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Artist’s impression of a compact neutron star, around which a disk of gas and dust swirls. Image: Raphael.concorde / Wikimedia Commons, CC BY-SA 4.0
- Occasionally the radiation emitted by neutron stars in a binary star increases by about ten times within a few seconds and drops sharply to the average intensity within a few minutes.
- Using data from the Indian space telescope AstroSat, scientists at IISER Mohali have gained important new insights into the astrophysics of these thermonuclear explosions.
- AstroSat’s specialty is to use its five instruments to simultaneously observe the radiation emitted by the same astronomical source in several frequencies.
Neutron stars are natural laboratories of physics like none on earth. But for all the power and spectacle they can muster, physicists struggled to fully understand them because of a loophole in their observations.
Now, a new study shows that India’s first and only space telescope, AstroSat, could fill that void and help solve a cosmic mystery.
All ordinary matter is made up of atoms, and in the centers all these atoms are nuclei made up of protons and neutrons. But neutron stars are only made up of neutrons packed against each other. That makes them extremely dense: a neutron star could be as massive as the sun, but only have a radius of about 10 km. As such, they could be more than 100,000,000,000,000 times as dense as the sun.
Only quantum mechanics can describe the properties and behavior of such ultra-dense matter. At the same time, the high density also offers an insight into the behavior of strong gravitational fields.
There is only one kind of cosmological object that is denser than neutron stars – black holes. And like neutron stars, their properties mysteriously combine quantum mechanics and gravity.
However, no light can escape from black holes, which makes them invisible except for specialized telescopes. Neutron stars distort the path of light but do not capture it, which enables physicists to study them directly.
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Sometimes neutron stars are gravitationally connected to other stars and orbit each other in a star dance. Scientists call such systems binary and examine them with the X-rays they emit.
Occasionally, the radiation emitted by neutron stars in a binary star increases by about ten times within a few seconds and drops to the average intensity within a few minutes. Astrophysicists call this lightning thermonuclear explosions.
A new study has shed light on the astrophysics of thermonuclear explosions. Scientists from India extracted data on a neutron star named 4U 1636-536 from the archives of AstroSat, the space telescope that India launched in 2015.
AstroSat’s specialty is to use its five instruments to simultaneously observe the radiation emitted by the same astronomical source in several frequencies.
Previously, astronomers examined 4U 1636-536 using the Rossi X-Ray Timing Explorer (RXTE), a NASA space telescope that observes the X-rays emitted by astronomical sources. RXTE detected photons in the energy range of 2-60 keV – about 1,000 times more energetic than typical yellow light.
By studying the photons emitted by neutron stars all MicroSecond, astronomers discovered high-frequency oscillations in the intensity of the radiation in the range of 300-600 times per second during the burst.
Experts have not yet been able to agree on what could be causing these vibrations.
A popular theory, also known as the flame propagation model, holds that nuclear reactions begin in localized parts of the neutron star, creating “hotspots” that then spread across the entire surface as the neutron star rotates.
Since neutron stars could rotate a few hundred times per second, the frequency of the X-rays coming from the hotspot could be similar, according to the model.
The RXTE ceased operations in 2012. Since then, only one other telescope, NASA’s Neutron Star Interior Composition Explorer (NICER), has been investigating the rapid variability of the X-ray light from neutron stars.
But NICER can detect photons with an energy as low as 12 keV, while astronomers need to understand X-ray emissions as high as 80 keV.
Scientists have now found that AstroSat can fill this gap. And based on his data, they have reason to believe that the flame spread model might be valid.
“Detecting burst oscillations requires X-ray telescopes with a large collection area and fine temporal resolution – properties not found in many instruments,” said Navin Sridhar, a graduate student in astrophysics at Columbia University in New York. He was not involved in the present study.
One such instrument is on board AstroSat, the so-called Large Area X-ray Proportional Counter (LAXPC). It detects photons with an energy of 3-80 keV every 10 microseconds.
The scientists behind the new study obtained LAXPC data on 12 thermonuclear outbursts in six separate observations of neutron star 4U 1636-536.
When they analyzed this data more closely, they discovered high-frequency oscillations with about 581 oscillations per second in three of these bursts. This is a major achievement as it signals that AstroSat can get in where RXTE left off.
“We observed burst oscillations [around 46 hours’] Archive data from this well-studied neutron star collected between 2016 and 2018, â€Pinaki Roy of the Indian Institute of Science Education and Research Mohali (IISER), Mohali, and one of the authors of the study.
Aru Beri, also from IISER-Mohali and another author of the study, added: “The results extend the timeline of the behavior of the neutron star from previous RXTE observations and are an excellent benchmark for further similar studies with AstroSat.”
Sridhar agreed. “The fact that observations with AstroSat–LAXPC was able to confirm burst oscillations in this source … further enhances the performance of the instrument, â€he said.
“The detection enables scientists to reliably use AstroSat for future observations and discoveries related to thermonuclear bursts and burst oscillations with greater reliability.”
Next, the IISER Mohali team investigated how that strength the burst oscillations – measured by their amplitude – changes according to the energy of each oscillation.
By studying how burst oscillations change, Sridhar says astronomers can understand how a burst forms, spreads, and fizzles out.
The flame propagation model predicts how the energy distribution of the radiation from the neutron star source will develop during the rise phase of the explosions. The researchers wanted to check whether these predictions matched AstroSat observations.
They did. They also found that a hotspot’s speed of propagation depends on its distance from the neutron star’s equator. This is the study’s second important finding.
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Although the study increases the prospects of the flame propagation model, it is still not a perfect model: to date, it has not been able to explain all thermonuclear explosion observations, just a few.
In many cases, observations have stood in the way of understanding the propagation of thermonuclear explosions across the surface of the neutron star.
“There are a lot of questions about the model,” says Manoneeta Chakraborty, an astrophysicist at IIT Indore, who was not also involved in the study. “What physical conditions determine, for example, the spread of the heat flame?”
She explained that several parameters determine what the X-ray emission looks like during a burst, including the spin frequency of the neutron star, the surface temperature, and the origin of the hotspot.
“That’s an important conclusion,” said Chakraborty, referring to the IISER team’s claim to have a relationship between the speed of a burst and its position on the surface.
At the moment the flame propagation model is adjusting the data of another neutron star.
We don’t yet know if we can expect a single model to explain the behavior of all bursts everywhere. However, astronomers and astrophysicists hope the study can lead to more detailed observations of thermonuclear burst oscillations using AstroSat.
Debdutta Paul received her PhD from the Department of Astronomy and Astrophysics at the Tata Institute of Fundamental Research in Mumbai. He is currently a freelance science communicator and journalist. He tweets under @dbdttpl.
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