SGR J1550–5418 BURSTS DETECTED WITH THEFERMIGAMMA-RAY BURST MONITOR DURING ITS MOST PROLIFIC ACTIVITY

Abstract

We have performed detailed temporal and time-integrated spectral analysis of 286 bursts from SGR J1550−5418 detected with the Fermi Gamma-ray Burst Monitor (GBM) in 2009 January, resulting in the largest uniform sample of temporal and spectral properties of SGR J1550−5418 bursts. We have used the combination of broadband and high time-resolution data provided with GBM to perform statistical studies for the source properties. We determine the durations, emission times, duty cycles, and rise times for all bursts, and find that they are typical of SGR bursts. We explore various models in our spectral analysis, and conclude that the spectra of SGR J1550−5418 bursts in the 8-200 keV band are equally well described by optically thin thermal bremsstrahlung (OTTB), a power law (PL) with an exponential cutoff (Comptonized model), and two blackbody (BB) functions (BB+BB). In the spectral fits with the Comptonized model, we find a mean PL index of −0.92, close to the OTTB index of −1. We show that there is an anti-correlation between the Comptonized E peak and the burst fluence and average flux. For the BB+BB fits, we find that the fluences and emission areas of the two BB functions are correlated. The low-temperature BB has an emission area comparable to the neutron star surface area, independent of the temperature, while the hightemperature BB has a much smaller area and shows an anti-correlation between emission area and temperature. We compare the properties of these bursts with bursts observed from other SGR sources during extreme activations, and discuss the implications of our results in the context of magnetar burst models.

BURST FLUENCE DISTRIBUTIONS OF SOFT GAMMA REPEATERS 1806–20 AND 1900+14 IN THEROSSI X-RAY TIMING EXPLORERPCA ERA

“…They are sometimes found in young supernova remnants and are characterized by recurrent emission of short duration, ∼0.1 s (Kouveliotou 1995), bursts of soft gamma-rays/hard X-rays with spectra described by optically thin thermal bremsstrahlung (OTTB) at kT ∼ 20-40 keV (Goǧus et al 2001). Recent studies at lower energies, <15 keV, have found that an OTTB model is no longer accurately fitted to the spectra and there is a rollover which can be best fitted with two blackbodies or alternatively a cutoff power law (Feroci et al 2004;Olive et al 2004;Nakagawa et al 2007;Israel et al 2008;Lin et al 2011;van der Horst et al 2012). There is evidence that these bursts are the result of starquakes that occur when the NS crust fractures due to a buildup of stress associated with the evolution of the strong magnetic field, greater than 10 14 G (Thompson & Duncan 1995).…”

Ionospheric response to magnetar flare: signature of SGR J1550–5418 on coherent ionospheric Doppler radar

“…This activity was observed by several high-energy missions, creating a good opportunity for investigating the broadband spectra of magnetar short bursts and intermediate flares in detail. Broadband spectral properties have been reported by several authors (e.g., van der Horst et al, 2012;Lin et al, 2012;Younes et al, 2014). …”

“…In early October 2008, both the Swift Burst Alert Telescope (BAT) and the Fermi Gamma Ray Burst Monitor (GBM) were triggered by numerous bursts from the source SGR 1550-5418 (Israel et al, 2010;von Kienlin et al, 2012). The source entered a second, even more active phase on 22 January 2009, during which a large number of bursts were observed by several satellites, as detected by Swift (Gronwall et al, 2009), Fermi GBM (Connaughton andBriggs, 2009;von Kienlin and Connaughton, 2009), Konus-Wind (Golenetskii et al, 2009), and RHESSI (Bellm et al, 2009).…”

“…Soft X-ray emissions from solar flares are the more common sources of ionospheric disturbances, which can be monitored using the very low frequency (VLF) technique (Bracewell and Straker, 1949;Thomson et al, 2005;Pacini and Raulin, 2006;Raulin et al, 2006Raulin et al, , 2010. In addition to these solarterrestrial events, the lower ionosphere is also affected by high-energy photons (X-rays and gamma rays) from extraterrestrial sources like gamma ray burst (GRB) and SGR (Inan et al, 1999(Inan et al, , 2007Tanaka et al, 2010).…”

Broadband Spectral Investigations of Magnetar Bursts

“…Consequently, the peak energy parameter of the Comptonized (often labeled as COMPT) model is interpreted in relation to the electron temperature. Time integrated spectral analysis of nearly 300 bursts from SGR J1550−5418 result in an average power law photon index of −0.92 and peak energy (E peak ) is typically around 40 keV (van der Horst et al 2012). …”