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Post Info TOPIC: GRB050911


Posts: 131433
RE: GRB050911

Title: GRB 050911: a black hole - neutron star merger or a naked GRB
Authors: K.L. Page (1), A.R. King (1), A.J. Levan (2), P.T. O'Brien (1), J.P Osborne (1), S.D. Barthelmy (5), A.P. Beardmore (1), D.N. Burrows (3), S. Campana (4), N. Gehrels (5), J. Graham (6), M.R. Goad (1), O. Godet (1), Y. Kaneko (7), J.A. Kennea (3), C.B. Markwardt (5), D.E. Reichart (8), T. Sakamoto (5), N.R. Tanvir (2) ((1) University of Leicester; (2) University of Hertfordshire; (3) PSU; (4) Osservatorio di Brera, Merate; (5) GSFC; (6)STScI; (7) NSSTC; (8) University of North Carolina)

GRB 050911, discovered by the Swift Burst Alert Telescope, was not seen 4.6 hr later by the Swift X-ray Telescope, making it one of the very few X-ray non-detections of a Gamma-Ray Burst (GRB) afterglow at early times.
The gamma-ray light-curve shows at least three peaks, the first two of which (~T_0 - 0.8 and T_0 + 0.2 s, where T_0 is the trigger time) were short, each lasting 0.5 s. This was followed by later emission 10-20 s post-burst. The upper limit on the unabsorbed X-ray flux was 1.7 x 10^-14 erg cm^-2 s^-1 (integrating 46 ks of data taken between 11 and 18 September), indicating that the decay must have been rapid.
All but one of the long bursts detected by Swift were above this limit at ~4.6 hr, whereas the afterglows of short bursts became undetectable more rapidly. Deep observations with Gemini also revealed no optical afterglow 12 hr after the burst, down to r=24.0 (5-sigma limit).
The researchers speculate that GRB 050911 may have been formed through a compact object (black hole-neutron star) merger, with the later outbursts due to a longer disc lifetime linked to a large mass ratio between the merging objects. Alternatively, the burst may have occurred in a low-density environment, leading to a weak, or non-existent, forward shock - the so-called 'naked GRB' model.

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Posts: 131433

The Swift Satellite has detected long burst on the 5th September.

At 15:59:34 UT, the Swift-BAT triggered and located GRB050911 (trigger=154630).
The BAT on-board calculated position(2000) is RA, 13.692, Dec -38.861, with an uncertainty of 3 arcmin (radius, 90% containment, stat+sys). The BAT light curve shows multi- peaks with short peaks (less than 500 msec) at T-1 and T+0 seconds, and a broader peak between T+10 to T+20 seconds. The peak count rate was about 1000 counts/sec (15-350 keV), at the time of the trigger.

Due to the Earth limb observing constraint, the spacecraft could not slew immediately. The burst became visible to the Narrow Field Instruments at 16.52 UT, at which point XRT data was obtained.

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