January 4, 2024

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In this STAN we describe COS posters to be presented at the 243rd AAS meeting.

COS Posters at the 243rd AAS Meeting

Several COS posters will be presented at the AAS meeting. We invite all COS users to review them to find out about the latest developments for COS. Below we include links to the posters. Additionally, all COS posters are archived on the COS poster page.

360.07 Improvements to the COS FUV Geometric Distortion and Walk Corrections 
N. Indriolo
The far ultraviolet (FUV) detector of the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope does not contain physical pixels. When a photon impacts the microchannel plate stack and produces a charge cloud, the location of that cloud is determined by cross delay line anodes. Based on that location, each photon event is then assigned to a digital pixel by the detector electronics. This digitization process was optimized for the more central regions of the detector segments and at certain gain levels, and raw pixel coordinates assigned by the instrument suffer from distortions, requiring a geometric correction. The walk correction shifts photon events at different gain levels impacting the same location on the detector so that they are co-aligned. The COS team has derived new reference files to correct the geometric, x-walk, and y-walk distortions that will be applied to all past and future FUV observations. For observations using the medium resolution gratings, the potential relative offset between lines observed at different locations on the detector is reduced from about 20 km/s to about 10 km/s. Full implementation of these new corrections requires updates to other reference files and to the calibration pipeline. This work is currently underway, and we expect the revised data products to be available to the community in Spring 2024.

360.31 Updated Status and Performance of the Cosmic Origins Spectrograph 
K. Rowlands
After 15 years of operations, the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope remains scientifically productive and continues to provide a high quality spectroscopic view of the ultraviolet Universe through the FUV and NUV channels, with the COS/FUV channel operating at five simultaneous lifetime positions. Here we present updates on the current status and performance of COS and summarize recent calibration and user support work of interest to HST Cycle 32 proposers and all COS users. Dark rates continue to be atypically high, as expected due to increased solar activity, and we continue to track how the sensitivities of the FUV and NUV channels are evolving with time. Furthermore, we are updating calibration reference files in response to the new geometric and walk corrections discussed in the poster by Indriolo et al. Additionally, we discuss both new and updated Jupyter notebooks for working with COS data that are executable with the new Conda Space Telescope Environment (stenv). Finally, we review the plan to maintain the unique abilities of COS beyond 2030.

360.32 Advancing Hubble's Spectroscopic Legacy: The Hubble Advanced Spectral Products (HASP) Program 
T. Fischer
The Hubble Advanced Spectral Products (HASP) program represents a significant advancement in leveraging the publicly available coadd code from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program to automate the coaddition of Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS) spectra within the Mikulski Archive for Space Telescopes (MAST). HASP employs a meticulous multi-stage filtering process to ensure data quality and facilitates the merging of spectra at the visit and program level. Specifically, it co-adds spectra obtained with specific central wavelength settings (cenwaves) across multiple visits within a program, as well as spectra obtained with multiple cenwaves within specific gratings in a program. It also combines spectra from different gratings and instruments into single 1D spectra. This results in the creation of coadded products at both the individual visit and entire program levels for each observed object for all COS and STIS programs. Further the HASP coadd code is available to the user community for additional, custom coaddition products. The release of HASP also lays the foundation for a future, updated Hubble Spectroscopic Legacy Archive (HSLA).


Additionally, two COS Interns will be presenting their work at the AAS Meeting:

202.13 Analyzing FUV Flux Variations in the O-Type Supergiant AV 75: Implications for COS Calibration 
J. Patel
We present an analysis of variability in the far ultraviolet (FUV) spectrum of AV 75, a blue supergiant star located in the Small Magellanic Cloud. Interstellar absorption lines in the spectrum of AV 75 are used to monitor the stability of the FUV wavelength solutions for the Cosmic Origins Spectrograph (COS), so AV 75 has been observed at least annually with COS between 2012 and 2023. Our analysis focuses on the wavelength range of 1425 – 1625 Å in the COS spectra, including the C IV line at 1550 Å, which has a P Cygni profile. We analyze the variability of both the continuum in this range and the C IV line. The continuum flux is nearly constant over the course of the observations, varying at the level of 1.5%. In contrast, the height of the red emission in the C IV line varies by 10%, and the blue absorption in the P Cygni profile is shallower when the red emission is stronger. The flux variation for the continuum is small, so AV 75 is useful as a validation target for COS flux calibration. 

360.08 Improving the COS Wavelength Calibration Through Cross-Correlation 
D. Savarino
Wavelength calibration (“wavecal”) shifts for spectra observed with the Cosmic Origins Spectrograph (COS) are found using a chi-squared fitting method that matches an observed calibration lamp spectrum to a template spectrum. However, this method is not ideal for observing modes in which the calibration lamp spectra are faint due to sensitivity loss, resulting in shifts that are reported as unreliable by the COS calibration pipeline (CalCOS). Using existing Python libraries and functions, we designed a more robust cross-correlation method to extract wavecal shifts for all COS FUV and NUV spectra. For exposures for which the existing method succeeds, we find the new method returns the same shift and for the faintest NUV wavecal spectra for which the existing method fails, the new method is able to successfully return a reliable shift. We also find that this new method is able to return a wavecal shift accurate to within 1 pixel for wavecal spectra that are 30% lower than the current S/N, simulating gain sag, and to within 2 pixels (1/3 of a resolution element) for wavecal spectra that are 50% lower than the current S/N for most COS FUV observing modes. 


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