Quasi-periodic X-ray eruptions years after a nearby tidal disruption event

\ua9 The Author(s) 2024.Quasi-periodic eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks1–5. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs) undergoing instabilities6–8 or interacting with a stellar object in a close orbit9–11. It has been suggested that this disk could be created when the SMBH disrupts a passing star8,11, implying that many QPEs should be preceded by observable tidal disruption events (TDEs). Two known QPE sources show long-term decays in quiescent luminosity consistent with TDEs4,12 and two observed TDEs have exhibited X-ray flares consistent with individual eruptions13,14. TDEs and QPEs also occur preferentially in similar galaxies15. However, no confirmed repeating QPEs have been associated with a spectroscopically confirmed TDE or an optical TDE observed at peak brightness. Here we report the detection of nine X-ray QPEs with a mean recurrence time of approximately 48 h from AT2019qiz, a nearby and extensively studied optically selected TDE16. We detect and model the X-ray, ultraviolet (UV) and optical emission from the accretion disk and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs

Investigating the theory of propagating fluctuations with numerical models of stochastic accretion discs

Across a large range of scales, accreting sources show remarkably similar patterns of variability, most notably the log-normality of the luminosity distribution and the linear root-mean square (rms)-flux relationship. These results are often explained using the theory of propagating fluctuations in which fluctuations in the viscosity create perturbations in the accretion rate at all radii, propagate inwards and combine multiplicatively. While this idea has been extensively studied analytically in a linear regime, there has been relatively little numerical work investigating the non-linear behaviour. In this paper, we present a suite of stochastically driven 1-d $\alpha$-disc simulations, exploring the behaviour of these discs. We find that the eponymous propagating fluctuations are present in all simulations across a wide range of model parameters, in contradiction to previous work. Of the model parameters, we find by far the most important to be the timescale on which the viscosity fluctuations occur. Physically, this timescale will depend on the underlying physical mechanism, thought to be the magnetorotational instability (MRI). We find a close relationship between this fluctuation timescale and the break frequency in the power spectral density (PSD) of the luminosity, a fact which could allow observational probes of the behaviour of the MRI dynamo. We report a fitting formula for the break frequency as a function of the fluctuation timescale, the disc thickness and the mass of the central object

A new 2D stochastic methodology for simulating variable accretion discs: propagating fluctuations and epicyclic motion

ABSTRACT Accretion occurs across a large range of scales and physical regimes. Despite this diversity in the physics, the observed properties show remarkable similarity. The theory of propagating fluctuations, in which broad-band variability within an accretion disc travel inwards and combine, has long been used to explain these phenomena. Recent numerical work has expanded on the extensive analytical literature but has been restricted to using the 1D diffusion equation for modelling the disc behaviour. In this work we present a novel numerical approach for 2D (vertically integrated), stochastically driven α-disc simulations, generalizing existing 1D models. We find that the theory of propagating fluctuations translates well to 2D. However, the presence of epicyclic motion in 2D (which cannot be captured within the diffusion equation) is shown to have an important impact on local disc dynamics. Additionally, there are suggestions that for sufficiently thin discs the log-normality of the light curves changes. As in previous work, we find that the break frequency in the luminosity power spectrum is strongly dependent on the driving time-scale of the stochastic perturbations within the disc, providing a possible observational signature for probing the magnetorotational instability dynamo. We also find that thinner discs are significantly less variable than thicker ones, providing a compelling explanation for the greater variability seen in the hard state versus the soft state of X-ray binaries. Finally, we consider the wide-ranging applications of our numerical model for use in other simulations.</jats:p

Modelling the distributions of white dwarf atmospheric pollution: A low Mg abundance for accreted planetesimals?

Abstract The accretion of planetesimals onto white dwarf atmospheres allows determination of the composition of this polluting material. This composition is usually inferred from observed pollution levels by assuming it originated from a single body. This paper instead uses a stochastic model wherein polluting planetesimals are chosen randomly from a mass distribution, finding that the single body assumption is invalid in {>20\%} of cases. Planetesimal compositions are modelled assuming parent bodies that differentiated into core, mantle and crust components. Atmospheric levels of Ca, Mg and Fe in the model are compared to a sample of 230 DZ white dwarfs for which such pollution is measured. A good fit is obtained when each planetesimal has its core, mantle and crust fractions chosen independently from logit-normal distributions which lead to average mass fractions of fCru = 0.15, fMan = 0.49 and fCor = 0.36. However, achieving this fit requires a factor 4 depletion of Mg relative to stellar material. This depletion is unlikely to originate in planetesimal formation processes, but might occur from heating while the star is on the giant branch. Alternatively the accreted material has stellar abundance, and either the inferred low Mg abundance was caused by an incorrect assumption that Mg sinks slower than Ca and Fe, or there are unmodelled biases in the observed sample. Finally, the model makes predictions for the timescale on which the observed pollutant composition varies, which should be the longer of the sinking and disc timescales, implying variability on decadal timescales for DA white dwarfs.</jats:p

The turbulent variability of accretion discs observed at high energies

We use numerical stochastic-viscosity hydrodynamic simulations and new analytical results from thin disc theory to probe the turbulent variability of accretion flows, as observed at high energies. We show that the act of observing accretion discs in the Wien tail exponentially enhances small-scale temperature variability in the flow, which in a real disc will be driven by magnetohydrodynamic turbulence, to large-amplitude luminosity fluctuations (as predicted analytically). In particular, we demonstrate that discs with more spatially coherent turbulence (as might be expected of thicker discs), and relativistic discs observed at larger inclinations, show significant enhancement in their Wien tail variability. We believe that this is the first analysis of relativistic viewing angle effects on turbulent variability in the literature. Using these results, we argue that tidal disruption events represent particularly interesting systems with which to study accretion flo w v ariability, and may in fact be the best astrophysical probes of small-scale disc turbulence. This is a result of a typical tidal disruption event disc being naturally observed in the Wien tail and likely having a somewhat thicker disc and cleaner X-ray spectrum than other sources. We argue for dedicated X-ray observational campaigns of tidal disruption events, with the aim of studying accretion flow variability

Quasi-periodic X-ray eruptions years after a nearby tidal disruption event.

Acknowledgements: We thank the Swift, AstroSat and NICER teams for scheduling our DDT requests. We thank the participants of the Kavli Institute for Theoretical Physics ‘TDE24’ meeting and C. Done for helpful discussions. M.N., A.A., C.R.A. and X.S. are supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 948381) and by UK Space Agency grant no. ST/Y000692/1. D.R.P. was supported by NASA grant 80NSSC19K1287. This work was supported by a Leverhulme Trust International Professorship grant (number LIP-202-014). E.C.F. is supported by NASA under award number 80GSFC21M0002. A.H. is supported by Carlsberg Foundation Fellowship Programme 2015. V.S.D. and ULTRACAM are financed by the UK Science and Technology Facilities Council (STFC, grant ST/Z000033/1). A.J. is supported by grant no. 2023/50/A/ST9/00527 from the Polish National Science Centre. E.J.R. and P.R. are supported by STFC studentships. K.D.A. acknowledges support from the National Science Foundation through award AST-2307668. K.A. is supported by the Australian Research Council Discovery Early Career Researcher Award (DECRA) through project number DE230101069. T.-W.C. acknowledges the Yushan Young Fellow Program by the Ministry of Education, Taiwan for the financial support. R.C. benefited from interactions with Theory Network participants that were supported by the Gordon and Betty Moore Foundation through grant GBMF5076. K.C.P. is financed in part by generous support from S. Nagaraj, L. Noll and S. Otellini. E.N. acknowledges support from NASA theory grant 80NSSC20K0540. A.I. acknowledges support from the Royal Society. S.G.D.T. acknowledges support under STFC grant ST/X001113/1. A.F.G. acknowledges support from the Department for the Economy (DfE) Northern Ireland postgraduate studentship scheme. This research was supported in part by grant NSF PHY-2309135 to the Kavli Institute for Theoretical Physics. This research has made use of data obtained from the Chandra Data Archive and the Chandra Source Catalog, and software provided by the Chandra X-ray Center (CXC) in the application packages CIAO and Sherpa. The AstroSat mission is operated by the Indian Space Research Organisation (ISRO) and the data are archived at the Indian Space Science Data Centre (ISSDC). The SXT data-processing software is provided by the Tata Institute of Fundamental Research (TIFR), Mumbai, India. The UVIT data were checked and verified by the UVIT POC at IIA, Bangalore, India. We acknowledge the use of public data from the Swift data archive. The Pan-STARRS telescopes are supported by NASA grants NNX12AR65G and NNX14AM74G. ZTF is supported by the National Science Foundation under grant nos. AST-1440341 and AST-2034437 and a collaboration including current partners Caltech, IPAC, the Oskar Klein Center at Stockholm University, the University of Maryland, University of California, Berkeley, the University of Wisconsin at Milwaukee, University of Warwick, Ruhr University, Cornell University, Northwestern University and Drexel University. Operations are conducted by COO, IPAC and UW. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK STFC.Quasi-periodic eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks1-5. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs) undergoing instabilities6-8 or interacting with a stellar object in a close orbit9-11. It has been suggested that this disk could be created when the SMBH disrupts a passing star8,11, implying that many QPEs should be preceded by observable tidal disruption events (TDEs). Two known QPE sources show long-term decays in quiescent luminosity consistent with TDEs4,12 and two observed TDEs have exhibited X-ray flares consistent with individual eruptions13,14. TDEs and QPEs also occur preferentially in similar galaxies15. However, no confirmed repeating QPEs have been associated with a spectroscopically confirmed TDE or an optical TDE observed at peak brightness. Here we report the detection of nine X-ray QPEs with a mean recurrence time of approximately 48 h from AT2019qiz, a nearby and extensively studied optically selected TDE16. We detect and model the X-ray, ultraviolet (UV) and optical emission from the accretion disk and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs