NYU ABUDHABI UNRAVEL THE SECRETS OF THE UNIVERSE, ONE COMPACT STAR AT A TIME

The universe continues to be a source of mystery for scientists, astronomers, physicists, and others, but each time a mystery is unraveled, the world learns more and in turn is able to enhance humanity’s knowledge.

This is exactly what transpired when a group of scientists, physicists, astronomers from New York University Abu Dhabi, and other astronomers across the globe studied the Compact Star or millisecond pulsar PSR J10230038+. The research report entitled, “Matter ejections behind the highs and lows of the transitional millisecond pulsar PSR J10230038+”

David Russell, Associate Professor of Physics at NYU Abu Dhabi, alongside Maria Cristina Baglio, who now has joined a fellowship position in Italy, and Postdoctoral Associates

Payaswini Saikia, Kevin Alabarta and Samuele Crespi (Center for Astro, Particle, and Planetary Physics, NYUAD) all worked to uncover the mystery behind the transitional millisecond pulsars, better known as dead stars.

What is a pulsar or compact star?

A star is alive when burning, once the amount of hydrogen is gone, it is extinguished and becomes what scientists call a white dwarf. A white dwarf is the stellar core left behind after a dying star has exhausted its nuclear fuel and expelled its outer layers to form a planetary nebula.

Once the star explodes, and no longer burns hydrogen, it becomes a compact star, such as white dwarfs, neutron stars, and black holes.

Herein comes the term Pulsars, which was the topic of the research. Pulsars are rapidly rotating neutron stars that emit radio waves in beams from their magnetic poles. The radio beam sweeps around as the neutron star rotates, in some cases almost a thousand times every second.

In more scientific terms, transitional millisecond pulsars are an emerging class of sources that link low-mass X-ray binaries to millisecond radio pulsars in binary systems. These pulsars alternate between a radio pulsar state and an active lowluminosity X-ray disc state.

During the active state, these sources exhibit two distinct emission modes (high and low) that alternate unpredictably, abruptly, and incessantly. X-ray to optical pulsations is observed only during the high mode. The root cause of this puzzling behavior remains elusive.

The research

The research was the biggest multi-wavelength campaign ever performed on J1023, a pulsar. It aimed at understanding the root cause of the mode changes while the pulsar was in the active sub-luminous X ray state.

The observations were carried out over two nights utilizing 12 telescopes from radio telescopes to X-Rays. The results provided a physical picture of the high-low mode switches in J1023, which involved a rotation-powered pulsar, an accretion disc, and discrete mass ejections on top of a relativistic compact jet.

By modeling the broadband spectral energy distributions in both emission modes, the report showed that the mode switches are caused by changes in the innermost region of the accretion disc. These changes trigger the emission of discrete mass ejections, which occur on top of a compact jet, as testified by the detection of at least one short-duration millimeter flare with ALMA (Atacama Large Millimeter/ submillimeter Array Astronomical Observatory) at the high-tolow mode switch. The pulsar is subsequently re-enshrouded, completing the picture of the mode switches.

Russell explains, “First it is important to understand that stars are formed when molecular clouds in interstellar space, known as stellar nurseries, collapse. Stars start burning their fuel and are considered dead when they have burned all the hydrogen into helium. While they are burning, they are bright and luminous.”

He notes, “To date we are still trying in labs to generate the same energy and power that stars do when burning hydrogen, as part of our green energy free power goals, including utilizing our Sun.”

Russell explains, “The pulsar we researched, which is a spinning neutron star, is about as big as the city of Abu Dhabi, and spins 592 times per second. In addition, it contains more mass than the sun, about 12- solar masses and as such is the densest form of matter. When these stars collapse even more, they become black holes.”

What is even more interesting about compact stars, pulsars in particular, is that because they contain a lot of mass, they have a strong gravitational pull that sucks matter in, and so matter orbiting around them gets sucked in.

Located about 4500 light-years away in the Sextans constellation, pulsar PSR J10230038+, closely orbits another star. Over the past decade, the pulsar has been actively pulling matter off this companion, which accumulates in a disc around the pulsar and slowly falls towards it.

Russell adds, “A normal star like our Sun orbits the pulsar, and sometimes these pulsars, or neutron stars, rip off matter on the edge of the stars that orbit them, and this matter plummets towards the neutron star while doing that it forms the swirling disc. So, the paper studied the behavior of this hot matter surrounding the neutron star.”

The Results

Understanding the strange behavior of the pulsar star was interesting for the scientific community because it would help them to understand the difference between a normal star and a neutron star. In addition, the pulsar becomes even more interesting because it is behaving differently than other neutron stars observed before.

Russell explains, “When we observed the pulsar using the 12 telescopes at the same time from around the world, we observed that matter goes into the stream onto the disc surrounding the neutron star, and then some matter gets ejected instead of hitting the neutron star, this is when the flare of gas is seen, spitting out. We see it in the form of a flare.”

The pulsar switches between two modes. In the ‘high’ mode, the pulsar gives off bright X-rays, ultraviolet and visible light, while in the ‘low’ mode, it is dimmer. When at these frequencies it emits more radio waves. The pulsar can stay in each mode for several seconds or minutes, and then switch to the other mode in just a few seconds. This switching is what has far puzzled astronomers.

In short, it was discovered that the mode switching stems from an intricate interplay between the pulsar wind, a flow of high-energy particles blowing away from the pulsar, as well as matter flowing towards the pulsar. In the low mode, matter flowing towards the pulsar is expelled in a narrow jet perpendicular to the disc. Gradually, this matter accumulates closer and closer to the pulsar and, as this happens, it is hit by the wind blowing from the pulsating star, causing the matter to heat up. The system is now in a high mode, glowing brightly in the X-ray, ultraviolet and visible light. Eventually, blobs of this hot matter are removed by the pulsar via the jet.

According to Russell, the research offered much information, and currently scientists are studying how much matter is being ejected and if that is consistent with the general relativity theory.

Russell explains, “For us studying a neutron star, like this pulsar, helps us to understand its behavior, the amount of matter blasted into the galaxy, which could influence the structure of the galaxy. For example, if you measure the pulses of the pulsar, you can infer the gravitational waves across the universe.

Gravitational changes were discovered, and gravitational waves are important for understanding physics.

How the UAE can benefit

While Russell spent 10 years in Abu Dhabi at NYU Abu Dhabi, impressed with both the UAE and the university, he never anticipated when he moved that the UAE would be launching their space program.

He states, “While astronomy departments were shrinking globally after 2008, the UAE was expanding it. I am pleased with the support offered to me by NYU Abu Dhabi not only in terms of allowing me to supervise undergraduates for research, or the traveling to seek more knowledge, but in the fact that while astronomy and space science is still in its infancy here, the potential is huge.”

In terms of how the research can benefit the UAE, Russell believes there are two ways to answer the question.

First, within the scientific community, the research helps understand the exotic behavior in space. Each discovery is also an opportunity to understand something in Physics. He explains, “For example Einstein’s theory of relativity is so good that even now we are trying to prove it’s true in certain conditions. One theory for example is that with black holes and stars, it’s all about gravity, so if the laws of physics can work in those extreme cases it can work in all cases.”

The UAE in its mission to send probes into space, and the moon, complement what is being done in terms of research at UAE universities.

Russell believes that the research allowed scientists to learn more about ejecting matter, and while most are so far away, they don’t affect the earth, some blast waves can travel through the galaxy and interact with our solar system. He gives the example of Betelgeuse (pronounced Beetlejuice) which marks one of the shoulders in the constellation Orion. Its name comes from the Arabic bat al-jawzāʾ, which means “the giant›s shoulder”.

The Betelgeuse now or in a hundred thousand years will explode and given that it is close to earth, when this happens it will be so bright that it will be seen in daytime. While earth will be safe because of its magnetic field, the blast of the explosion will travel to our solar system. After it explodes, it could end up as a black hole, and the shock wave that travels through space might mean that astronauts on the moon will have to stay inside because of radiation exposure.”

Future Research

According to Russell, the team is currently carrying out several projects to study black holes in our galaxy. The importance of this research is that when a black hole becomes active, it sucks up matter nearby, this matter becomes bright Xrays.

He explains, “Using optical telescopes we can spot when an outburst like that starts. This is important to study. For example, last year a black hole got brighter, we discovered heat waves traveling through the disc got hotter and brighter, which helped us once again to understand black holes better.”

NYU Abu Dhabi has PHD students, studying black holes 3000 light years away. If studied thoroughly, scientists can estimate how much matter and energy they give out, helping to understand extreme gravitational objects.

For Russell, the research and sharing it with the general public can increase students’ interest in the topic of space and exploration. He ends, “We do have high school students who are helping us to carry out data analysis. All this will grow the space industry in the UAE and attract youth to it.”

Title of Published Paper:

BusinessMatter ejections behind the highs and lows of the transitional millisecond pulsar PSR J1023+0038

Published in: Astronomy and Astrophysics

The Impact Factor: 6.5

It is published by EDP Sciences

The journal is indexed in ADS · CAS · CDS; Compendex; Current Contents · INIST · IET INSPEC; PaperChem; Physics Abstracts;

The (SJR) SCImago Journal Rank is 9.937.