Hannah Middleton
Ground-based gravitational-wave observatories such as Advanced LIGO and Advanced Virgo have observed the mergers of black hole and neutron star systems, but they are also looking for long duration signals called continuous gravitational waves. Continuous waves are long lasting, periodic signals that are expected to be present in the detector data all the time. Rotating neutron stars in low mass X-ray binaries (LMXBs) are prime continuous wave targets. Traditional continuous wave LMXB targets, such as Scorpius X-1, have been subject to thorough searches, but these searches are hampered by a lack of electromagnetic observation of the neutron star rotation frequency. However, X-ray observations of some LMXBs have high-precision measurements for their rotation frequency (and also of their orbital parameters). We present a continuous wave search for five LMXBs which is finely targeted using information from X-ray observations. This search (Middleton et al. 2020) focuses on data from the second observation run which lasted from November 2016 to August 2017.
So far, gravitational waves have been detected from the mergers of black holes and neutron stars. These are transient signals which are typically seconds in length. Continuous gravitational waves on the other hand, are long lived signals which are expected to be weaker than the transient signals seen so far. They are yet to be detected. The data we use comes from LIGO (Laser Interferometer Gravitational-wave Observatory) and Virgo, which are ground-based observatories.
The source of continuous waves that we are interested in is spinning neutron stars. If a spinning neutron star has some asymmetry, such as a mountain on its crust, it will emit gravitational waves. The schematic represents a neutron star emitting gravitational waves on each rotation to produce a sinusoidal signal. These continuous waves pass through the Earth and we search from their signal in detectors. The continuous wave signal should be present in the detector data all the time.
Slide 2: Search targets
We focus the search on low mass X-ray binaries. These are systems where a low mass star (which is less massive than our Sun) is in orbit with a compact object such as a neutron star or a black hole. We concentrate on low mass X-ray binaries that contain neutron stars.
The schematic represents continuous waves being emitted from a neutron star in a binary. The continuous waves are Doppler shifted due to the binary motion.
We use information from X-ray observations to inform our continuous wave search. Good measurements of the neutron star rotation frequency as well as the orbital parameters of the binary help to target the search and reduces computational cost. We target five low mass X-ray binaries which are: HETE J1900.1-2455, IGR J00291+5934, SAX J1808.4-3658, XTE J0929-314, and XTE J1814-338.
Slide 3 Search Method
Although we have good information from X-ray observations about the rotation frequency of the neutron stars, it is possible that the frequency of the continuous wave signal wanders slowly over time. So we need a search method that can keep track of a signal over time and frequency. The schematic shows a grid in time and frequency where the brighter grid points trace out a signal. We use an efficient path finding method called the Viterbi algorithm to find the most probable path of a signal through the time-frequency grid.
This method has been used for continuous wave searches for several different targets. One of the prime targets for continuous waves is the low mass X-ray binary Scorpius X-1, which is particularly bright in X-rays. However one challenging aspect for the Scorpius X-1 search is that X-ray observations have not been able to measure the rotation frequency of the neutron star. This means that a wide range of frequencies need to be searched.
The targets we select here good estimates of the rotation frequency and orbital parameters from X-ray observations. This allows us to target the search more efficiently and reduces the computational cost of the analysis.
Slide 4 Results and next steps
We use data from LIGO's Observing Run 2, which lasted approximately 230 days. For this search we use only LIGO data due to the shorter duration of Virgo data in Observing Run 2. We make no detection of continuous waves in this first search for these targets. A follow-up search is planned for the new longer data set of Observing Run 3.