Searching for Neutron Star Mergers in the Absence of Gravitational Waves with Optical Afterglow Emission

Abstract

With the forth observing run of the LIGO-Virgo-KAGRA gravitational-wave network, which enabled the discovery of the kilonova (KN) counterpart to GW170817, ending with no new confirmed neutron star mergers, the intrinsic rate of these events must be even lower than previously estimated. As a result, building a sample of KNe will remain challenging even with continued GW observations, motivating complementary discovery strategies that do not rely on gravitational-wave triggers. In this work, we consider how leveraging bright short gamma-ray burst afterglows can aid in the discovery on KNe with the Rubin Observatory's upcoming Legacy Survey of Space and Time (LSST), whose unprecedented depth will make such detections feasible. We find that nearly on-axis ($θ_{\rm view} \leq 30°$) afterglows can enhance KN detection rates in the LSST $g$-band from $29^{+51}_{-21} \ \rm yr^{-1}$ to $91^{+160}_{-65} \ \rm yr^{-1}$. We further show how the colors of the observed events can be used to distinguish between neutron star merger counterparts with and without KN emission. This study demonstrates how critical multi-wavelength and multi-survey observations are for these rare events, especially without context from gravitational waves. Fortunately, detectable events will likely be discovered near peak with LSST, allowing for rapid follow-up and confirmation. We discuss key uncertainties in our study, particularly volume rate of merger events, and the degeneracy between the empirically determined explosion energy and ambient medium density.

Figures (9)

• Figure 1: The parameter space of $E_0$ and $n_0$ as covered by the sample of afterglows reported in fong_decade_2015 (dark red points). The red star marks the location of the median values reported for the sample in Table 4 of fong_decade_2015. The right and upper histograms show this distribution of $E_0$ values of the simulated sample of afterglows. Since $n_0$ is determined directly from the sampled distribution of $E_0$ and the fit to the fong_decade_2015 data (orange line), the distribution is identical, so it has been omitted.
• Figure 2: KN (orange) and KN and AG (blue) median light curves $u$ and $g$-bands LSST bands for events at a distance of $160 \ \rm Mpc$, corresponding to the LIGO range of NSMs with two $1.4 \ \rm M_{\odot}$ neutron stars. The respective transparent regions represent the area between the 16th and 84th percentiles and the LSST 5-$\sigma$ depths are indicated with the gray lines. $rizy$-band light curves are omitted here due to the lack of significant enhancement from the AG.
• Figure 3: The $68\%$ and $95\%$ contours of $g-r$ color vs $r-i$ color of only afterglow (black), kilonova 1-day post-merger (purple), and kilonova 5-days post-merger (red) emission. Along with the combined emission at 1 (orange) and 5 days (teal) post-merger. The afterglow evolution is negligible compared to that of the KNe and thus only occupies a line in this color-color space. For the combined emission at later times, there is a small region separate from the majority due to combined events which are dominated by AG emission, rather than KN emission, leading to the contour appearing to stretch between the regions of the color space where the AG only and KN only lie; however, for the majority of events the color-color information is able to separate most events for which there is KN emission present. Additionally, a reddening vector (red arrow) shows the direction a GW170817-like KN dietrich_multi-messenger_2020 at 1 day post-merge would move starting at $A_V=0$ to $A_V=1$.
• Figure 4: Top: The fractions of simulated KN and combined events detected in two LSST observations as a function of time of 1st observation relative to time of merger, $t_0$. Bottom: The median change in observed magnitude from 1st to 2nd observation in $u$-band (right) and $g$-band (left).
• Figure 5: The median change in absolute magnitude from peak to 1st observation in $u$-band (right) and $g$-band (left) over time of first observation.