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Science

Arcus's primary science objectives directly answer questions in the themes set out by the 2020 Decadal Survey; it will meet these objectives within the first five years after launch. During this time it will also operate as a general observatory, allowing astronomers to do ground-breaking work in a broad range of community science and usher in a new era for X-ray and far-UV spectroscopy. Below are the themes that Arcus science objectives are based on, and the questions that they aim to answer.

Cosmic Ecosystems
Worlds and Suns in Context
New Messengers and New Physics
Community-Driven Science

Cosmic Ecosystems

Galactic feedback is the process by which mass and energy are transferred from condensed material like stars and planets to hot gas via supernovae, jets, and stellar winds, and it plays a crucial role in shaping a galaxy's evolution. While JWST and upcoming missions will provide revolutionary insights into the earliest phases of galaxy formation, they will be unable to provide a full picture of how galaxies grow, as high resolution X-ray spectra are required to investigate the hottest, most energetic phases of feedback.

What powers the supermassive black hole winds that drive the evolution of entire galaxies and clusters?
Accreting supermassive black holes at the cores of active galactic nuclei (AGN) are a key part of galactic feedback. They are known to influence their environments through their outflows, but how these outflows depend on the black hole's physical parameters (like mass and spin) are poorly understood. While current X-ray observatories have made progress toward understanding the mass and energy carried away by outflows, they do not have the sensitivity and resolution to constrain launch radii and reveal mass outflow rates and kinetic power.

Arcus will unveil how physics behaves at the extremes of space and time near a black hole (see Figure 1), and trace the ionization states, densities, and total mass flux in the winds. It will determine wind momentum from the response time of the wind to changes in the ionizing flux on timescales from 10 ks to 10 Ms. This will break the degeneracy between the density of the outflowing wind and its radius. The wind's kinetic power is proportional to v3 r NH , so the role of AGN wind feedback in shaping host galaxies will be quantified as it flows from black hole's vicinity into the interstellar and circumgalactic media.

Figure 1. A schematic of the inner-most regions of the accretion disk of a black hole. Arcus will measure the kinetic power of AGN outflows, and so will quantify the role of AGN wind feedback in shaping host galaxies.

How does matter cycle in and out of galaxies and galaxy clusters?
Bright background sources act like flashlights, illuminating the highly tenuous, hot material at and beyond the edges of galaxies and their local environments. The origin, distribution, and impact of this hot gas cannot be determined without knowing its mass, composition, thermal structure, and dynamics. However, its extreme diffuseness makes it exceedingly difficult to measure.

By observing several lines of sight through this hot material, Arcus will allow us to map the locations, motions, and temperatures of the hot gases and metals there (see Figure 2). This will provide a detailed picture of accretion and gas and metal recycling, a crucial yet poorly understood part of feedback.

Figure 2. Sight lines to background AGN pass through different regions of galaxies' hot halos. The absorption features in the spectra reveal the mass, temperature, and chemical abundances of the intervening material, as well as the motion of hot gas in the Milky Way's halo.

Worlds and Suns in Context

Stellar and exoplanetary astronomy have been invigorated by several recent missions which allowed detailed studies of physical phenomena in the stars' atmospheres and the planetary systems. While these leaps are set to continue and grow when the next generation of telescopes comes online, they will not tell the full story of stellar and planetary system development. High energy spectra provide unique insight into star systems at all phases in their lifecycle, from birth to death.

How do stellar coronae arise?
Studies have shown great variation in stellar characteristics, many of which are driven by magnetic fields. This includes coronae, which are common in cool main sequence stars, but have unkown origins. UV and X-ray observations can examine the outer layers of cool stars, above the photosphere, where magnetic fields begin to drive activity. Currently, coronal diagnostics exist for only the brightest, most active coronae.

Arcus will increase the number of stars for which we have coronal diagnostics by an order of magnitude, uniquely measuring temperature, density, and ionization equilibrium emission line diagnostics for a large sample of stars. It will be able to detect dielectronic recombination (DR) satellite lines (see Figure 3). The flux ratios of these lines to their parent resonance line are extremely sensitive charge-state-independent diagnostics of temperature. Arcus will easily measure multiple weak DR lines, finding temperatures accurate to 10%. These will be combined with DR ratio predictions from emission distribution analysis, and allow us to distinguish between competing coronal heating models.

Figure 3. A simulated Arcus spectrum showing emission lines from a young star. The DR lines, which are required to distinguish between coronal heating models, are highlighted in yellow; neither Chandra nor XRISM can detect them.
How do stars and protoplanetary disks form and evolve?
The final stage of star formation is driven by the accretion of a protoplanetary disk onto the nascent star, which follows the basic accretion shock model: after the shocked gas impacts the stellar corona, it gently slows and cools. However, a variety of data suggest a more turbulent interaction between the accretion stream and the stellar atmosphere. Further, the line diagnostics for some stars are contradictory: stars that are known to be accreting show enhanced post-shock emission, but do not show evidence of the shock.

Arcus has the sensitivity and resolution to differentiate between line signatures formed at different temperatures and densities, measuring shock line velocity shifts as low as 20 km/s; see (Figure 4). This will permit lines from the corona to be distinguished from lines from the shock, and thus resolve the inconsistencies.

Figure 4. A sketch of accretion onto and stellar wind from a young star, with a depiction of how ionized neon emission from the corona and shocks near the surface will be easily differentiated in Arcus spectra.

New Messengers and New Physics

A deep understanding of some of the most extreme objects and phenomena in the Universe can only be obtained by observing the full range of the electromagnetic spectrum, plus high-energy particles and gravitational waves — "multimessenger" studies. These studies, focusing on the transient signals of white dwarfs, neutron stars, and black holes, will transform our comprehension of these systems and the physical mechanisms that generate them.

How do accretion disks form around black holes?
Arcus is designed to be responsive to targets of opportunty, such as tidal disruption events (TDEs; see Figure 5, left), supernovae, and gravitational wave events. Planned observations can be easily interrupted, and the telescope can be on-target within 4 hours of notification. TDEs typically have a rapid initial rise, followed by a flux decay in UV and X-rays, and can have long-lasting super-Eddington phases that offer a rare window into the accretion flows that may have allowed the first quasars to quickly gain mass. The evolution of TDEs can show how accretion disks and winds evolve and how structures like the AGN broad line region may form. Arcus is perfectly suited to observe TDEs, as most are dominated by thermal emission that peaks between 28-83 Å (see Figure 5, right), and most are so faint that the absorption lines would not be observable with current X-ray gratings. Moreover, the cool, thermal spectrum is challenging to observe with XRISM/Resolve.

Figure 5. Left: An artist's depiction of the tidal disruption of a star by a black hole (credit: NASA/CXC/M.Weiss). Right: A simulated Arcus spectrum of a TDE with the same wind properties as ASASSN-14li, but only 10% of its flux.

Community-Driven Science

Arcus is an observatory for the entire astronomical community, and we are excited to support a robust and varied general observer science program. With its revolutionary X-ray and FUV spectroscopic capabilities, Arcus will be ideally suited to observe a broad range of science targets. Examples include:

  • Stars of all types (accreting, coronal, flaring)
  • Exoplanet atmospheres (see Figure 6, left)
  • X-ray binary systems
  • AGN (see Figure 6, right)
  • ISM studies, including dust grain composition
  • Novae
  • Ultra-luminous X-ray sources
  • FUV spectral imaging (e.g., mapping starburst galaxies like M82)

We'd love to hear your ideas! Please join us for the online-only Arcus Community Science Meeting on May 4-5, 2023.

Figure 6. Arcus will be able to address a broad set of science topics. Left: It will be able to measure the density and extent of high-altitude layers in transiting exoplanets by exploiting the energy-dependent transmission of elemental absorption edges. Right: It will also be able to place strong density constraints on the narrow line region of AGNs through observations of the O VII triplet. The inset panel shows a simulated 60 ks Arcus spectrum of the obscured AGN Mrk 3, assuming n = 3x109 cm-3. The O VII line ratios constrain the density to lie within 2-5 x 109 cm-3, an improvement of >10x over the precision possible with current telescopes.