Browsing by Author "Watts, A.L."

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    SGR J1550–5418 BURSTS DETECTED WITH THE FERMI GAMMA-RAY BURST MONITOR DURING ITS MOST PROLIFIC ACTIVITY
    (The American Astronomical Society, 2012) Van Der Horst, A.J.; Kouveliotou, C.; Gorgone, N.M.; Kaneko, Y.; Baring, M.G.; Guiriec, S.; Gogus, E.; Granot, J.; Watts, A.L.; Lin, L.; Bhat, P.N.; Bissaldi, E.; Chaplin, V.L.; Finger, M.H.; Gehrels, N.; Gibby, M.H.; Giles, M.M.; Goldstein, A.; Gruber, D.; Harding, A.K.; Kaper, L.; Von Kienlin, A.; Van Der Klis, M.; McBreen, S.; Mcenery, J.; Meegan, C.A.; Paciesas, W.S.; Pe'er, A.; Preece, R.D.; Ramirez-Ruiz, E.; Rau, A.; Wachter, S.; Wilson-Hodge, C.; Woods, P.M.; Wijers, R.A.M.
    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 Epeak 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.
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    Time resolved spectroscopy of SGR J1550−5418 bursts detected with Fermi/gamma-ray burst monitor
    (The American Astronomical Society, 2014) Younes, G.; Kouveliotou, C.; van der Horst, A.J.; Baring, M.G.; Granot, J.; Watts, A.L.; Bhat, P.N.; Collazzi, A.; Gehrels, N.; Gorgone, N.; Gogus, E.; Gruber, D.; Grunblatt, S.; Huppenkothen, D.; Kaneko, Y.; von Kienlin, A.; van der Klis, M.; Lin, L.; Mcenery, J.; van Putten, T.; Wijers, R.A.M.J.
    We report on a time-resolved spectroscopy of the 63 brightest bursts of SGR J1550–5418, detected with the Fermi/Gamma-ray Burst Monitor during its 2008-2009 intense bursting episode. We performed spectral analysis down to 4 ms timescales 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 the flux reaching ~1 at the highest fluxes. Using a two blackbody 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 the lines of constant luminosity at the lowest fluxes, R 2vpropkT –4, with a break at the higher fluxes (F > 10–5.5 erg s–1 cm–2). The area of the high-kT component increases with the flux while its temperature decreases, which we interpret as being due to an adiabatic cooling process. The area of the low-kT component, on the other hand, appears to saturate at the highest fluxes, toward R max ≈ 30 km. Assuming that crust quakes are responsible for soft gamma repeater (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 we put a lower limit on the internal magnetic field of SGR J1550–5418, B int gsim 4.5 × 1015 G.