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INTEGRAL IBIS catalog of magnetar bursts

Dominik P. Pacholski, Sandro Mereghetti, Martin Topinka

TL;DR

This study presents a comprehensive search of two decades of INTEGRAL/IBIS data to assemble a uniform catalog of magnetar bursts, yielding 1349 confirmed bursts from 21 magnetars. Burst durations follow a log-normal distribution centered near $10^{-1}$ s, while fluences obey power-law distributions with slopes around $0.7$–$1$, suggesting self-organized critical energy release. Spectral modeling shows most bursts are well described by a cutoff power law with $E_{ m peak}$ typically in the $20$–$60$ keV range, with 1E 1547--5408 exhibiting harder spectra ($E_{ m peak}\,\,\sim\,35$–$100$ keV). Time-resolved spectroscopy for bright bursts reveals limited evolution, except for the SGR 1935+2154 event associated with FRB-like radio emission, which shows notable spectral changes; moreover, SGR 1806--20 shows a clear anti-correlation between $E_{ m peak}$ and fluence. These results, enabled by the imaging capability of ISGRI, provide important constraints on magnetar emission mechanisms and the population statistics of magnetar bursts across multiple sources and activity states.

Abstract

One of the distinctive properties of magnetars, young neutron stars powered mainly by magnetic energy, is the emission of short ($\lesssim$1 s) bursts of hard X-rays. Such bursts have been observed in nearly all the known magnetars, although at different and time-variable rates of occurrence. In the last two decades, the INTEGRAL satellite has extensively covered with good imaging capabilities the Galactic plane, where most magnetars reside. We present the results of a comprehensive search for magnetar bursts in more than twenty years of archival data of the INTEGRAL IBIS instrument (15 keV - 1 MeV). This led to the detection of 1349 bursts with 30-150 keV fluence in the $\sim2\times10^{-9} - 3\times10^{-6}$ erg cm$^{-2}$ range from 21 of the 34 examined magnetars and candidate magnetars with well known positions. The durations of the bursts, in terms of $T_{90}$, follow a lognormal distribution centered at $\sim0.1$ s. Most of the detected bursts originated from three particularly active sources: 1E 1547-5408, SGR 1806-20, and SGR 1935+2154. The integral distributions of their burst fluences follow power laws with slopes $β$= 0.76$\pm$0.04, 0.95$\pm$0.06, and 0.92$\pm$0.10, respectively. The burst spectra are generally well fit with an exponentially cut-off power law with peak energy $E_{peak}$ in the range $\sim20-60$ keV for SGR 1806-20 and SGR 1935+2154, while the bursts of 1E 1547-5408 are slightly harder ($E_{peak}\sim35-100$ keV). A significant anti-correlation between $E_{peak}$ and fluence is found for SGR 1806-20, which provided the largest number of bursts among the sources of our sample.

INTEGRAL IBIS catalog of magnetar bursts

TL;DR

This study presents a comprehensive search of two decades of INTEGRAL/IBIS data to assemble a uniform catalog of magnetar bursts, yielding 1349 confirmed bursts from 21 magnetars. Burst durations follow a log-normal distribution centered near s, while fluences obey power-law distributions with slopes around –, suggesting self-organized critical energy release. Spectral modeling shows most bursts are well described by a cutoff power law with typically in the – keV range, with 1E 1547--5408 exhibiting harder spectra (– keV). Time-resolved spectroscopy for bright bursts reveals limited evolution, except for the SGR 1935+2154 event associated with FRB-like radio emission, which shows notable spectral changes; moreover, SGR 1806--20 shows a clear anti-correlation between and fluence. These results, enabled by the imaging capability of ISGRI, provide important constraints on magnetar emission mechanisms and the population statistics of magnetar bursts across multiple sources and activity states.

Abstract

One of the distinctive properties of magnetars, young neutron stars powered mainly by magnetic energy, is the emission of short (1 s) bursts of hard X-rays. Such bursts have been observed in nearly all the known magnetars, although at different and time-variable rates of occurrence. In the last two decades, the INTEGRAL satellite has extensively covered with good imaging capabilities the Galactic plane, where most magnetars reside. We present the results of a comprehensive search for magnetar bursts in more than twenty years of archival data of the INTEGRAL IBIS instrument (15 keV - 1 MeV). This led to the detection of 1349 bursts with 30-150 keV fluence in the erg cm range from 21 of the 34 examined magnetars and candidate magnetars with well known positions. The durations of the bursts, in terms of , follow a lognormal distribution centered at s. Most of the detected bursts originated from three particularly active sources: 1E 1547-5408, SGR 1806-20, and SGR 1935+2154. The integral distributions of their burst fluences follow power laws with slopes = 0.760.04, 0.950.06, and 0.920.10, respectively. The burst spectra are generally well fit with an exponentially cut-off power law with peak energy in the range keV for SGR 1806-20 and SGR 1935+2154, while the bursts of 1E 1547-5408 are slightly harder ( keV). A significant anti-correlation between and fluence is found for SGR 1806-20, which provided the largest number of bursts among the sources of our sample.

Paper Structure

This paper contains 15 sections, 1 equation, 6 figures.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Data analysis
  4. Burst Search
  5. Timing Analysis
  6. Spectral analysis
  7. Results
  8. Burst durations
  9. Burst spectra
  10. Log N -- Log S
  11. Discussion
  12. 1E 1547--5408
  13. SGR 1806--20
  14. SGR 1935+2154
  15. Conclusions

Figures (6)

  • Figure 1: Time distribution of the bursts (blue histograms) and exposure time (red histograms) of 1E 1547--5408 (upper), SGR 1806--20 (middle), and SGR 1935+2154 (bottom). The bursts are in bins of 30 days, while the exposure is for bins of 180 days.
  • Figure 2: Top panels: Distributions of the burst durations fitted with lognormal functions for 1E 1547--5408 (left), SGR 1806--20 (middle), and SGR 1935+2154 (right). Solid lines refer to $T_{\mathrm{BB}}$ and dashed lines to $T_{90}$. Bottom panels: $T_{\mathrm{BB}}$ duration as a function of the number of counts for the same three magnetars.
  • Figure 3: $T_{90}$ versus $T_{\mathrm{BB}}$ of all the bursts for which $T_{90}$ could be estimated.
  • Figure 4: E$_{peak}$ versus $\alpha$ (upper panels) and versus fluence (bottom panels) for the spectra fitted by the cut-off power law model. The figure show results for three sources: 1E 1547--5408 (left), SGR 1806--20 (middle), and SGR 1935+2154 (right). The fluence refers to the 30--150 keV range. The 2020 April 28 burst from SGR 1935+2154 with associated FRB-like emission is indicated in red.
  • Figure 5: Time resolved spectral analysis of one burst (n. 193) from SGR 1806--20 and four bursts from SGR 1935+2154. The upper panel in each figure shows the background subtracted light curve of the burst, while the bottom panel shows the time evolution of $E_{peak}$. The gaps in the light curve of burst n. 193 are due to telemetry saturation.
  • ...and 1 more figures