Optical Transient Source AT2021lfa:A Possible“Dirty Fireball”  

作　　者：Xue-Mei Ye Da-Ming Wei Yi-Ming Zhu Zhi-Ping Jin 

机构地区：[1]Key Laboratory of Dark Matter and Space Astronomy,Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210023,China [2]School of Astronomy and Space Science,University of Science and Technology of China,Hefei 230026,China [3]National Astronomical Observatories/Nanjing Institute of Astronomical Optics&Technology,Chinese Academy of Sciences,Nanjing 210042,China [4]CAS Key Laboratory of Astronomical Optics&Technology,Nanjing Institute of Astronomical Optics&Technology,Nanjing 210042,China

出　　处：《Research in Astronomy and Astrophysics》2024年第4期124-133,共10页天文和天体物理学研究（英文版）

基　　金：supported by the National Natural Science Foundation of China(NSFC,grant Nos.12073080,11933010,11921003,12233011,and 12225305)。

摘　　要：AT2021lfa,also known as ZTF21aayokph,was detected by the Zwicky Transient Facility on 2021 May 4,a 05:34:48 UTC.Follow-up observations were conducted using a range of ground-based optical telescopes,as wel as Swift/XRT and VLA instruments.AT2021lfa is classified as an“orphan afterglow”candidate due to its rapid flux decline and its reddened color(g-r=0.17±0.14 mag).For an optical transient source without promp gamma-ray detection,one key point is to determine its burst time.Here we measure the burst time through fitting the initial bump feature of AT2021lfa and obtain its burst time as 2021 May 3,at 22:09:50 UTC.Using afterglowpy,we model the multi-band afterglow of AT2021lfa and find that the standard model canno reproduce the late radio observations well.Considering that the microphysical parameters ε_(e),ε_(B)(the energy fraction given to electrons and magnetic field),andξN(the fraction of accelerated electrons)may vary with time we then model the afterglow of AT2021lfa taking into account the temporal evolution of the physical parameters ε_(e),ε^(B),and ξ_(N) and find in this case the multi-wavelength observations can be reproduced well.The initial Lorentz factor of AT2021lfa can be estimated from the peak time of the early afterglow,which yields a value of about 18 suggesting that AT2021lfa should be classified as a“dirty fireball.”From the upper limit for the prompt emission energy of AT2021lfa,we obtain that the radiation efficiency is less than 0.02%,which is much smaller than that of ordinary gamma-ray bursts(GRBs).It is also interesting that the fitted values of jet angle and viewing angle are very large,θ_(c)~0.66 rad,θ_(v)~0.53 rad,which may lead to the low Lorentz factor and radiation efficiency.When compared with GRB afterglow samples,it is evident that the onset bump timescale of AT2021lfa satisfies the empirical relationships observed in GRB samples.Additionally,the luminosity of AT2021lfa falls within the range of observations for GRB samples;however,approximately 1 day after the

关 键 词：gamma-rays:ISM methods:numerical radiation mechanisms:non-thermal 

分 类 号：O57[理学—粒子物理与原子核物理]