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Making waves in Stars: Bridging modern asteroseismology and numerical simulations

Stars and their evolution underpin most of modern astrophysics, enriching the galaxy with heavy elements and producing the progenitors of gravitational wave sources. Yet modern asteroseismology has shown that standard 1D stellar evolution models, used widely in astrophysics fall short when confronted with these detailed observations. The discrepancies lie in the 1D treatment of 3D processes, such as convective overshoot, waves and magnetism. In this talk I will review some of the important results from asteroseismology, my groups work over the years on these processes and the various ways in which we can make contact with the observations and (possibly) improve 1D models.

• Speaker: Tamara Rogers (Newcastle)
• Monday 17 February 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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arXiv:2502.06016v1 Announce Type: new Abstract: At cosmic dawn, the first stars and galaxies are believed to form from and be deeply embedded in clouds of dense, pristine gas. Here we present a study of the JWST/NIRSpec data of the most distant, spectroscopically confirmed galaxy observed to date, JADES-GS-z14-0 (GS-z14 for short), at $z=14.179$, combined with recent far-infrared measurements of the [OIII]-$88\mu$m and [CII]-$158\mu$m line transitions and underlying dust-continuum emission. Based on the observed prominent damped Lyman-$\alpha$ (DLA) absorption profile, we determine a substantial neutral atomic hydrogen (HI) column density, $\log (N_{\rm HI} / {\rm cm^{-2}}) = 22.27^{+0.08}_{-0.09}$, consistent with previous estimates though seemingly at odds with the dynamical and gas mass of the galaxy. Using various independent but complementary approaches, considering the implied neutral gas mass from the DLA measurement, the star-formation rate surface density, and the metal abundance, we demonstrate that the total gas mass of GS-z14 is of the order $\log (M_{\rm gas} / M_\odot) = 9.8\pm 0.3$. This implies a substantial gas mass fraction, $f_{\rm gas} \gtrsim 0.9$ and that the bulk of the interstellar medium (ISM) is in the form of HI. We show that the derived gas mass is fully consistent with the non-detection of [CII]-$158\mu$m, assuming an appropriate scaling to the neutral gas. The low dust-to-gas ratio, $A_V/N_{\rm HI} = (1.3\pm 0.6)\times 10^{-23}$\,mag\,cm$^2$, derived in the line-of-sight through the DLA further indicates that the absorbing gas is more pristine than the central, star-forming regions probed by the [OIII]-$88\mu$m emission. These results highlight the implications for far-infrared line-detection searchers attainable with ALMA and demonstrate that the bright, relatively massive galaxy GS-z14 at $z=14.179$ is deeply embedded in a substantial, pristine HI gas reservoir dominating its baryonic matter content.

Exoplanet Clouds and Chemistry: A WASP-17b case study

In the short time since the start of JWST ’s science operations, it has caused a paradigm shift in the information and understanding of giant planet atmospheres. The spectroscopic IR capabilities have revealed absorption from H2O , CO2, and CO with exquisite precision, provided the first look at elusive methane absorption, and shown a diversity of photochemistry and disequilibrium processes at play in giant planet atmospheres. Previously obscuring aerosols that plagued UV-optical spectra are revealing themselves via distinct absorption and emission in the mid-IR confirming for the first time in irradiated exoplanets theoretical predictions of cloud formation. I will present a case study of one such planet which is revealing the roles of clouds and chemistry in exoplanet atmospheres and the feedback imparted between composition, dynamics, and detectability.

• Speaker: Hannah Wakeford
• Thursday 20 March 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Matthew Grayling.

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The Death Throes of Massive Stars

Core collapse supernovae play many important roles in astronomy and astrophysics. They trigger and regulate star formation through the energy they inject into the interstellar medium and they forge and disperse elements that seed the next generation of stars. On much more compact scales, which is the focus area of this talk, core collapsing stars are the birth sites of neutron stars and black holes, and therefore they are the gateway to the compact Universe. Numerical simulations of the core collapse have rapidly progressed in the last decade. Explosions are now readily obtained, the key ingredient being multidimensionality. This colloquium will review recent progress in understanding the central engines at the heart of core-collapse supernovae. I’ll touch upon how we can still use 1D simulations to understand the population as a whole, 2D simulations to systematically study theoretical uncertainties and explore the multimessenger signals, and 3D simulations to push our understanding of these extreme events.

• Speaker: Evan O'Connor
• Thursday 06 March 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Matthew Grayling.

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Exoplanet Clouds and Chemistry: A WASP-17b case study

In the short time since the start of JWST ’s science operations, it has caused a paradigm shift in the information and understanding of giant planet atmospheres. The spectroscopic IR capabilities have revealed absorption from H2O , CO2, and CO with exquisite precision, provided the first look at elusive methane absorption, and shown a diversity of photochemistry and disequilibrium processes at play in giant planet atmospheres. Previously obscuring aerosols that plagued UV-optical spectra are revealing themselves via distinct absorption and emission in the mid-IR confirming for the first time in irradiated exoplanets theoretical predictions of cloud formation. I will present a case study of one such planet which is revealing the roles of clouds and chemistry in exoplanet atmospheres and the feedback imparted between composition, dynamics, and detectability.

• Speaker: Dr Hannah Wakeford
• Thursday 20 March 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Matthew Grayling.

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Instabilities and the remarkable survivability of AGN jets.

Jets powered by active galactic nuclei (AGN) appear impressively stable in comparison to their terrestrial and laboratory counterparts – they can be traced from their origin to distances exceeding their injection radius by up to a billion times. However some of them, the Fanaroff-Riley class I (FR-I) jets in particular, get disrupted and lose their coherence on the scale of host galaxy. It has been suggested that the survivability of AGN jets is related to their rapid expansion, and that the instabilities develop only when they eventually become confined/recollimated by the surrounding plasma.

Motivated by this hypothesis, we carried out 3D computer simulations of jets propagating through plasma with rapidly declining pressure and jets recollimated by galactic coronas with flat pressure distribution. The results support the stabilising role of jet expansion and show that the recollimation of unmagnetized jets by external pressure is indeed accompanied by a rapid development of instability and transition to a turbulent state. This instability is driven by the inertial force associated with the curved streamlines of recollimated jets and hence relates to the well-known centrifugal instability. Simulations of magnetised jets show that even relatively weak azimuthal magnetic field can suppress the recollimation instability, with the critical relativistic magnetisation parameter sigma about 0.01. These results are consistent with our heuristic analysis of the relativistic centrifugal instability (CFI) and the computer simulations of relativistic rotating fluids.

• Speaker: Serguei Komissarov (Leeds)
• Monday 24 February 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Thomas Jannaud.

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Towards Habitable Worlds: Detailed Characterization of the Nearest Planetary Systems

Over the past thirty years, astronomers have made extraordinary progress in detecting planets around other stars. We now know that stars commonly host planets with a wider range of physical properties and system architectures than exist in our own solar system, and that planets likely outnumber stars in our galactic neighborhood. Now, planet detection and characterization technologies have advanced to the point that it should be possible to search for signs of life in the atmospheres of Earth-like exoplanets around Sun-like stars within a few decades. These observations will give us our first glimpse at how common—or rare—life is in the universe. However, before we can carry out these observations and understand the implications for the abundance of life outside the Solar system, we must first find the nearest habitable planets to observe, learn their detailed properties, and refine our understanding of habitability. In this talk, I will describe my group’s work to fill in these knowledge gaps by developing new tools and methods to detect and characterize exoplanets. First, I will show how cutting-edge machine learning methods could help reveal the closest potentially habitable planets to Earth—ideal for biosignature searches in the 2040s. Next, I will show how we can learn about extrasolar geochemistry by studying planetary accretion onto white dwarf stars—allowing us to see whether geological processes important for habitability on Earth take place in other systems. And finally, I will describe our work to understand what happens to planets when stars run out of nuclear fuel and find out whether life can continue in a system after the host star’s death.

Note new Zoom details

• Speaker: Prof. Andrew Vanderburg, Massachusetts Institute of Technology
• Wednesday 12 February 2025, 10:00-11:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr. Mariona Badenas Agusti.

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arXiv:2406.08547v2 Announce Type: replace Abstract: We analyze ionized gas emission lines in deep rest-frame optical spectra of 16 quiescent galaxies at redshift $1.7

The Quipu superstructure is enormous, spanning 1.4 billion light years – and it could violate one of our fundamental assumptions about the universe

arXiv:2409.07805v2 Announce Type: replace Abstract: The James Webb Space Telescope (JWST) has uncovered low-luminosity active galactic nuclei (AGNs) at high redshifts of $z\gtrsim 4-7$, powered by accreting black holes (BHs) with masses of $\sim 10^{6-8}~M_\odot$. One remarkable distinction of these JWST-identified AGNs, compared to their low-redshift counterparts, is that at least $\sim 20\%$ of them present H$\alpha$ and/or H$\beta$ absorption, which must be associated with extremely dense ($\gtrsim 10^9~{\rm cm}^{-3}$) gas in the broad-line region or its immediate surroundings. These Balmer absorption features unavoidably imply the presence of a Balmer break caused by the same dense gas. In this Letter, we quantitatively demonstrate that a Balmer break can form in AGN spectra without stellar components, when the accretion disk is heavily embedded in dense neutral gas clumps with densities of $\sim 10^{9-11}~{\rm cm}^{-3}$, where hydrogen atoms are collisionally excited to the $n=2$ states and effectively absorb the AGN continuum at the bluer side of the Balmer limit. The non-stellar origin of a Balmer break offers a potential solution to the large stellar masses and densities inferred for little red dots (LRDs) when assuming that their continuum is primarily due to stellar light. Our calculations indicate that the observed Balmer absorption blueshifted by a few hundreds ${\rm km~s}^{-1}$ suggests the presence of dense outflows in the nucleus at rates exceeding the Eddington value. Other spectral features such as higher equivalent widths of broad H$\alpha$ emission and presence of OI lines observed in high-redshift AGNs including LRDs align with the predicted signatures of a dense super-Eddington accretion disk.

arXiv:2502.03085v1 Announce Type: new Abstract: The shape of the Ly-$\alpha$ transmission in the near zone of the redshift $z=5.9896$ quasar ULAS J0148$+$0600 (hereafter J0148) is consistent with a damping wing arising from an extended neutral hydrogen island in the diffuse intergalactic medium (IGM). Here we use simulations of late-ending reionisation from Sherwood-Relics to assess the expected incidence of quasars with Ly-$\alpha$ and Ly-$\beta$ absorption similar to the observed J0148 spectrum. We find a late end to reionisation at $z=5.3$ is a necessary requirement for reproducing a Ly-$\alpha$ damping wing consistent with J0148. This occurs in $\sim3$ per cent of our simulated spectra for an IGM neutral fraction $\langle x_{\rm HI}\rangle=0.14$ at $z=6$. However, using standard assumptions for the ionising photon output of J0148, the a priori probability of drawing a simulated quasar spectrum with a Ly-$\alpha$ damping wing profile and Ly-$\alpha$ near zone size that simultaneously match J0148 is very low, $p<10^{-3}$. We speculate this is because the ionising emission from J0148 is variable on timescales $t<10^{5}\rm\,yr$, or alternatively that the Ly-$\alpha$ transmission in the J0148 near zone is impacted by the transverse proximity effect from nearby star-forming galaxies or undetected quasars. We also predict the IGM temperature should be $T\sim 4\times 10^{4}\rm\,K$ within a few proper Mpc of the Ly-$\alpha$ near zone edge due to recent HI and HeII photo-heating. Evidence for enhanced thermal broadening in the Ly-$\alpha$ absorption near the damping wing edge would provide further evidence that the final stages of reionisation are occurring at $z<6$.

arXiv:2502.03021v1 Announce Type: new Abstract: We present the stellar properties of 2908 galaxies at 0.6 < z < 1.0 from the LEGA-C survey. We emphasize the importance of high signal-to-noise, high spectral resolution spectroscopy in the inference of stellar population properties of galaxies. We estimate the galaxy properties with the SED fitting code Prospector, by fitting spectroscopy and broadband photometry together, drawn from the LEGA-C DR3 and UltraVISTA catalogs respectively. We report a positive correlation between light-weighted ages and stellar velocity dispersion ($\sigma_\star$). The trend with $\sigma_\star$ is weaker for the mass-weighted ages and stellar metallicity ($Z_\star$). On average, quiescent galaxies are characterized by high $Z_\star$, they are \sim 1.1 Gyr older, less dusty, with steeper dust attenuation slopes compared to star-forming galaxies. Conversely, star-forming galaxies are characterized by significantly higher dust optical depths and shallower (grayer) attenuation slopes. Low mass (high mass) star-forming galaxies have lower (higher) $Z_\star$, while their stellar populations are on average younger (older). A key pragmatic result of our study is that a linear-space metallicity prior is preferable to a logarithmic-space one when using photometry alone, as the latter biases the posteriors downward. Spectroscopy greatly improves stellar population measurements and is required to provide meaningful constraints on age, metallicity, and other properties. Pairing spectroscopy with photometry helps resolving the dust-age-metallicity degeneracy, yielding more accurate mass- and light-weighted ages, with ages inferred from photometry alone suffering such large uncertainties. Stellar metallicities are constrained by our spectroscopy, but precise measurements remain challenging (and impossible with photometry alone), particularly in the absence of Mg and Fe lines redward of 5000 $\AA$ in the observed spectrum.

arXiv:2502.02983v1 Announce Type: new Abstract: Recent work has suggested that, during reionisation, spatial variations in the ionising radiation field should produce enhanced Ly ${\alpha}$ forest transmission at distances of tens of comoving Mpc from high-redshift galaxies. We demonstrate that the Sherwood-Relics suite of hybrid radiation-hydrodynamical simulations are qualitatively consistent with this interpretation. The shape of the galaxy--Ly ${\alpha}$ transmission cross-correlation is sensitive to both the mass of the haloes hosting the galaxies and the volume averaged fraction of neutral hydrogen in the IGM, $\bar{x}_{\rm HI}$. The reported excess Ly ${\alpha}$ forest transmission on scales r ~ 10 cMpc at $\langle z \rangle \approx 5.2$ -- as measured using C IV absorbers as proxies for high-redshift galaxies -- is quantitatively reproduced by Sherwood-Relics at z = 6 if we assume the galaxies that produce ionising photons are hosted in haloes with mass $M_{\rm h}\geq 10^{10}~h^{-1}\,{\rm M}_\odot$. However, this redshift mismatch is equivalent to requiring $\bar{x}_{\rm HI}\sim 0.1$ at $z\simeq 5.2$, which is inconsistent with the observed Ly ${\alpha}$ forest effective optical depth distribution. We speculate this tension may be partly resolved if the minimum C IV absorber host halo mass at z > 5 is larger than $M_{\rm h}=10^{10}~h^{-1}\,{\rm M}_\odot$. After reionisation completes, relic IGM temperature fluctuations will continue to influence the shape of the cross-correlation on scales of a few comoving Mpc at $4 \leq z \leq 5$. Constraining the redshift evolution of the cross-correlation over this period may therefore provide further insight into the timing of reionisation.

The capacity of Constant Torsion Emergent Gravity to resolve cosmological tensions

Despite the success of the ΛCDM model in explaining a wide range of cosmological phenomena, observational discrepancies such as the Hubble tension and curvature tension, as well as theoretical challenges such as the inability to unify General Relativity with other fundamental forces in particle physics, have prompted a reevaluation of our current cosmological model and an exploration of other theories of gravity. We investigate the constant torsion emergent gravity (CTEG) model, a specific case of Poincare Gauge Theory (PGT) of gravity. This model introduces two extra cosmological parameters in addition to the six parameters in ΛCDM, and can be treated as a standard extension to ΛCDM with modified dark energy. This modification is then implemented with CAMB and the constraints of the cosmological parameters of PGT is obtained through nested sampling using Polychord and Cobaya. Our results offer a comparative analysis against the ΛCDM model, looking at the possibility of CTEG to resolve key observational tensions.

• Speaker: Sinah Legner (University of Cambridge)
• Tuesday 11 February 2025, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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arXiv:2502.02647v1 Announce Type: new Abstract: The field of high redshift galaxy formation has been revolutionised by JWST, which is yielding unprecedented insights on galaxy assembly at early times. Our key aim is to study the physical mechanisms that can explain the unexpected abundance of bright galaxies at $z \geq 11$, as well as their metal enrichment and spectral properties. We also use recent data to determine the key sources of reionisation. To do so, we implement cold gas fractions and star formation efficiencies derived from the SPHINX20 high-resolution radiation-hydrodynamics simulation into DELPHI, a semi-analytic model that tracks the assembly of dark matter halos and their baryonic components from $z \sim 4.5-40$. In addition, we explore two different methodologies to boost galaxy luminosities at $z \geq 11$: a stellar initial mass function (IMF) that becomes increasingly top-heavy with decreasing metallicity and increasing redshift (eIMF model), and star formation efficiencies that increase with increasing redshift (eSFE model). Our key findings are: (i) both the eIMF and eSFE models can explain the abundance of bright galaxies at $z \geq 11$; (ii) dust attenuation plays an important role for the bright-end of the UV LF at $z \leq 11$; (iii) the mass-metallicity relation is in place as early as $z \sim 17$ in all models although its slope is model-dependent; (iv) within the spread of both models and observations, all of our models are in good agreement with current estimates of $\beta$ slopes at $z \sim 5-17$ and Balmer break strengths at $z \sim 6-10$; (v) in the eIMF model, galaxies at $z\geq12$ or with $\rm{M_{UV}}\geq-18$ show values of $\xi_{\rm{ion}} \sim 10^{25.55}~{\rm [Hz~erg^{-1}]}$, twice larger than in other models; (vi) star formation in galaxies below $10^{9}\rm{M_{\odot}}$ is the key driver of reionisation, providing the bulk ($\sim 85\%$) of ionising photons down to its midpoint at $z \sim 7$.

Third law of black hole mechanics for supersymmetric black holes

The third law of black hole mechanics asserts that it is impossible for a non-extremal black hole to become extremal in finite time (in classical General Relativity). A proof of this law was claimed in the 1980s. However, counterexamples to this law were found recently: gravitational collapse of a massless charged scalar field can produce an exactly extremal Reissner-Nordstrom black hole in finite time, passing through an intermediate phase in which the solution is exactly Schwarzschild at the horizon. These examples involve matter with a large charge to mass ratio. What about theories, such as supersymmetric theories, with an upper bound on the charge to mass ratio of matter? In this case I have proved that one cannot form a supersymmetric black hole (such as extremal Reissner-Nordstrom) in finite time. Thus a third law holds for supersymmetric black holes. The proof involves ideas related to quasi-local energy. In this talk I shall review all of these developments.

• Speaker: Harvey Reall, DAMTP, Cambridge
• Friday 14 February 2025, 13:00-14:00
• Venue: Potter room/Zoom: https://cam-ac-uk.zoom.us/j/86544434784?pwd=dE3XiqwmaWGPz9w5pXNkKk93XsmtJv.1.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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Kaleidoscope of irradiated disks: VLT/MUSE observations of proplyds

The evolutionary pathways of protoplanetary disks differ depending on the surrounding environment. In massive star clusters, UV radiation affects disks via external photoevaporative winds, depleting the disks of their material and shortening their lifetimes. Known as proplyds, such irradiated disks are typically surrounded by a teardrop-shaped cloud of ionized gas and observed in forbidden emission lines.

While external photoevaporation of disks is unique to clusters such as the Orion Nebula Cluster (ONC), internal photoevaporative winds may be present in both high UV environments, and low-mass star forming regions with weak external UV fields. In the latter case, the winds arise due to radiation from the central star and can also be studied via forbidden line emission. It is therefore crucial to determine how to disentangle external winds from internal ones.

I will present the results based on the visually striking VLT /MUSE IFU data of a dozen proplyds in the ONC . This sample allows us to study the morphology of proplyds in a wealth of emission lines and determine their physical parameters. Among the results, I will present a proxy for unambiguously identifying externally driven winds with a forbidden line of neutral atomic carbon.

• Speaker: Mari-Liis Aru
• Wednesday 12 February 2025, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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The history of the Milky Way's bar

The emergence of barred galaxies is a sensitive indicator of the conditions in the early Universe. Current cosmological simulations now routinely produce realistic barred galaxies but reproducing the precise distributions of morphology and properties remains challenging. One route to understanding how bars form and evolve is to look close to home at our own Galaxy’s bar. I will describe work on measuring the epoch of bar formation in the Milky Way using data from Gaia and a ground-based astrometric catalogue VVV /VIRAC from the VISTA telescope. I will describe how the bar formation can be dated by studying the star formation history in the Milky Way’s nuclear stellar disc, a central structure intimately related to the bar, and describe the broader implications of the derived bar formation epoch for the history of our Galaxy.

• Speaker: Jason Sanders (UCL)
• Friday 14 February 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Fact or FABLE: predictions for SMBH merger rates from cosmological simulations

The co-evolution of massive black holes and their host galaxies is well established within ΛCDM cosmology. The repeated mergers, accretion, and feedback that conspire to regulate this process can be studied in large-scale cosmological simulations, such as Illustris, FABLE , MillenniumTNG and Flamingo. These simulations resolve key galaxy formation processes at ~kpc scales, but are plagued with numerical inaccuracies at the smaller scales of black holes. This scale discrepancy presents significant challenges for investigating black hole properties and generating testable predictions, e.g. for future JWST , Gaia, LISA and IPTA observations of isolated and binary black holes. In this talk I will discuss the black hole population in FABLE . Our results show that the numerical treatment of black holes in cosmological simulations leads to a misleading picture, even at the well-resolved large scale of galaxies. In particular, a large fraction of black holes coalesce well before their host galaxies merge and thus require extra delays on the order of a few Gyrs. These delays, governed by the dynamical timescale of the merging host galaxies, occur before and in addition to any delays arising from unresolved ‘sub-grid’ physics describing BH hardening mechanisms on parsec scales. This effect has profound implications for the black hole merger rates predicted from these large-scale cosmological simulations as well as for the multi-messenger predictions, once black hole growth during these dynamical galaxy merger delays is accounted for.

• Speaker: Stephanie Buttigieg / IoA
• Wednesday 12 February 2025, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Uncovering the physical origin of quasi-periodic oscillations from black hole X-ray binaries

Black hole X-ray binary systems consist of a black hole stripping material from its stellar companion to generate a large X-ray flux. Quasi-periodic oscillations (QPOs) with a period in the range 0.1 to 10 seconds are routinely observed in the X-ray flux from these systems. Despite being strong signals that have been detectable for around 40 years, the physical origin of these QPOs is still debated. The leading theory associates the QPO with the relativistic effect of Lense-Thirring precession. This model requires the black hole spin axis to be moderately misaligned with the binary rotation axis, in which case the frame dragging effect is expected to induce precession in the inner accretion flow. I will review the precession model and the observational evidence in support of it, as well as discussing alternative theories. I will focus on two key predictions of the precession model. First, I will show that the iron line emitted from the inner accretion disk oscillates between red and blue shifted with QPO phase. Second, I will describe our work to determine whether the X-ray polarization angle oscillates with QPO phase, which has recently been made possible by the launch of the Imaging X-ray Polarimetry Explorer (IXPE).

• Speaker: Adam Ingram (Newcastle)
• Monday 10 February 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Thomas Jannaud.

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