arXiv subjects

Andrew P. Rasmussen

Publications and source records attributed to Andrew P. Rasmussen.

9 recordsLinked to original sources

arXiv2026

The "dark dips" phenomenon in the LSST Camera on-sky images

When the commissioning camera (ComCam), and then the LSST Camera, started taking on-sky images at the Vera Rubin Observatory, some of the ITL STA3800 CCDs exhibited a previously undocumented effect. When a sufficiently bright star is superimposed over the sky background, the sensor columns that contain the star appear slightly darker than the background, both above and below the position of the star. The visual appearance of these "dark dips" in the background is enhanced by a slight excess of flux within the neighboring columns, suggesting that they are caused by a lateral field distortion that shifts charges away from the central columns. This effect is not uniform across sensors, reaching an amplitude of up to seven percent of the background in the worst cases, but being undetectable in other sensors under the same conditions. For the affected sensors, the threshold required for the dips to become detectable also varies, from a few hundred to a few thousands saturated pixels within the star footprint. We developed means to quantify the dips, and studied the effect statistically over hundreds of frames, in order to categorize each sensor. In particular, we found no significant dependence on the filter color. We then developed a strategy to dynamically mask the affected columns during the Instrument Signature Removal process, to avoid any potential effect on photometric and astrometric performance.

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arXiv2026

Modeling of the diffuse background produced by the Vera C. Rubin Observatory M2 baffle scattered light

The Vera C. Rubin Observatory, with its unprecedented field of view and fast focal ratio, will survey the entire sky every 3.5 nights. This unique capacity requires dealing with off axis light that can produce stray light artefacts on the images. The secondary mirror (M2) baffle restricts the light that reaches the LSSTCam detector and it contributes to shaping the inner edge of the telescope optical pupil. This work studies the contribution to the background from the light scattered by the M2 baffle itself. The evanescence of this feature, together with the challenge of isolating it from the sky background, led to the necessity of performing in dome tests using a Collimated Beam Projector (CBP), normally used for calibration purposes. To complete the analysis, in addition to the in dome tests, an on sky observational campaign was conducted. This campaign employed both stellar targets and the Moon as illumination sources in order to determine the actual energy associated with the feature. The test data have been retro fitted thanks to the combination of ray tracing simulation, CBP and on sky data to infer the intensity and spatial distribution of the background scattered light within the different LSSTCam filters. We quantified the on sky impact of scattered light from the M2 baffle, both for light coming from bright and red stars and from the Moon. We also developed an approximate relation to transform the in dome measurements into predictions of on sky behavior. This transformation was achieved by comparing the illumination footprint produced by an off axis star with that generated by the CBP and by mapping the stellar Spectral Energy Distribution (SED) onto the CBP's set of discrete wavelengths. Finally, we extrapolated the scattered light behavior of the Moon to stellar sources, in order to build a compplete description of the M2 baffle contribution over the full range of magnitudes.

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arXiv2026

The On-Sky Performance of the LSST Camera CCD Array

The focal plane of the LSST Camera contains 189 individual science CCDs, arranged into 21 raft tower modules, along with 4 wavefront and 8 guider CCDs located in 4 additional corner RTMs. Altogether, the LSST Camera CCDs compose the largest focal plane ever constructed. The LSST Camera is the primary instrument of Rubin Observatory, which will begin the Legacy Survey of Space and Time in 2026. In this paper, we describe the on-sky performance of the LSST Camera CCDs, from receipt at NSF/DOE Vera C. Rubin Observatory in May 2024 to on-sky observations during the first year of operations. We discuss the process to establish functionality of several CCDs which were affected by an electrical short and faulty analog-digital converter, optimizations of readout timing in response to changes in the survey strategy, and implementation of enhanced focal plane safety measures through an active clearing mechanism on the CCDs. Finally, we discuss sensor features observed on-sky, and global performance during the first year of operations. The operations to date of the LSST Camera CCDs have demonstrated the capability of performing a wide, fast, and deep optical imaging survey of the entire southern sky at the Rubin Observatory.

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arXiv2026

Mechanical Studies of an Additional Light Baffle for the LSST Camera

Commissioning the NSF-DOE Vera C. Rubin Observatory consisted of engineering operation and on-sky data-taking, initially with the Commissioning Camera followed by the commissioning run of the LSST Camera (LSSTCam). As with other wide-field astronomical projects, the Rubin team anticipated a significant amount of stray light effects which would necessitate investigation and systematic mitigation. This led the Rubin stray light working group to develop tools, including a robust model of the entire observatory in Zemax, to trace the light paths of stray light artifacts back to their sources. This model along with the other efforts of the working group enabled significant improvements in stray light mitigation leading up to the commencement of the Legacy Survey of Space and Time (LSST). One such potential source was identified as a small chamfer on the L3 lens, for which it was hypothesized that a simple baffle added inside of the LSSTCam near the L3 should prove beneficial to the quality of data being collected in the LSST. Initial Zemax models proved this hypothesis to be correct, but it is important to weigh the improvements made versus the effort, risk, and cost especially when considering any hardware modifications to an instrument that is already running and collecting immense amounts of data each night. This paper investigates the impacts of installing an L3 baffle via a collection of mechanically focused studies, where the principal areas of focus are installation feasibility, baffle geometry, materials & coating selection, and potential impacts to the purge system.

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arXiv2006

On the Putative Detection of z>0 X-ray Absorption Features in the Spectrum of Markarian 421

In a series of papers, Nicastro et al. have reported the detection of z>0 OVII absorption features in the spectrum of Mrk421 obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). We evaluate this result in the context of a high quality spectrum of the same source obtained with the Reflection Grating Spectrometer (RGS) on XMM-Newton. The data comprise over 955ks of usable exposure time and more than 26000 counts per 50 milliAngstrom at 21.6 Angstroms. We concentrate on the spectrally clean region (21.3 < lambda < 22.5 Angstroms) where sharp features due to the astrophysically abundant OVII may reveal an intervening, warm--hot intergalactic medium (WHIM). We do not confirm detection of any of the intervening systems claimed to date. Rather, we detect only three unsurprising, astrophysically expected features down to the Log(N_i)~14.6 (3 sigma) sensitivity level. Each of the two purported WHIM features is rejected with a statistical confidence that exceeds that reported for its initial detection. While we can not rule out the existence of fainter, WHIM related features in these spectra, we suggest that previous discovery claims were premature. A more recent paper by Williams et al. claims to have demonstrated that the RGS data we analyze here do not have the resolution or statistical quality required to confirm or deny the LETGS detections. We show that our careful analysis resolves the issues encountered by Williams et al. and recovers the full resolution and statistical quality of the RGS data. We highlight the differences between our analysis and those published by Williams et al. as this may explain our disparate conclusions.

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arXiv2004

NLTE Model Atmosphere Analysis of the LMC Supersoft X-ray Source CAL 83

We present a non-LTE model atmosphere analysis of Chandra HRC-S/LETG and XMM-Newton RGS spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. Taken with a 16-month interval, the Chandra and XMM-Newton spectra are very similar. They reveal a very rich absorption line spectrum from the hot white dwarf photosphere, but no spectral signatures of a wind. We also report a third X-ray off-state during a later Chandra observation, demonstrating the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. We completed the analysis of the LETG and RGS spectra of CAL 83 with new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We successfully matched the Chandra and XMM-Newton spectra assuming a model composition with LMC metallicity. We derived the basic stellar parameters of the hot white dwarf, but the current state of atomic data in the soft X-ray domain precludes a detailed chemical analysis. We have obtained the first direct spectroscopic evidence that the white dwarf is massive (Mwd > 1 Msun). The short timescale of the X-ray off-states is consistent with a high white dwarf mass. Our analysis thus provides direct support for supersoft sources as likely progenitors of SN Ia.

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arXiv2003

A Long Look at NGC 3783 with the XMM-Newton Reflection Grating Spectrometers

A long 280 ks observation of the Seyfert 1 galaxy NGC 3783 with XMM-Newton is reported. We focus on the oxygen line complex between 17 and 24 A as measured with the RGS spectrometers. Accurate absorption column densities and emission line fluxes are obtained. We explore several options for the geometry and physical form of the emitting and absorbing gas. The lack of change in ionization in the absorber despite an increase in continuum flux during the observation restricts the high-ionization (O-K) and the low-ionization (Fe-M) gas to distances of at least 0.5 pc and 2.8 pc, respectively, away from the central source. Given the P-Cygni type profiles in the resonance spectral lines and the similar velocity widths, column densities, and ionization structure inferred separately from the emission and absorption lines, it is tempting to relate the X-ray narrow-line emitting plasma with the X-ray absorbing gas. Under this assumption, the scenario of dense clumped clouds can be ruled out. Conversely, extended ionization cones (r > 10 pc) are consistent with the observation independent of this assumption. These findings are in stark contrast with the picture of numerous clumpy (n_e > 10^9 cm^-3) clouds drawn recently from UV spectra, but it is consistent with the extended X-ray emission cones observed directly in Seyfert 2 galaxies.

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arXiv2000

High-resolution X-ray spectroscopy and imaging of supernova remnant N132D

The observation of the supernova remnant N132D by the scientific instruments on board the XMM-Newton satellite is presented. The X-rays from N132D are dispersed into a detailed line-rich spectrum using the Reflection Grating Spectrometers. Spectral lines of C, N, O, Ne, Mg, Si, S, and Fe are identified. Images of the remnant, in narrow wavelength bands, produced by the European Photon Imaging Cameras reveal a complex spatial structure of the ionic distribution. While K-shell Fe emission seems to originate near the centre, all of the other ions are observed along the shell. A high O VII / O VIII emission ratio is detected on the northeastern edge of the remnant. This can be a sign of hot ionising conditions, or it can reflect relatively cool gas. Spectral fitting of the CCD spectrum suggests high temperatures in this region, but a detailed analysis of the atomic processes involved in producing the O VII spectral lines leads to the conclusion that the intensities of these lines alone cannot provide a conclusive distinction between the two scenarios.

astro-ph↗

arXiv2000

A High Resolution Spectroscopic Observation of CAL 83 with XMM-Newton/RGS

We present the first high resolution photospheric X-ray spectrum of a Supersoft X-ray Source, the famous CAL~83 in the Large Magellanic Cloud. The spectrum was obtained with the Reflection Grating Spectrometer on XMM-Newton during the Calibration/Performance Verification phase of the observatory. The spectrum covers the range 20-40 A at an approximately constant resolution of 0.05 A, and shows very significant, intricate detail, that is very sensitive to the physical properties of the object. We present the results of an initial investigation of the spectrum, from which we draw the conclusion that the spectral structure is probably dominated by numerous absorption features due to transitions in the L-shells of the mid-$Z$ elements and the M-shell of Fe, in addition to a few strong K-shell features due to CNO.

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