The Broad-band Noise Characteristics of Selected Anomalous X-Ray Pulsars and Soft Gamma-ray Repeaters

TL;DR

This study analyzes the broad-band noise in selected Anomalous X-Ray Pulsars and Soft Gamma-ray Repeaters, revealing variable noise characteristics and their potential independence from glitches or flux changes, using archival X-ray data.

Contribution

First comprehensive analysis of broad-band noise in AXPs and SGRs across 2-60 keV, identifying source-specific noise behaviors and their relation to outbursts and glitches.

Findings

Detected band-limited noise at low frequencies in all sources.

Found noise increase in 1E 2259+586 after a major outburst.

Observed gradual noise rise in 1E 1048.1-5937 over nearly two years.

Abstract

We present the broad-band noise structure of selected Anomalous X-Ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs) in the 2-60 keV energy band. We have analyzed Rossi X-ray Timing Explorer Proportional Counter Array archival light curves for four AXPs and one SGR. We detect that the persistent emission of these sources show band limited noise at low frequencies in the range 0.005-0.05 Hz varying from 2.5% to 70% integrated rms in times of prolonged quiescence and following outbursts. We discovered band-limited red noise in 1E 2259+586 only for $\sim$2 years after its major 2002 outburst. The system shows no broad-band noise otherwise. Although this rise in noise in 1E 2259+586 occurred following an outburst which included a rotational glitch, the other glitching AXPs showed no obvious change in broad-band noise, thus it does not seem that this noise is correlated with glitches. The only source that showed significant variation in broad-band noise was 1E 1048.1-5937, where the noise gradually rose for 1.95 years at a rate of $\sim$3.6% per year. For this source the increases in broad-band noise was not correlated with the large increases in persistent and pulsed flux, or its two short SGR-like bursts. This rise in noise did commence after a long burst, however given the sparsity of this event, and the possibility that similar bursts went unnoticed the trigger for the rise is noise in 1E 1048.1-5937 is not as clear as for 1E 2259+586. The other three sources indicate a persistent band-limited noise at low levels in comparison.