NICER / ISS Science Nugget for October 25, 2018

Modeling of X-ray background critical to NICER success

Critical to understanding the data from the faintest targets of NICER is understanding the background. The ISS orbit includes the South Atlantic Anomaly (SAA) as well as extreme northern and southern latitudes where the Earth's magnetic field lines begin to converge. These regions are where the highest background radiation is located. In addition, space weather modulates this background, especially in the extreme latitudes.

To approach this, the NICER team has been developing several methods to model the background. One is to use the space weather "KP" index derived from ground based magnetometers. In the attached figure, we can see how this background model does for one of our prime science targets, PSR J0437-4715. At the highest energies, the pulsar source spectrum is background dominated and we see the background model matches the spectrum well. At low energies, the source spectrum dominates over the background, but the background model allows us to estimate its effect.

Comparison of observed data set and modeled background for PSR J0437-4715
Figure: Observed data and background model for the pulsar PSR J0437-4715. The red curve is raw NICER data accumulated looking at this pulsar, while the green curve is the background model.

This model and other models of the background are being further developed to enable NICER science.

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