In the past two decades, high amplitude electromagnetic outbursts have been detected from dormant galaxies and often attributed to the tidal disruption of a star by the central black hole 1,2 . X-ray emission from the Seyfert 2 galaxy GSN 069 (2MASX J01190869-3411305) at redshift z = 0.018 was first detected in 2010 July and implies an X-ray brightening of more than a factor of 240 over ROSAT observations performed 16 years earlier 3,4 . The emission has smoothly decayed over time since 2010, possibly indicating a long-lived tidal disruption event 5 . The X-ray spectrum is ultra-soft and can be described by accretion disc emission with luminosity proportional to the fourth power of the disc temperature during long-term evolution. Here we report observations of X-ray quasi-periodic eruptions from the nucleus of GSN 069 over the course of 54 days, 2018 December onwards. During these eruptions, the X-ray count rate increases by up to two orders of magnitude with event duration of just over 1 hour and recurrence time of about 9 hours. These eruptions are associated with fast spectral transitions between a cold and a warm phase in the accretion flow around a low-mass black hole (of approximately 4 × 10 5 solar masses) with peak Xray luminosity of ~ 5 × 10 42 ergs per second. The warm phase has a temperature of about 120 electronvolts, reminiscent of the typical soft X-ray excess, an almost universal thermal-like feature in the X-ray spectra of luminous active nuclei 6,7,8 . If the observed properties are not unique to GSN 069, and assuming standard scaling of timescales with black hole mass and accretion properties, typical active galactic nuclei with more massive black holes can be expected to exhibit high-amplitude optical to X-ray variability on timescales as short as months or years 9 .Since 2018 December 24, GSN 069 has exhibited peculiar high amplitude, short timescale X-ray variability, first detected during an XMM-Newton observation (XMM3, see Extended Data Table 1).The XMM-Newton light curve is characterized by two bright flares (bursts) with count rate increases by factors of ~ 22 and ~ 31 respectively in the 0.4-2 keV band ( Figure 1a). The bursts are separated by ~ 29.8 ks, their profile is close to symmetric with similar rise and decay times of ~ 1.8 ks, and the total event duration is ~ 4.5 ks. This unexpected X-ray variability prompted us to request a longer XMM-Newton Director Discretionary Time (DDT) observation performed on 2019 January 16/17 (XMM4).Five bursts are detected with varying amplitudes, corresponding to count rate increases by factors of ~ 19 to ~ 28, and longer recurrence time (~ 32.15 ks) than during XMM3 (Figure 1b). Finally, a Chandra DDT observation was performed on 2019 February 14/15 during which three further bursts are detected with count rate variations by factors of ~ 13 to ~ 22, and with recurrence time of ~ 32.7 ks (Figure 1c).We point out that no bursts were observed in a potentially long enough XMM-Newton exposure (83 ks) on 2014 December 5 (XMM2), i.e. 4 years before the X...

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 (~10(6) to 10(9) M(⊙)) found in the centers of most galaxies. Jets should also be produced by intermediate-mass black holes (~10(2) to 10(5) 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 × 10(3) M(⊙) and ~9 × 10(4) M(⊙).

MAXI J1535−571 is a Galactic black hole candidate X-ray binary that was discovered going into outburst in 2017 September. In this paper, we present comprehensive radio monitoring of this system using the Australia Telescope Compact Array (ATCA), as well as the MeerKAT radio observatory, showing the evolution of the radio jet during its outburst. Our radio observations show the early rise and subsequent quenching of the compact jet as the outburst brightened and then evolved towards the soft state. We constrain the compact jet quenching factor to be more than 3.5 orders of magnitude. We also detected and tracked (for 303 days) a discrete, relativistically-moving jet knot that was launched from the system. From the motion of the apparently superluminal knot, we constrain the jet inclination (at the time of ejection) and speed to ≤ 45 • and ≥ 0.69c, respectively. Extrapolating its motion back in time, our results suggest that the jet knot was ejected close in time to the transition from the hard intermediate state to soft intermediate state. The launching event also occurred contemporaneously with a short increase in X-ray count rate, a rapid drop in the strength of the X-ray variability, and a change in the type-C quasi-periodic oscillation (QPO) frequency that occurs >2.5 days before the first appearance of a possible type-B QPO.