XACT is a new suborbital X-ray payload being developed at GSFC to test and advance Technology Readiness Levels (TRL) of several technologies that could enable future missions. The scientific objective of XACT is to measure the X-ray polarization properties of the Crab Nebula and pulsar and of the accreting binary Her X-1. Polarimetry is a powerful tool for astrophysical investigation that has yet to be exploited in the X-ray band, where it could provide unique insights into neutron stars, black holes, and other extreme-physics environments.

With powerful new enabling technologies, XACT will demonstrate X-ray polarimetry as a practical and flight-ready astronomical technique. The technologies XACT will bring to flight readiness will also provide important new capabilities for NASA missions in space-based X-ray spectroscopy, timing, and photometry. XACT combines new ultra-lightweight optics, advanced photoelectric X-ray polarimeters, and novel calibration sources in a standard sounding-rocket payload.

The XACT optics - lightweight concentrators optimized for unresolved sources - will provide the largest focused X-ray-collecting area ever achieved on a sounding rocket. Currently under development at GSFC, they will have the largest focused collectingarea-to-mass ratio ever flown, enabling future spacebased missions that require truly enormous throughput and low background. As of early 2012, the XACT payload’s optical bench has been fully fabricated, and one of three concentrators is complete. The foil shells for the second and third concentrators are in production, as are the spider assemblies that hold them in place. Integration of the optical bench will be complete by Summer 2012.

The XACT polarimeters are the latest innovation in photoelectron tracking devices, combining good polarization response with high quantum efficiency to achieve unprecedented sensitivity. ASD scientists originated this polarimetry technique based on Time Projection Chambers (TPCs), and are currently developing it for astrophysics and heliophysics missions in the 1–100 keV band. XACT leverages these efforts to deliver the polarimeters at a low incremental cost.

XACT will demonstrate in-flight calibration using a novel electronic X-ray source that can be pulsed for arbitrary and commandable intervals. Such a modulated source can be used to provide calibration information on demand, minimizing the associated background and resulting in both higher sensitivity and observatory-scheduling freedom for future missions. The modulated X-ray source has been successfully used to calibrate drift velocities in several laboratory polarimeters.

The XACT program has included students from several schools. Three high-school students have participated in the design of the X-ray source. Eight undergraduate students from Olin College have worked on in-flight alignment monitoring system designs. One undergraduate from the U.S. Naval Academy has further refined this alignment system. One undergraduate from the University of Kentucky has helped with automation of X-ray mirror placement and alignment. A high school student developed code to provide robotic assistance to the assembly of the mirrors incorporating video capture software and mechanism controls. An undergraduate from the University of Alabama used this to assemble two mirrors. A graduate student has assisted in the design of the collimator that will be used to align the instrument and has also been working to streamline many aspects of the foil production and testing.

The XACT goals are accomplished with a simple flight plan that is well within the launch and recovery envelopes of a standard Black Brant IX at the White Sands Missile Range. The first launch will be in December 2012 to observe the Crab. Subsequent launches are anticipated with future funding.