XRISM and radio arrays place the 3C 120 black hole jet base inside a hot inner flow
Observations of the radio galaxy 3C 120 show a cold disk truncated at about 20 gravitational radii, with the jet tracing back to a hot inner flow, even at a high accretion rate.
A team led by Hirofumi Noda of Tohoku University and Kazuhiro Hada of Nagoya City University combined X-ray and radio observations of the broad-line radio galaxy 3C 120. In a paper in The Astrophysical Journal Letters, the authors report that its jet is linked to a hot inner accretion flow rather than to a cold disk iop.org
The Astrophysical Journal Lettersiop.orgOpen source ↗. The galaxy lies in the constellation Taurus, and its black hole has a mass of about 63 million suns. It shines at roughly 10% to 20% of the brightest level a black hole of that size can reach, and it still launches a powerful jet.
Why fast-feeding black holes are a puzzle
Jet production is widely attributed to the Blandford–Znajek mechanism. It extracts the rotational energy of the black hole through large-scale magnetic fields sustained by the accretion flow. Observations support this well for hot, geometrically thick flows. Highly accreting systems with strong jets are a paradox, because they are typically dominated by cold, geometrically thin disks that cannot sustain large-scale magnetic fields.
What the X-ray and radio data show
The Japanese X-ray satellite XRISM observed 3C 120 for about 55 hours in March 2025. Its microcalorimeter spectroscopy revealed a relativistically broadened Fe Kα line. The line indicates that the cold, thin disk is truncated at about 20 gravitational radii, and that the inner region is replaced by a hot, geometrically thick flow.
About five weeks later, 18 radio dishes observed the same galaxy for 20 hours as the Global Millimeter VLBI Array, with baselines of more than 9,500 km. The Very Long Baseline Array and the East Asian VLBI Network traced the jet farther out. Extrapolating the jet profile toward the horizon gives a radius comparable to, or even narrower than, the disk truncation radius iop.org
The Astrophysical Journal Lettersiop.orgOpen source ↗. According to the study, the beam is no wider than about 7 gravitational radii at its base. This places the jet origin inside the hot zone.
The authors conclude that, even at high accretion rates, jet launching is linked to a hot inner flow, possibly through the Blandford–Znajek process.
The X-ray data also give an outer-disk inclination of about 31 degrees to the line of sight, against about 19 degrees for the jet. The team saw a possible X-ray flicker repeating about once a day, but only four or five cycles were caught, and the authors call it marginal.
The result rests on one galaxy. The black hole spin could not be measured, because the disk ends too far out for the X-rays to carry that information. Some fast-feeding galaxies show a similar truncated disk and launch no jet. The team is now observing a heavier black hole in the same way.
