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NuSTAR (Nuclear Spectroscopic Telescope Array, also named Explorer 93 and SMEX-11) is a
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
space-based
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
telescope that uses a conical approximation to a Wolter telescope to focus high energy X-rays from astrophysical sources, especially for
nuclear spectroscopy Nuclear spectroscopy is a superordinate concept of methods that uses properties of a Nuclide, nucleus to probe material properties. By Radiation emission, emission or Absorption (electromagnetic radiation), absorption of radiation from the nucleus ...
, and operates in the range of 3 to 79 keV. NuSTAR is the eleventh mission of NASA's Small Explorer (SMEX-11) satellite program and the first space-based direct-imaging
X-ray telescope An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. X-rays are absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets ...
at energies beyond those of the
Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...
and
XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second corners ...
. It was successfully launched on 13 June 2012, having previously been delayed from 21 March 2012 due to software issues with the launch vehicle. The mission's primary scientific goals are to conduct a deep survey for
black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...
s a billion times more massive than the Sun, to investigate how particles are accelerated to very high energy in active galaxies, and to understand how the elements are created in the explosions of massive stars by imaging
supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
s. Having completed a two-year primary mission, NuSTAR is in its year of operation.


History

NuSTAR's predecessor, the High Energy Focusing Telescope (HEFT), was a balloon-borne version that carried telescopes and detectors constructed using similar technologies. In February 2003, NASA issued an Explorer program Announcement of Opportunity (AoO). In response, NuSTAR was submitted to NASA in May 2003, as one of 36 mission proposals vying to be the tenth and eleventh Small Explorer missions. In November 2003, NASA selected NuSTAR and four other proposals for a five-month implementation feasibility study. In January 2005, NASA selected NuSTAR for flight pending a one-year feasibility study. The program was cancelled in February 2006 as a result of cuts to science in NASA's 2007 budget. On 21 September 2007, it was announced that the program had been restarted, with an expected launch in August 2011, though this was later delayed to June 2012. The principal investigator is Fiona A. Harrison of the
California Institute of Technology The California Institute of Technology (branded as Caltech) is a private research university in Pasadena, California, United States. The university is responsible for many modern scientific advancements and is among a small group of institutes ...
(Caltech). Other major partners include the
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center (FFRDC) in La Cañada Flintridge, California, Crescenta Valley, United States. Founded in 1936 by Cali ...
(JPL),
University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California), is a Public university, public Land-grant university, land-grant research university in Berkeley, California, United States. Founded in 1868 and named after t ...
,
Technical University of Denmark The Technical University of Denmark (), often simply referred to as DTU, is a polytechnic university and school of engineering. It was founded in 1829 at the initiative of Hans Christian Ørsted as Denmark's first polytechnic, and it is today ran ...
(DTU),
Columbia University Columbia University in the City of New York, commonly referred to as Columbia University, is a Private university, private Ivy League research university in New York City. Established in 1754 as King's College on the grounds of Trinity Churc ...
,
Goddard Space Flight Center The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C., in Greenbelt, Maryland, United States. Established on May 1, 1959, as NASA's first space flight center, GSFC ...
(GSFC),
Stanford University Leland Stanford Junior University, commonly referred to as Stanford University, is a Private university, private research university in Stanford, California, United States. It was founded in 1885 by railroad magnate Leland Stanford (the eighth ...
,
University of California, Santa Cruz The University of California, Santa Cruz (UC Santa Cruz or UCSC) is a public university, public Land-grant university, land-grant research university in Santa Cruz, California, United States. It is one of the ten campuses in the University of C ...
,
Sonoma State University Sonoma State University (SSU, Sonoma State, or Sonoma) is a public university in Sonoma County, California, United States. It is part of the California State University system. Sonoma State offers 92 bachelor's degree programs, 19 master's de ...
,
Lawrence Livermore National Laboratory Lawrence Livermore National Laboratory (LLNL) is a Federally funded research and development centers, federally funded research and development center in Livermore, California, United States. Originally established in 1952, the laboratory now i ...
, and the
Italian Space Agency The Italian Space Agency (; ASI) is a government agency established in 1988 to fund, regulate and coordinate space exploration activities in Italy. The agency cooperates with numerous national and international entities who are active in aerospac ...
(ASI). NuSTAR's major industrial partners include
Orbital Sciences Corporation Orbital Sciences Corporation (commonly referred to as Orbital) was an American company specializing in the design, manufacture, and launch of small- and medium- class space and launch vehicle systems for commercial, military and other governmen ...
and ATK Space Components.


Launch

NASA contracted with Orbital Sciences Corporation to launch NuSTAR (mass ) on a Pegasus XL launch vehicle on 21 March 2012. It had earlier been planned for 15 August 2011, 3 February 2012, 16 March 2012, and 14 March 2012. After a launch meeting on 15 March 2012, the launch was pushed further back to allow time to review flight software used by the launch vehicle's flight computer. The launch was conducted successfully at 16:00:37
UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...
on 13 June 2012 about south of
Kwajalein Atoll Kwajalein Atoll (; Marshallese language, Marshallese: ) is part of the Marshall Islands, Republic of the Marshall Islands (RMI). The southernmost and largest island in the atoll is named Kwajalein Island, which its majority English-speaking re ...
. The Pegasus launch vehicle was dropped from the L-1011 'Stargazer' aircraft. On 22 June 2012, it was confirmed that the mast was fully deployed.


Optics

Unlike visible light telescopes – which employ mirrors or lenses working with normal incidence – NuSTAR has to employ grazing incidence optics to be able to focus X-rays. For this two conical approximation Wolter telescope design optics with focal length are held at the end of a long deployable mast. A laser
metrology Metrology is the scientific study of measurement. It establishes a common understanding of Unit of measurement, units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to stan ...
system is used to determine the exact relative positions of the optics and the focal plane at all times, so that each detected photon can be mapped back to the correct point on the sky even if the optics and the focal plane move relative to one another during an exposure. Each focusing optic consists of 133 concentric shells. One particular innovation enabling NuSTAR is that these shells are coated with depth-graded multilayers (alternating atomically thin layers of a high-density and low-density material); with NuSTAR's choice of Pt/SiC and W/Si multilayers, this enables reflectivity up to 79 keV (the platinum K-edge energy). The optics were produced, at
Goddard Space Flight Center The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C., in Greenbelt, Maryland, United States. Established on May 1, 1959, as NASA's first space flight center, GSFC ...
, by heating thin () sheets of flexible glass in an oven so that they slumped over precision-polished cylindrical quartz
mandrel A mandrel, mandril, or arbor is a tapered tool against which material can be forged, pressed, stretched or shaped (e.g., a ring mandrel - also called a triblet - used by jewellers to increase the diameter of a wedding ring), or a flanged or t ...
s of the appropriate radius. The
coating A coating is a covering that is applied to the surface of an object, or substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. powder coatings. Paints ...
s were applied by a group at the Danish Technical University. The shells were then assembled, at the Nevis Laboratories of Columbia University, using graphite spacers machined to constrain the glass to the conical shape, and held together by epoxy. There are 4680 mirror segments in total (the 65 inner shells each comprise six segments and the 65 outer shells twelve; there are upper and lower segments to each shell, and there are two telescopes); there are five spacers per segment. Since the epoxy takes 24 hours to cure, one shell is assembled per day – it took four months to build up one optic. The actual telescope consists of two separate Focal Plane Modules (FPMs) labelled FPMA and FPMB. These two FPMs are built to be similar, though they are not identical. Depending on the source and on the observation, one of the modules will usually report higher counts. This is corrected for in the science results step, usually by apply a constant multiplier during spectral fitting and light curve analysis. The expected point spread function for the flight mirrors is 43
arcseconds A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...
, giving a spot size of about two millimeters at the focal plane; this is unprecedentedly good resolution for focusing hard X-ray optics, though it is about one hundred times worse than the best resolution achieved at longer wavelengths by the
Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...
.


Detectors

Each focusing optic has its own focal plane module, consisting of a solid state
cadmium zinc telluride Cadmium zinc telluride, (CdZnTe) or CZT, is a compound of cadmium, zinc and tellurium or, more strictly speaking, an alloy of cadmium telluride and zinc telluride. A direct bandgap semiconductor, it is used in a variety of applications, including S ...
(CdZnTe) pixel detector surrounded by a cesium iodide (CsI) anti-coincidence shield. One detector unit — or focal plane — comprises four (two-by-two) detectors, manufactured b
eV Products
Each detector is a rectangular crystal of dimension and thickness ~ that have been gridded into
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
s (each pixel subtending 12.3 arcseconds) and provides a total of 12 arcminutes
field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...
(FoV) for each focal plane module. The cadmium zinc telluride (CdZnTe) detectors are
state of the art The state of the art (SOTA or SotA, sometimes cutting edge, leading edge, or bleeding edge) refers to the highest level of general development, as of a device, technique, or scientific field achieved at a particular time. However, in some contex ...
room temperature
semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
s that are very efficient at turning high energy photons into
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s. The electrons are digitally recorded using custom
application-specific integrated circuit An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-efficienc ...
s (ASICs) designed by the NuSTAR
California Institute of Technology The California Institute of Technology (branded as Caltech) is a private research university in Pasadena, California, United States. The university is responsible for many modern scientific advancements and is among a small group of institutes ...
(CalTech) Focal Plane Team. Each pixel has an independent discriminator and individual X-ray interactions trigger the readout process. On-board processors, one for each telescope, identify the row and column with the largest pulse height and read out pulse height information from this pixel as well as its eight neighbors. The event time is recorded to an accuracy of 2 μs relative to the on-board clock. The event location, energy, and depth of interaction in the detector are computed from the nine-pixel signals. The focal planes are shielded by cesium iodide (CsI) crystals that surround the detector housings. The crystal shields, grown by Saint-Gobain, register high energy photons and cosmic rays which cross the focal plane from directions other than the along the NuSTAR optical axis. Such events are the primary background for NuSTAR and must be properly identified and subtracted in order to identify high energy photons from cosmic sources. The NuSTAR active shielding ensures that any CZT detector event coincident with an active shield event is ignored.


Major scientific results

NuSTAR has demonstrated its versatility, opening the way to many new discoveries in a wide variety of areas of astrophysical research since its launch.


Spin measurement of a supermassive black hole

In February 2013, NASA revealed that NuSTAR, along with the
XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second corners ...
space observatory, has measured the spin rate of the
supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...
at the center of the galaxy NGC 1365. By measuring the frequency change of X-ray light emitted from the black hole corona, NuSTAR was able to view material from the corona be drawn closer to the
event horizon In astrophysics, an event horizon is a boundary beyond which events cannot affect an outside observer. Wolfgang Rindler coined the term in the 1950s. In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive c ...
. This caused inner portions of the black hole's
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...
to be illuminated with X-rays, allowing this elusive region to be studied by astronomers for spin rates.


Tracing radioactivity in a supernova remnant

One of NuSTAR's main goals is to characterize stars' explosions by mapping the radioactive material in a
supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
s. The NuSTAR map of Cassiopeia A shows the titanium-44 isotope concentrated in clumps at the remnant's center and points to a possible solution to the mystery of how the star exploded. When researchers simulate supernova blasts with computers, as a massive star dies and collapses, the main shock wave often stalls and the star fails to shatter. The latest findings strongly suggest the exploding star literally sloshed around, re-energizing the stalled shock wave and allowing the star to finally blast off its outer layers.


Nearby supermassive black holes

In January 2017, researchers from
Durham University Durham University (legally the University of Durham) is a collegiate university, collegiate public university, public research university in Durham, England, founded by an Act of Parliament (UK), Act of Parliament in 1832 and incorporated by r ...
and the
University of Southampton The University of Southampton (abbreviated as ''Soton'' in post-nominal letters) is a public university, public research university in Southampton, England. Southampton is a founding member of the Russell Group of research-intensive universit ...
, leading a coalition of agencies using NuSTAR data, announced the discovery of supermassive black holes at the center of nearby galaxies NGC 1448 and IC 3639.


Measurement of temperature variations of AGN wind

In March 2nd of 2017, NuSTAR published an article to Nature detailing observations of wind temperature variations around AGN IRAS 13224−3809. By detecting periodic absences of absorption lines in the X-ray spectrum from the accretion disk winds, NuSTAR and
XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second corners ...
observed heating and cooling cycles of the relativistic winds leaving the
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...
.


Detection of light reflecting behind a black hole

NuSTAR and
XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second corners ...
detected X-rays emitted behind the supermassive black hole within Seyfert 1 galaxy I Zwicky 1. Upon studying the flashes of light emitted by the corona of the black hole, researchers noticed that some detected light arrived to the detector later than the rest, with a corresponding change in frequency. The Stanford University team of scientists that led the study concluded that this change was directly attributable to radiation from the flash reflecting off of the accretion disk on the opposing side of the black hole. The path of this reflected light was bent by the high spacetime curvature, directed to the detector after the initial flash.


Ultra-luminous neutron star violating the Eddington limit

In April 6th of 2023, the NuSTAR team confirmed that neutron star M82 X-2 was emitting more radiation than was physically thought possible due to the Eddington limit, officially labeling it as an Ultraluminous X-ray source (ULX).


See also

* Explorer program *
Gravity and Extreme Magnetism Gravity and Extreme Magnetism Small Explorer (GEMS or SMEX-13) mission was a NASA space observatory mission. The main scientific goal of GEMS was to be the first mission to systematically measure the Polarization (waves), polarization of X-ray ...
, hard X-ray telescope measuring polarization (cancelled 2012) *
James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...
, infrared telescope launched on 25 December 2021 * XRISM, joint Japanese and American X-ray telescope * List of X-ray space telescopes


References


External links


NuSTAR website
at nasa.gov
NuSTAR website
at caltech.edu
Building, Launching, and Using the NuSTAR X-ray Observatory
talk by Melania Nynka of the MIT Kavli Institute for Astrophysics and Space Research ;Further reading * * {{Orbital launches in 2012 X-ray telescopes Explorers Program Space telescopes Spacecraft launched in 2012 Spacecraft launched by Pegasus rockets