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Compelling Science, Mature Technology

Arcus is a high energy mission that will be proposed to NASA as an Astrophysics Probe Explorer in 2023. It is comprised of a high resolution soft X-ray grating spectrometer and medium resolution far-UV spectrometer that work simultaneously.

The idea for Arcus originated with the proposed International X-ray Observatory (IXO), which would have had a 4 meter diameter mirror and a large suite of instruments, including X-ray imagers and a high-resolution spectrometer. IXO was ranked fourth among large missions in NASA's 2010 Astrophysics Decadal Survey, "New Worlds, New Horizons." This ranking reflected the technical, cost, and programmatic uncertainties associated with it at the time that the assessment was undertaken. However, the science questions that IXO would have addressed were deemed "high-priority", and it was recommended that NASA initiate "an aggressive program to mature the mission and develop the technology" so that the mission could be realized.

In response, a team of scientists and engineers continued to work on the grating and optics techonologies necessary to build the high-resolution spectrometer originally envisioned on IXO. In 2016, Arcus was proposed to NASA as a MIDEX and selected for a Phase A concept study. Since then, the technology upon which it relies was developed further, and it was redesigned to respond more completely to the goals of the 2020 Decadal Survey. In addition to the high-resolution soft X-ray spectrometer, it includes a far-UV spectrometer, and can respond rapidly to targets of opportunity, with a response time under four hours. Its key characteristics are summarized in Table 1.

Table 1: The Arcus key characteristics.

A Unique View of the Universe

Arcus will observe astrophysical phenomena over an incredibly broad range, in sorely underutilized bandpasses. Its mission includes investigations from the smallest scales to the largest, including the composition of minute interstellar dust grains and the physical conditions which affect their evolution, stellar evolution, feedback from supermassive black holes, and structure formation in galaxy clusters. Using technologies developed for ESA's Athena mission and in the world-class laboratories of MIT/Lincoln Labs, the X-ray telescope is expected to achieve resolution R (= λ/Δλ) = 3,500 over its projected 10-60 Å bandpass, an improvement of an order of magnitude over existing or approved X-ray observatories, such as Chandra, XMM-Newton, XRISM, and Athena. The effective area is shown in Figure 1 (left). These X-ray capabilities will work simultaneously with the co-aligned 0.6m far-UV telescope. The far-UV spectrometer will provide moderate resolution spectra (R=20,000; current best estimate is > 22,000) covering the 1,000-1,250 Å baseline bandpass; the current best estimate bandpass is 970-1,350 Å. It will provide 6 times the effective area of FUSE (Figure 1, middle), with high sensitivity (Figure 1, right).

No other mission, current or planned, offers these capabilities.

Figure 1. The X-ray effective area for each order is shown (left). The FUV spectrometer will have about 6 times the effective area of FUSE (middle), and high sensitivity (right).