Title: Luminous Millimeter, Radio, and X-ray Emission from ZTF20acigmel (AT2020xnd)
Authors: Anna Y. Q. Ho, Ben Margalit, et al.
First author’s institution: Department of Astronomy, University of California, Berkeley
Journal: Submitted to the Astrophysical Journal. Open access on ArXiv.
Disclaimer: The author would first like to publicly state that Black lives and Black Trans lives matter. Secondly, the author condemns all police brutality against people of color. Lastly, the author recognizes that the writing of this article was performed on the stolen land of indigenous people.
A Cow, a Koala and a Camel Walk into a Bar
Despite our ability to discover thousands of astrophysical explosions each year, the turbulent demise of a star is still a pretty unique event. However, a stellar explosion becomes even more special when it has a fun name like AT2018cow (“the Cow”) or ZTF18abvkwla (“the Koala”). Both of these transients belong to the small, but growing, class of rare explosions called FBOTs (Fast Blue Optical Transients), whose acronym was designed to reflect their unique observational signatures. These events are “fast” because they rise to peak brightness in only a few days and “blue” because they have extremely hot temperatures. This strange observational behavior has also kept astrophysicists guessing as to where these atypical transients come from: Does a massive star implode to form an accreting black hole or magnetar? Or maybe a massive star explosion impacts high density gas? Or a black hole shreds a white dwarf?
Today’s paper seeks to add to the FBOT origin story by presenting a brand new event called AT2020xnd. Keeping with tradition, this transient was nicknamed the “Camel” after some clever rearranging of the letters in its ZTF transient name ZTF20acigmel. The Camel was discovered in a very distant galaxy (a few billion light years away!) and faded from view for optical telescopes in less than a month. However, the authors of today’s paper present a unique dataset of observations across the electromagnetic spectrum that revealed luminous photons coming from the Camel for hundreds of days after explosion. Another fascinating fuzzy mammal has joined this herd of extragalactic explosions!
The Synchrotron Emission That Broke the Camel’s Back
Unlike the slow-moving pack animals of Earth, this Camel wasted no time exploring its local environment. The authors of today’s paper observed the Camel in radio (like in your car), millimeter (like in airport scanners), and X-ray (like at the doctor) wavebands, finding that the explosion was exceptionally luminous at many frequencies across the electromagnetic spectrum. As shown in Figure 1, the Camel shone bright in radio and millimeter frequencies, indicating that the explosion was blasting through space at a few tenths of the speed of light (~17 MILLION times faster than a camel on Earth!). The author’s concluded that the source of the Camel’s luminous radio and millimeter emission was a process for creating radiation called synchrotron emission, which arises when particles are accelerated in a magnetic field. These observations also revealed that there was a likely a large amount of high density gas around the star at the time of explosion. These early-time, high frequency millimeter observations of the Camel are very unique and have only been obtained in this detail once before in an FBOT.
On the other end of the electromagnetic spectrum, this long-legged transient was also quite luminous in the X-rays as its high energy photons quickly faded from view after a few months of observation (Figure 2). Like other FBOTs, the origin of the X-rays in the Camel are somewhat ambiguous because they appear to be derived from a physically distinct process than what is driving the radio emission. The authors posit that the excess of X-rays could be linked to the formation of a central engine like a black hole or rapidly rotating neutron star.
Lastly, the authors try to understand the unique evolution of the Camel at radio and millimeter frequencies, specifically the sharp slope of the Camel’s spectral energy distribution at higher frequencies. As shown in Figure 3, the authors accomplish this by a novel modeling approach that invokes a physical process where synchrotron emission is not derived from the uniform acceleration of all of the particles in the expanding blastwave. Instead, some of the particles are accelerated to cause non-thermal emission while others are accelerated through thermal processes. The authors demonstrate for the first time that this special particle configuration within the explosion nicely reproduces the observations of the Camel in radio and millimeter bands. All humps accounted for!
The Camel represents a breakthrough discovery in our understanding of FBOTs and semi-relativistic explosions. We can only hope that telescopes surveys will continue to discover more of these fascinating events for us to study. Maybe an aquatic animal next time…?