arXiv subjects

Anthony M. Brown

Publications and source records attributed to Anthony M. Brown.

At least 19 recordsLinked to original sources

arXiv2026

Multi-sensor fusion for fine-guidance and milliarcsecond-level attitude estimation of balloon-borne telescope

Balloon-borne telescopes rely on fine-guidance systems to achieve milliarcsecond image stability despite residual disturbances from the balloon environment. In these systems, the Fast Steering Mirror (FSM) stabilizes the image in two focal-plane axes, but leaves systematic, field-dependent residual motion induced by boresight roll. This effect, referred to as roll leakage, becomes more important for wider fields of view. In this work, roll leakage is characterized using data from the 2023 Superpressure Balloon-borne Imaging Telescope (SuperBIT) science flight. SuperBIT is a 0.5-m near-ultraviolet to near-infrared telescope that demonstrated milliarcsecond-level image stability during its 45-night science flight. We find that passive focal-plane star-camera measurements correlate strongly with independent roll measurements across a large set of science exposures, showing that boresight roll frequently drives residual focal-plane motion. We then develop a simulation framework combining optical ray tracing, asynchronous guide-star measurements, estimation, and FSM control to study this behavior. The framework is used to compare single-star and multi-star guidance architectures under realistic flight disturbances. For the SuperBIT geometry, we find that multi-star estimation reduces average roll-induced science-field image motion by 31.8%, increasing to 77.4% for a representative geometry of GigaBIT, SuperBIT's planned larger-aperture successor. These results motivate further investigation of multi-star fine-guidance architectures for GigaBIT.

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arXiv2026

Lensing in the Blue III: Weak Lensing Shape Catalogs of 30 Merging Galaxy Clusters

We present the weak gravitational lensing dataset from the Super-pressure Balloon-Borne Imaging Telescope (SuperBIT), which imaged 30 galaxy clusters during its 45 night flight in April to May 2023. SuperBIT is a first-of-its-kind balloon-borne imaging telescope that achieved near diffraction-limited observations in near-space conditions above 98% of the Earth's atmosphere. We use the metacalibration algorithm to obtain calibrated galaxy shapes for our target clusters and several calibration fields, enabling unbiased reconstruction of the weak-lensing signal. We employ several diagnostics throughout the pipeline, including assessments of point-spread function (PSF) modeling residuals and their impact on weak-lensing measurements, as well as tests for correlations between galaxy shapes and measured galaxy and PSF properties. To assess the multiplicative shear bias of the pipeline, we analyze a parallel set of simulated images that incorporate the real observing conditions from the flight, including measured SuperBIT PSFs, observed sky backgrounds, and detector noise, yielding a bias of $(1.1 \pm 7.8)$~per~cent.

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arXiv2025

Commissioning and Performance of the Trinity Demonstrator

The Trinity Demonstrator is a proof of concept prototype for the Trinity Neutrino Observatory, which is sensitive to astrophysical neutrinos above PeV energies. The Demonstrator is a one-square meter class imaging atmospheric Cherenkov telescope deployed on Frisco Peak, Utah, and remotely operated. The light-collection surface is equipped with 77 mirror facets with 15 cm diameter, and its $3.87^\circ\times3.87^\circ$ field of view is instrumented with a 256-pixel camera yielding a $0.24^\circ$ angular resolution. The camera signals are digitized with a 100 MS/s, and 12-bit resolution switched capacitor array readout. We discuss the Demonstrator's design, the telescope's deployment on Frisco Peak, and its commissioning.

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arXiv2024

The Effect of Pulsar Geometry on the Observed Gamma-ray Spectrum of Millisecond Pulsars

We analyse 13 yrs of $\textit{Fermi}$-LAT PASS 8 events from 127 gamma-ray emitting millisecond pulsars (MSPs) in the energy range 0.1$-$100 GeV and significantly detect 118 MSPs. We fit the stacked emission with a log parabola (LP) spectral model which we show is preferred to two previously published models. We consider the influence of pulsar properties and observer geometric effects on spectral features by defining energy flux colours for both the individual MSPs, and our stacked model as a baseline. There is no correlation of colours with pulsar luminosity, $\dot{E}$, surface magnetic field or magnetic impact angle. We also find that pulsar geometry has little effect on the observed gamma-ray spectrum which is in tension with previous modelling of gamma-ray emission with respect to pulsar geometry. Our LP MSP model is applicable to problems where an ensemble of gamma-ray MSPs is considered, such as that of the Galactic centre excess or in the case of emission from globular clusters.

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arXiv2024

Chasing Gravitational Waves with the Cherenkov Telescope Array

The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA.

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arXiv2023

Data downloaded via parachute from a NASA super-pressure balloon

In April to May 2023, the superBIT telescope was lifted to the Earth's stratosphere by a helium-filled super-pressure balloon, to acquire astronomical imaging from above (99.5% of) the Earth's atmosphere. It was launched from New Zealand then, for 40 days, circumnavigated the globe five times at a latitude 40 to 50 degrees South. Attached to the telescope were four 'DRS' (Data Recovery System) capsules containing 5 TB solid state data storage, plus a GNSS receiver, Iridium transmitter, and parachute. Data from the telescope were copied to these, and two were dropped over Argentina. They drifted 61 km horizontally while they descended 32 km, but we predicted their descent vectors within 2.4 km: in this location, the discrepancy appears irreducible below 2 km because of high speed, gusty winds and local topography. The capsules then reported their own locations to within a few metres. We recovered the capsules and successfully retrieved all of superBIT's data - despite the telescope itself being later destroyed on landing.

astro-ph.IM↗

arXiv2023

Lensing in the Blue II: Estimating the Sensitivity of Stratospheric Balloons to Weak Gravitational Lensing

The Superpressure Balloon-borne Imaging Telescope (SuperBIT) is a diffraction-limited, wide-field, 0.5 m, near-infrared to near-ultraviolet observatory designed to exploit the stratosphere's space-like conditions. SuperBIT's 2023 science flight will deliver deep, blue imaging of galaxy clusters for gravitational lensing analysis. In preparation, we have developed a weak lensing measurement pipeline with modern algorithms for PSF characterization, shape measurement, and shear calibration. We validate our pipeline and forecast SuperBIT survey properties with simulated galaxy cluster observations in SuperBIT's near-UV and blue bandpasses. We predict imaging depth, galaxy number (source) density, and redshift distribution for observations in SuperBIT's three bluest filters; the effect of lensing sample selections is also considered. We find that in three hours of on-sky integration, SuperBIT can attain a depth of b = 26 mag and a total source density exceeding 40 galaxies per square arcminute. Even with the application of lensing-analysis catalog selections, we find b-band source densities between 25 and 30 galaxies per square arcminute with a median redshift of z = 1.1. Our analysis confirms SuperBIT's capability for weak gravitational lensing measurements in the blue.

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arXiv2022

Weak lensing in the blue: a counter-intuitive strategy for stratospheric observations

The statistical power of weak lensing measurements is principally driven by the number of high redshift galaxies whose shapes are resolved. Conventional wisdom and physical intuition suggest this is optimised by deep imaging at long (red or near IR) wavelengths, to avoid losing redshifted Balmer break and Lyman break galaxies. We use the synthetic Emission Line EL-COSMOS catalogue to simulate lensing observations using different filters, from various altitudes. Here were predict the number of exposures to achieve a target z > 0.3 source density, using off-the-shelf and custom filters. Ground-based observations are easily better at red wavelengths, as (more narrowly) are space-based observations. However, we find that SuperBIT, a diffraction-limited observatory operating in the stratosphere, should instead perform its lensing-quality observations at blue wavelengths.

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arXiv2022

Design and Performance of the Prototype Schwarzschild-Couder Telescope Camera

The prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a novel dual mirror optics design which reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels), providing a 2.68$^{\circ}$ field of view (FoV). The camera electronics are based on custom TARGET (TeV array readout with GSa/s sampling and event trigger) application specific integrated circuits. Field programmable gate arrays sample incoming signals at a gigasample per second. A single backplane provides camera-wide triggers. An upgrade of the pSCT camera is in progress, which will fully populate the focal plane. This will increase the number of pixels to 11,328, the number of backplanes to 9, and the FoV to 8.04$^{\circ}$. Here we give a detailed description of the pSCT camera, including the basic concept, mechanical design, detectors, electronics, current status and first light.

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arXiv2021

Trinity: An Imaging Air Cherenkov Telescope to Search for Ultra-High-Energy Neutrinos

Earth-skimming neutrinos are those which travel through the Earth's crust at a shallow angle. For Ultra-High-Energy (E > 1 PeV; UHE) earth-skimming tau neutrinos, there is a high-probability that the tau lepton created by a neutrino-Earth interaction will emerge from the ground before it decays. When this happens, the decaying tau particle initiates an air shower of relativistic sub-atomic particles which emit Cherenkov radiation. To observe this Cherenkov radiation, we propose the Trinity Observatory. Using a novel optical structure design, pointing at the horizon, Trinity will observe the Cherenkov radiation from upward-going neutrino-induced air showers. Being sensitive to neutrinos in the 1-10,000 PeV energy range, Trinity's expected sensitivity will have a unique role to play filling the gap between the observed astrophysical neutrinos observed by IceCube and the expected sensitivity of radio UHE neutrino detectors.

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arXiv2020

Axion Constraints from Quiescent Soft Gamma-ray Emission from Magnetars

Axion-like-particles (ALPs) emitted from the core of a magnetar can convert to photons in its magnetosphere. The resulting photon flux is sensitive to the product of $(i)$ the ALP-nucleon coupling $G_{an}$ which controls the production cross section in the core and $(ii)$ the ALP-photon coupling $g_{aγγ}$ which controls the conversion in the magnetosphere. We study such emissions in the soft-gamma-ray range (300 keV to 1 MeV), where the ALP spectrum peaks and astrophysical backgrounds from resonant Compton upscattering are expected to be suppressed. Using published quiescent soft-gamma-ray flux upper limits in 5 magnetars obtained with $CGRO$ COMPTEL and $INTEGRAL$ SPI/IBIS/ISGRI, we put limits on the product of the ALP-nucleon and ALP-photon couplings. We also provide a detailed study of the dependence of our results on the magnetar core temperature. We further show projections of our result for future $Fermi$-GBM observations. Our results motivate a program of studying quiescent soft-gamma-ray emission from magnetars with the $Fermi$-GBM.

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arXiv2020

Locating the gamma-ray emission region in the brightest Fermi-LAT Flat Spectrum Radio Quasars

We present a temporal and spectral analysis of the gamma-ray flux from nine of the brightest flat spectrum radio quasars (FSRQs) detected with the Fermi Large Area Telescope (LAT) during its first eight years of operation, with the aim of constraining the location of the emission region. Using the increased photon statistics provided from the two brightest flares of each source, we find evidence of sub-hour variability from B2 1520+31, PKS 1502+106 and PKS 1424-41, with the remaining sources showing variability on timescales of a few hours. These indicate gamma-ray emission from extremely compact regions in the jet, potentially compatible with emission from within the broad line region (BLR). The flare spectra show evidence of a spectral cut-off in 7 of the 18 flares studied, further supporting the argument for BLR emission in these sources. An investigation into the energy dependence of cooling timescales finds evidence for both BLR origin and emission from within the molecular torus (MT). However, Monte Carlo simulations show that the very high energy (VHE) emission from all sources except 3C 279, 3C 454.3 and 4C 21.35 is incompatible with a BLR origin. The combined findings of all the approaches used suggest that the gamma-ray emission in the brightest FSRQs originates in multiple compact emission regions throughout the jet, within both the BLR and the MT.

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arXiv2020

Optical night sky brightness measurements from the stratosphere

This paper presents optical night sky brightness measurements from the stratosphere using CCD images taken with the Super-pressure Balloon-borne Imaging Telescope (SuperBIT). The data used for estimating the backgrounds were obtained during three commissioning flights in 2016, 2018, and 2019 at altitudes ranging from 28 km to 34 km above sea level. For a valid comparison of the brightness measurements from the stratosphere with measurements from mountain-top ground-based observatories (taken at zenith on the darkest moonless night at high Galactic and high ecliptic latitudes), the stratospheric brightness levels were zodiacal light and diffuse Galactic light subtracted, and the airglow brightness was projected to zenith. The stratospheric brightness was measured around 5.5 hours, 3 hours, and 2 hours before the local sunrise time in 2016, 2018, and 2019 respectively. The $B$, $V$, $R$, and $I$ brightness levels in 2016 were 2.7, 1.0, 1.1, and 0.6 mag arcsec$^{-2}$ darker than the darkest ground-based measurements. The $B$, $V$, and $R$ brightness levels in 2018 were 1.3, 1.0, and 1.3 mag arcsec$^{-2}$ darker than the darkest ground-based measurements. The $U$ and $I$ brightness levels in 2019 were 0.1 mag arcsec$^{-2}$ brighter than the darkest ground-based measurements, whereas the $B$ and $V$ brightness levels were 0.8 and 0.6 mag arcsec$^{-2}$ darker than the darkest ground-based measurements. The lower sky brightness levels, stable photometry, and lower atmospheric absorption make stratospheric observations from a balloon-borne platform a unique tool for astronomy. We plan to continue this work in a future mid-latitude long duration balloon flight with SuperBIT.

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arXiv2020

A search for Centaurus A-like features in the spectra of Fermi-LAT detected radio galaxies

Motivated by the detection of a hardening in the gamma-ray spectrum of the radio galaxy Centaurus A, we have analysed ~10 years of Fermi-LAT observations of 26 radio galaxies to search for similar spectral features. We find that the majority of the radio galaxies' gamma-ray spectral energy distributions are best fitted with a simple power-law model, and no spectral hardening similar to that found in Centaurus A was detected. We show that, had there been any such spectral features present in our sample of radio galaxies, they would have been seen, but note that 7 of the radio galaxies (3C 111, 3C 120, 3C 264, IC 4516, NGC 1218, NGC 2892 and PKS 0625-35) show evidence for flux variability on 6-month timescales, which makes the detection of any steady spectral features difficult. We find a strong positive correlation (r = 0.9) between the core radio power at 5 GHz and the gamma-ray luminosity and, using a simple extrapolation to TeV energies, we expect around half of the radio galaxies studied will be detectable with the forthcoming Cherenkov Telescope Array.

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arXiv2019

Robust diffraction-limited NIR-to-NUV wide-field imaging from stratospheric balloon-borne platforms -- SuperBIT science telescope commissioning flight & performance

At a fraction the total cost of an equivalent orbital mission, scientific balloon-borne platforms, operating above 99.7% of the Earth's atmosphere, offer attractive, competitive, and effective observational capabilities -- namely space-like resolution, transmission, and backgrounds -- that are well suited for modern astronomy and cosmology. SuperBIT is a diffraction-limited, wide-field, 0.5 m telescope capable of exploiting these observing conditions in order to provide exquisite imaging throughout the near-IR to near-UV. It utilizes a robust active stabilization system that has consistently demonstrated a 1 sigma sky-fixed pointing stability at 48 milliarcseconds over multiple 1 hour observations at float. This is achieved by actively tracking compound pendulations via a three-axis gimballed platform, which provides sky-fixed telescope stability at < 500 milliarcseconds and corrects for field rotation, while employing high-bandwidth tip/tilt optics to remove residual disturbances across the science imaging focal plane. SuperBIT's performance during the 2019 commissioning flight benefited from a customized high-fidelity science-capable telescope designed with exceptional thermo- and opto-mechanical stability as well as tightly constrained static and dynamic coupling between high-rate sensors and telescope optics. At the currently demonstrated level of flight performance, SuperBIT capabilities now surpass the science requirements for a wide variety of experiments in cosmology, astrophysics and stellar dynamics.

astro-ph.IM↗

arXiv2019

Reply to a comment on `Understanding the gamma-ray emission from the globular cluster 47 Tuc: evidence for dark matter?'

Analysing 9 years of Fermi-LAT observations, we recently studied the spectral properties of the prominent globular cluster 47 Tuc (Brown et al. 2018). In particular, we investigated several models to explain the observed gamma-ray emission, ranging from millisecond pulsars (MSP) to Dark Matter (DM), with the motivation for the latter model driven by recent evidence that 47 Tuc harbours an intermediate-mass black hole (IMBH). This investigation found evidence that the observed gamma-ray emission from 47 Tuc is due to two source populations of MSPs and DM. In Bartels \& Edwards (2018), the authors comment that this evidence is an artifact of the MSP spectra used in (Brown et al. 2019). Here we reply to this comment and argue that the authors of Bartels \& Edwards (2018) (i) do not give due consideration to a very important implication of their result and (ii) there is tension between our MSP fit and their MSP fit when taking uncertainties into consideration. As such, we still conclude there is evidence for a DM component which motivates a deeper radio study of the prominent globular cluster 47 Tuc.

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arXiv2019

Constraining Axion Mass through Gamma-ray Observations of Pulsars

We analyze 9 years of PASS 8 $\textit{Fermi}$-LAT data in the 60$-$500 MeV range and determine flux upper limits (UL) for 17 gamma-ray dark pulsars as a probe of axions produced by nucleon-nucleon Bremsstrahlung in the pulsar core. Using a previously published axion decay gamma-ray photon flux model for pulsars which relies on a high core temperature of 20 MeV, we improve the determination of the UL axion mass ($m_a$), at 95 percent confidence level, to 9.6 $\times$ 10$^{-3}$ eV, which is a factor of 8 improvement on previous results. We show that the axion emissivity (energy loss rate per volume) at realistic lower pulsar core temperatures of 4 MeV or less is reduced to such an extent that axion emissivity and the gamma-ray signal becomes negligible. We consider an alternative emission model based on energy loss rate per mass to allow $m_a$ to be constrained with $Fermi$-LAT observations. This model yields a plausible UL $m_a$ of 10$^{-6}$ eV for pulsar core temperature $<$ 0.1 MeV but knowledge of the extent of axion to photon conversion in the pulsar $B$ field would be required to make a precise UL axion mass determination. The peak of axion flux is likely to produce gamma-rays in the $\leq$ 1 MeV energy range and so future observations with medium energy gamma-ray missions, such as AMEGO and e-ASTROGAM, will be vital to further constrain UL $m_a$.

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arXiv2019

Trinity: An Air-Shower Imaging System for the Detection of Ultrahigh Energy Neutrinos

Efforts to detect ultrahigh energy neutrinos are driven by several objectives: What is the origin of astrophysical neutrinos detected with IceCube? What are the sources of ultrahigh energy cosmic rays? Do the ANITA detected events point to new physics? Shedding light on these questions requires instruments that can detect neutrinos above $10^7$ GeV with sufficient sensitivity - a daunting task. While most ultrahigh energy neutrino experiments are based on the detection of a radio signature from shower particles following a neutrino interaction, we believe that the detection of Cherenkov and fluorescence light from shower particles is an attractive alternative. Imaging air showers with Cherenkov and fluorescence light is a technique that is successfully used in several ultrahigh energy cosmic ray and very-high energy gamma-ray experiments. We performed a case study of an air-shower imaging system for the detection of earth-skimming tau neutrinos. The detector configuration we consider consists of an imaging system that is located on top of a mountain and is pointed at the horizon. From the results of this study we conclude that a sensitivity of $3\cdot10^{-9}$ GeV cm$^{-2}$s$^{-1}$sr$^{-1}$ can be achieved at $2\cdot10^8$ GeV with a relatively small and modular system after three years of observation. In this presentation we discuss key findings of our study and how they translate into design requirements for an imaging system we dub Trinity.

astro-ph.IM↗