A team of astronomers from the Raman Research Institute (RRI) has investigated a rare signal emanating from a bright X-ray source, which is repeating in nature but not at a consistent interval. Their findings suggest that the wobbling accretion disk around the object may lead to intriguing physical phenomena.

Ultraluminous X-ray sources (ULXs) consist of compact objects, including some of the universe’s most dense entities, such as black holes and neutron stars, which draw in material from a companion star. These systems are classified as accreting binary systems.

Every celestial object has a brightness limit, referred to as the Eddington limit, which primarily depends on the object’s mass. ULXs can consume material at such high rates that they exceed the Eddington limit by more than 100 times. The precise physical mechanisms that allow ULXs to achieve such luminosity remain a subject of extensive debate within the scientific community.

Aman Upadhyay, a PhD student in the Astronomy and Astrophysics division at RRI, along with colleagues, utilised observations from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton from 2001 to 2021 to analyse a ULX in the spiral galaxy M74, designated ULX M74 X-1. Their findings were published in The Astrophysical Journal. This ULX gained attention in 2005 when another research group reported observing rare energy bursts, known as flares, emanating from the source. During these flares, the brightness of the ULX fluctuates significantly over a short duration—approximately thirty minutes. Although the flares exhibited a repeating pattern, they did not follow a regular rhythm. Upadhyay’s research focused on examining both flaring and non-flaring data from this unique source.

The researchers initiated their study by analysing the flaring spectrum of the source, which illustrates the distribution of intensity across various energy levels. They detected a noticeable peak in the spectrum around one kilo-electronvolt (keV), a measurement unit for X-ray energy. This one keV feature has been observed in other ULXs and suggests the presence of a wind generated by radiation pressure, which strips layers from the inner regions of the accretion disk.

Beyond a funnel-shaped region surrounding the rotation axis of the accretion disk, the wind expelled by the object permeates the surrounding area. The size of this wind-free funnel depends on the rate at which the object consumes gas and dust. When viewed from a low inclination angle, the Chandra telescope can observe the accretion disk from above the funnel, while a high inclination angle allows it to see the disk edge-on through the wind, as indicated by the one keV peak in the flaring spectrum.

In contrast, the non-flaring spectrum provided a different perspective. The count of high-energy photons in the non-flaring spectrum was eight times greater than that of low-energy photons. These high-energy emissions are believed to originate from the most luminous section of the accretion disk, which is not affected by the wind, indicating that Chandra was viewing the system from a low inclination angle.

There appears to be a discrepancy between the flaring and non-flaring spectra. While the flaring spectrum suggests a high inclination angle, the non-flaring spectrum contradicts this observation. Prof. Vikram Rana, co-author of the study and Upadhyay’s PhD supervisor, proposes that the wobbling of the accretion disk may explain this phenomenon. The wobbling motion could cause the wind to shift in and out of Chandra’s line of sight, resulting in variations in brightness at irregular intervals, which accounts for the erratic flares observed.

Earlier analyses had applied models to observations of X-ray sources with standard luminosity, leading to the conclusion that the compact object was an intermediate-mass black hole. However, Upadhyay and his team employed updated spectral models, fitting a double disk blackbody to their observations. Upadhyay clarifies that a double disk model signifies a single accretion disk with at least two temperature zones. The outer zone is cooler, while the inner zone experiences super-Eddington accretion, meaning it accretes material at a rate exceeding the Eddington limit. This updated model revealed that the object has a mass approximately seven times that of the Sun, categorising it as a stellar mass black hole.

Interestingly, the observations align with those of neutron star ULXs, raising the possibility that the compact object may be a neutron star rather than a stellar mass black hole. If this hypothesis is validated, the study could significantly enhance our understanding of the nature of the compact object powering ULX M74 X-1.

Future research is planned to employ advanced techniques to search for pulsations from this source, which would further confirm the presence of a neutron star.

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