Paper Title: The Fastest Path to Discovering the Second Electromagnetic Counterpart to a Gravitational Wave Event
Authors: Ved Shah, Ryan J. Foley, Gautham Narayan
Lead Author Institute: Center for Interdisciplinary Exploration and Research in Astrophysics, Northwestern University
Status: [Submitted] AAS Journals
What is a Kilonova and How Are They Discovered?
We talk about Kilonovae very often on astrobites. It has been the subject of these: [1, 2, 3, 4] bites in the past! Today’s paper focuses on how Kilonovae are discovered using gravitational wave detectors. First, a bit of background on this front:
Kilonovae are the result of the merger of either a binary neutron star system or a neutron star-black hole binary system (I will use the phrase “Neutron Star Merger” to account for both of these cases). Such a binary system shrinks over time, pulling the two objects in orbit closer to each other as orbital energy is lost over time via gravitational wave (GW) radiation. This emission is strongest at the exact moment of the merger, creating the famous GW “chirp” signal. We can detect this GW signal using the four (LIGO has two sites: Hanford and Livingston) GW detectors we have: LIGO, Virgo, and KAGRA (LVK). Because GWs are emitted at the exact moment of the merger, the GW emission is an indicator that the kilonova is starting. As such, as soon as such a GW signal is detected, you could point a telescope to the location of the source of this GW signal and start detecting light from the kilonova. Read this bite to learn much more about the gravitational waves emitted by kilonovae!
As such, the combination of the two LIGO sites, Virgo, and KAGRA gives us four different michelson interferometers for detecting GWs from astrophysical sources, each with different sensitivities and detection thresholds. GWs coming from a neutron star merger can tell a lot about the system before it merged, like the mass of each body in the binary; whereas the electromagnetic (EM) emission from the kilonova can reveal a lot about the system after the merger, like the amount of radioactive material created and the energy of the explosion.
The four detectors attempt to coincide their maintenance and upgrade schedules, so that they overlap their active observing time as much as possible. Each period of active observing is called an “Observing Run” (i.e. O1 and O2 are the first and second observing runs, respectively). See Figure 1, which shows all previous and planned observing runs for each of LVK.
How Many Kilonovae?
So far, only two neutron star mergers have been detected from GW emission: GW170817, whose EM counterpart was extensively followed; and GW190425, whose EM counterpart was never found. As such, the last (and only) kilonova with extensive GW and EM observations was during O2-in 2017-over 7 years ago! None were detected in O3, and none still since O4 began in May 2023. O4 is planned to continue until June 9, 2025, after which a two year downtime is planned before O5. After this, O5 is planned to run for several years.
Given the quite rare occurrence of Neutron Star merger detections through LVK, today’s paper attempts to estimate the number of days expected to pass before the next detection. Specifically, the authors of this paper ask: are the expected upgrades to LVK for O5 worth the planned downtime between O4 and O5? To answer this, the authors use simulations to predict the number of detectable neutron star mergers over the course of O4 and O5..
Such a simulation is complicated, however, because we have large uncertainties on the intrinsic rate of these events. Given that only two have ever been detected (GW170817 and GW190425), and that only one (GW170817) has been extensively studied, the diversity and rate of these explosions is poorly understood.
Simulating LVK Detections
To determine the number of detectable neutron star mergers to LVK, the authors need to know 1) the strength of the GW emission for a given merger event and 2) how many merger events are expected in a given time period over a given region in space. In addition, the authors only consider mergers with kilonovae that would be bright enough (r < 23 apparent magnitude) to be observed optically. The strength of the GW signal is controlled by the masses of the binary objects (influencing the intrinsic GW emitted signal) and the distance to the merger from Earth (farther away events have weaker signals). The optical brightness of the kilonova is controlled by the mass of the emitted ejecta and the angle at which the kilonova is pointed relative to Earth.
To generate samples of neutron star merger events, the authors sample from theoretically expected distributions of binary masses and emitted ejecta masses, and randomly choose merger locations within a region 910 Megaparsecs across. For each sampled event, they check whether it could be detected by the O4 and O5 versions of LVK. By running many such simulations, they measure how long it could take before a kilonova is detected by LVK.
Results
Overall, they find that even with the planned upgrades to LVK for O5, it would not be worth the two year wait. With their measurements of the amount of time before the first detection in both O4 and O5, they find the difference between the amount of time expected to pass before the first detection in O5 versus in O4. They then show a cumulative probability distribution of this value in Figure 2. They find that the probability that this difference is 125 days or less is 50%. As such, if the shutdown is longer than ~125 days, it would be more worthwhile to keep O4 running than shutting LVK down to upgrade to O5. In fact, they find that the difference in the amount of time before the first detection of a kilonova in O5 versus O4 has a ~90% probability of being less than 2 years. Therefore, the expected upgrades to LVK may not reduce the time until the next kilonova detection by a substantial amount. In figure 2, they show the cumulative distribution of the difference in the amount of time before first detection in O5 versus O4.
In addition, they find that the distribution of expected kilonovae detected in O5 would be farther and fainter, making them harder to track and observe using EM telescopes (see figure 3).
Finally, the paper ends with a call to action to change the current plans of LVK to try to reduce the amount of downtime between O4 and O5, or to simply extend O4. They point out that it may well be worth it to hold off on LVK upgrades and to just wait and watch the sky for new events!
Future Avenues
This paper hits on a somewhat surprising question about O4 and about neutron star mergers: Where are all the kilonovae? We expected so many more than we have so far detected. The fact that we have detected so few potentially suggests that our expectations need to be revised about how binary neutron star systems form, and how long they take to merge after formation. This is potentially still a wide-open question with far-reaching implications for how stars form and die and about how heavy elements are formed in our universe!
Astrobite edited by Skylar Grayson
Featured image credit: ESO
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