Fermi/Gamma-ray Burst Monitor detection of SGR J1550-5418

TL;DR

This paper reports on Fermi/GBM observations of SGR J1550-5418, revealing hundreds of bursts, a unique enhanced emission with pulsations, spectral features, and a very small hot spot on the neutron star surface.

Contribution

First detailed analysis of SGR J1550-5418's burst activity and persistent emission using Fermi/GBM data, including spectral and timing properties and hot spot size estimation.

Findings

Detected ~450 bursts during peak activity

Identified a 150-s enhanced emission with pulsations

Measured the smallest hot spot for a magnetar

Abstract

SGR J1550-5418 exhibited three active bursting episodes in 2008 October and in 2009 January and March, emitting hundreds of typical Soft Gamma Repeater (SGR) bursts in soft gamma-rays. The second episode was especially intense, and our untriggered burst search on Fermi/GBM data (8-1000 keV) revealed ~450 bursts emitted over 24 hours during the peak of this activity. Using the GBM data, we identified a ~150-s-long enhanced persistent emission during 2009 January 22 that exhibited intriguing timing and spectral properties: (i) clear pulsations up to ~110 keV at the spin period of the neutron star, (ii) an additional (to a power-law) blackbody component required for the enhanced emission spectra with kT ~ 17 keV, (iii) pulsed fraction that is strongly energy dependent and highest in the 50-74 keV energy band. A total isotropic-equivalent energy emitted during this enhanced emission is estimated to be 2.9 x 10^{40}(D/5 kpc)^2 erg. The estimated area of the blackbody emitting region of ~0.046(D/5 kpc)^2 km^2 is the smallest "hot spot" ever measured for a magnetar and most likely corresponds to the size of magnetically-confined plasma near the neutron star surface.