Effect of a High Opacity on the Light Curves of Radioactively Powered Transients from Compact Object Mergers

TL;DR

This paper investigates how the presence of heavy r-process elements, especially lanthanides, significantly increases the opacity of ejecta from compact object mergers, leading to longer, infrared-dominated light curves and distinctive spectral features.

Contribution

It introduces higher opacity values for lanthanide-rich ejecta into radiative transfer models, revealing their impact on the duration and spectral characteristics of merger transients.

Findings

Higher opacities extend light curve durations to over a week.

Infrared emission dominates due to optical line blanketing.

A two-component spectral energy distribution is produced with 56Ni and r-process ejecta.

Abstract

The coalescence of compact objects are a promising astrophysical sources of gravitational wave (GW) signals. The ejection of r-process material from such mergers may lead to a radioactively-powered electromagnetic counterpart which, if discovered, would enhance the science return of a GW detection. As very little is known about the optical properties of heavy r-process elements, previous light curve models have adopted opacities similar to those of iron group elements. Here we report that the presence of heavier elements, particularly the lanthanides, increase the ejecta opacity by several orders of magnitude. We include these higher opacities in time dependent, multi-wavelength radiative transport calculations to predict the broadband light curves of one-dimensional models over a range of parameters (ejecta masses from 0.001 to 0.1 solar masses and velocities from 0.1 to 0.3c). We find that the higher opacities lead to much longer duration light curves which can last a week or more. The emission is shifted toward the infrared bands due to strong optical line blanketing, and the colors at later times are representative of a blackbody near the recombination temperature of the lanthanides (T ~ 2500K). We further consider the case in which a second mass outflow, composed of 56Ni, is ejected from a disk wind, and show that the net result is a distinctive two component spectral energy distribution, with a bright optical peak due to 56Ni and an infrared peak due to r-process ejecta. We briefly consider the prospects for detection and identification of these transients.