Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50Å bandpass with unprecedented sensitivity: effective areas of >450 cm2 and spectral resolution >2500. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned, although there will be capabilities to observe sources such as tidal disruption events or supernovae with a ~3 day turnaround. Following the 2nd year of operation, Arcus will transition to a proposal-driven guest observatory facility.
Arcus will be proposed to the NASA Explorer program as a free-flying satellite mission that will enable high-resolution soft X-ray spectroscopy (8-50) with unprecedented sensitivity – effective areas of >500 sq cm and spectral resolution >2500. The Arcus key science goals are (1) to determine how baryons cycle in and out of galaxies by measuring the effects of structure formation imprinted upon the hot gas that is predicted to lie in extended halos around galaxies, groups, and clusters, (2) to determine how black holes influence their surroundings by tracing the propagation of out-flowing mass, energy and momentum from the vicinity of the black hole out to large scales and (3) to understand how accretion forms and evolves stars and circumstellar disks by observing hot infalling and outflowing gas in these systems. Arcus relies upon grazing-incidence silicon pore X-ray optics with the same 12m focal length (achieved using an extendable optical bench) that will be used for the ESA Athena mission. The focused X-rays from these optics will then be diffracted by high-efficiency off-plane reflection gratings that have already been demonstrated on sub-orbital rocket flights, imaging the results with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. The majority of mission operations will not be complex, as most observations will be long (~100 ksec), uninterrupted, and pre-planned, although there will be limited capabilities to observe targets of opportunity, such as tidal disruption events or supernovae with a 3-5 day turnaround. After the end of prime science, we plan to allow guest observations to maximize the science return of Arcus to the community.
Arcus is a NASA/MIDEX mission under development in response to the anticipated 2016 call for proposals. It is a freeflying, soft X-ray grating spectrometer with the highest-ever spectral resolution in the 8-51 Å (0.24 – 1.55 keV) energy range. The Arcus bandpass includes the most sensitive tracers of diffuse million-degree gas: spectral lines from O VII and O VIII, H- and He-like lines of C, N, Ne and Mg, and unique density- and temperature-sensitive lines from Si and Fe ions. These capabilities enable an advance in our understanding of the formation and evolution of baryons in the Universe that is unachievable with any other present or planned observatory. The mission will address multiple key questions posed in the Decadal Survey1 and NASA’s 2013 Roadmap2: How do baryons cycle in and out of galaxies? How do black holes and stars influence their surroundings and the cosmic web via feedback? How do stars, circumstellar disks and exoplanet atmospheres form and evolve? Arcus data will answer these questions by leveraging recent developments in off-plane gratings and silicon pore optics to measure X-ray spectra at high resolution from a wide range of sources within and beyond the Milky Way. CCDs with strong Suzaku heritage combined with electronics based on the Swift mission will detect the dispersed X-rays. Arcus will support a broad astrophysical research program, and its superior resolution and sensitivity in soft X-rays will complement the forthcoming Athena calorimeter, which will have comparably high resolution above 2 keV.
We discuss concepts for high-throughput (effective area 250-1400 cm2), high-resolution (spectral resolving power R >
3500) soft X-ray grating spectroscopy in missions of moderate (probe-class or smaller) scale. Such missions can
achieve high-priority scientific objectives identified by the Astro2010 Decadal Survey attainable in no other way, and
would provide an essential complement to any future large-area X-ray observatory equipped with non-dispersive
spectrometers. We enumerate key science drivers and discuss consequences of various alternative design choices for
scientific capability and overall mission size.
High-energy astrophysics is a relatively young scientific field, made possible by space-borne telescopes. During the
half-century history of x-ray astronomy, the sensitivity of focusing x-ray telescopes-through finer angular resolution
and increased effective area-has improved by a factor of a 100 million. This technological advance has enabled
numerous exciting discoveries and increasingly detailed study of the high-energy universe-including accreting (stellarmass
and super-massive) black holes, accreting and isolated neutron stars, pulsar-wind nebulae, shocked plasma in
supernova remnants, and hot thermal plasma in clusters of galaxies. As the largest structures in the universe, galaxy
clusters constitute a unique laboratory for measuring the gravitational effects of dark matter and of dark energy. Here,
we review the history of high-resolution x-ray telescopes and highlight some of the scientific results enabled by these
telescopes. Next, we describe the planned next-generation x-ray-astronomy facility-the International X-ray
Observatory (IXO). We conclude with an overview of a concept for the next next-generation facility-Generation X.
The scientific objectives of such a mission will require very large areas (about 10000 m2) of highly-nested lightweight
grazing-incidence mirrors with exceptional (about 0.1-arcsecond) angular resolution. Achieving this angular resolution
with lightweight mirrors will likely require on-orbit adjustment of alignment and figure.
Generation-X is required to be an X-ray observatory with 50 m2 effective collecting area and 0.1 arcsec half-power
diameter (HPD) angular resolution at 1 keV. It is conceived that a launch vehicle such as that studied for the
Ares V will carry a monolithic 16-m-diameter mirror to the earth-sun L2 point. Even with such a vehicle, the
reflectors comprising the ≈ 250 nested shells must be extremely light-weight. Therefore their figure and alignment
cannot be achieved on the ground, and likely could not be maintained through the launch environment. We
will present a conceptual solution to those constraints: adjustable X-ray optics, as a case of "adaptive" optics
where the stability once in orbit should require adjustments no more frequently than yearly. The figure would
be adjusted via thin-film actuators deposited directly to the back (non-reflecting) side of each element. This
bi-morph configuration would impart in-plane strains via the piezoelectric or electrostrictive effect. Requirements
of the adjustment are to the order of a few nanometer precision. Each shell, and each module, must also be
aligned, to tolerances of about 0.1 micrometer. We conceive that on-orbit data would be acquired by a built-in
Hartmann system for the alignment adjustments and low-order figure, and by ring profile measurements of a
very bright celestial X-ray source to correct figure errors up to the mid-frequency range of several hundredths
We report on the prospects for the study of the first stars, galaxies and black holes with the Generation-X Mission.
Generation-X is a NASA "Vision Mission" which completed preliminary study in lat e2006. Generation-X was approved
in February 2008 as an Astrophysics Strategic Mission Concept Study (ASMCS) and is baselined as an X-ray
observatory with 50 square meters of collecting area at 1 keV (500 times larger than Chandra) and 0.1 arcsecond angular
resolution (several times better than Chandra and 50 times better than the Constellation-X resolution goal). Such a high
energy observatory will be capable of detecting the earliest black holes and galaxies in the Universe, and will also study
the chemical evolution of the Universe and extremes of density, gravity, magnetic fields, and kinetic energy which
cannot be created in laboratories. A direct signature of the formation of the first galaxies, stars and black holes is
predicted to be X-ray emission at characteristic X-ray temperatures of 0.1-1 keV from the collapsing proto-galaxies
before they cool and form the first stars.
Generation-X will be an X-ray observatory with 50 m2
collecting area at 1 keV and 0.1" angular resolution. A key
concept to enable such a dramatic improvement in angular resolution is
that the mirror figure will be adjusted on-orbit; e.g., via piezo-electric
actuators deposited on the back side of very thin glass and imparting
strains in a bi-morph configuration. To make local adjustments to the
individual mirror shells we must employ an imaging detector far
forward of the focal surface, so that rays from the individual shells
can be measured as distinct rings. We simulate this process on a few
representative shells via ray-traces of perfect optics, perturbed
axially by low order Legendre polynomial terms. This elucidates some of
the requirements for the on-orbit measurements, and on possible
algorithms to perform the on-orbit adjustment with acceptably rapid
Soon after the start of science operations of the Chandra X-ray Observatory, it became apparent that weakly penetrating
(0.1-0.5 MeV) protons in the Earth's radiation belt were causing an unexpectedly rapid increase in the charge-transfer
inefficiency of Chandra's front-illuminated CCDs. Fortunately, the Chandra team developed, implemented, and
maintains a radiation-protection program that successfully reduced the rate of degradation of the CCDs' performance to
acceptable levels. Since implementing this program, the average rate of increase of the charge-transfer inefficiency has
slowed to 3.2×10-6/y (2.3%/y) for the front-illuminated CCDs and 1.0×10-6/y (5.8%/y) for the back-illuminated CCDs.
This paper reviews the Chandra radiation-management program, reports the current status, and describes changes
planned or implemented since the previous paper on this topic.
The Chandra X-Ray Observatory was launched in July, 1999 and has yielded extraordinary scientific results. Behind the scenes, our Monitoring and Trends Analysis (MTA) approach has proven to be a valuable resource in providing telescope diagnostic information and analysis of scientific data to access Observatory performance. We have created and maintain real-time monitoring and long-term trending tools. This paper will update our 2002 SPIE paper on the design of the system and discuss lessons learned.
X-rays provide one of the few bands through which we can study the epoch of reionization, when the first galaxies,
black holes and stars were born. To reach the sensitivity required to image these first discrete objects in the
universe needs a major advance in X-ray optics. Generation-X (Gen-X) is currently the only X-ray astronomy
mission concept that addresses this goal. Gen-X aims to improve substantially on the Chandra angular resolution
and to do so with substantially larger effective area. These two goals can only be met if a mirror technology
can be developed that yields high angular resolution at much lower mass/unit area than the Chandra optics,
matching that of Constellation-X (Con-X). We describe an approach to this goal based on active X-ray optics
that correct the mid-frequency departures from an ideal Wolter optic on-orbit. We concentrate on the problems
of sensing figure errors, calculating the corrections required, and applying those corrections. The time needed
to make this in-flight calibration is reasonable. A laboratory version of these optics has already been developed
by others and is successfully operating at synchrotron light sources. With only a moderate investment in these
optics the goals of Gen-X resolution can be realized.
The CCDs on the Chandra X-ray Observatory are vulnerable to radiation damage from low-energy protons scattered off the telescope's mirrors onto the focal plane. Following unexpected damage incurred early in the mission, the Chandra team developed, implemented, and maintains a radiation-protection program. This program - involving scheduled radiation safing during radiation-belt passes, intervention based upon real-time space-weather conditions and radiation-environment modeling, and on-board radiation monitoring with autonomous radiation safing - has successfully managed the radiation damage to the CCDs. Since implementing the program, the charge-transfer inefficiency (CTI)
has increased at an average annual rate of only 3.2×10-6 (2.3%) for the front-illuminated CCDs and 1.0×10-6 (6.7%) for the back-illuminated CCDs. This paper describes the current status of the Chandra radiation-management program, emphasizing enhancements implemented since the original paper.
We have implemented a system to automatically analyze Chandra x-ray observations of point sources for use in monitoring telescope parameters such as point spread function, spectral resolution, and pointing accuracy, as well as for use in scientific studies. The Chandra archive currently contains at least 50 observations of star cluster-like objects, yielding 5,000+ sources of all spectral types well-suited for cataloging. The system incorporates off-the-shelf tools to perform the steps from source detection to temporal and spectral analyses. Our software contribution comes from wrapper scripts to autonomously run each step in turn, verify intermediate results, apply any logic required to set parameters, decide best-fit results, merge in data from other catalogs and to format convenient text and web-based output. We will outline this processing pipeline design and challenges, discuss the scientific applications, and focus on its role in monitoring on-orbit observatory performance.
The CCDs on the Chandra X-ray Observatory are sensitive to radiation damage, particularly from low-energy protons scattering off the telescope's mirrors onto the focal plane. In its highly elliptical orbit, Chandra passes through a spatially and temporally varying radiation environment, ranging from the radiation belts to the solar wind. Translating the Advanced CCD Imaging Spectrometer (ACIS) out of the focal position during radiation-belt passages has prevented loss of scientific utility. However, carefully managing the radiation damage during the remainder of the orbit, without unnecessarily sacrificing observing time, is essential to optimizing the scientific value of this exceptional observatory throughout its planned 10-year mission. In working toward this optimization, the Chandra team developed and applied a radiation-management strategy. This strategy includes autonomous instrument safing triggered by the on-board radiation monitor, as well as monitoring, alerts, and intervention based upon real-time space environment data from NOAA and NASA spacecraft. Furthermore, because Chandra often spends much of its orbit out of the solar wind (in the Earth's outer magnetosphere and magnetosheath), the team developed the Chandra Radiation Model to describe the complete low-energy-proton environment. Management of the radiation damage has thus far succeeded in limiting degradation of the charge-transfer inefficiency (CTI) to less than 3.5(10-6) and 1.3(10-6) per year for the front-illuminated and back-illuminated CCDs, respectively. This rate of degradation is acceptable for maintaining the scientific viability of all ACIS CCDs for more than ten years.
The Chandra X-ray Observatory was launched in July, 1999 and has yielded extraordinary scientific results. Behind the scenes, our Monitoring and Trends Analysis (MTA) system has proven to be a valuable resource. With three years worth of on-orbit data, we have available a vast array of both telescope diagnostic information and analysis of scientific data to access Observatory performance. As part of Chandra's Science Operations Team (SOT), the primary goal of MTA is to provide tools for effective decision making leading to the most efficient production of quality science output from the Observatory. We occupy a middle ground between flight operations, chiefly concerned with the health and safety of the spacecraft, and validation and verification, concerned with the scientific validity of the data taken and whether or not they fulfill the observer's requirements. In that role we provide and receive support from systems engineers, instrument experts, operations managers, and scientific users. MTA tools, products, and services include real-time monitoring and alert generation for the most mission critical components, long term trending of all spacecraft systems, detailed analysis of various subsystems for life expectancy or anomaly resolution, and creating and maintaining a large SQL database of relevant information. This is accomplished through the use of a wide variety of input data sources and flexible, accessible programming and analysis techniques. This paper will discuss the overall design of the system, its evolution and the resources available.
The Chandra X-ray Observatory was launched in July 1999, and is returning exquisite sub-arc second X-ray images of star groups, supernova remnants, galaxies, quasars, and clusters of galaxies. In addition to being the premier X-ray observatory in terms of angular and spectral resolution, Chandra is the best calibrated X-ray facility ever flown. We discuss here the calibration of the on-axis effective area of the High Resolution Mirror Assembly. Because we do not know the absolute X-ray flux density of any celestial source, this must be based primarily on ground measurements and on modeling. We use celestial sources which may be assumed to have smoothly varying spectra, such as the BL Lac object Markarian 421, to verify the continuity of the area calibration as a function of energy across the Ir M-edges. We believe the accuracy of the HRMA area calibration is of order 2%.