Continuous gravitational waves should be easier to detect because they last much longer than signals from compact object collisions. One possible source of continuous waves is neutron stars, which are the stellar “corpses” left behind by supernova explosions of massive stars. After the initial explosion, the star collapses on itself, then compresses atoms into an ultra-dense ball of subatomic particles called “neutrons” — hence the name “neutron star.” Since the CW signal is related to the rotational speed of the neutron star, using more conventional telescopes to make precise measurements of the spin frequency would greatly increase the chances of detecting these elusive waves.
In a recent study, a team of researchers led by OzGrav PhD student Shanika Galaudage from Monash University sought to determine the rotational frequency of neutron stars to help detect continuous gravitational waves.
Possible source of continuous gravitational waves
In the study, the scientists hypothesized that continuous gravitational waves arise indirectly from the gradual accumulation of matter from low-mass companion stars onto neutron stars — binary systems of neutron stars and companion stars known as low-mass X-ray binaries (LMXBs).
If a neutron star can maintain a “mountain” of accumulated matter, even if it’s only a few centimeters high, then it will produce a continuous wave. The frequency of these waves is related to the rotational speed of the neutron star. The faster the accumulation of this material, the larger the “mountain”, creating a larger continuous wave. Systems that accumulate this material faster also appear brighter in X-ray light. Therefore, the brightest LMXBs are the most promising targets for continuous wave detection.
Scorpius X-1 (Sco X-1) and Cygnus X-1 (Cyg X-2) are the two brightest LMXB systems — Sco X-1 is the second brightest in X-rays compared to the sun. In addition to their extreme brightness, scientists also know a lot about these two LMXB systems, which makes them ideal sources for studying continuous waves. But their spin frequencies are still unknown.
Study leader Shanika Galaudage said: “One way we can determine the rotational speed of these neutron stars is by searching for X-ray pulses. X-ray pulses from neutron stars are like cosmic beacons. If we can determine the timing of the pulses, we will immediately be able to reveal Their rotational frequencies are closer to the detection of a continuous gravitational wave signal.”
“Sco X-1 is one of our best prospects for the first detection of continuous gravitational waves, but it’s a very difficult data analysis problem,” said OzGrav researcher and paper co-author Karl Wette from the Australian National University. Finding a spin frequency in the X-ray data is like shining a spotlight on the gravitational wave data. ‘Here, this is where we should be looking’. Then, Sco X-1 will be a hit in detecting continuous gravitational waves.”
Search for X-ray pulsations
The team searched for X-ray pulses from Sco X-1 and Cyg X-2. They processed more than 1,000 hours of X-ray data collected by the Rossi X-ray Timing Explorer instrument. The search used a total of about 500 hours of computing time on the OzSTAR supercomputer.
Unfortunately, however, this study did not find any clear evidence of pulsation from these LMXB sources. There could be a number of reasons for this: LMXBs could have weak magnetic fields, not strong enough to support detectable pulses, or it could be that pulses appear and disappear over time, which would make them difficult to detect. In the case of Sco X-1, it’s likely a black hole, so it shouldn’t be expected to produce X-ray pulsations.
The study does find an optimal limit on how bright these X-ray pulsations can be if they do occur, but these results could also mean that neutron stars cannot sustain mountains of matter under their strong gravitational pull. Future research could build on this research by employing better search techniques and more sensitive data.