Multiwavelength Analysis of Six Luminous, Fast Blue Optical Transients

TL;DR

We present six luminous fast blue optical transients (LFBOTs) discovered in ZTF and followed with optical, X-ray, radio, and millimeter observations. LFBOTs are identified by rapid light-curve evolution ($t_{1/2}\leq 12$ d) and blue peak colors ($g-r\leq -0.5$ mag), and exhibit high optical luminosities ($M_g<-20$) with X-ray or radio detections. Radio emission is modeled as synchrotron radiation from fast shocks in a dense circumburst medium, with peak luminosities $10^{38}-10^{40}$ erg s$^{-1}$ at rest-frame times $t_{rest}\approx 50-100$ d, and X-ray luminosities spanning $10^{42}-10^{44}$ erg s$^{-1}$ around $t_{rest}\approx 20$ d. Host galaxies are predominantly non-nuclear, star-forming systems with stellar masses $10^{9}-10^{11} M_\odot$; AT2024aehp shows a luminous optical plateau ($M<-19$ mag) with a blue continuum and a late-time radio brightening ($t_{rest}\approx 70-130$ d), supporting a common circumburst environment and a progenitor channel involving recent mass loss, such as a massive star merging with a compact object.

Abstract

We present multiwavelength observations and analysis of six luminous fast blue optical transients (LFBOTs) discovered in Zwicky Transient Facility (ZTF) survey data. We identified these LFBOTs from their fast light-curve evolution ($t_{1/2}\leq 12 $d), blue colors at peak brightness ($g-r\leq-0.5 $mag), a visible host galaxy, high optical luminosity ($M_g<-20$), and an X-ray or radio detection. With the exception of AT2024aehp (ZTF24abygbss), these transients exhibit peaks in their $10\,$GHz radio light curves at $t_{\text{rest}} \approx 50-100$ d, with peak radio luminosities ranging from $10^{38}-10^{40}$ erg s$^{-1}$. Modeling the radio emission as synchrotron radiation indicates a fast ($v=0.1-0.3c$) shock in a dense ($n_e\approx10^{3}-10^{4}$ cm$^{-3}$) medium. The X-ray emission varies by $\approx2$ orders of magnitude in luminosity ($10^{42}-10^{44}$ erg s$^{-1}$) at $t_{\text{rest}}\sim20 $d. Analysis of the host-galaxy photometry and spectroscopy for each transient shows that they are predominantly nonnuclear (a few kpc offset) with star-forming host galaxies of stellar masses $10^{9}-10^{11} ,M_\odot$. Unlike all other LFBOTs to date, AT2024aehp exhibited a luminous ($M<-19 $mag) plateau in the optical light curve; spectra during this plateau phase showed a featureless blue continuum. The $6-15$ GHz radio emission of AT2024aehp brightened by over an order of magnitude from $t_{\text{rest}} \approx70 $d to $t_{\mathrm{rest}} \approx130 $d. The mostly consistent radio behavior between optically selected LFBOTs implies a similar circumburst medium, leading us to prefer a progenitor scenario in which mass is lost in a consistent way shortly prior to the terminal event, such as a massive star merging with a compact object.