arXiv subjects

Sean Lockwood

Publications and source records attributed to Sean Lockwood.

5 recordsLinked to original sources

arXiv2026

Wavelength Calibration Accuracy Across the STIS CCD: Pipeline Update and User Guidance

We investigate the wavelength calibration accuracy of STIS CCD spectra as a function of detector position, focusing on the E1/E2 pseudo-apertures. Comparison between measured lamp line centroids and laboratory wavelengths shows that, while the standard calstis solution is stable at the detector center, significant offsets are present toward the CCD edges and increase with time. We used a test version of calstis in which the wavelength calibration step employs a row-selected cross-correlation, leading to improved wavelength calibration accuracy for edge extractions. Motivated by these results, we implemented an update to calstis4 that applies a row-selected wavecal procedure to observations taken with the E1/E2 pseudo-apertures, while preserving the standard procedure for nominal extractions. The updated pipeline was then used to reprocess the entire STIS archive in MAST. Validation tests confirm improved agreement between nominal and E1/E2 spectra without significant side effects for more than 97% of the datasets in the MAST archive. The main exceptions are the majority of G230MB and a few G230LB and G430M datasets, where the lower signal-to-noise at the CCD edge makes the lamp lines harder to detect and cosmic-ray residuals can dominate the cross-correlation, leading to incorrect wavelength shifts. A Jupyter Notebook is also provided to enable similar corrections in case the cross-correlation fails, for extractions at positions other than E1/E2, or for multiple extractions along the slit. Finally, we discuss the potential impact of the wavelength calibration accuracy on science results.

astro-ph.IM↗

arXiv2026

Barycentric Corrections for HST/STIS Data

We describe $\texttt{stistools.barycentric_correction}$, a new Python utility for calculating barycentric timing corrections for HST/STIS observations. This tool replaces the deprecated $\texttt{stsdas.hst_calib.stis.odelaytime}$ IRAF function that was previously used for HST barycentric corrections. Our new utility uses $\texttt{astropy}$ for conversion between time formats and standards and introduces a new way to calculate HST's position through JPL Horizons, replacing the need to download separate HST orbital ephemeris files. Here, we describe the methods used in the new utility, the tests that were carried out to verify its accuracy, and explain some of the complexities involved in determining light travel times to accuracies down to a millisecond for HST. We also summarize the current understanding of the absolute accuracy of STIS time stamps.

astro-ph.IM↗

arXiv2026

Uncertainties in Low-Count STIS Spectra

We evaluate uncertainty calculations in the calstis pipeline for data in the low-count regime. Due to the low dark rate and read-noise free nature of MAMA detectors, observations of UV-dim sources can result in exposures with 0 or 1 counts in some pixels. In this regime, the "root-N" approximation widely used to calculate uncertainties breaks down, and one must compute Poisson confidence intervals for more accurate uncertainty calculations. The CalCOS pipeline was updated in 2020 to account for these low-count uncertainties. Here, we assess how STIS observations are currently affected by this phenomenon, describe a new Jupyter notebook exploring the issue, and introduce a new utility, stistools.poisson_err, to manually calculate Poisson confidence intervals for 1D STIS spectra. Additionally, we describe a related software bug in the stistools$.$inttag utility, which splits TIME-TAG data into sub-exposures. This newly fixed bug serves as a useful case-study for the proper use of Poisson confidence intervals.

astro-ph.IM↗

arXiv2025

The UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Large Director's Discretionary Program with Hubble. I. Goals, Design, and Initial Results

Specifically selected to leverage the unique ultraviolet capabilities of the Hubble Space Telescope, the Hubble Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) is a Director's Discretionary program of approximately 1000 orbits - the largest ever executed - that produced a UV spectroscopic library of O and B stars in nearby low metallicity galaxies and accreting low mass stars in the Milky Way. Observations from ULLYSES combined with archival spectra uniformly sample the fundamental astrophysical parameter space for each mass regime, including spectral type, luminosity class, and metallicity for massive stars, and the mass, age, and disk accretion rate for low-mass stars. The ULLYSES spectral library of massive stars will be critical to characterize how massive stars evolve at different metallicities; to advance our understanding of the production of ionizing photons, and thus of galaxy evolution and the re-ionization of the Universe; and to provide the templates necessary for the synthesis of integrated stellar populations. The massive star spectra are also transforming our understanding of the interstellar and circumgalactic media of low metallicity galaxies. On the low-mass end, UV spectra of T Tauri stars contain a plethora of diagnostics of accretion, winds, and the warm disk surface. These diagnostics are crucial for evaluating disk evolution and provide important input to assess atmospheric escape of planets and to interpret powerful probes of disk chemistry, as observed with ALMA and JWST. In this paper we motivate the design of the program, describe the observing strategy and target selection, and present initial results.

astro-ph.SR↗

arXiv2025

Time-resolved Hubble Space Telescope UV observations of an X-ray quasi-periodic eruption source

X-ray quasi-periodic eruptions (QPEs) are a novel mode of variability in nearby galactic nuclei whose origin remains unknown. Their multi-wavelength properties are poorly constrained, as studies have focused almost entirely on the X-ray band. Here we report on time-resolved, coordinated Hubble Space Telescope far ultraviolet and XMM-Newton X-ray observations of the shortest period X-ray QPE source currently known, eRO-QPE2. We detect a bright UV point source ($L_{\rm FUV} \approx {\rm few} \times 10^{41}$ erg s$^{-1}$) that does not show statistically significant variability between the X-ray eruption and quiescent phases. This emission is unlikely to be powered by a young stellar population in a nuclear stellar cluster. The X-ray-to-UV spectral energy distribution can be described by a compact accretion disk ($R_{\rm out} = 343^{+202}_{-138} \ R_{\rm g}$). Such compact disks are incompatible with typical disks in active galactic nuclei, but form naturally following the tidal disruption of a star. Our results rule out models (for eRO-QPE2) invoking i) a classic AGN accretion disk and ii) no accretion disk at all. For orbiter models, the expected radius derived from the timing properties would naturally lead to disk-orbiter interactions for both quasi-spherical and eccentric trajectories. We infer a black hole mass of log$_{10}(M_{\rm BH}) = 5.9 \pm 0.3$ M$_{\odot}$ and Eddington ratio of 0.13$^{+0.18}_{-0.07}$; in combination with the compact outer radius this is inconsistent with existing disk instability models. After accounting for the quiescent disk emission, we constrain the ratio of X-ray to FUV luminosity of the eruption component to be $L_{\rm X} / L_{\rm FUV} > 16-85$ (depending on the intrinsic extinction).

astro-ph.HE↗