Scientists have uncovered evidence of a rare structure around Centaurus X-3, one of the brightest X-ray pulsars known to astronomers. The discovery suggests that the system draws in material through an accretion disk, a process considered uncommon among high-mass X-ray binaries.
The research was led by Kinjal Roy of the Raman Research Institute (RRI), an autonomous institution under the Department of Science and Technology (DST). By examining four secondary emission lines in detail, the team found that the structure responsible for variations in these emissions must rotate in coordination with the pulsar.
The findings could help scientists develop more accurate models of Centaurus X-3, one of the most extensively studied binary pulsars. They may also improve understanding of the physical processes operating in binary pulsar systems more broadly.
Centaurus X-3 is a high-mass X-ray binary (HMXB) located approximately 24,000 light-years from Earth in the constellation Centaurus. Its companion star has more than 10 times the mass of the Sun. The system was among the first X-ray pulsars discovered and was the third X-ray source identified in the Centaurus constellation.
How Pulsars Produce X-rays and Secondary Emission
An X-ray binary pulsar consists of a pulsar, which is a highly magnetised neutron star, and a companion star bound to it by gravity. A neutron star contains roughly the mass of the Sun compressed into an extremely small object, approximately the size of Bengaluru.
The pulsar’s intense gravitational field pulls in material expelled by its companion star. As this material falls towards the neutron star, the process of accretion generates X-rays.
Pulsars emit radiation from their magnetic poles. This primary emission sweeps through space as the neutron star rotates, allowing satellites positioned to observe the system to detect periodic signals.
Some of this radiation also strikes surrounding structures, including the pulsar’s surface, the surface of its companion star and the stellar wind released by the companion. These structures absorb and re-emit the radiation, producing what scientists call secondary radiation.
Studying this secondary emission can help researchers investigate structures that cannot be directly resolved through conventional observations. Such measurements can reveal information about a binary system’s physical characteristics, including its orientation and magnetic field strength.
However, X-ray binaries are so distant that they appear as point sources to observers. This makes it difficult to distinguish their individual components and understand the structures surrounding them. Changes in secondary emission provide an alternative way to investigate these otherwise hidden features.
Previous studies had established that the intensity of secondary emission lines in Centaurus X-3 varied with the pulsar’s rotation. Building on these observations, Roy and the RRI team investigated four such emission lines to determine what was causing the variations.
Accretion Stream Points to a Rare Disk Structure
The researchers found that the structure responsible for the changing secondary emission lines must be rotating along with the pulsar. Their analysis pointed to the accretion stream, which channels material from an accretion disk towards the neutron star.
An accretion stream carries matter from the disk and directs it towards the pulsar’s magnetic poles, where the material follows the magnetic field lines. As this stream rotates, it can periodically obstruct a satellite’s view of the pulsar.
When the accretion stream moves into the line of sight, the detected primary emission from the pulsar decreases, while the secondary emission from the stream increases. This changing pattern provides evidence that the accretion stream is responsible for the observed variability.
The existence of such a stream also indicates that an accretion disk must be present in the system. This is significant because high-mass X-ray binaries generally accumulate matter through the direct capture of stellar wind from their companion stars, rather than through an accretion disk.
Roy highlighted the implications for existing models of the system.
“Accretion disk is not expected to occur in [HMXBs] So, if an accretion disk is forming in this system, emission models have to be modified,” says Roy.
He further explained that future modelling of Centaurus X-3 would need to account for radiation from the disk.
“When Centaurus X-3 modelling happens in future work, emission from the disk must be accounted for,” he added.
The findings suggest that the mechanisms governing matter accretion in Centaurus X-3 may be more complex than previously expected. By identifying the accretion stream as the source of the changing secondary emission lines, the researchers have provided evidence for a disk-mediated process in this high-mass binary system.
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