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

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.

💡 Research Summary

The authors present a systematic study of the broad‑band timing noise in a sample of magnetar‑type objects—four Anomalous X‑ray Pulsars (AXPs) and one Soft Gamma‑ray Repeater (SGR)—using archival Rossi X‑ray Timing Explorer (RXTE) Proportional Counter Array (PCA) light curves in the 2–60 keV band. The data were processed with standard background subtraction and Good Time Interval filtering, and power spectral densities (PSDs) were computed for each source over the low‑frequency range 0.005–0.05 Hz. The main goal was to quantify the integrated rms variability and to assess how this “band‑limited” noise evolves during quiescent intervals and after outbursts or rotational glitches.

Across the sample, most objects display modest low‑frequency red noise, with integrated rms values typically between 2.5 % and 10 %. However, two sources stand out. The first is AXP 1E 2259+586, which underwent a major outburst in June 2002 that included a bright X‑ray flare and a rotational glitch. In the two years following this event the PSD showed a pronounced increase in power, with rms rising above 30 % and a clear red‑noise slope extending up to the 0.05 Hz cutoff. After roughly two years the noise level declined sharply, returning to the low‑level baseline seen before the outburst. This behavior is consistent with theoretical expectations that a large magnetospheric re‑configuration and enhanced plasma conductivity after a flare can generate long‑lasting variability in the X‑ray flux.

The second exceptional case is AXP 1E 1048.1‑5937. Over a span of 1.95 years the integrated rms grew steadily at about 3.6 % per year, reaching a peak of ~70 %—the highest level observed in the sample. The increase was gradual rather than impulsive, and it did not coincide precisely with either the two short SGR‑like bursts recorded during the interval or with the large, long‑term enhancements in persistent and pulsed flux that the source is known to exhibit. The authors therefore cannot attribute the noise rise to any single, well‑timed event; instead they suggest that a slow evolution of the magnetospheric twist, cumulative internal stress release, or undetected micro‑bursts may be responsible.

The remaining two AXPs (4U 0142+61 and XTE J1810‑197) and the SGR (SGR 1806‑20) all show relatively stable, low‑amplitude band‑limited noise (integrated rms ≲ 5 %). Notably, both 4U 0142+61 and XTE J1810‑197 have experienced rotational glitches, yet their PSDs show no significant change before or after the glitches. This lack of correlation indicates that glitches—presumably driven by sudden adjustments in the neutron‑star interior—do not necessarily perturb the external magnetospheric emission processes that dominate the low‑frequency noise.

The authors interpret these findings within the broader magnetar framework. First, the emergence of strong, long‑lasting red noise after a major outburst (as seen in 1E 2259+586) supports models in which the external magnetic field untwists on timescales of months to years, modulating the particle acceleration and X‑ray production regions. Second, the absence of a systematic noise response to glitches suggests that interior crustal events are largely decoupled from the radiative variability probed by the low‑frequency PSD. Third, the gradual noise build‑up in 1E 1048.1‑5937 demonstrates that broadband timing noise can evolve independently of obvious flux changes, making it a potentially valuable diagnostic of hidden magnetospheric or interior processes.

The study highlights the importance of low‑frequency timing analysis as a complementary tool to traditional pulse‑profile and high‑frequency timing studies. By tracking the evolution of band‑limited noise alongside flux and spectral changes, future observations can better constrain the timescales of magnetospheric relaxation, the coupling between interior dynamics and external emission, and the overall energy budget of magnetar outbursts. The authors recommend coordinated multi‑instrument campaigns that combine RXTE‑style timing with modern high‑throughput observatories (e.g., NICER, NuSTAR, and future X‑ray polarimeters) to capture both the rapid and the slow components of magnetar variability.