An Empirical Background Model for the NICER X-ray Timing Instrument

TL;DR

This paper presents an empirical three-parameter background model for NICER X-ray data, enabling improved background prediction and source detection, especially for faint sources, by analyzing observations across multiple pointing directions.

Contribution

The paper introduces a novel empirical background model for NICER based on observations of source-free regions, improving background estimation and flare detection capabilities.

Findings

Median background rate of 0.87 c/s in 3556 GTIs

Filtering criteria can flag GTIs with unsatisfactory background predictions

Detection limit of 1.20 c/s at 0.4-12 keV for single GTI

Abstract

NICER has a comparatively low background rate, but it is highly variable, and its spectrum must be predicted using measurements unaffected by the science target. We describe an empirical, three-parameter model based on observations of seven pointing directions that are void of detectable sources. An examination of 3556 good time intervals (GTIs), averaging 570 s, yields a median rate (0.4-12 keV; 50 detectors) of 0.87 c/s, but in 5 percent (1 percent) of cases, the rate exceeds 10 (300) c/s. Model residuals persist at 20-30 percent of the initial rate for the brightest GTIs, implying one or more missing model parameters. Filtering criteria are given to flag GTIs likely to have unsatisfactory background predictions. With such filtering, we estimate a detection limit, 1.20 c/s (3 sigma, single GTI) at 0.4-12 keV, equivalent to 3.6e-12 erg/cm^2/s for a Crab-like spectrum. The corresponding limit for soft X-ray sources is 0.51 c/s at 0.3-2.0 keV, or 4.3e-13 erg/cm^2/s for a 100 eV blackbody. Faint-source filtering selects 85 percent of the background GTIs, and higher rates are expected for targets scheduled more favorably. An application of the model to 1 s timescale makes it possible to distinguish source flares from possible surges in the background.