Radio Detections During Two State Transitions of the Intermediate-Mass Black Hole HLX-1

Big Black Holes Black holes come in two sizes: stellar-mass black holes, with masses just above that of the Sun, and supermassive black holes, with masses up to a billion times that of the Sun. The hyperluminous x-ray source HLX-1 in the spiral galaxy ESO 243-49 is the best candidate to host a black hole of intermediate mass. Webb et al. (p. 554, published online 5 July) now report the detection of transient radio emission from this source, which may represent a jet ejection event. The radio flares indicate a mass that is consistent with that of an intermediate mass black hole. Jets have been seen to emanate from both supermassive and stellar-mass black holes. Intermediate mass black holes thus seem to behave like other black holes. Observations of a candidate intermediate-mass black hole support the scale invariance of jets in black holes. Relativistic jets are streams of plasma moving at appreciable fractions of the speed of light. They have been observed from stellar-mass black holes (~3 to 20 solar masses, M☉) as well as supermassive black holes (~106 to 109 M☉) found in the centers of most galaxies. Jets should also be produced by intermediate-mass black holes (~102 to 105 M☉), although evidence for this third class of black hole has, until recently, been weak. We report the detection of transient radio emission at the location of the intermediate-mass black hole candidate ESO 243-49 HLX-1, which is consistent with a discrete jet ejection event. These observations also allow us to refine the mass estimate of the black hole to be between ~9 × 103 M☉ and ~9 × 104 M☉.

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