 NuSTAR (Explorer 93) satellite | |
| Names | Explorer 93 Nuclear Spectroscopic Telescope Array SMEX-11 |
|---|---|
| Mission type | X-ray astronomy |
| Operator | NASA / JPL |
| COSPAR ID | 2012-031A |
| SATCATno. | 38358 |
| Website | www |
| Mission duration | 2 years (planned) 12 years, 9 months(in progress) |
| Spacecraft properties | |
| Spacecraft | Explorer XCIII |
| Spacecraft type | Nuclear Spectroscopic Telescope Array |
| Bus | LEOStar-2 |
| Manufacturer | Orbital ATK (formerlyOrbital Sciences Corporation andATK Space Components) |
| Launch mass | 350 kg (770 lb)[1] |
| Payload mass | 171 kg (377 lb) |
| Dimensions | 1.2 × 10.9 m (3 ft 11 in × 35 ft 9 in) |
| Power | 750watts[2] |
| Start of mission | |
| Launch date | 13 June 2012, 16:00:37UTC[3] |
| Rocket | Pegasus XL (F41) |
| Launch site | Kwajalein Atoll,Stargazer |
| Contractor | Orbital Sciences Corporation |
| Orbital parameters | |
| Reference system | Geocentric orbit |
| Regime | Near-equatorial orbit |
| Perigee altitude | 596.6 km (370.7 mi) |
| Apogee altitude | 612.6 km (380.7 mi) |
| Inclination | 6.027° |
| Period | 96.8 minutes |
| Main telescope | |
| Type | Wolter type I |
| Focal length | 10.15 m (33.3 ft)[2] |
| Collecting area | 9 keV: 847 cm2 (131.3 sq in) 78 keV: 60 cm2 (9.3 sq in) |
| Wavelengths | 3–79 keV |
| Resolution | 9.5 arcseconds |
| Instruments | |
| Dual X-ray telescope | |
Explorer program | |
NuSTAR (Nuclear Spectroscopic Telescope Array, also namedExplorer 93 andSMEX-11) is aNASA space-basedX-ray telescope that uses aconical approximation to aWolter telescope to focus high energy X-rays fromastrophysical sources, especially fornuclear spectroscopy, and operates in the range of 3 to 79keV.[4]
NuSTAR is the eleventh mission of NASA'sSmall Explorer (SMEX-11) satellite program and the first space-based direct-imagingX-ray telescope at energies beyond those of theChandra X-ray Observatory andXMM-Newton. It was successfully launched on 13 June 2012, having previously been delayed from 21 March 2012 due to software issues with the launch vehicle.[5][6]
The mission's primary scientific goals are to conduct a deep survey forblack holes a billion times more massive than the Sun, to investigate how particles are accelerated to very high energy inactive galaxies, and to understand how the elements are created in the explosions of massive stars by imagingsupernova remnants.
Having completed a two-year primary mission,[7] NuSTAR is in its twelfth year of operation.
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.[5] 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.[8] 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.[6][9][10][11]
The principal investigator isFiona A. Harrison of theCalifornia Institute of Technology (Caltech). Other major partners include theJet Propulsion Laboratory (JPL),University of California, Berkeley,Technical University of Denmark (DTU),Columbia University,Goddard Space Flight Center (GSFC),Stanford University,University of California, Santa Cruz,Sonoma State University,Lawrence Livermore National Laboratory, and theItalian Space Agency (ASI). NuSTAR's major industrial partners includeOrbital Sciences Corporation andATK Space Components.
NASA contracted with Orbital Sciences Corporation to launch NuSTAR (mass 350 kg (770 lb))[12] on aPegasus XL launch vehicle on 21 March 2012.[6] It had earlier been planned for 15 August 2011, 3 February 2012, 16 March 2012, and 14 March 2012.[13] 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.[14] The launch was conducted successfully at 16:00:37UTC on 13 June 2012[3] about 117 mi (188 km) south ofKwajalein Atoll.[15] The Pegasus launch vehicle was dropped from theL-1011 'Stargazer' aircraft.[12][16]
On 22 June 2012, it was confirmed that the 10 m (33 ft) mast was fully deployed.[17]
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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 approximationWolter telescope design optics with 10.15 m (33.3 ft) focal length are held at the end of a longdeployable mast. A lasermetrology 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 withdepth-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 platinumK-edge energy).[18][19]
The optics were produced, atGoddard Space Flight Center, by heating thin (210 μm (0.0083 in)) sheets of flexible glass in an oven so that they slumped over precision-polished cylindrical quartzmandrels of the appropriate radius. Thecoatings were applied by a group at theDanish Technical University.
The shells were then assembled, at theNevis 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.[20]
The expected point spread function for the flight mirrors is 43arcseconds, 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 theChandra X-ray Observatory.
Each focusing optic has its own focal plane module, consisting of a solid statecadmium zinc telluride (CdZnTe) pixel detector[21] surrounded by acesium iodide (CsI)anti-coincidence shield. One detector unit — or focal plane — comprises four (two-by-two) detectors, manufactured byeV Products. Each detector is a rectangular crystal of dimension 20 × 20 mm (0.79 × 0.79 in) and thickness ~2 mm (0.079 in) that have been gridded into 32 × 32 × 0.6 mm (1.260 × 1.260 × 0.024 in)pixels (each pixel subtending 12.3 arcseconds) and provides a total of 12 arcminutesfield of view (FoV) for each focal plane module.
The cadmium zinc telluride (CdZnTe) detectors arestate of the art room temperaturesemiconductors that are very efficient at turninghigh energy photons intoelectrons. The electrons are digitally recorded using customapplication-specific integrated circuits (ASICs) designed by the NuSTARCalifornia Institute of Technology (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.[22][23]
The focal planes are shielded bycesium iodide (CsI) crystals that surround the detector housings. The crystal shields, grown bySaint-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.
NuSTAR has demonstrated its versatility, opening the way to many new discoveries in a wide variety of areas of astrophysical research since its launch.
In February 2013, NASA revealed that NuSTAR, along with theXMM-Newton space observatory, has measured the spin rate of thesupermassive black hole at the center of the galaxyNGC 1365.[24] 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 theevent horizon. This caused inner portions of the black hole'saccretion disk to be illuminated with X-rays, allowing this elusive region to be studied by astronomers for spin rates.[24]
One of NuSTAR's main goals is to characterize stars' explosions by mapping the radioactive material in asupernova remnants. The NuSTAR map ofCassiopeia A shows thetitanium-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.[26]
In January 2017, researchers fromDurham University and theUniversity of Southampton, leading a coalition of agencies using NuSTAR data, announced the discovery of supermassive black holes at the center of nearby galaxiesNGC 1448 and IC 3639.[27][28][29]
In March 2nd of 2017, NuSTAR published an article to Nature detailing observations of wind temperature variations aroundAGNIRAS 13224−3809. By detecting periodic absences of absorption lines in the X-ray spectrum from the accretion disk winds, NuSTAR andXMM-Newton observed heating and cooling cycles of the relativistic winds leaving theaccretion disk.[30][31]
NuSTAR andXMM-Newton detected X-rays emitted behind the supermassive black hole withinSeyfert 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 correspondingchange 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.[32][33]
In April 6th of 2023, the NuSTAR team confirmed that neutron starM82 X-2 was emitting more radiation than was physically thought possible due to theEddington limit, officially labeling it as anUltraluminous X-ray source (ULX).[34][35]
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