Optical Transient Source AT2021lfa: A Possible “Dirty Fireball”

Abstract

AT2021lfa, also known as ZTF21aayokph, was detected by the Zwicky Transient Facility on May 4, 2021, 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. AT2021lfa is classified as an ``orphan afterglow'' candidate due to its rapid flux decline and its reddened color ($g-r = 0.17 \pm 0.14$ mag). For 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 AT2021lfa and obtain its burst time as May 3, 2021, at 22:09:50 UTC. Using \texttt{Afterglowpy}, we model the multi-band afterglow of AT2021lfa and find that the standard model cannot reproduce the late radio observations well. Considering that the microphysical parameters $\epsilon_e$, $\epsilon_B$ (the energy fraction given to electrons and magnetic field), and $\xi_N$ (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 $\epsilon_e$, $\epsilon_B$, and $\xi_N$ 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 the radiation efficiency is less than 0.02\%, which is much smaller than that of ordinary gamma-ray bursts. It is also interesting that the fitted values of jet angle and viewing angle are very large, $\theta_c \sim$ 0.66 rad, $\theta_v \sim$ 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. Additionally, the luminosity of AT2021lfa 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.