Data-oriented Diagnostics of Pileup Effects on the Suzaku XIS

TL;DR

This paper systematically analyzes pileup effects in the Suzaku XIS instrument, quantifying how pileup impacts spectral measurements and providing practical guidelines and tools for data correction.

Contribution

It introduces a method to quantify pileup effects using the pileup fraction and offers practical criteria and software for mitigating pileup in X-ray CCD data.

Findings

Spectral hardening increases with pileup fraction.

Events outside a certain radius are suitable for spectral analysis.

Provided tools facilitate pileup correction in data reduction.

Abstract

We present the result of a systematic study of pileup phenomena seen in the X-ray Imaging Spectrometer, an X-ray CCD instrument, onboard the Suzaku observatory. Using a data set of observed sources in a wide range of brightness and spectral hardness, we characterized the pileup fraction, spectral hardening, and grade migration as a function of observed count rate in a frame per pixel. Using the pileup fraction as a measure of the degree of pileup, we found that the relative spectral hardening (the hardness ratio normalized to the intrinsic spectral hardness), branching ratio of split events, and that of detached events increase monotonically as the pileup fraction increases, despite the variety of brightness and hardness of the sample sources. We derived the pileup fraction as a function of radius used for event extraction. Upon practical considerations, we found that events outside of the radius with a pileup fraction of 1% or 3% are useful for spectral analysis. We present relevant figures, tables, and software for the convenience of users who wish to apply our method for their data reduction of piled-up sources.