Kristopher Mortensen: Northwestern Univerisity Undergraduate

Kristopher Mortensen

Undergraduate Student
Northwestern University Department of Physics and Astronomy

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What Is A Kilonova?

The Large Synoptic
Survey Telescope

Target of Opportunity
Strategy

Conclusions

About Me

Target of Opportunity Strategy

The Target of Opportunity Strategy is the method that LIGO and LSST will use to seek out kilonovae in the sky. The strategy is as follows:

1. LIGO will observe the sky for gravitational wave sources.
2. Once a source is detected, LIGO will map the sky and determine what type of merger the source is.
3. If it is an NS-NS or NS-BH merger, LIGO will inform LSST where in the sky the source is.
4. Once LSST receives word of a source, it will search through the mapped area and use its "discovery metric" to locate the kilonova.

Problems for LSST

While it may seem trivial to create a proper cadence for LSST to measure a portion of the sky, the problems increase when the telescope needs to discover a kilonova. Some of the issues are:

Kilonova light curves diminish from view over the course of 7-10 days.
There is a 12-hour delay for LSST to receive information from a source that LIGO detected.
There are two different types of kilonova light curves depending on the type of merger.
The light of a kilonova spans from the UV to the NIR.
Kilonovae are scattered in areas filled with supernovae, gamma ray bursts, and other transient sources that may confuse LSST to think they are kilonovae.
The weather may be too poor for proper observing.

Therefore, our plan of action, or what will now be called our "discovery metric," for finding kilonovae must take into account all of these different variables. If we can simulate these variables and find a way to discover kilonovae at a high efficiency, then we will have a strategy that LSST should use.

Discovery Metrics

To optimize the success rate of the LSST, we decided to concentrate on the kilonova's defining features: it's relatively cold explosions, and it's short visibility time. Since kilonovae are typically colder than other stellar explosions, they tend to have a "red" color (f-z > 0.5 where f is a filter other than the z-band). However, other theories suggest there are "blue" kilonovae (f-z < -0.5) also may exist, so we included blue kilonova in the 1-day discovery metric. Apart from color, the short visibility time means a large change in magnitude (Δm) in one of LSST's filters.

Using theoretical light curves from Tanaka and Hotokezaka (2013), we designed and simulated three discovery metrics (see table below) for various NS-NS and NS-BH mergers. Each discovery metric simulation has the following criteria:

Maximum of 4 observations by LSST.
First observation happens at a random time in a 24-hour period.
If LSST cannot observe due to bad weather (20% probability), then it will observe 24 hours later.
The farther away the kilonova is, the harder it is for LSST to detect it.

We then created efficiency plots for how well LSST was able to discover kilonovae at different distances with these metrics. The histogram shows the efficiency of LSST at a particular distance and the data points shows the total number of kilonovae that LSST could have discovered.

1-Day Discovery Metric

NS-NS Efficiency Plot	NS-BH Efficiency Plot

3-Day Discovery Metric

NS-NS Efficiency Plot	NS-BH Efficiency Plot

7-Day Discovery Metric

NS-NS Efficiency Plot	NS-BH Efficiency Plot