arXiv subjects

C. E. Grant

Publications and source records attributed to C. E. Grant.

At least 19 recordsLinked to original sources

arXiv2026

Enhancing the sensitivity of next-generation X-ray imaging detectors with artificial intelligence and advanced event reconstruction algorithms

Advanced algorithms incorporating artificial intelligence and machine learning (AI/ML) enhance the sensitivity of X-ray imaging detectors and the scientific capabilities of future X-ray missions. In orbit, current instruments are limited in their sensitivity by (1) the instrumental background, induced by cosmic rays which produce signals that can be confused for genuine, astrophysical X-rays, and (2) the ability to reconstruct the detected photon events, degrading the quantum efficiency and energy resolution at the lowest energies, where much discovery space resides. We report on the development of prototype algorithms designed to operate on the raw frame-level data to provide improved identification of particle-induced background events and enhanced energy reconstruction. These algorithms consider the contextual information from all signals in a frame, and are built upon physics-motivated models of charge diffusion and signal generation within the detector. Using high fidelity simulations, we show that following recent developments, prototype ML algorithms can reduce the unrejected particle background by up to 68 per cent compared with traditional filtering methods when operated in an aggressive mode suitable for source detection in imaging surveys, or up to 40 per cent in a conservative mode designed to prioritize accurate measurements of the spectrum. We find that next-generation event reconstruction algorithms improve the sensitivity and energy resolution of CCD-like detectors at event energies below 1keV, and can aid in background filtering, and reduce the impact of photon pile-up. We present new laboratory data that demonstrates the performance of the algorithm on the MIT-LL CCID-93 CCD detector. Together with the capabilities of next-generation high-speed, low-noise detectors, these algorithms can satisfy the requirements for future X-ray flagship missions.

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arXiv2025

International Astrophysical Consortium for High-energy Calibration: Summary of the 16th IACHEC Workshop

In this report we summarize the activities of the International Astronomical Consortium for High Energy Calibration (IACHEC) from the 16th IACHEC Workshop at Parador de La Granja, Spain. Sixty-one scientists directly involved in the calibration of operational and future high-energy missions gathered during 3.5 days to discuss the status of the cross-calibration between the current international complement of X-ray observatories, and the possibilities to improve it. This summary consists of reports from the Working Groups with topics ranging across: the identification and characterization of standard calibration sources, multi-observatory cross-calibration campaigns, appropriate and new statistical techniques, calibration of instruments and characterization of background, preservation of knowledge, and results for the benefit of the astronomical community.

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arXiv2024

Augmenting astronomical X-ray detectors with AI for enhanced sensitivity and reduced background

Bringing artificial intelligence (AI) alongside next-generation X-ray imaging detectors, including CCDs and DEPFET sensors, enhances their sensitivity to achieve many of the flagship science cases targeted by future X-ray observatories, based upon low surface brightness and high redshift sources. Machine learning algorithms operating on the raw frame-level data provide enhanced identification of background vs. astrophysical X-ray events, by considering all of the signals in the context within which they appear within each frame. We have developed prototype machine learning algorithms to identify valid X-ray and cosmic-ray induced background events, trained and tested upon a suite of realistic end-to-end simulations that trace the interaction of cosmic ray particles and their secondaries through the spacecraft and detector. These algorithms demonstrate that AI can reduce the unrejected instrumental background by up to 41.5 per cent compared with traditional filtering methods. Alongside AI algorithms to reduce the instrumental background, next-generation event reconstruction methods, based upon fitting physically-motivated Gaussian models of the charge clouds produced by events within the detector, promise increased accuracy and spectral resolution of the lowest energy photon events.

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arXiv2024

International Astrophysical Consortium for High-energy Calibration: Summary of the 15th IACHEC Workshop

In this report, we summarize the activities of the International Astronomical Consortium for High Energy Calibration (IACHEC) from the 15th IACHEC Workshop in Pelham, Germany. Sixty scientists directly involved in the calibration of operational and future high-energy missions gathered for 3.5 days to discuss the status of the cross-calibration between the current international complement of X-ray observatories and the possibilities to improve it. This summary consists of reports from the Working Groups with topics ranging across the identification and characterization of standard calibration sources, multi-observatory cross-calibration campaigns, appropriate and new statistical techniques, calibration of instruments and characterization of background, preservation of knowledge, and results for the benefit of the astronomical community.

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arXiv2022

Reducing the background in X-ray imaging detectors via machine learning

The sensitivity of astronomical X-ray detectors is limited by the instrumental background. The background is especially important when observing low surface brightness sources that are critical for many of the science cases targeted by future X-ray observatories, including Athena and future US-led flagship or probe-class X-ray missions. Above 2keV, the background is dominated by signals induced by cosmic rays interacting with the spacecraft and detector. We develop novel machine learning algorithms to identify events in next-generation X-ray imaging detectors and to predict the probability that an event is induced by a cosmic ray vs. an astrophysical X-ray photon, enabling enhanced filtering of the cosmic ray-induced background. We find that by learning the typical correlations between the secondary events that arise from a single primary, machine learning algorithms are able to successfully identify cosmic ray-induced background events that are missed by traditional filtering methods employed on current-generation X-ray missions, reducing the unrejected background by as much as 30 per cent.

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arXiv2021

IACHEC 2020/2021 Pandemic Report

In this report we summarize the activities of the International Astronomical Consortium for High Energy Calibration (IACHEC) and the work done since the last in-person meeting in Japan (Shonan Village Center), May 2019, through two virtual meetings that were held in November 2020 and May 2021. The on-line only meetings divided the contents of the usual in-person workshop between mission updates and working group updates. The November meeting was dedicated to mission calibration updates and the current status of the cross-calibration between NuSTAR, Swift, and NICER, which frequently join together in observations of bright transients, and a review of the XMM-Newton and Chandra cross-calibration. Results between \nustar\ and \swift\ overall show good agreement, but issues persist in the overlap region 3--5 keV for bright source with large dust scattering halos. The NICER cross-calibration is still progressing and evolving, while for the XMM-Newton and Chandra cross-calibration systematic differences both in the absolute flux and spectral shape determination still exists on different classes of sources. The meeting in May was focused on the Working Group progress and reports summarized here.

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arXiv2020

Identifying charged particle background events in X-ray imaging detectors with novel machine learning algorithms

Space-based X-ray detectors are subject to significant fluxes of charged particles in orbit, notably energetic cosmic ray protons, contributing a significant background. We develop novel machine learning algorithms to detect charged particle events in next-generation X-ray CCDs and DEPFET detectors, with initial studies focusing on the Athena Wide Field Imager (WFI) DEPFET detector. We train and test a prototype convolutional neural network algorithm and find that charged particle and X-ray events are identified with a high degree of accuracy, exploiting correlations between pixels to improve performance over existing event detection algorithms. 99 per cent of frames containing a cosmic ray are identified and the neural network is able to correctly identify up to 40 per cent of the cosmic rays that are missed by current event classification criteria, showing potential to significantly reduce the instrumental background, and unlock the full scientific potential of future X-ray missions such as Athena, Lynx and AXIS.

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arXiv2020

Summary of the 14th IACHEC Meeting

We summarize the 14th meeting of the International Astronomical Consortium for High Energy Calibration (IACHEC) held at \textit{Shonan Village} (Kanagawa, Japan) in May 2019. Sixty scientists directly involved in the calibration of operational and future high-energy missions gathered during 3.5 days to discuss the status of the cross-calibration between the current international complement of X-ray observatories, and the possibilities to improve it. This summary consists of reports from the various WGs with topics ranging from the identification and characterization of standard calibration sources, multi-observatory cross-calibration campaigns, appropriate and new statistical techniques, calibration of instruments and characterization of background, communication and preservation of knowledge, and results for the benefit of the astronomical community.

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arXiv2019

Summary of the 13th IACHEC Meeting

We summarize the outcome of the 13th meeting of the International Astronomical Consortium for High Energy Calibration (IACHEC), held at Tenuta dei Ciclamini (Avigliano Umbro, Italy) in April 2018. Fifty-one scientists directly involved in the calibration of operational and future high-energy missions gathered during 3.5 days to discuss the current status of the X-ray payload inter-calibration and possible approaches to improve it. This summary consists of reports from the various working groups with topics ranging from the identification and characterization of standard calibration sources, multi-observatory cross-calibration campaigns, appropriate and new statistical techniques, calibration of instruments and characterization of background, and communication and preservation of knowledge and results for the benefit of the astronomical community.

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arXiv2017

Summary of the 12th IACHEC Meeting

We summarize the outcome of the 12th meeting of the International Astronomical Consortium for High Energy Calibration (IACHEC), held at the UCLA conference center in Lake Arrowhead (California) in March 2017. 56 scientists directly involved in the calibration of operational and future high-energy missions gathered during 3.5 days to discuss the status of the X-ray payload inter-calibration, as well as possible ways to improve it. The "Thermal Supernovas Remnant" (SNR) Working Group presented a recently published paper on 1E0102.2-7219 as a calibration standard in the 0.5-1.0 keV band. A new method to measure the high-energy spectrum of the Crab Nebula and pulsar with NuSTAR without using its optics may yield a new absolute flux standard in the 3-7 keV band. A new ACIS contamination model - released with CALDB version 4.7.3 - leads to a significant improvement in modeling the spectral, spatial, and temporal properties of the contaminant. The first calibration results of the scientific payload on board Hitomi confirm the excellent performance of the instruments before the spacecraft operation problems leading to its loss. Finally, the meeting discussed extensively a novel statistic approach to formally identify in which direction the effective areas of different instruments would need to be changed to bring them into concordance. This method could inform future further calibration efforts.

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arXiv2012

Using ACIS on the Chandra X-ray Observatory as a particle radiation monitor II

The Advanced CCD Imaging Spectrometer is an instrument on the Chandra X-ray Observatory. CCDs are vulnerable to radiation damage, particularly by soft protons in the radiation belts and solar storms. The Chandra team has implemented procedures to protect ACIS during high-radiation events including autonomous protection triggered by an on-board radiation monitor. Elevated temperatures have reduced the effectiveness of the on-board monitor. The ACIS team has developed an algorithm which uses data from the CCDs themselves to detect periods of high radiation and a flight software patch to apply this algorithm is currently active on-board the instrument. In this paper, we explore the ACIS response to particle radiation through comparisons to a number of external measures of the radiation environment. We hope to better understand the efficiency of the algorithm as a function of the flux and spectrum of the particles and the time-profile of the radiation event.

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arXiv2010

Using ACIS on the Chandra X-ray Observatory as a particle radiation monitor

The Advanced CCD Imaging Spectrometer (ACIS) is one of two focal-plane instruments on the Chandra X-ray Observatory. During initial radiation-belt passes, the exposed ACIS suffered significant radiation damage from trapped soft protons scattering off the x-ray telescope's mirrors. The primary effect of this damage was to increase the charge-transfer inefficiency (CTI) of the ACIS 8 front-illuminated CCDs. Subsequently, the Chandra team implemented procedures to remove the ACIS from the telescope's focus during high-radiation events: planned protection during radiation-belt transits; autonomous protection triggered by an on-board radiation monitor; and manual intervention based upon assessment of space-weather conditions. However, as Chandra's multilayer insulation ages, elevated temperatures have reduced the effectiveness of the on-board radiation monitor for autonomous protection. Here we investigate using the ACIS CCDs themselves as a radiation monitor. We explore the 10-year database to evaluate the CCDs' response to particle radiation and to compare this response with other radiation data and environment models.

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arXiv2008

Physics of reverse annealing in high-resistivity Chandra ACIS CCDs

After launch, the Advanced CCD Imaging Spectrometer (ACIS), a focal plane instrument on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. An effect of the damage was to increase the charge transfer inefficiency (CTI) of the front illuminated CCDs. As part of the initial damage assessment, the focal plane was warmed from the operating temperature of -100C to +30C which unexpectedly further increased the CTI. We report results of ACIS CCD irradiation experiments in the lab aimed at better understanding this reverse annealing process. Six CCDs were irradiated cold by protons ranging in energy from 100 keV to 400 keV, and then subjected to simulated bakeouts in one of three annealing cycles. We present results of these lab experiments, compare them to our previous experiences on the ground and in flight, and derive limits on the annealing time constants.

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arXiv2006

Temperature dependence of charge transfer inefficiency in Chandra X-ray CCDs

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arXiv2005

Long-term trends in radiation damage of Chandra X-ray CCDs

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. Current operations require ACIS to be protected during radiation belt passages to prevent this type of damage, but there remains a much slower and more gradual increase. We present the history of ACIS charge transfer inefficiency (CTI), and other measures of radiation damage, from January 2000 through June 2005. The rate of CTI increase is low, of order 1e-6 per year, with no indication of step-function increases due to specific solar events. Based on the time history and CCD location of the CTI increase, we speculate on the nature of the damaging particles.

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arXiv2005

An Overview of the Performance of the Chandra X-Ray Observatory

The Chandra X-ray Observatory is the X-ray component of NASA's Great Observatory Program which includes the recently launched Spitzer Infrared Telescope, the Hubble Space Telescope (HST) for observations in the visible, and the Compton Gamma-Ray Observatory (CGRO) which, after providing years of useful data has reentered the atmosphere. All these facilities provide, or provided, scientific data to the international astronomical community in response to peer-reviewed proposals for their use. The Chandra X-ray Observatory was the result of the efforts of many academic, commercial, and government organizations primarily in the United States but also in Europe. NASA's Marshall Space Flight Center (MSFC) manages the Project and provides Project Science; Northrop Grumman Space Technology (NGST -- formerly TRW) served as prime contractor responsible for providing the spacecraft, the telescope, and assembling and testing the Observatory; and the Smithsonian Astrophysical Observatory (SAO) provides technical support and is responsible for ground operations including the Chandra X-ray Center (CXC). Telescope and instrument teams at SAO, the Massachusetts Institute of Technology (MIT), the Pennsylvania State University (PSU), the Space Research Institute of the Netherlands (SRON), the Max-Planck Institut fur extraterrestrische Physik (MPE), and the University of Kiel also provide technical support to the Chandra Project. We present here a detailed description of the hardware, its on-orbit performance, and a brief overview of some of the remarkable discoveries that illustrate that performance.

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arXiv2004

A charge transfer inefficiency correction model for the Chandra Advanced CCD Imaging Spectrometer

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. The primary effect of the damage was to increase the charge transfer inefficiency (CTI) of the eight front illuminated CCDs by more than two orders of magnitude. The ACIS instrument team is continuing to study the properties of the damage with an emphasis on developing techniques to mitigate CTI and spectral resolution degradation. We will discuss the characteristics of the damage, the detector and the particle background and how they conspire to degrade the instrument performance. We have developed a model for ACIS CTI which can be used to correct each event and regain some of the lost performance. The correction uses a map of the electron trap distribution, a parameterization of the energy dependent charge loss and the fraction of the lost charge re-emitted into the trailing pixel to correct the pixels in the event island. This model has been implemented in the standard Chandra data processing pipeline. Some of the correction algorithm was inspired by the earlier work on ACIS CTI correction by Townsley et al. (2000; 2002). The details of the CTI model and how each parameter improves performance will be discussed, as well as the limitations and the possibilities for future improvement.

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arXiv2004

Detection of x-rays from galaxy groups associated with the gravitationally lensed systems PG 1115+080 and B1422+231

Gravitational lenses that produce multiple images of background quasars can be an invaluable cosmological tool. Deriving cosmological parameters, however, requires modeling the potential of the lens itself. It has been estimated that up to a quarter of lensing galaxies are associated with a group or cluster which perturbs the gravitational potential. Detection of X-ray emission from the group or cluster can be used to better model the lens. We report on the first detection in X-rays of the group associated with the lensing system PG 1115+080 and the first X-ray image of the group associated with the system B1422+231. We find a temperature and rest-frame luminosity of 0.8 +/- 0.1 keV and 7 +/- 2 x 10^{42} ergs/s for PG 1115+080 and 1.0 +infty/-0.3 keV and 8 +/- 3 x 10^{42} ergs/s for B1422+231. We compare the spatial and spectral characteristics of the X-ray emission to the properties of the group galaxies, to lens models, and to the general properties of groups at lower redshift.

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