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The black hole in the binary system 4U 1755-338 had been silent for 25 years, but in 2020 it suddenly flared up. Astronomers pointed two X-ray telescopes at it simultaneously, NICER on the space station and NuSTAR in orbit, and watched every change in its glow.

From the spectrum, they measured how much interstellar gas lies between us and the system: the hydrogen column turned out to be as thin as a clear film. The outburst itself started small: matter fell onto the black hole slowly, so the brightness was low. The inner edge of the hot disk stayed at a constant distance, as if the black hole wouldn't let it come any closer.

Usually, such outbursts produce a q-shaped curve on the hardness–intensity diagram. Not here: hardness and intensity changed together, as if someone were smoothly turning both the volume and tone knobs at once. This suggested that the light came from the disk itself, not from a corona above it.

The scientists fitted two different models to derive the system's parameters. Both agreed that we see the disk nearly edge-on — at an inclination of about 67–75 degrees. The black hole weighs roughly 3.4 solar masses and spins moderately fast: its spin parameter is 0.78 in one model and 0.50 in the other. The discrepancy isn't alarming: both numbers indicate that the hole isn't static, but it's not spinning at its limit either.

The spectrum showed no signs of X-ray reflection from the disk. Such echoes usually appear when a bright corona illuminates the disk and the disk reflects its light. Here they're absent — meaning the corona was weak or hidden behind the disk. Taken together (the high temperature, the smooth rise in brightness, the lack of reflection, and the weak variability) this suggests that the outburst never left the soft state: the disk remained the primary source of light.

The black hole woke up, but it didn't go wild. It quietly heated its disk and shone steadily, without sharp jumps. Such calm outbursts are rare, and they help us understand how matter behaves right at the edge of a black hole.