the position reconstruction methods and surface background model for the PandaX-4T dark matter direct search experiment. This work develops two position reconstruction algorithms: template matching (TM) method and photon acceptance function (PAF) method. Both methods determine the horizontal position of events based on the light pattern of secondary scintillation collected by the light sensors. After a comprehensive evaluation of resolution, uniformity, and robustness, the PAF method was selected for position reconstruction, while the TM method was employed for verification. The PAF method achieves a bulk event resolution of 1.0 mm and a surface event resolution of 4.4 mm for a typical S2 signal with a bottom charge of 1500 PE (about 14 keV). The uniformity is around 20%. Robustness studies reveal average deviations of 5.1 mm and 8.8 mm for the commissioning run (Run0) and the first science run (Run1), respectively, due to the deactivation of certain PMTs. A data-driven surface background model is developed based on the PAF method. The surface background is estimated to be 0.09 ± 0.06 events for Run0 (0.54 tonne·year) and 0.17 ± 0.11 events for Run1 (1.00 tonne·year). © 2025, CC BY.

关键词： Template matching

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Quark Transverse Spin-Momentum Correlation of the Nucleon from Lattice QCD: The Boer-Mulders Function

arXiv

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arXiv 2025年

作者： Ma, Lingquan Hua, Jun Schäfer, Andreas Shu, Hai-Tao Su, Yushan Sun, Peng Walter, Lisa Wang, Wei Xiong, Xiaonu Yang, Yi-Bo Zhang, Jian-Hui Zhang, Qi-An Center of Advanced Quantum Studies Department of Physics Beijing Normal University Beijing100875 China School of Science and Engineering The Chinese University of Hong Kong Shenzhen518172 China Guangdong Basic Research Center of Excellence for Structure and Fundamental Interactions of Matter Institute of Quantum Matter South China Normal University Guangzhou510006 China Guangdong-Hong Kong Joint Laboratory of Quantum Matter Guangdong Provincial Key Laboratory of Nuclear Science Southern Nuclear Science Computing Center South China Normal University Guangzhou510006 China Institut für Theoretische Physik Universität Regensburg RegensburgD-93040 Germany Institute of Particle Physics Central China Normal University Wuhan430079 China Department of Physics University of Maryland College ParkMD20742 United States Institute of Modern Physics Chinese Academy of Sciences Gansu Province Lanzhou730000 China Shanghai Key Laboratory for Particle Physics and Cosmology School of Physics and Astronomy Shanghai Jiao Tong University Shanghai200240 China Institute of Modern Physics Chinese Academy of Sciences Guangdong Province Huizhou516000 China School of Physics Central South University Changsha418003 China CAS Key Laboratory of Theoretical Physics Institute of Theoretical Physics Chinese Academy of Sciences Beijing100190 China School of Fundamental Physics and Mathematical Sciences Hangzhou Institute for Advanced Study UCAS Hangzhou310024 China International Centre for Theoretical Physics Asia-Pacific Hangzhou Beijing China University of Chinese Academy of Sciences School of Physical Sciences Beijing100049 China School of Physics Beihang University Beijing102206 China

We present the first lattice QCD calculation of the quark transverse spin-momentum correlation, i.e., the naive time-reversal-odd Boer-Mulders function, of the nucleon, using large-momentum effective theory (LaMET). The calculation is carried out on an ensemble with lattice spacing a = 0.098 fm and pion mass 338 MeV, at various proton momenta up to 2.11 GeV. We have implemented perturbative matching up to the next-to-next-to-leading order together with a renormalization-group resummation improvement. The result exhibits a decay behavior with increasing transverse separation b̝. We also compare the results in the nucleon and pion. Copyright © 2025, The Authors. All rights reserved.

关键词： Momentum

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Search for Double Beta Decay of 136Xe to the 0+1 Excited State of 136Ba with PandaX-4T

arXiv

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arXiv 2025年

作者： Luo, Lingyin Bo, Zihao Chen, Wei Chen, Xun Chen, Yunhua Cheng, Zhaokan Cui, Xiangyi Fan, Yingjie Fang, Deqing Gao, Zhixing Geng, Lisheng Giboni, Karl Guo, Xunan Guo, Xuyuan Guo, Zichao Han, Chencheng Han, Ke He, Changda He, Jinrong Huang, Di Huang, Houqi Huang, Junting Hou, Ruquan Hou, Yu Ji, Xiangdong Ji, Xiangpan Ju, Yonglin Li, Chenxiang Li, Tao Li, Jiafu Li, Mingchuan Li, Shuaijie Li, Zhiyuan Lin, Qing Liu, Jianglai Lu, Congcong Lu, Xiaoying Luo, Yunyang Ma, Wenbo Ma, Yugang Mao, Yajun Meng, Yue Ning, Xuyang Pang, Binyu Qi, Ningchun Qian, Zhicheng Ren, Xiangxiang Shan, Dong Shang, Xiaofeng Shao, Xiyuan Shen, Guofang Shen, Manbin Sun, Wenliang Tao, Yi Wang, Anqing Wang, Guanbo Wang, Hao Wang, Jiamin Wang, Lei Wang, Meng Wang, Qiuhong Wang, Shaobo Wang, Siguang Wang, Wei Wang, Xiuli Wang, Xu Wang, Zhou Wei, Yuehuan Wu, Weihao Wu, Yuan Xiao, Mengjiao Xiao, Xiang Xiong, Kaizhi Xu, Yifan Yao, Shunyu Yan, Binbin Yan, Xiyu Yang, Yong Ye, Peihua Yu, Chunxu Yuan, Ying Yuan, Zhe Yun, Youhui Zeng, Xinning Zhang, Minzhen Zhang, Peng Zhang, Shibo Zhang, Shu Zhang, Tao Zhang, Wei Zhang, Yang Zhang, Yingxin Zhang, Yuanyuan Zhao, Li Zhou, Jifang Zhou, Jiaxu Zhou, Ning Zhou, Xiaopeng Zhou, Yubo Zhou, Zhizhen New Cornerstone Science Laboratory Tsung-Dao Lee Institute Shanghai Jiao Tong University Shanghai201210 China State Key Laboratory of Nuclear Physics and Technology School of Physics Peking University Beijing100871 China Shanghai Key Laboratory for Particle Physics and Cosmology Shanghai200240 China Jinping Deep Underground Frontier Science and Dark Matter Key Laboratory of Sichuan Province China Yalong River Hydropower Development Company Ltd. 288 Shuanglin Road Chengdu610051 China Sino-French Institute of Nuclear Engineering and Technology Sun Yat-Sen University Zhuhai519082 China Department of Physics Yantai University Yantai264005 China Institute of Modern Physics Fudan University Shanghai200433 China School of Physics Beihang University Beijing102206 China Peng Huanwu Collaborative Center for Research and Education Beihang University Beijing100191 China International Research Center for Nuclei and Particles in the Cosmos Beijing Key Laboratory of Advanced Nuclear Materials and Physics Beihang University Beijing100191 China Institute of Modern Physics Chinese Academy of Sciences Huizhou516000 China SJTU Paris Elite Institute of Technology Shanghai Jiao Tong University Shanghai200240 China Shanghai Jiao Tong University Sichuan Research Institute Chengdu610213 China School of Mechanical Engineering Shanghai Jiao Tong University Shanghai200240 China Department of Physics University of Maryland College ParkMD20742 United States School of Physics Nankai University Tianjin300071 China School of Physics Sun Yat-Sen University Guangzhou510275 China State Key Laboratory of Particle Detection and Electronics University of Science and Technology of China Hefei230026 China Department of Modern Physics University of Science and Technology of China Hefei230026 China Research Center for Particle Science and Technology Institute of Frontier and Interdisciplinary Science Shandong University Shandong Qingdao266237 China Key Laboratory o

We perform a search of double beta decay of 136Xe to the excited state, 0+1 , of 136Ba (2νββ-0+1 ), using the dual-phase xenon detector of PandaX-4T with the first 94.9-day commissioning data. The multi-site events are reconstructed up to the MeV energy scale, which helps to improve the background model significantly. The background contribution from the stainless steel platform outside PandaX-4T cryostat is evaluated for the first time. No significant evidence for 2νββ-0+1 is observed, resulting in a lower limit on half-life of 7.5×1022 yr at the 90% confidence level. This is the first experimental limit on such a rare decay in a natural xenon-based detector. © 2025, CC BY.

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Impact of the reionization history on constraining primordial gravitational waves in future all-sky cosmic microwave background experiments

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Physical Review D 2025年第8期111卷 083555-083555页

作者： Hanchun Jiang Toshiya Namikawa Department of Physics Graduate School of Science The University of Tokyo Tokyo 113- 0033 Japan Department of Applied Mathematics and Theoretical Physics University of Cambridge Wilberforce Road Cambridge CB3 0WA United Kingdom Center for Data-Driven Discovery Kavli IPMU (WPI) UTIAS The University of Tokyo Kashiwa 277-8583 Japan Kavli Institute for Cosmology Cambridge Madingley Road Cambridge CB3 0HA United Kingdom

We explore the impact of the reionization history on examining the shape of the power spectrum of the primordial gravitational waves (PGWs) with the cosmic microwave background (CMB) polarization. The large-scale CMB generated from the reionization epoch is important in probing the PGWs from all-sky experiments, such as LiteBIRD. The reionization model has been constrained by several astrophysical observations. However, its uncertainty could impact constraining models of the PGWs if we use large-scale CMB polarization. Here, by expanding the analysis of Mortonson and Hu (2007), we estimate how reionization uncertainty impacts constraints on a generic primordial tensor power spectrum. We assume that CMB polarization is measured by a LiteBIRD-like experiment and the tanh model is adopted for a theoretical template when we fit data. We show that constraints are almost unchanged even if the true reionization history is described by an exponential model, where all parameters are within 68% confidence level (CL). We also show an example of the reionization history that the constraints on the PGWs are biased more than 68% CL. Even in that case, using E-mode power spectrum on large scales would exclude such a scenario and make the PGW constraints robust against the reionization uncertainties.

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Gravitational Wave Decoupling in Retrograde Circumbinary Disks

arXiv

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arXiv 2025年

作者： O’Neill, David Tiede, Christopher D’Orazio, Daniel J. Haiman, Zoltán MacFadyen, Andrew Niels Bohr International Academy Niels Bohr Institute Blegdamsvej 17 Copenhagen ØDK-2100 Denmark Space Telescope Science Institute 3700 San Martin Drive BaltimoreMD21 United States Department of Astronomy Columbia University 550 West 120th Street New YorkNY10027 United States Department of Physics Columbia University 550 West 120th Street New YorkNY10027 United States Am Campus 1 Klosterneuburg3400 Austria Center for Cosmology and Particle Physics Physics Department New York University New YorkNY10003 United States

We present a study of the late-time interaction between supermassive black hole binaries and retrograde circumbinary disks during the period of gravitational wave-driven inspiral. While mergers in prograde disks have received extensive study, retrograde disks offer distinct dynamics that could promote mergers and produce unique observational signatures. Through numerical simulations, we explore the process of binary-disk decoupling, where the binary’s orbital decay rate is faster than the disk’s viscous response rate. We find the point of decoupling to be comparable in prograde and retrograde disks, suggesting that any associated electromagnetic (EM) signatures will be produced at comparable times preceding merger. However, we find smaller central cavities for retrograde disks, likely leading to higher-frequency EM emissions and shorter post-merger rebrightening timescales compared to their prograde counterparts. Additionally, we identify quasi-periodic flaring due to instabilities unique to low-viscosity retrograde disks, which may produce distinctive EM signatures. © 2025, CC BY.

关键词： Gravitational effects

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Spectro-polarimetry of GRB 180427A: evidence for distinct emission sites with varying polarisation

arXiv

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arXiv 2025年

作者： Sonawane, Rushikesh Iyyani, Shabnam Gupta, Soumya Chattopadhyay, Tanmoy Bhattacharya, Dipankar Bhalerao, Varun B. Vadawale, Santosh V. Dewangan, G.C. Centre for High Performance Computing Indian Institute of Science Education and Research Thiruvananthapuram Thiruvananthapuram695551 India School of Physics Indian Institute of Science Education and Research Thiruvananthapuram Thiruvananthapuram695551 India Homi Bhabha National Institute Anushakti Nagar Maharashtra Mumbai400094 India Kavli Institute of Particle Astrophysics and Cosmology Stanford University 452 Lomita Mall StanfordCA94305 United States Department of Physics Ashoka University Haryana Sonepat131029 India Indian Institute of Technology Bombay Mumbai India Physical Research Laboratory Gujarat Ahmedabad380009 India Inter-University Center for Astronomy and Astrophysics Maharashtra Pune411007 India

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 4.82 ± 0.12 s. Spectral analysis indicates that the first pulse is dominated by blackbody radiation, while the second pulse exhibits a 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 ∼ 0.05, the blackbody emission originates from the jet photosphere at 1011 cm, whereas the non-thermal emission arises from an optically thin region at 1015 cm. The changing dominance of these emissions explains the observed PA shift of 60◦ ±22.3◦. The spectral cutoff at 1 MeV suggests pair opacity due to the jet’s low bulk Lorentz factor (Γ ∼ tens). The high polarisation and hard 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 radiation within the viewing cone (1/Γ), accounting for the observed polarisation. © 2025, CC BY.

关键词： Gamma rays

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