arXiv subjects

M. Chu

Publications and source records attributed to M. Chu.

8 recordsLinked to original sources

arXiv2026

SN 2019vxm: A luminous and long-lived Type IIn supernova with early flash-ionisation features

We present the photometric and spectroscopic analysis of the luminous and long-lasting Type IIn supernova (SN) 2019vxm. The SN reaches a peak V-band absolute magnitude of MV = -20.01 +/- 0.13 mag in 35.0 days, and displays slow evolution in both the light curves and spectra, resembling that of long-lived SNe IIn. A mid-infrared (MIR) excess is detected starting from seven months after maximum brightness, suggesting a few 10^-3 solar masses of dust are newly formed at >= 210 days (and up to 0.01 solar masses at +4.5 yr). The spectra are dominated by a blue continuum at early stages, with narrow, symmetric Balmer lines and flash-ionisation emission lines of C III, N III, and He II. Comparing our flash-ionised spectrum with early interacting SN spectral models, we estimate a lower limit for the mass-loss rate of the progenitor of >= 0.01 solar masses per year. A weak P Cygni absorption feature is detected in the H-beta profile of the high-resolution Echelle spectrum at +19.7 d, suggesting the presence of slow-moving (60 +/- 10 km/s), unshocked circumstellar material (CSM) arising from the pre-SN wind of the progenitor. The H-alpha and H-beta profiles gradually evolve and become broader and asymmetric, showing a progressively increasing blueshift, with a clear flux deficit in the red wings of the broad velocity component after +102 days. Our observed bolometric light curve before about 100 days can be well fitted by a power-law function (L(t) = 2 x 10^44 (t/day)^-0.49 erg/s), which is very similar to SN 2010jl.

astro-ph.SR↗

arXiv2025

The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes

We present updated forecasts for the scientific performance of the degree-scale (0.5 deg FWHM at 93 GHz), deep-field survey to be conducted by the Simons Observatory (SO). By 2027, the SO Small Aperture Telescope (SAT) complement will be doubled from three to six telescopes, including a doubling of the detector count in the 93 GHz and 145 GHz channels to 48,160 detectors. Combined with a planned extension of the survey duration to 2035, this expansion will significantly enhance SO's search for a $B$-mode signal in the polarisation of the cosmic microwave background, a potential signature of gravitational waves produced in the very early Universe. Assuming a $1/f$ noise model with knee multipole $\ell_{\rm knee} = 50$ and a moderately complex model for Galactic foregrounds, we forecast a $1\sigma$ (or 68% confidence level) constraint on the tensor-to-scalar ratio $r$ of $\sigma_r = 1.2\times10^{-3}$, assuming no primordial $B$-modes are present. This forecast assumes that 70% of the $B$-mode lensing signal can ultimately be removed using high resolution observations from the SO Large Aperture Telescope (LAT) and overlapping large-scale structure surveys. For more optimistic assumptions regarding foregrounds and noise, and assuming the same level of delensing, this forecast constraint improves to $\sigma_r = 7\times10^{-4}$. These forecasts represent a major improvement in SO's constraining power, being a factor of around 2.5 times better than what could be achieved with the originally planned campaign, which assumed the existing three SATs would conduct a five-year survey.

astro-ph.CO↗

arXiv2025

The Simons Observatory: Science Goals and Forecasts for the Enhanced Large Aperture Telescope

We describe updated scientific goals for the wide-field, millimeter-wave survey that will be produced by the Simons Observatory (SO). Significant upgrades to the 6-meter SO Large Aperture Telescope (LAT) are expected to be complete by 2028, and will include a doubled mapping speed with 30,000 new detectors and an automated data reduction pipeline. In addition, a new photovoltaic array will supply most of the observatory's power. The LAT survey will cover about 60% of the sky at a regular observing cadence, with five times the angular resolution and ten times the map depth of Planck. The science goals are to: (1) determine the physical conditions in the early universe and constrain the existence of new light particles; (2) measure the integrated distribution of mass, electron pressure, and electron momentum in the late-time universe, and, in combination with optical surveys, determine the neutrino mass and the effects of dark energy via tomographic measurements of the growth of structure at $z < 3$; (3) measure the distribution of electron density and pressure around galaxy groups and clusters, and calibrate the effects of energy input from galaxy formation on the surrounding environment; (4) produce a sample of more than 30,000 galaxy clusters, and more than 100,000 extragalactic millimeter sources, including regularly sampled AGN light-curves, to study these sources and their emission physics; (5) measure the polarized emission from magnetically aligned dust grains in our Galaxy, to study the properties of dust and the role of magnetic fields in star formation; (6) constrain asteroid regoliths, search for Trans-Neptunian Objects, and either detect or eliminate large portions of the phase space in the search for Planet 9; and (7) provide a powerful new window into the transient universe on time scales of minutes to years, concurrent with observations from Rubin of overlapping sky.

astro-ph.IM↗

arXiv2018

Production and Integration of the ATLAS Insertable B-Layer

During the shutdown of the CERN Large Hadron Collider in 2013-2014, an additional pixel layer was installed between the existing Pixel detector of the ATLAS experiment and a new, smaller radius beam pipe. The motivation for this new pixel layer, the Insertable B-Layer (IBL), was to maintain or improve the robustness and performance of the ATLAS tracking system, given the higher instantaneous and integrated luminosities realised following the shutdown. Because of the extreme radiation and collision rate environment, several new radiation-tolerant sensor and electronic technologies were utilised for this layer. This paper reports on the IBL construction and integration prior to its operation in the ATLAS detector.

physics.ins-det↗

arXiv2007

The Role of Reconstructed Surfaces in the Intrinsic Dissipative Dynamics of Silicon Nanoresonators

Dissipation in the flexural dynamics of doubly clamped nanomechanical bar resonators is investigated using molecular dynamics simulation. The dependence of the quality factor Q on temperature and the size of the resonator is calculated from direct simulation of the oscillation of a series of Si <001> bars with bare {100} dimerized surfaces. The bar widths range from 3.3 to 8.7nm, all with a fixed length of 22nm. The fundamental mode frequencies range from 40 to 90GHz and Q from 10^2 near 1000K to 10^4 near 50K. The quality factor is shown to be limited by defects in the reconstructed surface.

cond-mat.mtrl-sci↗

arXiv2004

Cosmological Parameter Constraints as Derived from the Wilkinson Microwave Anisotropy Probe Data via Gibbs Sampling and the Blackwell-Rao Estimator

We study the Blackwell-Rao (BR) estimator of the probability distribution of the angular power spectrum, P(C_l|d), by applying it to samples of full-sky no-noise CMB maps generated via Gibbs sampling. We find the estimator, given a set of samples, to be very fast and also highly accurate, as determined by tests with simulated data. We also find that the number of samples required for convergence of the BR estimate rises rapidly with increasing l, at least at low l. Our existing sample chains are only long enough to achieve convergence at l less than about 40. In comparison with P(C_l|d) as reported by the WMAP team we find significant differences at these low l values. These differences lead to up to approximately 0.5 sigma shifts in the estimates of parameters in a 7-parameter Lambda CDM model with non-zero dn_s/dln(k). Fixing dn_s/dln(k)= 0 makes these shifts much less significant. Unlike existing analytic approximations, the BR estimator can be straightforwardly extended for the case of power spectra from correlated fields, such as temperature and polarization. We discuss challenges to extending the procedure to higher l and provide some solutions.

astro-ph↗

arXiv2002

Probing the Reionization History of the Universe using the Cosmic Microwave Background Polarization

The recent discovery of a Gunn--Peterson (GP) trough in the spectrum of the redshift 6.28 SDSS quasar has raised the tantalizing possibility that we have detected the reionization of the universe. However, a neutral fraction (of hydrogen) as small as 0.1% is sufficient to cause the GP trough, hence its detection alone cannot rule out reionization at a much earlier epoch. The Cosmic Microwave Background (CMB) polarization anisotropy offers an alternative way to explore the dark age of the universe. We show that for most models constrained by the current CMB data and by the discovery of a GP trough (showing that reionization occurred at z > 6.3), MAP can detect the reionization signature in the polarization power spectrum. The expected 1-sigma error on the measurement of the electron optical depth is around 0.03 with a weak dependence on the value of that optical depth. Such a constraint on the optical depth will allow MAP to achieve a 1-sigma error on the amplitude of the primordial power spectrum of 6%. MAP with two years (Planck with one year) of observation can distinguish a model with 50% (6%) partial ionization between redshifts of 6.3 and 20 from a model in which hydrogen was completely neutral at redshifts greater than 6.3. Planck will be able to distinguish between different reionization histories even when they imply the same optical depth to electron scattering for the CMB photons.

astro-ph↗

arXiv1994

Quantisation of the SU(N) WZW Model at Level $k$

The quantisation of the Wess-Zumino-Witten model on a circle is discussed in the case of $SU(N)$ at level $k$. The quantum commutation of the chiral vertex operators is described by an exchange relation with a braiding matrix, $Q$. Using quantum consistency conditions, the braiding matrix is found explicitly in the fundamental representation. This matrix is shown to be related to the Racah matrix for $U_t(SL(N))$. From calculating the four-point functions with the Knizhnik-Zamolodchikov equations, the deformation parameter $t$ is shown to be $t=\exp({i\pi /(k+N)})$ when the level $k\ge 2$. For $k=1$, there are two possible types of braiding, $t=\exp({i\pi /(1+N)})$ or $t=\exp(i\pi)$. In the latter case, the chiral vertex operators are constructed explicitly by extending the free field realisation a la Frenkel-Kac and Segal. This construction gives an explicit description of how to chirally factorise the $SU(N)_{k=1}$ WZW model.

hep-th↗