Supergiant fast x-ray transients (SGXT) are in the news. SGXT is the name for a certain type of binary star — a pair of stars revolving around each other.
To be specific, they are a pair in which one partner is a big bright star and the other is a highly condensed dark companion — a black hole or a neutron star — which attracts mass from the bright star.
As the material spirals into the dark star, it emits x-rays. Hence, it appears to the onlooker that they are shining brightly in x-rays.
Suddenly, without warning, the pair dims to a fraction of its brightness within minutes. This behaviour of shining and dimming, called “fasting” and “feasting” has puzzled astronomers for a decade now.
This puzzle has now been partly resolved by means of a breakthrough, thanks to the work of an international team led by Varun Bhalerao of the Inter University Centre for Astronomy and Astrophysics, Pune. The results were published recently in the journal Monthly Notices of the Royal Astronomical Society .
There were several competing theories as to why the fasting and feasting behaviour happens. One is that the large star gives out a clumpy wind, and when this wind hits the dense star, it would glow.
The other theory is that the dense star has a high magnetic field and this served as a barrier that would dam the wind until the pressure built up and broke the “dam” and the matter carried by the wind would suddenly fall into the compact star, causing a glow.
Varun Bhalerao’s team observed the magnetic field of the dark companion and actually measured it, finding it to be too weak for the damming mechanism to work. “We knew that the key to the puzzle was to measure the neutron star's magnetic field,” he says.
Dr. Bhalerao’s team observed the binary using a space x-ray telescope known as NuStar, a NASA space mission. It is the first x-ray space telescope that can focus on very high energy x-rays.
“NuSTAR is used to study the most extreme environments in the universe, which emit x-rays.
“The x-rays that NuSTAR is sensitive to are similar to the x-rays used in hospitals for diagnoses. Astronomers call them ‘Hard x-rays’. During my Ph.D at Caltech [California Institute of Technology], I was part of the team that built NuSTAR's detectors,” he says.
The actual star-pair they observed, IGR J17544-2619, is an example of such an SGXT. It is a binary located about 12,000 light years away from the earth. It contains a supergiant star, about 25 times as massive as our Sun, and a compressed dead partner, about twice as massive as the Sun but compressed to a diameter of just about 30 km. the stars orbit around each other in 4.9 days.
The binary shines in x-rays and over a period of months can sporadically become bright or faint. The brightest known state was about one lakh times brighter than the dim state.
The discovery of the mechanism of fasting and feasting process is the breakthrough that many were looking forward to and has given important inputs for further theoretical understanding of these binaries. Says Dr Bhalerao: “This allows us to better understand how massive stars form, to study how binaries evolve and to calculate details of supernova explosions, where a neutron star is born in the death of a massive star.”
