Timeline analysis and wavelet multiscale analysis of the AKARI All-Sky Survey at 90 micron

TL;DR

This paper analyzes the detection limits of the AKARI 90 micron survey using timeline and wavelet multiscale methods, identifying real sources and distinguishing them from noise and cirrus emission.

Contribution

It introduces a combined timeline and wavelet multiscale analysis approach to improve point source detection and noise discrimination in the AKARI survey data.

Findings

Point source detection limit of ~0.4 Jy at S/N > 5

IRAS sources above 4σ are confirmed as real sources

Non-IRAS sources are likely noise or cirrus artifacts

Abstract

We present a careful analysis of the point source detection limit of the AKARI All-Sky Survey in the WIDE-S 90 $\mu$m band near the North Ecliptic Pole (NEP). Timeline Analysis is used to detect IRAS sources and then a conversion factor is derived to transform the peak timeline signal to the interpolated 90 $\mu$m flux of a source. Combined with a robust noise measurement, the point source flux detection limit at S/N $>5$ for a single detector row is $1.1\pm0.1$ Jy which corresponds to a point source detection limit of the survey of $\sim$0.4 Jy. Wavelet transform offers a multiscale representation of the Time Series Data (TSD). We calculate the continuous wavelet transform of the TSD and then search for significant wavelet coefficients considered as potential source detections. To discriminate real sources from spurious or moving objects, only sources with confirmation are selected. In our multiscale analysis, IRAS sources selected above $4\sigma$ can be identified as the only real sources at the Point Source Scales. We also investigate the correlation between the non-IRAS sources detected in Timeline Analysis and cirrus emission using wavelet transform and contour plots of wavelet power spectrum. It is shown that the non-IRAS sources are most likely to be caused by excessive noise over a large range of spatial scales rather than real extended structures such as cirrus clouds.