NuSTAR will continue both Galactic and Extragalactic Legacy Surveys during the extended mission with programs selected based on community input. The legacy surveys listed below have been chosen to be observed during the extended mission (2015-2018).

Last updated: 2017/07/19

Why Legacy Surveys?

Two of NuSTAR’s key mission goals based on which the mission was selected were to conduct deep, wide-field surveys in both Galactic and extragalactic fields. While a significant amount of time has already been invested in these surveys during the baseline mission, it is clear that maximizing the legacy of the NuSTAR satellite will require significantly extending both of these surveys.

Legacy Surveys in Action

The Legacy Survey programs have been designed in consultation with the community. The detailed observations are planned, scheduled, and executed by the NuSTAR science team. This team has the expertise and the experience in the intricacies of such observations – e.g., avoiding stray light from (transient) bright sources, scheduling repeat observations to minimize roll angle changes, optimal background subtraction, and source identification. Such data will have no exclusive use period and will be obtained by the NuSTAR science team as a service to the community. The NuSTAR team, with its extensive experience with background subtraction and source identification that are specific to the NuSTAR instrument, will also provide high level data products (such as source positions and spectra in electronic form) for the community’s use.

One of the principal goals of the NuSTAR extragalactic surveys is to constrain the high-energy X-ray luminosity function and trace its evolution as a function of redshift and AGN obscuration. We will continue to pursue a multi-layer survey strategy, adding ultra-deep observations within the current deep fields (e.g., ECDFS, EGS, and COSMOS), expanding the number of deep fields (e.g., adding fields such as GOODS-N), and increasing the area of shallow observations through serendipitous sources and potentially a wider-area, shallow survey (e.g., mapping fields such as Stripe 82, XBoötes, and/or the eROSITA deep polar fields). Besides the science afforded by studies of samples of individual sources, such data, with a focus on improving statistics at the least well-constrained portions of L-z space, will significantly improve our understanding of the full census of AGN, the cosmic history of black hole growth, and the role of AGN feedback in galaxy evolution.

There are three survey fields and four samples of AGN approved for observations in 2017 and 2018 in addition to the continuation of the Swift-BAT AGN survey. 

       1)   Deeper exposures around sources discovered in three sky surveys, XMMLSS, XBootes, and SDSS stripe 82.   50 ks exposures for each position are planned for 2017-2018 (Total 1650 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) Survey of hard X-ray sources in M33 (3 positions, 200 ks each, total 600 ks)

A NuSTAR survey of M33 will be conducted to study the distribution of X-ray binary (XRB) accretion states in an actively star-forming environment, and constrain the 3-40 keV spectrum of the most luminous XRB in the Local Group, M33 X-8. Three 100 ks NuSTAR exposures will be obtained over an area covering ~70% of the star-formation activity within the M33 disk in 2017 and an additional three 100 ks exposures will be obtained in the summer of 2017 coordinated with XMM-Newton cycle-16 observations (proposal 080035, PI Lehmer).

The goal is to characterize XRB accretion states for ~15 sources, most of which are expected to be high-mass XRBs (HMXBs). By comparison, the number of visible Milky Way and Magellanic Cloud HMXBs above the luminosity limit of this survey (L_X ~ 10³⁷ erg/s) is ~6 (depending on variability). In addition, this data set will contain a factor of 8-15 times more HMXBs than the NuSTAR survey of M31. Taken together, these observations will provide a significant improvement in our knowledge of HMXB accretion states that will prove valuable for theoretical XRB population synthesis studies.

2) Hard X-ray observations of sources identified in Swift Galactic Plane Survey Key Project (Multiple targets, 500 ks total)

* Observation Completed

3) Survey of hard X-ray sources in the SMC (3 positions, 250 ks each, total 750 ks)

A NuSTAR survey of three fields in the SMC will be conducted in 2017. Observations of each position will be 150 + 100 ks separated by at least two months to examine variability of sources in each field of view.

4) SMC transients  - followup of targets detected during the Swift SMC Survey (S-CUBED) (Multiple targets, 200 ks total)

* Observation Completed

5) Sgr A molecular clouds (300 ks)

*First of two observations was performed in 2016 October

6) Sgr A* monitoring (350 ks)  

Coordinated with Chandra/Spitzer cycle-18 proposal 18620763 (PI Baganoff, 2 x 24 hour duration observations in the summer of 2017) and with Chandra GTO proposal 18620742 (PI Garmire) coordinated with the Event Horizon Telescope in the spring of 2017. Further observations will also be performed in 2018 coordinated with Chandra cycle-19 programs.

7) TeV gamma-ray source survey (450 ks)

* Observation completed
(1) 50ks completed so far
(2) Observations postponed

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.

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