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• Speaker: Rajsekhar Mohapatra (Princeton)
• Friday 10 May 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Which universes does the no-boundary wave function favour?

Please notice the unusual schedule (9:45am) and location (MR9) due to previous overlaps with the Dirac lunch and Dirac lecture.

• Speaker: Jean-Luc Lehners (MPI for Gravitational Physics, Potsdam)
• Monday 13 May 2024, 09:45-10:45
• Venue: CMS, Pav. B, MR9 (B0.09) .
• Series: Cosmology Lunch; organiser: Thomas Colas.

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Which universes does the no-boundary wave function favour?

Please notice the unusual schedule due to previous overlaps with the Dirac lunch and Dirac lecture.

• Speaker: Jean-Luc Lehners (MPI for Gravitational Physics, Potsdam)
• Monday 13 May 2024, 09:45-10:45
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Thomas Colas.

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arXiv:2404.09825v1 Announce Type: new Abstract: We use Legacy Survey photometric data to probe the stellar halo in multiple directions of the sky using a probabilistic methodology to identify Blue Horizontal Branch (BHB) stars. The measured average radial density profile follows a double power law in the range $ 5

arXiv:2404.09564v1 Announce Type: new Abstract: We use numerical stochastic-viscous 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 significantly enhancement in their Wien-tail variability. We believe 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 flow variability, 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.

arXiv:2404.08746v1 Announce Type: new Abstract: The detection of low-mass planets orbiting the nearest stars is a central stake of exoplanetary science, as they can be directly characterized much more easily than their distant counterparts. Here, we present the results of our long-term astrometric observations of the nearest binary M-dwarf Gliese 65 AB (GJ65), located at a distance of only 2.67 pc. We monitored the relative astrometry of the two components from 2016 to 2023 with the VLTI/GRAVITY interferometric instrument. We derived highly accurate orbital parameters for the stellar system, along with the dynamical masses of the two red dwarfs. The GRAVITY measurements exhibit a mean accuracy per epoch of 50-60 microarcseconds in 1.5h of observing time using the 1.8m Auxiliary Telescopes. The residuals of the two-body orbital fit enable us to search for the presence of companions orbiting one of the two stars (S-type orbit) through the reflex motion they imprint on the differential A-B astrometry. We detected a Neptune-mass candidate companion with an orbital period of p = 156 +/- 1 d and a mass of m = 36 +/- 7 Mearth. The best-fit orbit is within the dynamical stability region of the stellar pair. It has a low eccentricity, e = 0.1 - 0.3, and the planetary orbit plane has a moderate-to-high inclination of i > 30{\deg} with respect to the stellar pair, with further observations required to confirm these values. These observations demonstrate the capability of interferometric astrometry to reach microarcsecond accuracy in the narrow-angle regime for planet detection by reflex motion from the ground. This capability offers new perspectives and potential synergies with Gaia in the pursuit of low-mass exoplanets in the solar neighborhood.

The renowned scientist came up with revolutionary ideas in the 1960s, sparking a 50-year search for evidence.

Chandra autonomously transitioned to Normal Sun Mode (NSM) during an observation late on Saturday, February 2, 2024. The transition was nominal, science instruments were safed properly, and all telemetry was as expected. The immediate cause of the anomaly was a trip of the Bright Star/Commanded Attitude Monitor. The root cause was identified as a transient high Aspect Camera Assembly (ACA) background event which caused the ACA to lose track of guide stars over approximately a 2-minute period. This anomaly has been seen earlier in the mission, the last time in 2019, most likely due to flaking MLI that reflects sun into the aspect camera and raises the background signature to a point that guide stars are not found by software. Chandra resumed science observation on February 7, 2024, following passage through orbital low-perigee and commanded S/C recovery as planned.

Nature, Published online: 15 April 2024; doi:10.1038/d41586-024-01115-3

New results could help to end a long standing disagreement over the rate of cosmic expansion. But scientists say more measurements are needed.

Nature, Published online: 15 April 2024; doi:10.1038/d41586-024-01109-1

The agency’s head calls the current plan for delivering samples collected by the Perseverance rover “too expensive” and its schedule “unacceptable.”

A black hole 33 times the mass of the sun is the largest stellar black hole ever spotted, and its strange companion star could help explain how it got so huge

Wading through the wealth of data from ESA’s Gaia mission , scientists have uncovered a ‘sleeping giant’. A large black hole, with a mass of nearly 33 times the mass of the Sun, was hiding in the constellation Aquila, less than 2000 light-years from Earth. This is the first time a black hole of stellar origin this big has...

Studies of the reionization-era intergalactic and circumgalactic media using cosmological simulations

TBC

• Speaker: Dr. Caitlin Doughty (University of Leiden/MPA Garching)
• Tuesday 22 October 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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The ionising properties of galaxies at the Epoch of Reionisation with JWST

TBC

• Speaker: Dr. Charlotte Simmonds (Kavli Institute for Technology, University of Cambridge)
• Tuesday 11 June 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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The Black Hole Mass Metallicity Relation and Insights into Galaxy Quenching

TBC

• Speaker: William Baker (University of Cambridge)
• Tuesday 28 May 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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Liquid Crystal based adaptive optics

TBC

• Speaker: Oana Niculescu (University of Cambridge)
• Tuesday 14 May 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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Interferometric measurements of the 21-cm signal with SKA

The Cosmic Dawn marks the first star formations and preceded the Epoch-of-Reionization, when the Universe underwent a fundamental transformation propelled by the radiation from these first stars and galaxies. Interferometric 21-cm experiments aim to probe redshifted neutral hydrogen signals from these periods, constraining the conditions of the early Universe. The SKA -LOW instrument of the Square Kilometre Array telescope is envisaged to be the largest and most sensitive radio telescope at m and cm wavelengths. In this talk we present a data analysis pipeline that was used in the SKA Science Data Challenge 3a: Epoch of Reionisation (SKA SDC3a) to process the novel data products expected from the SKA . To determine whether a successful 21-cm detection is possible with the envisaged SKA , we implement predictive foreground and Bayesian Gaussian Process Regression models alongside a foreground avoidance strategy to isolate the 21-cm signal from that of the astrophysical radio frequency (RF) foregrounds.

• Speaker: Yuchen Liu and Oscar O'Hara (Cavendish Astrophysics)
• Tuesday 07 May 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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Radio observations of extra-galactic transients with the AMI-LA telescope

TBC

• Speaker: Dr. Lauren Rhodes (University of Oxford)
• Tuesday 30 April 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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arXiv:2310.01763v2 Announce Type: replace Abstract: We use semi-analytical models to study the effects of primordial black hole (PBH) accretion on the cosmic radiation background during the epoch of reionization ($z\gtrsim 6$). We consider PBHs floating in the intergalactic medium (IGM), and located inside haloes, where star formation can occur. For stars with a mass $\gtrsim 25 \rm\ M_{\odot}$, formed in suitable host haloes, we assume they quickly burn out and form stellar remnant black holes (SRBHs). Since SRBHs also accrete material from their surroundings, we consider them to have similar radiation feedback as PBHs in the halo environment. To estimate the background radiation level more accurately, we take into account the impact of PBHs on structure formation, allowing an improved modeling of the halo mass function. We consider the radiation feedback from a broad suite of black holes: PBHs, SRBHs, high-mass X-ray binaries (HMXBs), and supermassive black holes (SMBHs). We find that at $z\gtrsim 30$, the radiation background energy density is generated by PBHs accreting in the IGM, whereas at lower redshifts, the accretion feedback power from haloes dominates. We also analyze the total power density by modeling the accretion spectral energy distribution (SED), and break it down into select wavebands. In the UV band, we find that for $f_{\rm PBH} \lesssim 10^{-3}$, the H-ionizing and Lyman-$\alpha$ fluxes from PBH accretion feedback do not violate existing constraints on the timing of reionization, and on the effective Wouthuysen-Field coupling of the 21-cm spin temperature of neutral hydrogen to the kinetic temperature of the IGM. However, in the X-ray band, with the same abundance, PBHs contribute significantly and could account for the unresolved part of the cosmic X-ray background.

Title to be confirmed

Abstract not available

• Speaker: Florian Lienhard (Zurich)
• Tuesday 30 April 2024, 13:00-14:00
• Venue: Battcock coffee area + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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