The rotational phase dependence of magnetar bursts

Abstract

The trigger for the short bursts observed in γ-rays from many magnetar sources remains unknown. One particular open question in this context is the localization of burst emission to a singular active region or a larger area across the neutron star. While several observational studies have attempted to investigate this question by looking at the phase dependence of burst properties, results have been mixed. At the same time, it is not obvious a priori that bursts from a localized active region would actually give rise to a detectable phase-dependence, taking into account issues such as geometry, relativistic effects, and intrinsic burst properties such brightness and duration. In this paper we build a simple theoretical model to investigate the circumstances under which the latter effects could affect detectability of a dependence of burst emission on rotational phase. We find that even for strongly phase-dependent emission, inferred burst properties may not show a rotational phase dependence depending on the geometry of the system and the observer. Furthermore, the observed properties of bursts with durations short as 10-20% of the spin period can vary strongly depending on the rotational phase at which the burst was emitted. We also show that detectability of a rotational phase dependence depends strongly on the minimum number of bursts observed, and find that existing burst samples may simply be too small to rule out a phase dependence.

NICER View of the 2020 Burst Storm and Persistent Emission of SGR 1935+2154

“…Finally, the lack of dependency of the burst peak arrival times with phase is also consistent with the majority of magnetar sources (e.g., Scholz & Kaspi 2011); see also the literature survey in Elenbaas et al (2018). This implies that magnetar bursts occur approximately randomly in magnetic colatitudes in the magnetosphere, perhaps close to the stellar surface.…”

Burst Phase Distribution of SGR J1935+2154 Based on Insight-HXMT

“…To date, except for the confirmed alignment phenomenon during the outburst of XTE J1810-197 (Woods et al 2008), the alignment trend found in other magnetar reports during flux enhancement has been found to be inconclusive after in-depth study (Göğüş et al 2017). According to Elenbaas et al (2018) simulations, burst phase dependence is often affected by a number of external factors, . The hardness ratio of the high energy 30-250 keV to the medium energy 10-30 keV of the Insight-HXMT bursts.…”

“…In addition to this tendency of the burst start times, the photon arrival times of each burst have a more obvious concentrated structure align with the 197 (Woods et al 2008b), the alignment trend found in other magnetar reports during flux enhancement has been found to be inconclusive after in-depth study (Gögüs ¸et al 2017). According to (Elenbaas et al 2018) simulations, burst phase dependence is often affected by a number of external factors, such as observer angle and the location of the radiation area, beam bunching, and also requires a sufficient number of bursts to ensure complete sampling. When the burst aligns near the maximum of the X-ray pulse profile, if it is a thermal burst, it may be generated from the surface of the neutron star.…”

Repeating Fast Radio Bursts from Magnetars with Low Magnetospheric Twist

“…Moreover, evidence for a spin-phase dependence of short bursts is generally weak (e.g., Collazzi et al 2015); the duty cycle of bursts over a spin period can be broad and weakly varying over a known rotational ephemeris. If the bursts are intrinsically beamed, strong evidence for phase dependence is not expected to emerge without significantly larger samples of bursts (Elenbaas et al 2018) owing to the large separation of timescales between the short burst durations and long spin period. The inverse problem of establishing periodicity from burst arrivals would clearly be challenging for a limited collection of bursts.…”