TIME RESOLVED SPECTROSCOPY OF SGR J1550–5418 BURSTS DETECTED WITHFERMI/GAMMA-RAY BURST MONITOR

Abstract

We report on time-resolved spectroscopy of the 63 brightest bursts of SGR J1550-5418, detected with Fermi/Gamma-ray Burst Monitor during its 2008-2009 intense bursting episode. We performed spectral analysis down to 4 ms time-scales, to characterize the spectral evolution of the bursts. Using a Comptonized model, we find that the peak energy, E peak , anti-correlates with flux, while the low-energy photon index remains constant at ∼ −0.8 up to a flux limit F ≈ 10 −5 erg s −1 cm −2 . Above this flux value the E peak −flux correlation changes sign, and the index positively correlates with flux reaching ∼1 at the highest fluxes. Using a two black-body model, we find that the areas and fluxes of the two emitting regions correlate positively. Further, we study here for the first time, the evolution of the temperatures and areas as a function of flux. We find that the area−kT relation follows lines of constant luminosity at the lowest fluxes, R 2 ∝ kT −4 , with a break at higher fluxes (F > 10 −5.5 erg s −1 cm −2 ). The area of the high−kT component increases with flux while its temperature decreases, which we interpret as due to an adiabatic cooling process. The area of the low−kT component, on the other hand, appears to saturate at the highest fluxes, towards R max ≈ 30 km. Assuming that crust quakes are responsible for SGR bursts and considering R max as the maximum radius of the emitting photon-pair plasma fireball, we relate this saturation radius to a minimum excitation radius of the magnetosphere, and put a lower limit on the internal magnetic field of SGR J1550-5418, B int 4.5 × 10 15 G.

Properties of Konus-Wind SGR bursts

“…For SGR1806-20 we find that the temperatures of the low-kT BB vs highkT BB are best fit with a broken power law (BPL) with the break located at ∼ 4 and 11 keV. This result is consistent with the time-resolved spectroscopy of GBM bursts from SGR 1550-5418 [10]. Other SGRs distributions do not require BPL and show only a positive correlation between the low-and high-kTs.…”

Broadband Spectral Investigations of Magnetar Bursts

“…However, we note that the flux range analyzed by Younes et al 2014 for SGR J1550−5418 (above 10 −6.5 erg/cm 2 /s) is much higher from the flux range covered in our investigation (∼ 10 −9 − 10 −8 erg/cm 2 /s). It is possible that the area vs. kT behavior of the two blackbody components differ more significantly with increasing flux than previously discussed or that the relationship differs between sources or burst episodes.…”

“…The dashed lines represent the R 2 ∝ T −3 trend proposed in previous studies (e.g. Israel et al 2008;Younes et al 2014) and the solid line represents the R 2 ∝ T −4 trend that is expected from blackbody emission. These trends are drawn for comparison only.…”

Scale-invariance in soft gamma repeaters

“…Due to the lack of bursting behavior, SGR J1550-5418 was initially classified as an AXP [17]. In 2008 the source began its burst-active episodes and hundreds of bursts have been observed by several instruments, including the Swift Burst Alert Telescope (BAT) and Fermi Gamma-ray Burst Monitor (GBM) [18][19][20][21][22]. The source shares SGRlike behavior with SGRs 1806-20, 1900+14 and 1627-41.…”