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On the Double: Two Luminous Flares from the Nearby Tidal Disruption Event ASASSN-22ci (AT2022dbl) and Connections to Repeating TDE Candidates

Recent IoA Publications - Fri, 09/05/2025 - 11:36
arXiv:2412.15326v2 Announce Type: replace Abstract: We present observations of ASASSN-22ci (AT2022dbl), a nearby tidal disruption event (TDE) discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of d$_L \simeq 125$ Mpc. Roughly two years after the initial ASAS-SN discovery, a second flare was detected coincident with ASASSN-22ci. UV/optical photometry and optical spectroscopy indicate that both flares are likely powered by TDEs. The striking similarity in flare properties suggests that these flares result from subsequent disruptions of the same star. Each flare rises on a timescale of $\sim$30 days, has a temperature of $\approx$30,000 K, a peak bolometric luminosity of $L_{UV/Opt} = 10^{43.6 - 43.9} \textrm{ erg} \textrm{ s}^{-1}$, and exhibits a blue optical spectrum with broad H, He, and N lines. No X-ray emission is detected during either flare, but X-ray emission with an unabsorbed luminosity of $L_{X} = 3\times10^{41} \textrm{ erg} \textrm{ s}^{-1}$ and $kT = 0.042$ eV is observed between the flares. Pre-discovery survey observations rule out the existence of earlier flares within the past $\approx$6000 days, indicating that the discovery of ASASSN-22ci likely coincides with the first flare. If the observed flare separation of $720 \pm 4.7$ days is the orbital period, the next flare of ASASSN-22ci should occur near MJD 61075 (2026 February 04). Finally, we find that the existing sample of repeating TDE candidates is consistent with Hills capture of a star initially in a binary with a total mass between $\sim$$1 - 4$ M$_{\odot}$ and a separation of $\sim$$0.01 - 0.1$ AU.

A data-driven approach for star formation parameterization using symbolic regression

Stars and stellar evolution - Fri, 09/05/2025 - 11:30
arXiv:2505.04681v1 Announce Type: new Abstract: Star formation (SF) in the interstellar medium (ISM) is fundamental to understanding galaxy evolution and planet formation. However, efforts to develop closed-form analytic expressions that link SF with key influencing physical variables, such as gas density and turbulence, remain challenging. In this work, we leverage recent advancements in machine learning (ML) and use symbolic regression (SR) techniques to produce the first data-driven, ML-discovered analytic expressions for SF using the publicly available FIRE-2 simulation suites. Employing a pipeline based on training the genetic algorithm of SR from an open software package called PySR, in tandem with a custom loss function and a model selection technique which compares candidate equations to analytic approaches to describing SF, we produce symbolic representations of a predictive model for the star formation rate surface density ($\Sigma_\mathrm{SFR}$) averaged over both 10 Myr and 100 Myr based on eight extracted variables from FIRE-2 galaxies. The resulting model that PySR finds best describes SF, on both averaging timescales, features equations that incorporates the surface density of gas, $\Sigma_\mathrm{gas}$, the velocity dispersion of gas $\sigma_{\mathrm{gas,~z}}$ and the surface density of stars $\Sigma_\mathrm{*}$. Furthermore, we find that the equations found for the longer SFR timescale all converge to a scaling-relation-like equation, all of which also closely capture the intrinsic physical scatter of the data within the Kennicutt-Schmidt (KS) plane. This observed convergence to physically interpretable scaling relations at longer SFR timescales demonstrates that our method successfully identifies robust physical relationships rather than fitting to stochastic fluctuations.

A data-driven approach for star formation parameterization using symbolic regression

Recent IoA Publications - Fri, 09/05/2025 - 11:30
arXiv:2505.04681v1 Announce Type: new Abstract: Star formation (SF) in the interstellar medium (ISM) is fundamental to understanding galaxy evolution and planet formation. However, efforts to develop closed-form analytic expressions that link SF with key influencing physical variables, such as gas density and turbulence, remain challenging. In this work, we leverage recent advancements in machine learning (ML) and use symbolic regression (SR) techniques to produce the first data-driven, ML-discovered analytic expressions for SF using the publicly available FIRE-2 simulation suites. Employing a pipeline based on training the genetic algorithm of SR from an open software package called PySR, in tandem with a custom loss function and a model selection technique which compares candidate equations to analytic approaches to describing SF, we produce symbolic representations of a predictive model for the star formation rate surface density ($\Sigma_\mathrm{SFR}$) averaged over both 10 Myr and 100 Myr based on eight extracted variables from FIRE-2 galaxies. The resulting model that PySR finds best describes SF, on both averaging timescales, features equations that incorporates the surface density of gas, $\Sigma_\mathrm{gas}$, the velocity dispersion of gas $\sigma_{\mathrm{gas,~z}}$ and the surface density of stars $\Sigma_\mathrm{*}$. Furthermore, we find that the equations found for the longer SFR timescale all converge to a scaling-relation-like equation, all of which also closely capture the intrinsic physical scatter of the data within the Kennicutt-Schmidt (KS) plane. This observed convergence to physically interpretable scaling relations at longer SFR timescales demonstrates that our method successfully identifies robust physical relationships rather than fitting to stochastic fluctuations.

Star formation and accretion rates within 500 pc as traced by Gaia DR3 XP spectra

Stars and stellar evolution - Fri, 09/05/2025 - 10:58
arXiv:2505.04699v1 Announce Type: new Abstract: Accretion rates from protoplanetary disks onto forming stars are a key ingredient in star formation and protoplanetary disk evolution. Extensive efforts surveying individual star forming regions with spectroscopy and narrow-band photometry have been performed to derive accretion rates on large populations of young stellar objects (YSOs). We use Gaia DR3 XP spectra to perform the first all-sky homogeneous analysis of YSO accretion within 500 pc. We characterise the H$\alpha$ line emission of YSOs by using the H$\alpha$ pseudo-equivalent widths and XP spectra from Gaia DR3. We derive accretion luminosities, mass accretion rates and stellar parameters for 145 975 candidate YSO H$\alpha$ emitters all-sky. We describe filtering strategies to select specific sub-samples of YSOs from this catalogue. We identify a large population of low-accreting YSO candidates untraced by previous surveys. The population of low accreting YSOs is mostly spatially dispersed, away from star forming regions or more clustered environments of star formation. Many YSOs appear disconnected from young populations, reminiscent of 'Peter Pan' YSOs. We find $L_{acc}\propto L_\star^{1.41\pm0.02}$ and $\dot M_{acc}\propto M_\star^{2.4\pm0.1}$ for the purest all-sky sample of YSO candidates. By fitting an exponential to the fraction of accreting stars in clusters of different ages in the Sco-Cen complex, we obtain an accretion timescale of 2.7$\pm$0.4 Myr. The percentage of accretors found by fitting a power-law is 70% at 2 Myr and 2.8% at 10 Myr. With this new catalogue of H$\alpha$ emitters we significantly increase the number of YSO candidates with accretion rate estimations in the local neighbourhood. This allows us to study accretion timescales and the spatial and physical properties of YSO accretion from a large, all-sky, and homogeneous sample for the first time. [abridged]

Star formation and accretion rates within 500 pc as traced by Gaia DR3 XP spectra

Recent IoA Publications - Fri, 09/05/2025 - 10:58
arXiv:2505.04699v1 Announce Type: new Abstract: Accretion rates from protoplanetary disks onto forming stars are a key ingredient in star formation and protoplanetary disk evolution. Extensive efforts surveying individual star forming regions with spectroscopy and narrow-band photometry have been performed to derive accretion rates on large populations of young stellar objects (YSOs). We use Gaia DR3 XP spectra to perform the first all-sky homogeneous analysis of YSO accretion within 500 pc. We characterise the H$\alpha$ line emission of YSOs by using the H$\alpha$ pseudo-equivalent widths and XP spectra from Gaia DR3. We derive accretion luminosities, mass accretion rates and stellar parameters for 145 975 candidate YSO H$\alpha$ emitters all-sky. We describe filtering strategies to select specific sub-samples of YSOs from this catalogue. We identify a large population of low-accreting YSO candidates untraced by previous surveys. The population of low accreting YSOs is mostly spatially dispersed, away from star forming regions or more clustered environments of star formation. Many YSOs appear disconnected from young populations, reminiscent of 'Peter Pan' YSOs. We find $L_{acc}\propto L_\star^{1.41\pm0.02}$ and $\dot M_{acc}\propto M_\star^{2.4\pm0.1}$ for the purest all-sky sample of YSO candidates. By fitting an exponential to the fraction of accreting stars in clusters of different ages in the Sco-Cen complex, we obtain an accretion timescale of 2.7$\pm$0.4 Myr. The percentage of accretors found by fitting a power-law is 70% at 2 Myr and 2.8% at 10 Myr. With this new catalogue of H$\alpha$ emitters we significantly increase the number of YSO candidates with accretion rate estimations in the local neighbourhood. This allows us to study accretion timescales and the spatial and physical properties of YSO accretion from a large, all-sky, and homogeneous sample for the first time. [abridged]

Euclid preparation. The impact of redshift interlopers on the two-point correlation function analysis

Instrumentation and Surveys - Fri, 09/05/2025 - 10:47
arXiv:2505.04688v1 Announce Type: new Abstract: The Euclid survey aims to measure the spectroscopic redshift of emission-line galaxies by identifying the H$\,{\alpha}$ line in their slitless spectra. This method is sensitive to the signal-to-noise ratio of the line, as noise fluctuations or other strong emission lines can be misidentified as H$\,{\alpha}$, depending on redshift. These effects lead to catastrophic redshift errors and the inclusion of interlopers in the sample. We forecast the impact of such redshift errors on galaxy clustering measurements. In particular, we study the effect of interloper contamination on the two-point correlation function (2PCF), the growth rate of structures, and the Alcock-Paczynski (AP) parameters. We analyze 1000 synthetic spectroscopic catalogues, the EuclidLargeMocks, designed to match the area and selection function of the Data Release 1 (DR1) sample. We estimate the 2PCF of the contaminated catalogues, isolating contributions from correctly identified galaxies and from interlopers. We explore different models with increasing complexity to describe the measured 2PCF at fixed cosmology. Finally, we perform a cosmological inference and evaluate the systematic error on the inferred $f\sigma_8$, $\alpha_{\parallel}$ and $\alpha_{\perp}$ values associated with different models. Our results demonstrate that a minimal modelling approach, which only accounts for an attenuation of the clustering signal regardless of the type of contaminants, is sufficient to recover the correct values of $f\sigma_8$, $\alpha_{\parallel}$, and $\alpha_{\perp}$ at DR1. The accuracy and precision of the estimated AP parameters are largely insensitive to the presence of interlopers. The adoption of a minimal model induces a 1%-3% systematic error on the growth rate of structure estimation, depending on the redshift. However, this error remains smaller than the statistical error expected for the Euclid DR1 analysis.

Euclid preparation. The impact of redshift interlopers on the two-point correlation function analysis

Recent IoA Publications - Fri, 09/05/2025 - 10:47
arXiv:2505.04688v1 Announce Type: new Abstract: The Euclid survey aims to measure the spectroscopic redshift of emission-line galaxies by identifying the H$\,{\alpha}$ line in their slitless spectra. This method is sensitive to the signal-to-noise ratio of the line, as noise fluctuations or other strong emission lines can be misidentified as H$\,{\alpha}$, depending on redshift. These effects lead to catastrophic redshift errors and the inclusion of interlopers in the sample. We forecast the impact of such redshift errors on galaxy clustering measurements. In particular, we study the effect of interloper contamination on the two-point correlation function (2PCF), the growth rate of structures, and the Alcock-Paczynski (AP) parameters. We analyze 1000 synthetic spectroscopic catalogues, the EuclidLargeMocks, designed to match the area and selection function of the Data Release 1 (DR1) sample. We estimate the 2PCF of the contaminated catalogues, isolating contributions from correctly identified galaxies and from interlopers. We explore different models with increasing complexity to describe the measured 2PCF at fixed cosmology. Finally, we perform a cosmological inference and evaluate the systematic error on the inferred $f\sigma_8$, $\alpha_{\parallel}$ and $\alpha_{\perp}$ values associated with different models. Our results demonstrate that a minimal modelling approach, which only accounts for an attenuation of the clustering signal regardless of the type of contaminants, is sufficient to recover the correct values of $f\sigma_8$, $\alpha_{\parallel}$, and $\alpha_{\perp}$ at DR1. The accuracy and precision of the estimated AP parameters are largely insensitive to the presence of interlopers. The adoption of a minimal model induces a 1%-3% systematic error on the growth rate of structure estimation, depending on the redshift. However, this error remains smaller than the statistical error expected for the Euclid DR1 analysis.

Failed Soviet probe will soon crash to Earth – and we don't know where

Astronomy News - Fri, 09/05/2025 - 10:43

Kosmos 482, a Soviet spacecraft that never made it beyond Earth’s orbit on its way to Venus, is due to come crashing down on 9 or 10 May

NASA Telescopes Tune Into a Black Hole Prelude, Fugue

Astronomy News - Fri, 09/05/2025 - 10:42
8 Min Read NASA Telescopes Tune Into a Black Hole Prelude, Fugue The first sonification features WR124, an extremely bright, massive star. Here, the star is shown in a short-lived phase preceding the possible creation of a black hole.

NASA released three new pieces of cosmic sound Thursday that are associated with the densest and darkest members of our universe: black holes. These scientific productions are sonifications — or translations into sound — of data collected by NASA telescopes in space including the Chandra X-ray Observatory, James Webb Space Telescope, and Imaging X-ray Polarimetry Explorer (IXPE).

This trio of sonifications represents different aspects of black holes and black hole evolution. WR124 is an extremely bright, short-lived massive star known as a Wolf-Rayet that may collapse into a black hole in the future. SS 433 is a binary, or double system, containing a star like our Sun in orbit with either a neutron star or a black hole. The galaxy Centaurus A has an enormous black hole in its center that is sending a booming jet across the entire length of the galaxy. Data from Chandra and other telescopes were translated through a process called “sonification” into sounds and notes.

This new trio of sonifications represents different aspects of black holes. Black holes are neither static nor monolithic. They evolve over time, and are found in a range of sizes and environments.

WR 124  Credit: X-ray: NASA/CXC/SAO; Infrared: (Herschel) ESA/NASA/Caltech, (Spitzer) NASA/JPL/Caltech, (WISE) NASA/JPL/Caltech; Infrared: NASA/ESA/CSA/STScI/Webb ERO Production Team; Image processing: NASA/CXC/SAO/J. Major; Sonification: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)

The first movement is a prelude to the potential birth of a black hole. WR124 is an extremely bright, short-lived massive star known as a Wolf-Rayet at a distance of about 28,000 light-years from Earth. These stars fling their outer layers out into space, creating spectacular arrangements seen in an image in infrared light from the Webb telescope. In the sonification of WR124, this nebula is heard as flutes and the background stars as bells. At the center of WR124, where the scan begins before moving outward, is a hot core of the star that may explode as a supernova and potentially collapse and leave behind a black hole in its wake. As the scan moves from the center outward, X-ray sources detected by Chandra are translated into harp sounds. Data from NASA’s James Webb Space Telescope is heard as metallic bell-like sounds, while the light of the central star is mapped to produce the descending scream-like sound at the beginning. The piece is rounded out by strings playing additional data from the infrared telescopic trio of ESA’s (European Space Agency’s) Herschel Space Telescope, NASA’s retired Spitzer Space Telescope, and NASA’s retired Wide Image Survey Explorer (WISE) as chords.

SS 433 Credit: X-ray: (IXPE): NASA/MSFC/IXPE; (Chandra): NASA/CXC/SAO; (XMM): ESA/XMM-Newton; IR: NASA/JPL/Caltech/WISE; Radio: NRAO/AUI/NSF/VLA/B. Saxton. (IR/Radio image created with data from M. Goss, et al.); Image Processing/compositing: NASA/CXC/SAO/N. Wolk & K. Arcand; Sonification: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)

In the second movement of this black hole composition, listeners can explore a duet. SS 433 is a binary, or double, system about 18,000 light-years away that sings out in X-rays. The two members of SS 433 include a star like our Sun in orbit around a much heavier partner, either a neutron star or a black hole. This orbital dance causes undulations in X-rays that Chandra, IXPE, and ESA’s XMM-Newton telescopes are tuned into. These X-ray notes have been combined with radio and infrared data to provide a backdrop for this celestial waltz. The nebula in radio waves resembles a drifting manatee, and the scan sweeps across from right to left. Light towards the top of the image is mapped to higher-pitch sound, with radio, infrared, and X-ray light mapped to low, medium, and high pitch ranges. Bright background stars are played as water-drop sounds, and the location of the binary system is heard as a plucked sound, pulsing to match the fluctuations due to the orbital dance.

Centarus A Credit: X-ray: (Chandra) NASA/CXC/SAO, (IXPE) NASA/MSFC; Optical: ESO; Image Processing: NASA/CXC/SAO/K. Arcand, J. Major, and J. Schmidt; Sonification: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)

The third and final movement of the black hole-themed sonifications crescendos with a distant galaxy known as Centaurus A, about 12 million light-years away from Earth. At the center of Centaurus A is an enormous black hole that is sending a booming jet across the entire length of the galaxy. Sweeping around clockwise from the top of the image, the scan encounters Chandra’s X-rays and plays them as single-note wind chimes. X-ray light from IXPE is heard as a continuous range of frequencies, producing a wind-like sound. Visible light data from the European Southern Observatory’s MPG telescope shows the galaxy’s stars that are mapped to string instruments including foreground and background objects as plucked strings.

For more NASA sonifications and information about the project, visit https://chandra.si.edu/sound/

These sonifications were led by the Chandra X-ray Center (CXC), with support from NASA’s Marshall Space Flight Center and NASA’s Universe of Learning program, which is part of the NASA Science Activation program. The collaboration was driven by visualization scientist Kimberly Arcand (CXC), astrophysicist Matt Russo, and musician Andrew Santaguida (both of the SYSTEM Sounds project), along with consultant Christine Malec.

NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts. NASA’s Universe of Learning materials are based upon work supported by NASA under cooperative agreement award number NNX16AC65A to the Space Telescope Science Institute, working in partnership with Caltech/IPAC, Center for Astrophysics | Harvard & Smithsonian, and NASA’s Jet Propulsion Laboratory.

The agency’s IXPE is a collaboration between NASA and the Italian Space Agency with partners and science collaborators in 12 countries. The IXPE mission is led by Marshall. BAE Systems, Inc., headquartered in Falls Church, Virginia, manages spacecraft operations together with the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder.

To learn more about NASA’s space telescopes, visit:

https://science.nasa.gov/universe

Read more from NASA’s Chandra X-ray Observatory

Learn more about the Chandra X-ray Observatory and its mission here:

https://www.nasa.gov/chandra

https://chandra.si.edu

Visual Description

This release features three sonifications related to black holes, presented as soundtracks to short videos. Each sonification video features a composite image representing a different aspect of the life of a black hole. These images are visualizations of data collected by NASA telescopes. During each video, a line sweeps through the image. When the line encounters a visual element, it is translated into sound according to parameters established by visualization scientist Kimberly Arcand, astrophysicist Matt Russo, musician Andrew Santaguida, and consultant Christine Malec.

The first sonification features WR124, an extremely bright, massive star. Here, the star is shown in a short-lived phase preceding the possible creation of a black hole. At the center of the composite image is the large gleaming star in white and pale blue. The star sits at the heart of a mottled pink and gold cloud, its long diffraction spikes extending to the outer edges. Also residing in the cloud are other large gleaming stars, glowing hot-pink dots, and tiny specks of blue and white light. In this sonification, the sound activation line is an ever-expanding circle which starts in the center of the massive star and continues to grow until it exits the frame.

The second sonification features SS 433, a binary star system at the center of a supernova remnant known as the Manatee Nebula. Visually, the translucent, blobby teal nebula does, indeed, resemble a bulbous walrus or manatee, floating in a red haze packed with distant specs of light. Inside the nebula is a violet streak, a blue streak, and a large bright dot. The dot, represented by a plucking sound in the sonification, is the binary system at the heart of the nebula. In this sonification, the vertical activation line begins at our right edge of the frame, and sweeps across the image before exiting at our left.

The third and final sonification features Centaurus A, a distant galaxy with an enormous black hole emitting a long jet of high-energy particles. The black hole sits at the center of the composite image, represented by a brilliant white light. A dark, grainy, oblong cloud cuts diagonally across the black hole from our lower left toward our upper right. A large, faint, translucent blue cloud stretches from our upper left to our lower right. And the long, thin jet, also in translucent blue, extends from the black hole at the center toward the upper lefthand corner. In this sonification, the activation line rotates around the image like the hand of a clock. It begins at the twelve o’clock position, and sweeps clockwise around the image.

News Media Contact

Megan Watzke
Chandra X-ray Center
Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu

Lane Figueroa
Marshall Space Flight Center, Huntsville, Alabama
256-544-0034
lane.e.figueroa@nasa.gov

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Moon dust 'rarer than gold' arrives in UK from China

Astronomy News - Fri, 09/05/2025 - 10:42

First Moon samples collected in nearly 50 years and loaned by China for the first time are now in the UK.

Tue 20 May 11:15: A 21-cm Cosmologist’s Journey: From Cambridge to North America and Back Again

Next External Talks - Thu, 08/05/2025 - 21:22
A 21-cm Cosmologist’s Journey: From Cambridge to North America and Back Again

In this talk, I’ll take you on a whistle-stop tour of my journey in 21-cm cosmology – from my PhD days in Cambridge to fellowship and research scientist positions in the USA and Canada. I’ll discuss the significance of 21-cm cosmology in understanding the Universe’s first billion years and describe key projects I’ve worked on, including the SKA , HERA, EDGES , and REACH . Along the way, I’ll share some personal highlights from my time in North America, including adventures in national parks and snow sports.

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Categories: Talks

Fri 11 Jul 11:30: Title to be confirmed

IoA Institute of Astronomy Talk Lists - Thu, 08/05/2025 - 17:58
Title to be confirmed

Abstract not available

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Wed 14 May 13:40: Gravitational Phase-Space Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power-Spectrum

IoA Institute of Astronomy Talk Lists - Thu, 08/05/2025 - 16:41
Gravitational Phase-Space Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power-Spectrum

The matter power spectrum is one of the fundamental quantities in the study of large-scale structure in cosmology. In this talk, I will describe its small-scale asymptotic limit, and give a theoretical argument to the effect that, for cold dark matter, P(k) has a universal asymptotic scaling with the wave-number k, for k >> k_nl, viz. P(k) ~ k^(-3). I will explain how gravitational collapse drives a turbulent phase-space flow of the quadratic Casimir invariant, where the linear and non-linear time scales are balanced, and how this balance dictates the k dependence of the power spectrum. The coldness of the dark-matter distribution function — its non-vanishing only on a 3-dimensional sub-manifold of phase-space — underpins the analysis. I will show Vlasov-Poisson simulations that support the theory, and if time permits, also describe a stationary-phase technique for deriving an equivalent result. 

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Wed 14 May 13:40: Gravitational Phase-Space Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power-Spectrum

Next Wednesday Seminars - Thu, 08/05/2025 - 16:41
Gravitational Phase-Space Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power-Spectrum

The matter power spectrum is one of the fundamental quantities in the study of large-scale structure in cosmology. In this talk, I will describe its small-scale asymptotic limit, and give a theoretical argument to the effect that, for cold dark matter, P(k) has a universal asymptotic scaling with the wave-number k, for k >> k_nl, viz. P(k) ~ k^(-3). I will explain how gravitational collapse drives a turbulent phase-space flow of the quadratic Casimir invariant, where the linear and non-linear time scales are balanced, and how this balance dictates the k dependence of the power spectrum. The coldness of the dark-matter distribution function — its non-vanishing only on a 3-dimensional sub-manifold of phase-space — underpins the analysis. I will show Vlasov-Poisson simulations that support the theory, and if time permits, also describe a stationary-phase technique for deriving an equivalent result. 

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Categories: Talks

Fri 16 May 13:00: Modified gravity and the atomic world

Next External Talks - Thu, 08/05/2025 - 13:47
Modified gravity and the atomic world

The existence of dark energy and dark matter hint that there is more to gravity than meets the eye.  A wide range of new theories, exhibiting a new scalar particle with a property called screening, indicate small-scale tests as the most promising route towards detection of new particles.  Atomic physics is especially promising.  I will discuss how pairs of atomic clocks are capable of searching for equivalence-principle violating scalar couplings to Standard Model particles, which hold the potential to detect quintessence, ultralight dark matter, and modified gravity.  Similarly, atom interferometry and atomic spectroscopy provide a window to detect new forces associated with new screened scalars as well.

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DAmodel: Hierarchical Bayesian Modelling of DA White Dwarfs for Spectrophotometric Calibration

Stars and stellar evolution - Thu, 08/05/2025 - 12:14
arXiv:2412.08809v2 Announce Type: replace Abstract: We use hierarchical Bayesian modelling to calibrate a network of 32 all-sky faint DA white dwarf (DA WD) spectrophotometric standards ($16.5 < V < 19.5$) alongside three CALSPEC standards, from 912 \r{A} to 32 $\mu$m. The framework is the first of its kind to jointly infer photometric zeropoints and WD parameters (surface gravity $\log g$, effective temperature $T_{\text{eff}}$, extinction $A_V$, dust relation parameter $R_V$) by simultaneously modelling both photometric and spectroscopic data. We model panchromatic Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) UVIS and IR photometry, HST/STIS UV spectroscopy and ground-based optical spectroscopy to sub-percent precision. Photometric residuals for the sample are the lowest yet yielding $<0.004$ mag RMS on average from the UV to the NIR, achieved by jointly inferring time-dependent changes in system sensitivity and WFC3/IR count-rate nonlinearity. Our GPU-accelerated implementation enables efficient sampling via Hamiltonian Monte Carlo, critical for exploring the high-dimensional posterior space. The hierarchical nature of the model enables population analysis of intrinsic WD and dust parameters. Inferred spectral energy distributions from this model will be essential for calibrating the James Webb Space Telescope as well as next-generation surveys, including Vera Rubin Observatory's Legacy Survey of Space and Time and the Nancy Grace Roman Space Telescope.

DAmodel: Hierarchical Bayesian Modelling of DA White Dwarfs for Spectrophotometric Calibration

Recent IoA Publications - Thu, 08/05/2025 - 12:14
arXiv:2412.08809v2 Announce Type: replace Abstract: We use hierarchical Bayesian modelling to calibrate a network of 32 all-sky faint DA white dwarf (DA WD) spectrophotometric standards ($16.5 < V < 19.5$) alongside three CALSPEC standards, from 912 \r{A} to 32 $\mu$m. The framework is the first of its kind to jointly infer photometric zeropoints and WD parameters (surface gravity $\log g$, effective temperature $T_{\text{eff}}$, extinction $A_V$, dust relation parameter $R_V$) by simultaneously modelling both photometric and spectroscopic data. We model panchromatic Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) UVIS and IR photometry, HST/STIS UV spectroscopy and ground-based optical spectroscopy to sub-percent precision. Photometric residuals for the sample are the lowest yet yielding $<0.004$ mag RMS on average from the UV to the NIR, achieved by jointly inferring time-dependent changes in system sensitivity and WFC3/IR count-rate nonlinearity. Our GPU-accelerated implementation enables efficient sampling via Hamiltonian Monte Carlo, critical for exploring the high-dimensional posterior space. The hierarchical nature of the model enables population analysis of intrinsic WD and dust parameters. Inferred spectral energy distributions from this model will be essential for calibrating the James Webb Space Telescope as well as next-generation surveys, including Vera Rubin Observatory's Legacy Survey of Space and Time and the Nancy Grace Roman Space Telescope.

Mon 09 Jun 14:00: Title to be confirmed

Next External Talks - Thu, 08/05/2025 - 12:11
Title to be confirmed

Abstract not available

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Mon 12 May 14:00: On the role of magnetic fluctuations in low magnetic Prandtl number plasmas

Next External Talks - Thu, 08/05/2025 - 11:55
On the role of magnetic fluctuations in low magnetic Prandtl number plasmas

Magnetic fields on small scales are ubiquitous in the universe. For example, the fluctuating magnetic fields in star-forming regions of galaxies are more than twice the strength of the magnetic fields coherent over large scales. On the solar surface, magnetic fields are mostly concentrated in medium and small-scale structures, while the proportion comprising the mean field strength is even lower than in galaxies. The generation mechanisms of the fluctuating magnetic fields are not fully understood. One possibility is the so-called small-scale dynamo (SSD), the other is tangling of the large-scale field structures through turbulence acting on them. In the interstellar medium of galaxies, the resistivity is much lower than the viscosity, such that magnetic instabilities are easier to excite relative to the turbulence. SSD in such high magnetic Prandtl number (Pm, i.e. the ratio between viscosity and resistivity) conditions has therefore been predicted to be easily excited. In the Sun and cool stars, Pm is much lower, namely in the range of 1e-6 to 1e-3. Both theoretically and especially numerically, SSD is more difficult to excite at such very low magnetic Prandtl numbers. Indeed, some recent numerical studies has indicated that the threshold for SSD excitation should systematically increase with decreasing Pm, concluding that SSD would be impossible in the Sun and cool stars.

Accelerating the magnetohydrodynamics solvers with graphics processing units has recently opened an avenue to numerically study low-Pm flows. With these tools we have been able to perform simulations that approach the solar Pm-values, studying both kinematic and non-linear regimes. Contrary to earlier findings, the SSD turns out not only to be possible for Pms down to 0.0031, but even to become increasingly easy to excite for Pm below approximately 0.05. We relate this behaviour to the known hydrodynamic phenomenon, referred to as the bottleneck effect. Extrapolating our results to solar values of Pm indicates that an SSD would be possible under such conditions. The saturation strength of the SSD is of the order of the turbulent kinetic energy independent of the Pm, when the magnetic Reynolds number (Rm) is moderate (up to a few thousands). For higher Rm the saturation strength rapidly diminishes and reaches levels of order of magnitude lower than turbulent kinetic energy, casting a new doubt of the SSD being important in the Sun and stars. Even higher resolution studies, however, would be required to verify this robustly. For such calculations, however, extraordinary resources/quantum computers are required.

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Early and Extensive Ultraviolet Through Near Infrared Observations of the Intermediate-Luminosity Type Iax Supernovae 2024pxl

Stars and stellar evolution - Thu, 08/05/2025 - 11:53
arXiv:2505.04610v1 Announce Type: new Abstract: We present ultraviolet (UV) through near-infrared (NIR) photometric and spectroscopic observations of the nearby SN 2024pxl, the third Type Ia supernova (SN Ia) in NGC 6384. SN 2024pxl is a Type Iax supernova (SN Iax) with an intermediate luminosity ($M_r = -16.99\pm0.32$ mag) and an average SN Iax light curve decline rate. SN 2024pxl was discovered $\sim$3 days after first light, and the rising light curve follows a single power law that is inconsistent with significant interaction with a companion star or circumstellar material. Our extensive NIR photometric coverage is comparable to that of the well-observed SNe Iax 2005hk and 2012Z, and we demonstrate that the $J-H$ colors of SNe Iax differ from normal SNe Ia and appear to be more homogeneous as a class. Spectroscopically, we report the earliest-ever NIR spectrum of a SN Iax as measured from maximum light ($t\approx-9$ days): a featureless continuum with similarities to a $\sim$9,000 K blackbody, and the line velocities are consistent with a mixed-ejecta structure, with C, Si, and Fe having similar velocities and velocity evolutions. We find a tentative correlation between the $H$-band break Co II velocity $\sim$20 days post-peak and absolute magnitude, with more luminous SNe Iax showing faster Co II velocities. Our observations suggest that SN 2024pxl resulted from the thermonuclear disruption of a CO white dwarf star that undergoes deflagration burning.