Astronomers search for X-ray signposts of the elusive continuous gravitational waves
In the last few years, astronomers have achieved an incredible milestone: the detection of gravitational waves, vanishingly weak ripples in the fabric of space and time emanating from some of the most cataclysmic events in the Universe, including collisions betweens black holes and neutron stars. So far there have been over 90 gravitational-wave detections of such events, observable for only ~0.1 to 100 seconds. However, there may be other sources of gravitational waves, and astronomers are still on the hunt for continuous gravitational waves.
Continuous gravitational waves should be easier to detect since they are much longer in duration compared to signals from compact-object collisions. A possible source of continuous waves is neutron stars, which are stellar “corpses” left over from supernova explosions of massive stars. After the initial explosion, the star collapses in on itself, crushing atoms down into a super-dense ball of subatomic particles called “neutrons” - hence the name “neutron star”. The continuous wave signal is related to how fast the neutron star is spinning, so precise measurements of the spin frequency using more conventional telescopes would greatly improve the chance of detection of these elusive waves.
In a recent study, led by OzGrav PhD student Shanika Galaudage from Monash University, scientists aimed to determine neutron stars’ spin frequencies to help detect continuous gravitational waves.
Possible sources of continuous gravitational waves
In this study, researchers hypothesised that continuous gravitational-waves indirectly come from the gradual accumulation of matter onto a neutron star from a low-mass companion star–these binary systems of a neutron star and companion star are called low mass X-ray binaries (LMXBs).
If the neutron star can maintain an accumulated "mountain" of matter, (even if only a few centimetres in height!), it will produce continuous waves. The frequency of these waves relate to how fast the neutron star is spinning. The faster you accumulate this matter, the bigger the "mountain", producing larger continuous waves. Systems that accumulate this matter more quickly are also brighter in X-ray light. Therefore the brightest LMXBs are the most promising targets for detecting continuous waves.
Scorpius X-1 (Sco X-1) and Cygnus X-1 (Cyg X-2) are two of the brightest LMXB systems–Sco X-1 ranks second in X-ray brightness compared to the Sun. In addition to their extreme brightness, scientists know a lot about these two LMXB systems, making them ideal sources of continuous waves to study. But, their spin frequencies are still unknown.
“A way we can determine how fast these neutron stars are spinning is by searching for X-ray pulsations,” says study lead Shanika Galaudage. “X-ray pulsations from neutron stars are like cosmic lighthouses. If we can time the pulse we would immediately be able to reveal their spin frequency and get closer to detecting the continuous gravitational-wave signal.”
“Sco X-1 is one of the best prospects we have for making a first detection of continuous gravitational waves, but it’s a very hard data analysis problem,” says OzGrav researcher and study co-author Karl Wette, from The Australian National University. “Finding a spin frequency in the X-ray data would be like shining a spotlight on the gravitational wave data: ‘here, this is where we should be looking’. Sco X-1 would then be a red-hot favourite to detect continuous gravitational waves.”
Searching for X-ray pulsations
The team performed a search for X-ray pulsations from Sco X-1 and Cyg X-2. They processed over 1000 hours of X-ray data collected by the Rossi X-ray Timing Explorer instrument. The search used a total of ~500 hours of computational time on the OzSTAR supercomputer!
Unfortunately, the study did not find any clear evidence of pulsations from these LMXB sources. There are a number of reasons why this could be: the LMXB could have weak magnetic fields which are not powerful enough to support detectable pulsations. Or it could be that the pulsations come and go over time, which would make them hard to detect. In the case of Sco X-1, it could possibly be a black hole, which we would not expect to produce X-ray pulsations.
The study does find the best limits on how bright these X-ray pulsations could be if they did occur; these results could mean that neutron stars cannot sustain mountains of matter under its strong gravity. Future research can build on this study by employing better search techniques and more sensitive data.
Written by OzGrav researcher Shanika Galaudage (Monash University)
Published in MNRAS: Deep searches for X-ray pulsations from Scorpius X-1 and Cygnus X-2 in support of continuous gravitational wave searches
Gravitational wave scientists from The University of Western Australia have led the development of a new laser modesensor with unprecedented precision that will be used to probe the interiors of neutron stars and test fundamentallimits of general relativity.
Research Associate from UWA’s Centre of Excellence for Gravitational Wave Discovery (OzGrav-UWA) Dr Aaron Jones,said UWA co-ordinated a global collaboration of gravitational wave, metasurface and photonics experts to pioneer anew method to measure structures of light called ‘eigenmodes’.
“Gravitational wave detectors like LIGO, Virgo and KAGRA store enormous amount of optical power and several pairs ofmirrors are used to increase the amount of laser light stored along the massive arms of the detector,” Dr Jones said.
“However, each of these pairs has small distortions that scatters light away from the perfect shape of the laser beamwhich can cause excess noise in the detector, limiting sensitivity and taking the detector offline.
“We wanted to test an idea that would let us zoom in on the laser beam and look for the small ‘wiggles’ in power thatcan limit the detectors’ sensitivity.”
Dr Jones said a similar problem is encountered in the telecoms industry where scientists are investigating ways to usemultiple eigenmodes to transport more data down optical fibres.
“Telecoms scientists have developed a way to measure the eigenmodes using a simple apparatus, but it’s not sensitiveenough for our purposes,” he said. “We had the idea to use a metasurface – an ultra-thin surface with a special patternencoded in sub-wavelength size – and reached out to collaborators who could help us make one.”
The proof-of-concept setup the team developed was over one thousand times more sensitive than the originalapparatus developed by telecoms scientists and the researchers will now look to translate this work into gravitational-wave detectors.
OzGrav-UWA Chief Investigator Associate Professor Chunnong Zhao said the development is another step forward in detecting and analysing the information carried by gravitational waves, allowing us to observe the universe in newways.
“Solving the mode sensing problem in future gravitational wave detectors is essential if we are to understand theinsides of neutron stars and further our observation of the universe in a way never before possible,” Associate ProfessorZhao said.
The breakthrough is detailed in a study published in Physical Review.
WRITTEN BY MILKA BUKILICIN - UWA RESEARCH