Fermi/GAMMA-RAY BURST MONITOR OBSERVATIONS OF SGR J0501+4516 BURSTS

Abstract

We present our temporal and spectral analyses of 29 bursts from SGR J0501+4516, detected with the gamma-ray burst monitor on board the Fermi Gamma-ray Space Telescope during 13 days of the source's activation in 2008 (August 22-September 3). We find that the T 90 durations of the bursts can be fit with a log-normal distribution with a mean value of ∼123 ms. We also estimate for the first time event durations of soft gamma repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T 90 values estimated in count space (following a log-normal distribution with a mean value of ∼124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two blackbody functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that E peak decreases with energy flux (and fluence) to a minimum of ∼30 keV at F = 8.7 × 10 −6 erg cm −2 s −1 , increasing steadily afterward. Two more sources exhibit a similar trend: SGRs J1550-5418 and 1806-20. The isotropic luminosity, L iso , corresponding to these flux values is roughly similar for all sources (0.4-1.5 × 10 40 erg s −1 ).

THE FERMI GBM GAMMA-RAY BURST SPECTRAL CATALOG: THE FIRST TWO YEARS

“…Their νF ν energy spectra typically peak around 40 keV with a very steep decline in flux above the peak. Analysis of all the SGR bursts detected by GBM is currently underway (see Lin et al 2011;A. J. van der Horst et al 2012, in preparation;A.…”

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

“…We calculate these quantities using TTE data binned at 2 ms resolution in the 8-100 keV energy range. We apply a two-step background subtraction method to the total number of counts during and around the burst time interval; details on the particular method used here are given in Göǧüş et al (2001) and Lin et al (2011).…”

“…Next, we used the T 90,50 values and their uncertainties to construct their probability density functions (PDFs) as described in Lin et al (2011). For each individual PDF, we adopted a twosided normal distribution with the widths given by the measured (asymmetric) uncertainties (Starling et al 2008).…”

“…The histograms shown in Figure 2 for the various temporal parameters are typical of magnetar bursts, although there may be variations between different sources (e.g., Göǧüş et al 2001;Gavriil et al 2004;Lin et al 2011). The mean T 90 values in the GBM energy band typically range between 0.1 and 0.2 s, while τ 90 ranges are 0.05-0.1 s, resulting in duty cycles of 0.4-0.6 (see also Lin et al 2011).…”

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).…”