"The innermost part is at enormous temperatures, up to tens of millions of degrees," Shaposhnikov said.
In prior work, Shaposhnikov and colleague Lev Titarchuk had calculated that fluctuations in x-ray emissions should be linked to the size of the black hole.
They tested their theory with black holes whose masses had been estimated with other techniques, such as measuring the wobble caused in a companion star's orbit by the object's gravity.
The researchers found that the new method produced similar mass figures with apparently higher precision.
In the case of XTE J1650-500, they were able to pinpoint its mass within half a solar mass—far more precisely than was possible by the wobble technique.
Because black holes like XTE J1650-500 are formed by the death of stars, the new find provides insights into stellar physics and evolution, researchers say.
For example, scientists believe that these black holes can only form from stars several times the mass of the sun. But nobody's sure where the mass cutoff is.
"This limit is somewhere between two and three solar masses," Shaposhnikov said. "So with a detection of 3.8 solar masses, we get quite close to that boundary."
Although the team's method of black hole measurement has yet to be accepted as a new standard, the results are impressive, said Vicky Kalogera, a theoretical astrophysicist from Northwestern University who was not involved in the study.
"The question of black hole masses has concerned us for more than a decade, particularly in the context of understanding how massive stars end their lives," she said.
Part of the concern was an apparent scarcity of relatively low-mass black holes, in the range of three to five solar masses, even though astrophysical theory said they should be common.
"The result Nicolai presented here is particularly interesting because it constrains the black hole to be in the low-mass range," she said.
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