GRB 050911: A black hole-neutron star merger or a naked GRB

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 nondetections of a gamma-ray burst (GRB) afterglow at early times. The g-ray light curve shows at least three peaks, the first two of which (∼ and s, where T 0.8 T 0.2 0 0 is the trigger time) were short, each lasting 0.5 s. This was followed by later emission 10–20 s postburst. The T0 upper limit on the unabsorbed X-ray flux was ergs cm 2 s 1 (integrating 46 ks of data taken between 14 1.7 # 10 September 11 and 18), 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 p (5 j limit). We speculate that GRB 050911 may have been formed through a compact object (black hole– 24.0 neutron star) merger, with the later outbursts due to a longer disk 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 nonexistent, forward shock—the so-called “naked GRB” model. Subject heading: gamma rays: bursts