{
  "bibcode": "2009GCN..9237....1M",
  "body": "R. Margutti (Univ. Bicocca & INAF-OAB), C. Guidorzi (Univ. Ferrara),\nJ. Mao (INAF-OAB)  and J. K. Cannizzo (NASA/UMBC) report on behalf of\nthe Swift-XRT team:\n\nWe have analysed the first 2 orbits of Swift-XRT data obtained for\nGRB 090424 (Cannizzo et al. GCN Circ. 9223), covering 1732 s of\nWindowed Timing (WT) and 1329 s of Photon Counting (PC) mode data,\nrespectively, between 88 s and 8 ks after the trigger.\nThe UVOT-enhanced XRT position was given by Goad et al. in GCN Circ. 9232.\n\nThe light-curve can be modelled by a double broken power-law, with\na first decaying index alpha_1= 1.29 +/- 0.05 and a first break time\nof about 260 s. The decay then flattens to alpha_2=  0.74  +/- 0.02.\nAfter 1450 s the decay is best modelled by a power-law index\nalpha_3= 1.15 +/- 0.05.\n\nThe very good statistics allows us to observe spectral evolution\nin WT data before the first break, with the simple power law photon index\nevolving from 2.6 to 2.1 . A spectrum extracted from WT mode data in the\ntime interval 0.4-1.4 ks can be modelled with an absorbed\npower-law with a photon index Gamma= 2.13 +/- 0.05 and best-fitting\nabsorption column NH = (0.20 +/- 0.01) x10^22 cm^-2, in excess of the \nGalactic\nvalue of 1.9 x 10^20 cm^-2 (Kalberla et al. 2005).\n\nThe spectrum extracted from PC data in the time interval 6.1-8 ks\ncan be modelled with an absorbed power-law, with Gamma = 2.06 +/- 0.09\nand a column density of NH = (0.27 +/- 0.03)x10^22 cm^-2.\nThe observed (unabsorbed) 0.3 -10 keV flux over this time interval is\n4.2x10^-11 (6.7x10^-11) erg cm^-2 count^-1.\nUncertainties are given at 90% confidence.\n\nIf the light-curve continues to decay with alpha ~1.15, the count rate 24\nhours after the burst is estimated to be 0.10 count s^-1, which\ncorresponds to an observed (unabsorbed) flux of 4.2x10^-12\n(6.7x10^-12) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00350311.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 9237,
  "createdOn": 1240614947000,
  "email": "raffaella.margutti@brera.inaf.it",
  "subject": "GRB 090424: Swift-XRT refined analysis",
  "submitter": "Raffaella Margutti at U. di Milano Bicocca  <raffaella.margutti@brera.inaf.it>",
  "eventId": "GRB 090424"
}