arXiv subjects

Sasha Brownsberger

Publications and source records attributed to Sasha Brownsberger.

11 recordsLinked to original sources

arXiv2024

StarDICE III: Characterization of the photometric instrument with a Collimated Beam Projector

The measurement of type Ia supernovae magnitudes provides cosmological distances, which can be used to constrain dark energy parameters. Large photometric surveys require a substantial improvement in the calibration precision of their photometry to reduce systematic uncertainties in cosmological constraints. The StarDICE experiment is designed to establish accurate broadband flux references for these surveys, aiming for sub-percent precision in magnitude measurements. This requires a precise measurement of the filter bandpasses of both the StarDICE and survey instruments with sub-nanometer accuracy. To that end, we have developed the Collimated Beam Projector (CBP), an optical device capable of calibrating the throughput of an astronomical telescope and of its filters. The CBP is built from a tunable laser source and a reversed telescope to emit a parallel monochromatic light beam that is continuously monitored in flux and wavelength. The CBP output light flux is measured using a large area photodiode, previously calibrated relative to a NIST photodiode. We derive the StarDICE telescope throughput and filter transmissions from the CBP measurements, anchoring it to the absolute calibration provided by the NIST. After analyzing the systematic uncertainties, we achieved sub-nanometer accuracy in determining filter central wavelengths, measured each filter transmission with a precision of 0.5% per 1nm bin, and detected out-of-band leakages at 0.01%. Furthermore, we have synthesized the equivalent transmission for full pupil illumination from four sample positions in the StarDICE telescope mirror, with an accuracy of approximately 0.2nm for central wavelengths and 7mmag for broadband fluxes. We demonstrated our ability to characterize a telescope throughput down to the mmag, and paved the way for future developments, such as a portable CBP version for in-situ transmission monitoring.

astro-ph.IM↗

arXiv2024

COOL-LAMPS VI: Lens model and New Constraints on the Properties of COOL J1241+2219, a Bright z = 5 Lyman Break Galaxy and its z = 1 Cluster Lens

We present a strong lensing analysis of COOL J1241+2219, the brightest known gravitationally lensed galaxy at $z \geq 5$, based on new multi-band Hubble Space Telescope (HST) imaging data. The lensed galaxy has a redshift of z=5.043, placing it shortly after the end of the Epoch of Reionization, and an AB magnitude z_AB=20.47 mag (Khullar et al. 2021). As such, it serves as a touchstone for future research of that epoch. The high spatial resolution of HST reveals internal structure in the giant arc, from which we identify 15 constraints and construct a robust lens model. We use the lens model to extract cluster mass and lensing magnification. We find that the mass enclosed within the Einstein radius of the z=1.001 cluster lens is M(<5.77'')=$1.079^{+0.023}_{-0.007}$, significantly lower than other known strong lensing clusters at its redshift. The average magnification of the giant arc is $<μ_{arc}>=76^{+40}_{-20}$, a factor of $2.4^{+1.4}_{-0.7}$ greater than previously estimated from ground-based data; the flux-weighted average magnification is $<μ_{arc}>=92^{+37}_{-31}$ We update the current measurements of the stellar mass and star formation rate (SFR) of the source for the revised magnification, $\log(M_\star/M_{\odot})=9.7\pm0.3$ and ${\rm SFR} = 10.3^{+7.0}_{-4.4}$ $ M_{\odot} $yr$^{-1}$. The powerful lensing magnification acting upon COOL J1241+2219 resolves the source and enables future studies of the properties of its star formation on a clump-by-clump basis. The lensing analysis presented here will support upcoming multiwavelength characterization with HST and JWST data of the stellar mass assembly and physical properties of this high-redshift lensed galaxy.

astro-ph.GA↗

arXiv2022

An Estimator for Lensing Potential from Galaxy Number Counts

We derive an estimator for the lensing potential from galaxy number counts which contains a linear and a quadratic term. We show that this estimator has a much larger signal-to-noise ratio than the corresponding estimator from intensity mapping. This is due to the additional lensing term in the number count angular power spectrum which is present already at linear order. We estimate the signal-to-noise ratio for future photometric surveys. Particularly at high redshifts, $z\gtrsim 1.5$, the signal to noise ratio can become of order 30. Therefore, the number counts in photometric surveys would be an excellent means to measure tomographic lensing spectra.

astro-ph.CO↗

arXiv2021

The Pantheon+ Analysis: Dependence of Cosmological Constraints on Photometric-Zeropoint Uncertainties of Supernova Surveys

Type Ia supernovae (SNe Ia) measurements of the Hubble constant, H$_0$, the cosmological mass density, $Ω_M$, and the dark energy equation-of-state parameter, $w$, rely on numerous SNe surveys using distinct photometric systems across three decades of observation. Here, we determine the sensitivities of the upcoming SH0ES+Pantheon+ constraints on H$_0$, $Ω_M$, and $w$ to unknown systematics in the relative photometric zeropoint calibration between the 17 surveys that comprise the Pantheon+ supernovae data set. Varying the zeropoints of these surveys simultaneously with the cosmological parameters, we determine that the SH0ES+Pantheon+ measurement of H$_0$ is robust against inter-survey photometric miscalibration, but that the measurements of $Ω_M$ and $w$ are not. Specifically, we find that miscalibrated inter-survey systematics could represent a source of uncertainty in the measured value of H$_0$ that is no larger than $0.2$ km s$^{-1}$ Mpc$^{-1}$. This modest increase in H$_0$ uncertainty could not account for the $7$ km s$^{-1}$ Mpc$^{-1}$ "Hubble Tension" between the SH0ES measurement of H$_0$ and the Planck $Λ$CDM-based inference of H$_0$. However, we find that the SH0ES+Pantheon+ best-fit values of $Ω_M$ and $w$ respectively slip, to first order, by $0.04$ and $-0.17$ per $25$ mmag of inter-survey calibration uncertainty, underscoring the vital role that cross-calibration plays in accurately measuring these parameters. Because the Pantheon+ compendium contains many surveys that share low-$z$ Hubble Flow and Cepheid-paired SNe, the SH0ES+Pantheon+ joint constraint of H$_0$ is robust against inter-survey photometric calibration errors, and such errors do not represent an impediment to jointly using SH0ES+Pantheon+ to measure H$_0$ to 1% accuracy.

astro-ph.CO↗

arXiv2021

Characterization and Quantum Efficiency Determination of Monocrystalline Silicon Solar Cells as Sensors for Precise Flux Calibration

As the precision frontier of astrophysics advances towards the one millimagnitude level, flux calibration of photometric instrumentation remains an ongoing challenge. We present the results of a lab-bench assessment of the viability of monocrystalline silicon solar cells to serve as large-aperture (up to 125mm diameter), high-precision photodetectors. We measure the electrical properties, spatial response uniformity, quantum efficiency (QE), and frequency response of 3$^{rd}$ generation C60 solar cells, manufactured by Sunpower. Our new results, combined with our previous study of these cells' linearity, dark current, and noise characteristics, suggest that these devices hold considerable promise, with QE and linearity that rival those of traditional, small-aperture photodiodes. We argue that any photocalibration project that relies on precise knowledge of the intensity of a large-diameter optical beam should consider using solar cells as calibrating photodetectors.

astro-ph.IM↗

arXiv2021

COOL-LAMPS I. An Extraordinarily Bright Lensed Galaxy at Redshift 5.04

We report the discovery of COOL J1241+2219, a strongly-lensed galaxy at redshift $z$=5.043$\pm$0.002 with observed magnitude $z_{AB}=20.47$, lensed by a moderate-mass galaxy cluster at $z$=1.001$\pm$0.001. COOL J1241+2219 is the brightest lensed galaxy currently known at optical and near-infrared wavelengths at $z$ $\gtrsim$ 5; it is $\sim$5 times brighter than the prior record-holder lensed galaxy, and several magnitudes brighter than the brightest unlensed galaxies known at these redshifts. It was discovered as part of COOL-LAMPS, a collaboration initiated to find strongly lensed systems in recent public optical imaging data. We characterise the lensed galaxy, as well as the central galaxy of the lensing cluster using ground-based $griz$JH imaging and optical spectroscopy. We report model-based magnitudes, and derive stellar masses, dust content, metallicity and star-formation rates via stellar-population synthesis modeling. Our lens mass modeling, based on ground-based imaging, implies a median source magnification of $\sim$30, which puts the stellar mass and star formation rate (in the youngest age bin, closest to the epoch of observation) at logM$_{*}$ = $10.11^{+0.21}_{-0.26}$ and SFR = $27^{+13}_{-9}$ M$_{\odot}$/yr, respectively. We constrain a star formation history for COOL J1241+2219 consistent with constant star formation across $\sim$1 Gyr of cosmic time, and that places this galaxy on the high-mass end of the star-forming main sequence. COOL J1241+2219 is 2-4 times more luminous than a galaxy with the characteristic UV luminosity at these redshifts. The UV continuum slope $β$= -2.2$\pm$0.2 places this galaxy on the blue side of the observed distribution of galaxies at $z$=5, although the lack of Ly$α$ emission indicates dust sufficient to suppress this emission.

astro-ph.GA↗

arXiv2020

Windowing Artifacts Likely Account for Recent Claimed Detection of Oscillating Cosmic Scale Factor

Using the Pantheon data set of Type Ia supernovae, a recent publication (R20 in this work) reports a $~2σ$ detection of oscillations in the expansion history of the universe. Applying the R20 methodology to simulated Pantheon data, we determine that these oscillations likely arise from analysis artifacts. The uneven spacing of Type Ia supernovae in redshift space and the complicated analysis method of R20 impose a structured throughput function. When analyzed with the R20 prescription, about $11\%$ of artificial $Λ$CDM data sets produce a stronger oscillatory signal than the actual Pantheon data. The study conducted by R20 is a wholly worthwhile endeavor. However, we believe that the detected oscillations are not due to an oscillating cosmic scale factor and are instead artifacts of the data processing. Our results underscore the importance of understanding the false `signals' that can be introduced by complicated data analyses.

astro-ph.CO↗

arXiv2020

The Most Rapidly Declining Type I Supernova 2019bkc/ATLAS19dqr

We report observations of the hydrogen-deficient supernova (SN) 2019bkc/ATLAS19dqr. With B- and r-band decline between peak and 10 days post peak of Delta m_10(B)=5.24+/-0.07 mag and Delta m_10(r)=3.85+/-0.10$ mag, respectively, SN 2019bkc is the most rapidly declining SN I discovered so far. While its closest matches are the rapidly declining SN 2005ek and SN 2010X, the light curves and spectra of SN 2019bkc show some unprecedented characteristics. SN 2019bkc appears "hostless," with no identifiable host galaxy near its location, although it may be associated with the galaxy cluster MKW1 at z = 0.02. We evaluate a number of existing models of fast-evolving SNe, and we find that none of them can satisfactorily explain all aspects of SN 2019bkc observations.

astro-ph.HE↗

arXiv2019

Stellar Profile Independent Determination of the Dark Matter Distribution of the Fornax Local Group Dwarf Spheroidal Galaxy

The local group dwarf spheroidal galaxies (LG dSphs) are among the most promising astrophysical targets for probing the small scale structure of dark matter (DM) subhalos. We describe a method for testing the correspondence between proposed DM halo models and observations of stellar populations within LG dSphs. By leveraging the gravitational potential of any proposed DM model and the available stellar kinematical data, we can derive a prediction for the observed stellar surface density of an LG dSph that can be directly compared with observations. Because we do not make any reference to an assumed surface brightness profile, our model can be applied to exotic DM distributions that produce atypical stellar density distributions. We use our methodology to determine that the DM halo of the Fornax LG dSph is more likely cored than cusped, ascertain that it is characterized by a semi-minor to semi-major axis ratio in minor tension with simulations, and find no substantial evidence of a disk within the dSph's larger DM halo.

astro-ph.GA↗

arXiv2019

Initial Assessment of Monocrystalline Silicon Solar Cells as Large-Area Sensors for Precise Flux Calibration

As the precision frontier of large-area survey astrophysics advances towards the one millimagnitude level, flux calibration of astronomical instrumentation remains an ongoing challenge. We describe initial testing of silicon solar cells as large-aperture precise calibration photodiodes. We present measurements of dark current, linearity, frequency response, spatial response uniformity, and noise characteristics of the Sunpower C60 solar cells, an interdigitated back-contact 125mm x 125mm monocrystalline solar cell. We find that these devices hold considerable promise as large-area flux calibration sensors and warrant further characterization.

astro-ph.IM↗

arXiv2014

A Survey for H$α$ Pulsar Bow Shocks

We report on a survey for H$α$ bow shock emission around nearby $γ$-detected energetic pulsars. This survey adds three Balmer-dominated neutron star bow shocks to the six previously confirmed examples. In addition to the shock around {\it Fermi} pulsar PSR J1741$-$2054, we now report H$α$ structures around two additional $γ$-ray pulsars, PSR J2030+4415 and PSR J1509$-$5850. These are the first known examples of H$α$ nebulae with pre-ionization halos. With new measurements, we show that a simple analytic model can account for the angular size and flux of the bow shocks' apices. The latter, in particular, provides a new pulsar probe and indicates large moments of inertia and smaller distances than previously assumed, in several cases. In particular we show that the re-measured PSR J0437$-$4715 shock flux implies $I = (1.7\pm 0.2) \times 10^{45}/(f_{HI} {\rm sin}i) {\rm g\,cm^2}$. We also derive a distance $d\approx 0.72$kpc for the $γ$-ray only pulsar PSR J2030+4415 and revised distances for PSRs J1959+2048 (1.4kpc) and J2555+6535 ($\sim 1$kpc), smaller than the conventional DM-estimated values. Finally we report upper limits for 94 additional LAT pulsars. An estimate of the survey sensitivity indicates that for a warm neutral medium filling factor $ϕ_{WNM}\sim 0.3$ there should be a total of $\sim$ nine H$α$ bow shocks in our LAT-targeted survey; given that seven such objects are now known, a much larger $ϕ_{WNM}$ seems problematic.

astro-ph.SR↗