mission control The Neutron star Interior Composition Explorer (NICER) is looking for neutron stars and pulsars from its perch on the International Space Station. Keith Gendreau and Zaven Arzoumanian provide an overview of its capabilities. Sunshades and IOB ICER is an International extremes encountered in orbit XRCs ( 56) Space Station (ISS) Mission of so that the instrument’s effective area TMDs (4) N Opportunity that was selected as is stable. Sunshades on the XRCs, part of NASA’s Explorers Program. It is thermal blankets and controlled heaters the first NASA mission dedicated to keep the instrument in a thermally XTI the study of neutron stars and has been benign state. in operation since its 13 June 2017 As the ISS orbits Earth, the XTI tracks installation on the ISS (following a celestial targets to an accuracy better than FPMs Star tracker 3 June launch), nearly 50 years after the 66 arcsec more than 99% of the time. ( 56) assembly discovery of neutron stars. NICER studies NICER’s pointing system uses a star tracker, the energetics, dynamics and structure with built-in microelectromechanical gyros Deployment and of neutron stars through observations pointing system and accelerometers, to provide an attitude in the soft X-ray band, exploring rapid solution at a 10 Hz rate. This high rate Contamination brightness and spectral fluctuations enables pointing control software running shield with high-precision time-stamping and in NICER’s onboard computer to command spectroscopy of individually detected EPIC an elevation-over-azimuth gimbal system X-ray photons. Compared with its to steer and maintain the XTI on target predecessor X-ray timing astrophysics while overcoming vibrations from the ISS Adapter plate mission, the Rossi X-ray Timing Explorer and the NICER payload itself. To minimize (RXTE), which launched over two decades some of the vibrations, passive tuned mass Fig. 1 | Illustration of the NICER payload without ago, NICER provides order-of-magnitude dampers (TMDs) attached to corners of thermal blankets installed. Credit: NASA improvements in spectral resolution, the XTI dissipate certain predicted absolute timing resolution and sensitivity. mechanical modes. Early observations with NICER are already The XTI and the pointing system providing exciting results. X-ray photons interact within the SDDs, are deployed by a boom from a plate The NICER ISS payload consists of an producing charge clouds proportional to that is connected to a standard ISS X-ray timing instrument (XTI) as well as the photon energy. The charge clouds payload adapter. This adapter provides supporting hardware that points the XTI then drift along electric field lines to a the mechanical and electrical interface at celestial objects while maintaining an central sensing anode where their content to the ISS as well as to the SpaceX Dragon interface to the ISS and its infrastructure is measured with a charge-sensitive spacecraft that delivered NICER to the (Fig. 1). The XTI comprises an instrument preamplifier followed by a pair of shaping ISS on top of a Falcon 9 rocket. The ISS optical bench (IOB) that holds an aligned amplifiers in parallel. The latter record the provides 28 V and 120 V power via the collection of 56 X-ray concentrators (XRCs) pulse amplitude using two different signal ExPA Power Interface Controller (EPIC), and associated focal plane modules (FPMs). smoothing times, which yields a measure as well as a 1 Mbps MIL-STD-1553 The XRCs are non-imaging optics that of the distance over which the charge cloud command and telemetry link and concentrate X-ray photons from a distant drifted from its interaction point in the a 6 Mbps ethernet downlink. target onto the FPMs, with a field of view silicon to the sensing anode; this enables Every two or three days, the NICER of approximately 30 arcmin . Within each discrimination between X-rays concentrated ground system uplinks commands that FPM is a commercially available silicon drift by the optics and much of the radiation- include a continuous timeline of celestial detector (SDD) that detects individual X-ray induced ‘background’ events also registered targets to track. Typically, NICER tracks photons, recording their energies with a by the detectors. two to four targets during each ISS orbit, precision of about 2% and their arrival times Detector readout electronics record with a long-term on-target science to better than 100 ns. Combined, the XTI the pulse height produced by each X-ray efficiency of greater than 50%. NICER’s provides an effective collecting area that peaks photon together with its arrival time baseline mission dedicated to neutron at nearly 1,900 cm for X-ray energies of 1,500 relative to a precise pulse-per-second stars will continue until January 2019, eV and ranges from 200 to 12,000 eV. signal generated by a global positioning after which it will transition into Each XRC consists of a collection of 24 system-steered onboard clock. NICER’s a general-use X-ray astrophysics single-bounce grazing-incidence reflective timing resolution has been calibrated observatory. ❐ parabolic shells with a focal length of to 84 ns (root mean squared), including 1.085 m. These shells are thin aluminium the uncertainty in the location of the Keith Gendreau* and Zaven Arzoumanian foils with smooth replicated gold surfaces. instrument. The charge-cloud drift NASA’s Goddard Space Flight Center, Greenbelt, MD, Because NICER’s neutron star targets are, speed in the SDD dominates the overall USA. *e-mail: email@example.com for the most part, isolated point sources, time-stamping resolution. concentrating optics are a mass-efficient way The IOB maintains the alignment of the Published online: 1 December 2017 https://doi.org/10.1038/s41550-017-0301-3 of providing a large collecting area. XTI’s components through all of the thermal NatuRE astR oNomy | VOL 1 | DECEMBER 2017 | 895 | www.nature.com/natureastronomy © 2017 Nature America Inc., part of Springer Nature. All rights reserved.
Nature Astronomy – Springer Journals
Published: Dec 1, 2017
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