Effect of a High Opacity on the Light Curves of Radioactively Powered Transients from Compact Object Mergers (1303.5787v1)

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.

• The paper shows that high lanthanide opacity alters NSM light curves, extending transient durations to over a week.
• It employs time-dependent, multi-wavelength radiative transport calculations to reveal an infrared-dominated spectral energy distribution.
• The analysis highlights a two-component mass outflow, offering a unique observational signature for electromagnetic counterparts to gravitational waves.

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

The paper by Barnes and Kasen offers an in-depth analysis on the effect of opacity on the light curves of radioactively-powered transients resulting from compact object mergers such as neutron star mergers (NSMs). The paper examines how these transients serve as potential electromagnetic counterparts to gravitational wave (GW) signals, which are significant for enhancing the scientific return from GW detections.

Key Contributions

The primary focus of this investigation was the opacity of heavy r-process elements, specifically lanthanides, within the ejected matter from compact object mergers. Previous models of such events had utilized opacity values akin to those of iron group elements. However, this paper identifies that the inclusion of lanthanides can increase the opacity by orders of magnitude, resulting in significant alterations to the resultant light curves.

Results and Findings

1. Opacity Increase: The paper demonstrates that the high opacity of lanthanides significantly alters the predicted light curves of NSMs. The enhanced opacity leads to longer-duration light curves, sometimes lasting for over a week, contrary to prior assumptions that the duration would peak sharply at about 1 day.
2. Spectral Energy Distribution (SED): With the use of time-dependent, multi-wavelength radiative transport calculations, the paper finds that emission is primarily in the infrared bands, with prominent optical line blanketing. The SED corresponds closely to a blackbody near the recombination temperature of the lanthanides at approximately 2500 K.
3. Two-Component Mass Outflow: A theoretical model is considered where a secondary outflow, rich in $^ {56} \text{Ni}$, is expelled from the system. This scenario results in a distinct two-component spectral energy distribution, displaying both optical and infrared peaks. Such composite light curves present a unique observational signature conducive for identification.

Implications and Future Directions

The implications of these results are substantial for observational astrophysics, particularly for the identification and paper of NSMs. The insights provided by Barnes and Kasen could guide the development of strategies for observing these events and understanding their contribution to heavy element nucleosynthesis.

Advanced observational facilities, particularly those with infrared capabilities, will be essential for detecting these transients. As new GW events are detected, efforts can be tailored to identify their electromagnetic counterparts using the characteristics outlined in this work. Moreover, the distinctive features of the two-component light curves if $^{56}\text{Ni}$ is present emphasize the need for rapid multi-band follow-up observations.

Future research should focus on reducing the uncertainties in opacity calculations by refining the treatment of lanthanide atomic structures. Additionally, more complex three-dimensional models and comprehensive radiative transfer calculations considering the geometry and interaction of mixed compositional ejecta will further elucidate the observational signatures of these astrophysical events. These efforts will bridge theoretical predictions and empirical observations, ultimately leading to a deeper understanding of the universe’s heavy element production processes.