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An unexpected detection of bifurcated blue straggler sequences in the young globular cluster ngc 2173

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arXiv 2018年

作者： Li, Chengyuan Deng, Licai Grijs, Richard De Jiang, Dengkai Xin, Yu Department of Physics and Astronomy Macquarie University Balaclava Road North RydeNSW2109 Australia Department of Astronomy China West Normal University Nanchong637002 China School of Astronomy and Space Science University of Chinese Academy of Sciences 20A Datun Road Beijing100012 China Kavli Institute for Astronomy and Astrophysics and Department of Astronomy Peking University Yi He Yuan Lu 5 Beijing100871 China International Space Science Institute-Beijing 1 Nanertiao Zhongguancun Beijing100190 China Yunnan Observatories Chinese Academy of Sciences Kunming650216 China Key Laboratory for Structure and Evolution of Celestial Objects Chinese Academy of Sciences Kunming650011 China Center for Astronomical Mega-Science Chinese Academy of Sciences Beijing100012 China

Bifurcated patterns of blue straggler stars in their color-magnitude diagrams have atracted significant attention. This type of special (but rare) pattern of two distinct blue straggler sequences is commonly interpreted as evidence of cluster core-collapse-driven stellar collisions as an efficient formation mechanism. Here, we report the detection of a bifurcated blue straggler distribution in a young LargeMagellanic Cloud cluster, NGC 2173. Because of the cluster's low central stellar number density and its young age, dynamical analysis shows that stellar collisions alone cannot explain the observed blue straggler stars. Therefore, binary evolution is instead the most viable explanation of the origin of these blue straggler stars. However, the reason why binary evolution would render the color-magnitude distribution of blue straggler stars bifurcated remains unclear. Copyright © 2018, The Authors. All rights reserved.

关键词： Stars

The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope - Data Release 3

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arXiv 2022年

作者： Ajello, Marco Baldini, Luca Ballet, Jean Bastieri, Denis Gonzalez, Joseff Becerra Bellazzini, Ronaldo Berretta, Alessandra Bissaldi, Elisabetta Bonino, Raffaella Brill, Ari Bruel, Philippe Buson, Sara Caputo, Regina Caraveo, Patrizia A. Cheung, C.C. Chiaro, Graziano Cibrario, Nicolo Ciprini, Stefano Crnogorcevic, Milena Cutini, Sara D’Ammando, Filippo De Gaetano, Salvatore Di Lalla, Niccolo Di Venere, Leonardo Domínguez, Alberto Ramazani, Vandad Fallah Ferrara, Elizabeth C. Fiori, Alessio Fukazawa, Yasushi Funk, Stefan Fusco, Piergiorgio Gammaldi, Viviana Gargano, Fabio Garrappa, Simone Gasparrini, Dario Giglietto, Nico Giordano, Francesco Giroletti, Marcello Green, David Grenier, Isabelle A. Guiriec, Sylvain Horan, Deirdre Hou, Xian Kayanoki, Taishu Kuss, Michael Larsson, Stefan Latronico, Luca Lewis, Tiffany Li, Jian Liodakis, Ioannis Longo, Francesco Loparco, Francesco Lott, Benoit Lovellette, Michael N. Lubrano, Pasquale Madejski, Grzegorz M. Maldera, Simone Manfreda, Alberto Martí-Devesa, Guillem Mazziotta, Mario Nicola Mereu, Isabella Michelson, Peter F. Mirabal, Nestor Mitthumsiri, Warit Mizuno, Tsunefumi Monzani, Maria Elena Morselli, Aldo Moskalenko, Igor V. Negro, Michela Ojha, Roopesh Orienti, Monica Orlando, Elena Ormes, Jonathan F. Pei, Zhiyuan Peña-Herazo, Harold A. Persic, Massimo Pesce-Rollins, Melissa Petrosian, Vahe Pillera, Roberta Poon, Helen Porter, Troy A. Principe, Giacomo Rainò, Silvia Rando, Riccardo Rani, Bindu Razzano, Massimiliano Razzaque, Soebur Reimer, Anita Reimer, Olaf Scotton, Lorenzo Serini, Davide Sgrò, Carmelo Siskind, Eric J. Spandre, Gloria Spinelli, Paolo Suson, Dan J. Tajima, Hiro Torres, Diego F. Valverde, Janeth Yassin, Hend Zaharijas, Gabrijela Department of Physics and Astronomy Clemson University Kinard Lab of Physics ClemsonSC29634-0978 United States Università di Pisa Istituto Nazionale di Fisica Nucleare Sezione di Pisa PisaI-56127 Italy AIM CEA CNRS Université Paris-Saclay Université de Paris Gif-sur-YvetteF-91191 France Istituto Nazionale di Fisica Nucleare Sezione di Padova PadovaI-35131 Italy Dipartimento di Fisica e Astronomia "G. Galilei" Università di Padova Via F. Marzolo 8 PadovaI-35131 Italy Center for Space Studies and Activities "G. Colombo" University of Padova Via Venezia 15 PadovaI-35131 Italy Instituto de Astrofísica de Canarias Observatorio del Teide C/Via Lactea s/n La Laguna TenerifeE-38205 Spain Istituto Nazionale di Fisica Nucleare Sezione di Pisa PisaI-56127 Italy Dipartimento di Fisica Università degli Studi di Perugia PerugiaI-06123 Italy Dipartimento di Fisica "M. Merlin" Università e del Politecnico di Bari via Amendola 173 BariI-70126 Italy Istituto Nazionale di Fisica Nucleare Sezione di Bari BariI-70126 Italy Istituto Nazionale di Fisica Nucleare Sezione di Torino TorinoI-10125 Italy Dipartimento di Fisica Università degli Studi di Torino TorinoI-10125 Italy NASA Postdoctoral Program United States NASA Goddard Space Flight Center GreenbeltMD20771 United States Laboratoire Leprince-Ringuet École polytechnique CNRS IN2P3 PalaiseauF-91128 France Institut für Theoretische Physik and Astrophysik Universität Würzburg WürzburgD-97074 Germany INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica Milano via E. Bassini 15 MilanoI-20133 Italy Space Science Division Naval Research Laboratory WashingtonDC20375-5352 United States Istituto Nazionale di Fisica Nucleare Sezione di Roma "Tor Vergata" RomaI-00133 Italy Space Science Data Center - Agenzia Spaziale Italiana Via del Politecnico snc RomaI-00133 Italy Department of Astronomy University of Maryland College ParkMD20742 United States Istituto Nazionale di Fisica Nucleare Sezio

An incremental version of the fourth catalog of active galactic nuclei (AGNs) detected by the Fermi-Large Area Telescope is presented. This version (4LAC-DR3) derives from the third data release of the 4FGL catalog based on 12 years of E>50 MeV gamma-ray data, where the spectral parameters, spectral energy distributions (SEDs), yearly light curves, and associations have been updated for all sources. The new reported AGNs include 587 blazar candidates and four radio galaxies. We describe the properties of the new sample and outline changes affecting the previously published one. We also introduce two new parameters in this release, namely the peak energy of the SED high-energy component and the corresponding flux. These parameters allow an assessment of the Compton dominance, the ratio of the Inverse-Compton to the synchrotron peak luminosities, without relying on X-ray data. © 2022, CC BY.

关键词： Active Galactic Nuclei

Stellar and agn feedback in isolated early-type galaxies: The role in regulating star formation and ism properties

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arXiv 2018年

作者： LI, YA-PING YUAN, FENG MO, HOUJUN YOON, DOOSOO GAN, ZHAOMING HO, LUIS C. WANG, BO OSTRIKER, JEREMIAH P. CIOTTI, LUCA Key Laboratory for Research in Galaxies and Cosmology Shanghai Astronomical Observatory Chinese Academy of Sciences 80 Nandan Road Shanghai200030 China School of Astronomy and Space Sciences University of Chinese Academy of Sciences No. 19A Yuquan Road Beijing100049 China Department of Astronomy University of Massachusetts AmherstMA01003-9305 United States Tsinghua Center of Astrophysics and Department of Physics Tsinghua University Beijing100084 China Kavli Institute for Astronomy and Astrophysics Peking University Beijing100871 China Department of Astronomy School of Physics Peking University Beijing100871 China Key Laboratory for the Structure and Evolution of Celestial Objects Yunnan Observatories CAS Kunming650216 China Department of Astronomy Columbia University 550 W 120th Street New YorkNY10027 United States Department of Physics and Astronomy University of Bologna via Piero Gobetti 93/2 Bologna40129 Italy

Understanding how galaxies maintain the inefficiency of star formation with physically self-consistent models is a central problem for galaxy evolution. Although numerous theoretical models have been proposed in recent decades, the debate still exists. By means of high-resolution two-dimensional hydrodynamical simulations, we study the three feedback effects (the stellar wind heating, SNe feedback, and AGN feedback) in suppressing star formation activities on the evolution of early-type galaxies with different stellar masses. AGN feedback models are updated based on Yuan et al. (2018). The gas sources comes exclusively from the mass losses of dying low-mass stars for most of our models. We find that SNe feedback can keep star formation at a significantly low level for low mass elliptical galaxies for a cosmological evolution time. For the high mass galaxies, AGN feedback can efficiently offset the radiative cooling and thus regulate the star formation activities. Such a suppression of star formation is extremely efficient in the inner region of the galaxies. AGB heating cannot account for this suppression for low and high mass galaxies. The X-ray temperature TX and luminosity LX of hot plasma can be in agreement with the observed data with the inclusion of effective feedback processes. These results thus suggest that we can use TX and LX to probe the role of different feedback processes. The inclusion of additional gas sources can make the mass scale between SNe and AGN feedback dominating in suppressing star formation decrease to an observationally inferred value of a few 1010 Mo. Copyright © 2018, The Authors. All rights reserved.

关键词： Galaxies

Synchrotron spectrum of fast cooling electrons in GRBs

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arXiv 2017年

作者： Zhao, Xiao-Hong Bai, Jin-Ming Yunnan Observatories Chinese Academy of Sciences 396 Yangfangwang Guandu District Kunming650216 China Center for Astronomical Mega-Science Chinese Academy of Sciences 20A Datun Road Chaoyang District Beijing100012 China Key Laboratory for the Structure and Evolution of Celestial Objects Chinese Academy of Sciences 396 Yangfangwang Guandu District Kunming650216 China

We discuss the synchrotron emission of fast cooling electrons in shocks. The fast cooling electrons behind the shocks can generate a position-dependent inhomogeneous electron distribution if they have not enough time to mix homogeneously. This would lead to a very different synchrotron spectrum in low frequency bands from that in the homogeneous case due to the synchrotron absorption. In this paper, we calculate the synchrotron spectrum in this inhomogeneous case in a gamma-ray burst (GRB). Both the forward shock and the reverse shock are considered. We find for the reverse shock dominated case, we would expect a "reverse shock bump" in the low frequency spectrum. The spectral bump is due to the combining synchrotron absorption in both the forward and reverse shock regions. In the forward shock spectrum in the low frequencies has two unconventional segments with spectral slopes of 1 and 11/8. The slope of 11/8 has been found by some authors, while the slope of 1 is new, which is due to the approximately constant electron temperature in the optically thick region. In the future, simultaneous observations in multiple bands (especially in the low frequency bands) in the GRB early afterglow or prompt emission phases will possibly reveal these spectral characteristics and enable us to identify the reverse shock component and distinguish between the forward and reverse shock emissions. This also may be as a method to diagnose the electron distribution status (homogeneous or inhomogeneous) after fast cooling in relativistic shock region. Copyright © 2017, The Authors. All rights reserved.

关键词： Cooling

A wide star-black-hole binary system from radial-velocity measurements

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arXiv 2019年

作者： Liu, Jifeng Zhang, Haotong Howard, Andrew W. Bai, Zhongrui Lu, Youjun Soria, Roberto Justham, Stephen Li, Xiangdong Zheng, Zheng Wang, Tinggui Belczynski, Krzysztof Casares, Jorge Zhang, Wei Yuan, Hailong Dong, Yiqiao Lei, Yajuan Isaacson, Howard Wang, Song Bai, Yu Shao, Yong Gao, Qing Wang, Yilun Niu, Zexi Cui, Kaiming Zheng, Chuanjie Mu, Xiaoyong Zhang, Lan Wang, Wei Heger, Alexander Qi, Zhaoxiang Liao, Shilong Lattanzi, Mario Gu, Wei-Min Wang, Junfeng Wu, Jianfeng Shao, Lijing Shen, Rongfeng Wang, Xiaofeng Bregman, Joel Stefano, Rosanne Di Liu, Qingzhong Han, Zhanwen Zhang, Tianmeng Wang, Huijuan Ren, Juanjuan Zhang, Junbo Zhang, Jujia Wang, Xiaoli Cabrera-Lavers, Antonio Corradi, Romano Rebolo, Rafael Zhao, Yongheng Zhao, Gang Chu, Yaoquan Cui, Xiangqun Key Laboratory of Optical Astronomy National Astronomical Observatories Chinese Academy of Sciences Beijing China School of Astronomy and Space Sciences University of Chinese Academy of Sciences Beijing China WHU-NAOC Joint Center for Astronomy Wuhan University Wuhan China Department of Astronomy Caltech PasadenaCA United States Sydney Institute for Astronomy School of Physics A28 University of Sydney SydneyNSW Australia The Anton Pannekoek Institute for Astronomy University of Amsterdam Amsterdam Netherlands School of Astronomy and Space Science Nanjing University Nanjing China Misnistry of Education Nanjing China Department of Physics and Astronomy University of Utah Salt Lake CityUT United States CAS Key Laboratory for Research in Galaxies and Cosmology Department of Astronomy University of Science and Technology of China Hefei China Nicolaus Copernicus Astronomical Centre Polish Academy of Sciences Warsaw Poland Instituto de Astrofísica de Canarias La Laguna Tenerife Spain Departamento de Astrofsica Universidad de La Laguna Santa Cruz de Tenerife Spain Astronomy Department University of California BerkeleyCA United States School of Physics and Technology Wuhan University Wuhan China Monash Centre for Astrophysics School of Physics and Astronomy Monash University 19 Rainforest Walk VIC Australia Shanghai Astronomical Observatory Chinese Academy of Sciences Shanghai China INAF–Osservatorio Astrofisico di Torino Pino Torinese Italy Department of Astronomy Xiamen University Xiamen China Kavli Institute for Astronomy and Astrophysics Peking University Beijing China School of Physics and Astronomy Sun Yat-Sen University Zhuhai China Physics Department and Tsinghua Center for Astrophysics Tsinghua University Beijing China Department of Astronomy University of Michigan 1085 South University Street Ann ArborMI United States Harvard-Smithsonian Center for Astrophysics CambridgeMA United States Key Laboratory of Dark Matter and Space Astr

All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M*1–4. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries5,6. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying Hα emission line require the presence of a dark companion with a mass of 68+11−13 M*, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes7, 8, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories9–11. Copyright © 2019, The Authors. All rights reserved.

关键词： Stars

Optical and near-infrared observations of the nearby type Ia supernova 2017cbv

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arXiv 2020年

作者： Wang, Lingzhi Contreras, Carlos Hu, Maokai Hamuy, Mario A. Hsiao, Eric Y. Sand, David J. Anderson, Joseph P. Ashall, Chris Burns, Christopher R. Chen, Juncheng Diamond, Tiara R. Davis, Scott Förster, Francisco Galbany, Lluís González-Gaitán, Santiago Gromadzki, Mariusz Hoeflich, Peter Li, Wenxiong Marion, G.H. Morrell, Nidia Pignata, Giuliano Prieto, Jose L. Phillips, Mark M. Shahbandeh, Melissa Suntzeff, Nicholas Valenti, Stefano Wang, Lifan Wang, Xiaofeng Young, D.R. Yu, Lixin Zhang, Jujia CAS Key Laboratory of Optical Astronomy National Astronomical Observatories Chinese Academy of Sciences Beijing100101 China Chinese Academy of Sciences South America Center for Astronomy National Astronomical Observatories CAS Beijing100101 China Carnegie Observatories Las Campanas Observatory Colina El Pino Casilla601 Chile Purple Mountain Observatory NanjingJiangsu201008 China Departamento de Astronomía Universidad de Chile Camino El Observatorio 1515 Las Condes Santiago Chile Millennium Institute of Astrophysics Santiago Chile Department of Physics Florida State University TallahasseeFL32306 United States Department of Astronomy and Steward Observatory University of Arizona 933 North Cherry Avenue TucsonAZ85721-0065 United States European Southern Observatory Alonso de Córdova 3107 Casilla 19 Santiago Chile Observatories of the Carnegie Institution for Science 813 Santa Barbara St. PasadenaCA91101 United States School of Electronics and Information Engineering Wuzhou University Wuzhou543002 China Departamento de Física Teórica y del Cosmos Universidad de Granada GranadaE-18071 Spain CENTRA/COSTAR Instituto Superior Técnico Universidade de Lisboa Av. Rovisco Pais 1 Lisboa1049-001 Portugal Astronomical Observatory University of Warsaw Al. Ujazdowskie 4 Warszawa00-478 Poland Physics Department/Tsinghua Center for Astrophysics Tsinghua University Beijing100084 China Department of Astronomy University of Texas at Austin 1 University Station C1400 AustinTX78712-0259 United States Departamento de Ciencias Físicas Universidad Andres Bello Avda. República 252 Santiago8320000 Chile Núcleo de Astronomía de la Facultad de Ingeniería Universidad Diego Portales Av. Ejército 441 Santiago Chile George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy Department of Physics and Astronomy Texas United States A&M University College Station TX77843 United States Department of Physics University of California 1 S

Supernova (SN) 2017cbv in NGC 5643 is one of a handful of type Ia supernovae (SNe Ia) reported to have excess blue emission at early times. This paper presents extensive BV RIY JHKs-band light curves of SN 2017cbv, covering the phase from −16 to +125 days relative to B-band maximum light. SN 2017cbv reached a B-band maximum of 11.710±0.006 mag, with a post-maximum magnitude decline ∆m15(B)=0.990±0.013 mag. The supernova suffered no host reddening based on Phillips intrinsic color, Lira-Phillips relation, and the CMAGIC diagram. By employing the CMAGIC distance modulus µ = 30.58±0.05 mag and assuming H0 = 72 km s−1 Mpc−1, we found that 0.73 Mo56Ni was synthesized during the explosion of SN 2017cbv, which is consistent with estimates using reddening-free and distance-free methods via the phases of the secondary maximum of the NIR-band light curves. We also present 14 near-infrared spectra from −18 to +49 days relative to the B-band maximum light, providing constraints on the amount of swept-up hydrogen from the companion star in the context of the single degenerate progenitor scenario. No Paβ emission feature was detected from our post-maximum NIR spectra, placing a hydrogen mass upper limit of 0.1 Mo. The overall optical/NIR photometric and NIR spectral evolution of SN 2017cbv is similar to that of a normal SN Ia, even though its early evolution is marked by a flux excess no seen in most other well-observed normal SNe Ia. We also compare the exquisite light curves of SN 2017cbv with some Mch DDT models and sub-Mch double detonation models. Copyright © 2020, The Authors. All rights reserved.

关键词： Supernovae

X-ray light curve in GRB 170714a: evidence for quark star?

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arXiv 2017年

作者： Hou, Shu-Jin Liu, Tong Xu, Ren-Xin Mu, Hui-Jun Song, Cui-Ying Lin, Da-Bin Gu, Wei-Min College of Physics and Electronic Engineering Nanyang Normal University Nanyang Henan473061 China Key Laboratory for the Structure and Evolution of Celestial Objects Chinese Academy of Sciences Kunming Yunnan650011 China Laboratory for Relativistic Astrophysics Department of Physics Guangxi University Nanning Guangxi530004 China Department of Astronomy Xiamen University XiamenFujian361005 China School of Physics Peking University Beijing100871 China

Two plateaus and one following bump in the X-ray light curve of GRB 170714A have been detected by the Swift/X-Ray Telescope, which could be very meaningful for the central engine of gamma-ray bursts (GRBs), implying that the origin of this burst might be different from that of other ultra-long GRBs. We propose that merging two neutron stars into a hyper-massive quark star (QS) and then collapsing into a black hole (BH), with a delay time around 104 s, could be responsible for those X-ray components. The hyper-massive QS is initially in a fluid state, being turbulent and differentially rotating, but would be solidified and release its latent heat injected into the GRB fireball (lasting about 103 s during the liquid-solid phase transition). Magnetic field as high as ∼ 1015 G could be created by dynamo action of the newborn liquid QS, and a magnetar-like central engine (after solidification) supplies significant energy for the second plateau. More energy could be released during a fall-back accretion after the post-merger QS collapses to a BH, and the X-ray bump forms. This post-merger QS model might be tested by future observations, with either advanced gravitational wave detectors (e.g., advanced LIGO and VIRGO) or X-ray/optical telescopes. Copyright © 2017, The Authors. All rights reserved.

关键词： Stars

Can turbulence dominate depolarization of optical blazars?

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arXiv 2017年

作者： Guo, Xiaotong Mao, Jirong Wang, Jiancheng Yunnan Observatories Chinese Academy of Sciences Kunming Yunnan Province650011 China Center for Astronomical Mega-Science Chinese Academy of Sciences 20A Datun Road Chaoyang Dis-trict Beijing100012 China Key Laboratory for the Structure and Evolution of Celestial Objects Chinese Academy of Sciences Kunming650011 China University of Chinese Academy of Sciences Beijing100049 China

We carefully examine the depolarization feature of blazars in the optical and near-infrared bands using the sample of Mead et al. Magnetohydrodynamics (MHD) turbulence could be one possible reason for the depolarization of optical/infrared blazars when we apply the theoretical analysis of Lazarian & Pogosyan. We further identify in the sample that the depolarization results shown in most blazars roughly obey the form of the three-dimensional anisotropic Kolmogorov scaling. The effective Faraday rotation window length scale is not small enough to resolve the polarization correlation length scale in the blazar sample. The depolarization and the related turbulent features show diversities in different blazar sources. We suggest more simultaneous observations in both the optical/infrared and the high-energy bands for the study of the blazar polarization. Copyright © 2017, The Authors. All rights reserved.

关键词： Depolarization

Spectropolarimetry and photometry of the early afterglow of the gamma-ray burst GRB 191221B

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arXiv 2020年

作者： Buckley, David A.H. Bagnulo, S. Britto, R.J. Mao, J. Kann, D.A. Cooper, J. Lipunov, V. Hewitt, D.M. Razzaque, S. Kuin, N.P.M. Monageng, I.M. Covino, S. Jakobsson, P. van der Horst, A.J. Wiersema, K. Böttcher, M. Campana, S. D'Elia, V. Gorbovskoy, E.S. Gorbunov, I. Groenewald, D.N. Hartmann, D.H. Kornilov, V.G. Mundell, C.G. Podesta, R. Thomas, J.K. Tyurina, N. Vlasenko, D. van Soelen, B. Xu, D. South African Astronomical Observatory PO Box 9 Observatory Road Observatory Cape Town7935 South Africa Department of Astronomy University of Cape Town Private Bag X3 Rondebosch7701 South Africa Armagh Observatory & Planetarium College Hill ArmaghBT61 9DG United Kingdom Department of Physics University of the Free State PO Box 339 Bloemfontein9300 South Africa Yunnan Observatories Chinese Academy of Sciences Yunnan Province Kunming650011 China Center for Astronomical Mega-Science Chinese Academy of Sciences 20A Datun Road Chaoyang District Beijing100012 China Key Laboratory for the Structure and Evolution of Celestial Objects Chinese Academy of Sciences Kunming650011 China Glorieta de la Astronomía s/n Granada18008 Spain M. V. Lomonosov Moscow State University SAI Physics Department 13 Univeristetskij pr-t Moscow119991 Russia M.V.Lomonosov Moscow State University Physics Department Leninskie gory GSP-1 Moscow119991 Russia Department of Physics University of Johannesburg PO Box 524 Auckland Park2006 South Africa Mullard Space Science Laboratory Dept. of Space and Climate Sciences University College London Holmbury St Mary DorkingRH5 6NT United Kingdom I-23807 Italy Centre for Astrophysics and Cosmology Science Institute University of Iceland Dunhagi 5 Reykjavík107 Iceland Department of Physics The George Washington University 725 21st St. NW WashingtonDC20052 United States The George Washington University WashingtonDC20052 United States Department of Physics University of Warwick Gibbet Hill Road CoventryCV4 7AL United Kingdom Department of Physics and Astronomy University of Leicester LE1 7RH United Kingdom Centre for Space Research North West University Potchefstroom South Africa ASI-Space Science Data Center via del Politecnico snc Rome00133 Italy INAF - Osservatorio Astronomico di Roma via Frascati 33 Monte Porzio Catone00040 Italy Southern African Large Telescope Foundation PO Box 9 Observatory Road Obse

We report on results of spectropolarimetry of the afterglow of the long gamma-ray burst GRB 191221B, obtained with SALT/RSS and VLT/FORS2, as well as photometry from two telescopes in the MASTER Global Robotic Network, at the MASTER-SAAO (South Africa) and MASTER-OAFA (Argentina) stations. Prompt optical emission was detected by MASTER-SAAO 38 s after the alert, which dimmed from a magnitude (white-light) of ∼ 10 to 16.2 mag over a period of ∼10 ks, followed by a plateau phase lasting ∼10 ks and then a decline to ∼18 mag after 80 ks. The light curve shows complex structure, with four or five distinct breaks in the power-law decline rate. SALT/RSS linear spectropolarimetry of the afterglow began ∼ 2.9 h after the burst, during the early part of the plateau phase of the light curve. Absorption lines seen at ∼ 6010 Å and 5490 Å are identified with the Mg II 2799 Å line from the host galaxy at z = 1.15 and an intervening system located at z = 0.96. The mean linear polarisation measured over 3400 − 8000 Å was ∼1.5% and the mean equatorial position angle (θ) ∼65◦. VLT/FORS2 spectropolarimetry was obtained ∼ 10 h postburst, during a period of slow decline (α = −0.44), and the polarisation was measured to be p = 1.2% and θ = 60◦. Two observations with the MeerKAT radio telescope, taken 30 and 444 days after the GRB trigger, detected radio emission from the host galaxy only. We interpret the light curve and polarisation of this long GRB in terms of a slow-cooling forward-shock. Copyright © 2020, The Authors. All rights reserved.

关键词： Stars