AT20211fa,also known as ZTF21aayokph,was detected by the Zwicky Transient Facility on 2021 May 4,at 05:34:48 UTC.Follow-up observations were conducted using a range of ground-based optical telescopes,as well as Swift/XRT and VLA instruments.AT20211fa 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 prompt 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 AT20211fa and obtain its burst time as 2021 May 3,at 22:09:50 UTC.Using afterglowpy,we model the multi-band afterglow of AT20211fa and find that the standard model cannot reproduce the late radio observations well.Considering that the microphysical parameters εe,εB(the energy fraction given to electrons and magnetic field),and ζN(the fraction of accelerated electrons)may vary with time,we then model the afterglow of AT20211fa taking into account the temporal evolution of the physical parametersεe,εB,and ζN and find in this case the multi-wavelength observations can be reproduced well.The initial Lorentz factor of AT20211fa can be estimated from the peak time of the early afterglow,which yields a value of about 18,suggesting that AT20211fa should be classified as a"dirty fireball."From the upper limit for the prompt emission energy of AT20211fa,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,θc～0.66 rad,θv～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 AT20211fa satisfies the empirical relationships observed in GRB samples.Additionally,the luminosity of AT20211fa falls within the range of observations for GRB samples;however,approximately 1 day after the burst,its luminosity exceeds that of the majority of GRB samples.
Key Laboratory of Dark Matter and Space Astronomy,Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210023,China
School of Astronomy and Space Science,University of Science and Technology of China,Hefei 230026,China
National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technology,Chinese Academy of Sciences,Nanjing 210042,China
CAS Key Laboratory of Astronomical Optics & Technology,Nanjing Institute of Astronomical Optics & Technology,Nanjing 210042,China
展开 >
National Natural Science Foundation of China(NSFC)National Natural Science Foundation of China(NSFC)National Natural Science Foundation of China(NSFC)National Natural Science Foundation of China(NSFC)National Natural Science Foundation of China(NSFC)
万方数据
