All response files represent the response of one NuSTAR telescope. The two NuSTAR telescopes (usually labeled by their focal plane modules, FPMA and FPMB) have subtly different responses and we do not recommend explicitly combining data from FPMA and FPMB when performing data analysis (i.e., combining individual counts from both telescopes to make a single spectrum). We recommend that science users simulate the source spectrum twice and use simultaneous fitting when determining the feasibility of an observation.
The response file (nustar.rmf) is constructed from real data for sources near the optical axis of either telescope and varies relatively little between FPMA and FPMB.
Background files should be paired with similar extraction region sources (e.g., use the 30-arcescond background region for the 30-arcsecond extraction region ARF, not with the 60-arcsecond extraction region ARF).
To facilitate more accurate simulations, we have generated a set of response files to be used with XSPEC fakeit - or another observatory simulator. We divide the sources into point-like sources and extended sources.
If the proposed target is a point source, the only distinction to be made is whether or not it qualifies as strong or weak. For stronger sources we recommend using a 60-arcsecond radius circular extraction region, while for a weaker sources we recommend using a 30-arcsecond radius circular extraction region. In real sources, we recommend varying the source extraction size to determine the optimal signal-to-noise. For simulating sources, it may not be apparent whether a source is "strong" or "weak", so we recommend simulating the source using both 30'' and 60'' extraction regions to get a handle on where the background crosses over for a particular exposure time.
We have found that, on average, point sources end up between 1 and 3 arcminutes away from the optical axis of the telescope (we refer to the distance between the optical axis and the source as the "off-axis angle", or OA). We have provided response files that span the likely off-axis angle distance at 1, 3 and 5 arcminutes off-axis. In most cases, if there is just one source in the FOV, we recommend using the 1arcmin response files and background.
See the included readme.txt file for examples of simulating a source.
If the proposed target is an extended source, then we need to specify the amount of area over which the photons are distributed on the sky. We have used the PWNe G21.5+0.9 and the SNRs Cas A and Tycho to generate representative response files for objects with a projected radius on the sky of 1, 3, and 5 arcminutes. All three sources are roughly spherical and have roughly constant surface brightness. We note that if a source is composed of "knots" superposed on an extended target we recommend separately simulating the knots using the point source response files above.
See the included readme.txt file for examples of simulating a source.
To use the provided NuSTAR ARF in ISIS the intrinsic variable Remove_Spectrum_Gaps has to be set to 1 before loading the ARF. We also note that ISIS assumes a background scaling factor of 1 for simulated spectra, therefore the BACKSCAL keyword in the provided background files needs to be set to 1 to have them scaled correctly.
The Nuclear Spectroscopic Telescope Array science operations center is located on the campus of the California Institute of Technology
