Study reveals clues to nature of ultraluminous X

Scientists from the Raman Research Institute (RRI) have analysed rare, repeating bursts of energy from a distant ultraluminous X-ray source (ULX), offering new insights into the behaviour of some of the universe’s most extreme objects.

The study focuses on ULX M74 X-1, located in the spiral galaxy M74, where astronomers observed irregular but recurring X-ray flares. These bursts, lasting around half an hour, show no fixed periodicity, making the source particularly intriguing.

ULXs are systems in which a compact object—such as a black hole or neutron star—pulls in matter from a companion star. This process, known as accretion, releases enormous amounts of energy. In some cases, these sources exceed the Eddington limit, the theoretical maximum brightness an object can achieve, by more than 100 times.

A research team led by PhD scholar Aman Upadhyay analysed data collected between 2001 and 2021 from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton telescope. Their findings have been published in The Astrophysical Journal.

The team studied both flaring and non-flaring phases of the source. During flares, the spectrum showed a distinct feature around one kilo-electronvolt (keV), indicating the presence of strong winds generated by radiation pressure from the accretion disk. These winds are believed to strip material from the inner regions of the disk.

However, observations during non-flaring periods presented a contrasting picture. The data showed a dominance of high-energy photons, suggesting a direct view of the central region of the accretion disk without interference from surrounding winds.

Researchers propose that this difference could be explained by a wobbling accretion disk. Similar to a spinning top that oscillates while rotating, the disk’s motion may cause the wind to move in and out of the telescope’s line of sight. This leads to irregular variations in brightness, resulting in the observed flares.

The study also revisits the nature of the compact object powering the system. Earlier models suggested the presence of an intermediate-mass black hole. However, using updated spectral models, the researchers estimate the object’s mass to be about seven times that of the Sun, placing it in the category of a stellar-mass black hole.

At the same time, the observed characteristics are consistent with those seen in neutron star ULXs. This raises the possibility that the object could instead be a neutron star. Scientists plan to conduct further studies to detect pulsations, which would help confirm its identity.

The findings are expected to contribute to a better understanding of ultraluminous X-ray sources and the physical processes that drive their extreme brightness.