Short gamma-ray bursts are extremely bright bursts of high-energy light that last for a couple of seconds. In many of these bursts, there is a mysterious material left behind: a prolonged ‘afterglow’ of radiation, including X-rays. Despite the efforts of many scientists over many years, we still don’t know where this afterglow comes from.
In our recently accepted paper, we investigated a simple model that proposes a rotating neutron star—an extremely dense collapsed core of a massive supergiant star—as the engine behind a type of lengthy X-ray afterglows, known as X-ray plateaux. Using a sample of six short gamma-ray bursts with an X-ray plateau, we worked out the properties of the central neutron star and the mysterious remnant surrounding it.
The model we used was inspired by remnants from young supernova. While remnants from short gamma-ray bursts and supernovae have many differences, the energy driving from a rotating neutron star has the same underlying physics. So, if the remnant of a short gamma-ray burst is a neutron star, it must have a similar energy outflow as a supernova remnant.
In our study, we borrowed the basic physics from previous short gamma-ray burst models to predict the luminosity and duration of the X-ray plateau. For each short gamma-ray burst, the results suggested that the remnant neutron star is a millisecond magnetar: a neutron star with an extraordinarily powerful magnetic field. All known magnetars have a very slow rotation frequency; similarly, all observed neutron stars with millisecond spins have weak magnetic fields. This gap in observations isn’t surprising because the magnetic field of the star converts the rotational energy into electromagnetic energy. For a magnetar-strength field, this process happens on a scale from seconds to days – exactly the duration of most X-ray plateaux.
This paper is the first attempt at estimating the source of X-ray afterglows using this kind of model. As the model matures and further data is collected, we’ll be able to make stronger conclusions about the source of X-ray plateaux and, if we’re lucky, discover what these mysterious remnants are.
Written by OzGrav PhD student Lucy Strang – The University of Melbourne
Direct link to online publications, a journal citation or any other websites
Current status of paper (submitted/accepted/published)
Acceptance and/or publication date
Accepted 27/07/21, publication TBC
RESEARCH HIGHLIGHT: Bayesian inference for gravitational waves from binary neutron star mergers in third-generation observatories
In the 2030’s, gravitational-wave detectors will be thousands of times more sensitive than Advanced LIGO, Virgo, and KAGRA. The network of “third generation” (3G) observatories will almost certainly include Cosmic Explorer (US), Einstein Telescope (EU), and may include a Southern-hemisphere Cosmic-Explorer like observatory. These amazing instruments will see every binary neutron star merger in the Universe, and most binary black holes out to redshifts beyond 10: hundreds of thousands, possibly millions, of resolvable signals per year. Many of these signals will be extremely loud, with signal to noise ratios in the thousands, facilitating breakthroughs in fundamental physics and cosmology. And herein lies a challenge! How do we extract all the information from these signals? On the surface it seems like a straightforward task: just keep on running parameter estimation like we’re already doing! But it turns out that our current parameter estimation methods don’t scale so well when signals are really loud, and very long in band.
To see why, we imagined a binary neutron star merger signal “GW370817”, which originated about 40 Mpc from Earth — roughly the distance of GW170817 (assuming 3G detectors are online in 2037, we’re guaranteed to observe a thousand or so binary neutron star mergers on August 17th, 2037!). A network of 3G detectors would observe GW370817 for 90 minutes, with a staggering signal to noise ratio of 2500. Analysing this signal is around a thousand times more computationally expensive than analysing a signal in today’s detectors — by our back of the envelope estimates, it would take around 1000 years! This prohibitive analysis time is a hurdle to astrophysics with 3G data, and it’s the problem we solve in our paper. To drive down the computation time, we developed “reduced order models” of gravitational-wave signals which allow us to infer binary neutron star properties using heavily compressed data, with almost no loss in accuracy. We reduced the computational cost of inference on 3G data by a factor of 13’000. Together with a pinch of parallel computing, we’re able to perform data analysis in a few hours. This is good news for astrophysics in the 3G era.
While the 2030’s and 3G detectors are a few years away, our results and methods are useful for a wide range of theoretical and design studies, which are ramping up in lockstep with the development of the detector technology. For those old enough to remember, the first LISA mock-data challenges began in 2005, which gives a sense of how much exploratory work takes place before a detector is operational. For the time being, there are plenty of interesting astrophysics questions we can start to think about in the context of 3G detectors: how well will we be able to measure the neutron star equation of state and the maximum mass of neutron stars? And what will this tell us about extreme matter? How well can neutron star spins be measured and can this tell us anything about supernova mechanisms? etc…Our results and method will facilitate this kind of theoretical work by enabling us to perform robust inferences on binary neutron star properties in mock 3G data.
Link to research paper: https://arxiv.org/abs/2103.12274
Written by Rory Smith, Monash University
Galaxies host supermassive black holes, which weigh millions to billions times more than our Sun. When galaxies collide, pairs of supermassive black holes at their centres also lie on the collision course. It may take millions of years before two black holes slam into each other. When the distance between them is small enough, the black hole binary starts to produce ripples in space-time, which are called gravitational waves.
Gravitational waves were first observed in 2015, but they were detected from much smaller black holes, which weigh like tens times our Sun. Gravitational waves from supermassive black holes are still a mystery to scientists. Their discovery would be invaluable to figuring out how galaxies and stars form and evolve, and finding the origin of dark matter.
A recent study led by Dr Boris Goncharov and Prof Ryan Shannon—both researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)—has tried to solve this puzzle. Using the most recent data from the Australian experiment known as the Parkes Pulsar Timing Array, the team of scientists searched for these mystery gravitational waves from supermassive black holes.
The experiment observed radio pulsars: extremely dense collapsed cores of massive supergiant stars (called neutron stars) that pulse out radio waves, like a lighthouse beam. The timing of these pulses is extremely precise, whereas the background of gravitational waves advances and delays pulse arrival times in a predicted pattern across the sky, by around the same amount in all pulsars. The researchers now found that arrival times of these radio waves do show deviations with similar properties as we expect from gravitational waves However, more data is needed to conclude whether radio wave arrival times are correlated in all pulsars across the sky, which is considered the “smoking gun”. Similar results have also been obtained by collaborations based in North America and Europe. These collaborations, along with groups based in India, China, and South Africa, are actively combining datasets under the International Pulsar Timing Array, to improve the sky coverage.
This discovery is considered a precursor to the detection of gravitational waves from supermassive blackholes. However, Dr Goncharov and colleagues pointed out that the observed variations in the radio wave arrival times might also be due to ipulsar-intrinsic noise. Dr Goncharov said: “To find out if the observed "common" drift has a gravitational wave origin, or if the gravitational-wave signal is deeper in the noise, we must continue working with new data from a growing number of pulsar timing arrays across the world”.