NuSTAR Legacy Survey observations began after the completion of the NuSTAR primary mission and were designed to complement the General Observer (GO) program with studies that required a larger alloation of exposure time than was available in the GO program. Legacy surveys were developed based on reccomendations from the scientific community and the NuSTAR Users Committee during each 2-year mission extension.

The NuSTAR Legacy Survey program is now complete. The observations performed between 2015 and 2019 are listed below and the data is available in the NuSTAR mission archive.

Last updated: 2019-09-24

There were two survey fields and three AGN samples approved for legacy survey observations in 2015 to 2019.

1) GOODS North  3 tiles centered on J2000 189.2 +62.2 degrees, 4 x 50 ks exposures each tile in 2015 (allocated 600 ks)

2) UDS  15 tiles centered on J2000 34.4 -5.2 degrees, 2 x 50 ks each tile in 2016 (allocated 1500 ks).

3-5) Deeper exposures around sources discovered in three sky surveys, XMMLSS, XBootes, and SDSS stripe 82. 50 ks exposures for each position were planned for 2017-2019 (allocated 1650 ks)

6) Swift-BAT detected galaxy mergers  -  Observations were coordinated in Chandra GO cycle-17 program (allocated 300 ks). Each target was observed for 20 ks within 2 weeks of the Chandra observation.

9) Swift Galactic Plane Survey Key Project  -  NuSTAR observations of sources identified during the Swift Galactic plane survey (allocated 500 ks). 

11) Survey of Intermediate Polars    (allocated 1020 ks)

Intermediate Polars (IP) are binary systems in which matter from a main sequence companion is accreted along magnetic field lines onto the poles of a magnetic white dwarf (WD). This systematic survey of 25 IPs selected from the Swift-BAT 70 month catalog aims to measure the masses of the WDs and constrain the mass distribution of these magnetic cataclysmic variable systems.

Cosmic X-ray Background (CXB) (allocated 200 ks)

The cosmic X-ray background (CXB) was discovered over 50 years ago, and represents an important integral of the total accretion power of supermassive black holes. However, despite many attempts to measure its spectrum, the normalization of the spectrum at ~3-20keV has considerable uncertainty. In this legacy survey we plan to use the Moon as a shutter, enabling a clear and unambiguous measurement of the CXB to be made with NuSTAR. During the observation we will track the Moon while it oscillates, due to parallax, over 4 degrees of sky. An exposure of 200ks is sufficient to achieve our goal of measuring the normalization and spectral shape of the CXB. - Observations suspended in 2018

Solar hard X-ray corona monitoring (allocated 90 ks)

The Sun currently is entering its minimum period of activity, with the absolute minimum expected to occur in about 2019, and global Solar activity will be monitored by NuSTAR through this period. An investigation of the solar-cycle variability of the global corona will be a novel and important scientific achievement. NuSTAR's spectral coverage makes it the most sensitive instrume\ nt ever for the detection of temperature variations and non-thermal bremsstrahlung/free-bound emission.

During this part of the cycle, the shear zones of the solar torsional oscillations, which relate globally to magnetic flux emergence, will jump from low latitudes to high latitudes according to the well-established Babcock-Leighton pattern. The NuSTAR observations can relate this to global coronal temperature variations for the first time. At 3 keV, relative to a greybody spectrum at\ kT < 0.1 keV, we would expect a 30-fold amplification of the temperature dependence: d(ln S)/d(ln T) ~ 30, where S is the spectral flux at 3 keV. This implies very powerful leverage on possible plasma temperature variations in the quiet corona, which NuSTAR mosaic imaging can characterize globally. In addition to thermal emission processes there is the possibility of non-thermal s\ ignatures associated with coronal heating (e.g., nanoflares or with cosmic-ray secondary products). This investigation will also contribute to other ongoing science priorities: the study of quiet Sun microflares (which also have an unknown variation through this part of the cycle) and the search for an axion signal from Sun center.

Two orbit mosaic maps of the Sun will be obtained at approximately three month intervals.