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The tidal forces close to massive black holes can rip apart stars that come too close to them. As the resulting stellar debris spirals toward the black hole, the debris heats up and emits x-rays. We report observations of a stable 131-second x-ray quasi-periodic oscillation from the tidal disruption event ASASSN-14li. Assuming the black hole mass indicated by host galaxy scaling relations, these observations imply that the periodicity originates from close to the event horizon and that the black hole is rapidly spinning. Our findings demonstrate that tidal disruption events can generate quasi-periodic oscillations that encode information about the physical properties of their black holes.

The brightest fast blue optical transients (FBOTs) are mysterious extragalactic explosions that may represent a new astrophysical phenomenon 1 . Their fast time to maximum brightness of less than a week, decline over several months, and atypical optical spectra and evolution are difficult to explain within the context of the core collapse of massive stars, which are powered by radioactive decay of 56 Ni and evolve more slowly 2 , 3 . AT2018cow (at a redshift of 0.014) is an extreme FBOT in terms of rapid evolution and high luminosity 4 – 7 . Here we present evidence for a high-amplitude quasiperiodic oscillation of AT2018cow’s soft X-rays with a frequency of 224 Hz (at a 3.7 σ significance level or false alarm probability of 0.02%) and fractional root-mean-squared amplitude of >30%. This signal is found in the average power density spectrum taken over the entire 60-day outburst and suggests a highly persistent signal that lasts for a billion cycles. The high frequency (rapid timescale) of 224 Hz (4.4 ms) argues for a compact object in AT2018cow, which could be a neutron star or black hole with a mass less than 850 solar masses. If the quasiperiodic oscillation is equivalent to the spin period of a neutron star, we can set limits on the star’s magnetic field strength. Our work highlights a new way of using high-time-resolution X-ray observations to study FBOTs. A high-frequency quasiperiodic oscillation in the soft X-rays from unusual transient AT2018cow points towards the presence of a compact object in the remnant: either a neutron star with spin period of 4 ms or a low-mass black hole.

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion 1 . It is not known whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces 2 . Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed 3 , 4 ) are thought to measure the stellar spin 2 , 5 , but direct proof of a rotational origin has been lacking. Here we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence 5 , 6 . The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron-star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars 7 , 8 , 9 .

Evidence suggests that, when compact objects such as black holes and neutron stars form, they may receive a ‘natal kick’, during which the stellar remnant gains momentum. Observational evidence for neutron star kicks is substantial 1 , 2 , yet is limited for black hole natal kicks, and some proposed black hole formation scenarios result in very small kicks 3 – 5 . Here we report that the canonical black hole low-mass X-ray binary (LMXB) V404 Cygni is part of a wide hierarchical triple with a tertiary companion at least 3,500 astronomical units ( au ) away from the inner binary. Given the orbital configuration, the black hole probably received a sub-5 km s −1 kick to have avoided unbinding the tertiary. This discovery lends support to the idea that at least some black holes form with nearly no natal kick. Furthermore, the tertiary in this system lends credence to evolutionary models of LMXBs involving a hierarchical triple structure 6 . Remarkably, the tertiary is evolved, indicating that the system formed 3–5 billion years ago and that the black hole has removed at least half a solar mass of matter from its evolved secondary companion. During the event in which the black hole formed, it is required that at least half of the mass of the black hole progenitor collapsed into the black hole; it may even have undergone a complete implosion, enabling the tertiary to remain loosely bound. Analysis of the black hole low-mass X-ray binary V404 Cygni shows that it is part of a wide hierarchical triple whose configuration provides evidence that some black holes form with nearly no natal kick.  

Planets with short orbital periods (roughly under 10 days) are common around stars like the Sun 1 , 2 . Stars expand as they evolve and thus we expect their close planetary companions to be engulfed, possibly powering luminous mass ejections from the host star 3 – 5 . However, this phase has never been directly observed. Here we report observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae 6 , 7 —a class of eruptions now confirmed 8 to arise from mergers of binary stars. Its exceptionally low optical luminosity (approximately 10 35  erg s −1 ) and radiated energy (approximately 6.5 × 10 41  erg) point to the engulfment of a planet of fewer than roughly ten Jupiter masses by its Sun-like host star. We estimate the Galactic rate of such subluminous red novae to be roughly between 0.1 and several per year. Future Galactic plane surveys should routinely identify these, showing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System. Observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright, long-lived infrared emission, show that the resulting light curve and spectra are consistent with the signatures of a planet being engulfed by its host star.

Fallback position The idea of ‘fallback’ in supernova explosions, where some of the explosion ejecta fail to escape, has been invoked to explain black-hole formation, the spin distribution of magnetars, and the origin of Earth-mass planets around pulsars. Until now there has been no direct evidence that the process occurs. But a cool dust disk around an isolated young X-ray pulsar, detected in data from NASA's Spitzer Space Telescope, may be the first supernova fallback disk to have been observed. By analogy with protoplanetary disks around ordinary stars, a passive debris disk like this around a neutron star could be a site of planet formation. Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star 1 or it may possess sufficient angular momentum to form a disk 2 . Such ‘fallback’ is both a general prediction of current supernova models 3 and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation 4 . Fallback disks could dramatically affect the early evolution of pulsars 2 , 5 , yet there are few observational constraints on whether significant fallback occurs or even the actual existence of such disks. Here we report the discovery of mid-infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of the order of 10 Earth masses, and its lifetime (≥ 10 6  years) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars 6 , suggesting the possibility of planet formation around young neutron stars.

Over a dozen millisecond pulsars are ablating low-mass companions in close binary systems. In the original ‘black widow’, the eight-hour orbital period eclipsing pulsar PSR J1959+2048 (PSR B1957+20) 1 , high-energy emission originating from the pulsar 2 is irradiating and may eventually destroy 3 a low-mass companion. These systems are not only physical laboratories that reveal the interesting results of exposing a close companion star to the relativistic energy output of a pulsar, but are also believed to harbour some of the most massive neutron stars 4 , allowing for robust tests of the neutron star equation of state. Here we report observations of ZTF J1406+1222, a wide hierarchical triple hosting a 62-minute orbital period black widow candidate, the optical flux of which varies by a factor of more than ten. ZTF J1406+1222 pushes the boundaries of evolutionary models 5 , falling below the 80-minute minimum orbital period of hydrogen-rich systems. The wide tertiary companion is a rare low-metallicity cool subdwarf star, and the system has a Galactic halo orbit consistent with passing near the Galactic Centre, making it a probe of formation channels, neutron star kick physics 6 and binary evolution. ZTF J1406+1222 is a wide hierarchical triple system that hosts a low-metallicity subdwarf star and a ‘black widow’ millisecond pulsar that has a highly varying optical flux and a 62-minute period.

Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes 1 – 9 . One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf 10 – 14 , and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs 13 – 18 . However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modelling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs. A 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a comparable temperature to that of the Sun but a 100 times greater density, accreting onto a white dwarf is reported.

Typical radio pulsars are magnetized neutron stars that are born rapidly rotating and slow down as they age on time scales of 10 to 100 million years. In contrast, millisecond radio pulsars spin very rapidly even though many are billions of years old 1 . The most compelling explanation is that they have been ‘spun up’ by the transfer of angular momentum during the accretion of material from a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs consist of a neutron star or black hole accreting matter from a companion with mass less than one solar mass 2 .) The recent detection of coherent X-ray pulsations with a millisecond period from a suspected low-mass X-ray binary system appears to confirm this link 3 . Here we report observations showing that the orbital period of this binary system is two hours, which establishes it as an LMXB. We also find an apparent modulation of the X-ray flux at the orbital period (at the two per cent level), with a broad minimum when the pulsar is behind the low-mass companion star. This system seems closely related to the ‘black-widow’ millisecond radio pulsars, which are evaporating their companions through irradiation 4 , 5 , 6 , 7 , 8 . It may appear as an eclipsing radio pulsar during periods of X-ray quiescence.

Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star or it may possess sufficient angular momentum to form a disk. Such 'fallback' is both a general prediction of current supernova models and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. Fallback disks could dramatically affect the early evolution of pulsars, yet there are few observational constraints on whether significant fallback occurs or even the actual existence of such disks. Here we report the discovery of mid- infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of the order of 10 Earth masses, and its lifetime (greater 106 years) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars, suggesting the possibility of planet formation around young neutron stars.