6)  Event Horizon Telescope (EHT) multiwavelength campaigns    (total 180 ks)

The Galactic and Nearby Galaxies Survey

NuSTAR is allowing us to probe more deeply into the Galaxy than ever before. The goals of these surveys are to expose the hidden Milky Way and reveal the wide range of astrophysics that creates high-energy emission. NuSTAR studies of flares from Sgr A* probing the physical conditions in the immediate vicinity of the nearest supermassive black hole. By studying high-mass X-ray binaries (HMXB) in a range of environments, we can learn about their formation soon after a supernova event and determine the fraction of HMXBs with black holes. The latter is important for answering questions about HMXB evolution, such as how many neutron star/black hole binaries exist, which has important implications for future gravity wave experiments. In addition to point source studies, NuSTAR is also investigating diffuse emission toward the Galactic Center, including the Galactic ridge emission and the time variation of the reflected emission from dense molecular clouds that probe the accretion history of Sgr A*. The coordinates and characteristics of any transient discovered will immediately be broadcast to the community via electronic circulars such as the Astronomer’s Telegram.

There are seven survey programs approved for observations in 2017 and 2018:

1) Swift Galactic Plane Survey Key Project  -  NuSTAR observations of sources identified during the Swift Galactic plane survey (Multiple targets, 500 ks total)

4) Sgr A* monitoring (100 ks)

Chandra and Spitzer for two 24 hour duration observations in the summer of 2018.

5) TeV gamma-ray source survey (250 ks)

7) Survey of Intermediate Polars    (total 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) (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.

Solar hard X-ray corona monitoring (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 instrument 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 signatures 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.

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