LMC X-3 grew brighter and quieter between X-ray observing campaigns

A black hole can reveal its surroundings through the X-rays emitted by gas falling toward it. In a black hole binary, a companion star supplies that gas, which forms a swirling accretion disk. The disk’s relatively steady thermal glow and emission from hotter, more variable regions can contribute differently as the system changes. By studying both the energy of the X-rays and how their brightness flickers, astronomers can look for signs of those changes.

A team led by Sayantan Bhattacharya examined LMC X-3, a persistently active black hole binary in the Large Magellanic Cloud. They used observations from XSPECT, the spectroscopy and timing instrument aboard India’s XPoSat satellite, collected during campaigns in 2024 and 2025. Their question was whether the system’s spectral and timing behavior changed between the campaigns—and whether the different measurements told a consistent story.

Reading the X-ray changes

The researchers analyzed light curves in soft and hard X-ray energy bands. A hardness ratio—here, the hard-band count rate divided by the soft-band count rate—provides a way to track whether the detected emission is relatively hard or soft. They also compared hardness with brightness and with fractional root-mean-square (rms) variability, a measure of how strongly the signal fluctuates around its average.

For a separate view of rapid changes, the team calculated power density spectra. These show how variability is distributed across timescales, including broad, irregular flickering. They also fitted models to spectra combined within each observing campaign to estimate properties of the emitting disk.

The overall pattern was clear: the 2024 observations covered a wider range of brightness and hardness, and included harder, more variable conditions. In 2025, the source was generally brighter and softer, with lower fractional variability. Within the 2024 campaign, the measurements also traced movement toward softer and brighter emission. The power spectra showed stronger broad-band variability in 2024, particularly at low frequencies; the team reported no statistically significant quasi-periodic oscillations in either campaign.

A disk-dominated interpretation

A thermal disk model fitted to the combined spectra gave an estimated inner-disk temperature of about 0.84 keV for 2024 and 1.17 keV for 2025. The reported unabsorbed X-ray flux rose from about 1.96 to 9.59 in units of 10⁻¹⁰ erg cm⁻² s⁻¹—roughly a fivefold increase. The 2024 spectral fit is an important qualification: it used only the softer observations, because the harder portion was too faint and brief to provide sufficient statistics for that combined analysis.

Taken together, the lower hardness and variability, higher brightness, and hotter fitted disk in 2025 are consistent with a greater contribution from a relatively stable thermal accretion disk. In this interpretation, stronger disk emission can make fluctuations from other parts of the flow less prominent in the total signal. The agreement among the timing and spectral indicators makes the disk-dominated explanation plausible, but does not demonstrate that disk emission alone caused the change.

The study also has limits. It compares two observing campaigns separated by roughly a year, rather than providing an uninterrupted record of the transition. The spectra were combined by epoch, and the 2024 fit does not represent its harder observations. A simple absorbed multicolour disk model described the fitted spectra; adding a power-law component did not improve the fits, but that result is not proof that no other emission component exists. The paper also reports no significant inter-band time lags, so these data do not establish a delay between changes in soft and hard X-rays.

The authors describe this as an early long-term spectral-timing study of LMC X-3 with XPoSat/XSPECT. It documents changes in a familiar black hole system and shows how one instrument can track both spectra and variability. The results are consistent with established interpretations of accretion-state changes, rather than evidence for a newly discovered state or a definitive account of the underlying physics. The article is an arXiv preprint; the supplied source does not establish that it has undergone peer review, and its findings should be read with that qualification.