Cosmic X-Ray Search for Continuous Mysterious Gravitational-Wave Signposts

The observation of gravity waves, vanishingly faint disturbances in the fabric of space and time originating from some of the most catastrophic events in the Universe, such as mergers between black holes and neutron stars, has been a major accomplishment for astronomers in recent years. Observable lasting only 0.1 to 100 seconds, there have been over 90 gravitational-wave detections of these events too far. Astronomers are still looking for continuous gravitational waves, therefore there may be more sources of gravitational waves.

Given that they last much longer than the signals from collisions of compact objects, continuous gravitational waves should be simpler to find. Neutron stars, stellar "corpses" left over following the supernova explosions of huge stars, are one potential source of continuous waves. The star collapses in on itself after the first explosion, smashing atoms into a super-dense ball of subatomic particles known as "neutrons," thus the term "neutron star." Since the continuous wave signal is dependent on the neutron star's spin frequency, accurate measurements made with more conventional telescopes would significantly increase the likelihood of finding these elusive waves.

These binary systems of a neutron star and companion star are known as low-mass X-ray binaries, and it was theorized in this study that continuous gravitational waves indirectly emerge from the steady buildup of stuff onto a neutron star from a partner star (LMXBs).

The neutron star will continuously emit waves if it can sustain an accumulated "mountain" of matter, even if it is just a few millimeters high. The neutron star's rotation rate is correlated with the frequency of these waves. The "mountain" grows greater as this substance is accumulated more quickly, resulting in longer, more powerful waves. In X-ray light, systems that acquire this stuff more quickly are likewise more visible. Therefore, the most promising targets for detecting continuous waves are the brightest LMXBs.

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.

Searching for X-ray pulsations allows us to estimate how quickly these neutron stars are spinning, according to research leader Shanika Galaudage. "Neutron star X-ray pulsations are like cosmic lighthouses. We would quickly be able to determine their spin frequency and move a step closer to finding the continuous gravitational-wave signal if we could clock the pulse.

According to OzGrav researcher and study co-author Karl Wette of The Australian National University, "Sco X-1 is one of the finest possibilities we have for making the first detection of continuous gravitational waves, but it's a really demanding data analysis task." It would be like to flashing a spotlight on the gravitational wave data and saying, "This is where we should be searching," if a spin frequency were to be discovered in the X-ray data. Then, Sco X-1 would be the hot bet to find persistent gravitational waves. 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!

Sadly, the investigation was unable to uncover any conclusive proof of pulsations coming from these LMXB sources. There are several explanations for this, including the possibility that the LMXB has magnetic fields that are too weak to allow visible pulsations. It's also possible that the pulsations are difficult to notice since they fluctuate throughout time. It is possible that Sco X-1 is a black hole, which we would not anticipate emitting X-ray pulsations. The study does discover the best limitations on how intense these X-ray pulsations might be if they did happen; this finding may imply that neutron stars are unable to support mountains of mass under their powerful gravitation. By utilizing more sensitive data and improved search methods, future research can build on these findings.