TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3411 SUBJECT: GRB 050509b Milagro GeV/TeV Observations DATE: 05/05/11 16:11:11 GMT FROM: Pablo Saz Parkinson at UCSC/Milagro Pablo Saz Parkinson (UC Santa Cruz) on behalf of the Milagro collaboration reports: We have searched Milagro data for emission at GeV/TeV energies from GRB 050509b (GCN Circ 3381, C. Hurkett et al.), during the burst duration (30 ms) reported by the Swift team (GCN Circ 3385, S. Barthelmy et al.). No evidence for prompt GeV/TeV emission was found. A preliminary analysis, assuming a differential photon spectral index of -2.4, gives an upper limit on E^2dN/dE at 99% confidence of: E^2dN/dE at 2.5 TeV < 5.4 * 10^(-8) erg cm^(-2) (No EBL absorption assumed) The spectrum of the host galaxy of the proposed afterglow of GRB 050509b implies a redshift of 0.225 (GCN Circ 3386, J. Bloom et al., GCN Circ 3392, J. Bloom et al.). We expect that TeV photons will be attenuated by pair production with infrared photons in intergalactic space so we also calculate upper limits assuming different extragalactic infrared background light (EBL) absorption models, a "fast evolution" model by Stecker & de Jager 1988 (A&A, 334, L85) and a model by Primack et al. 2004 (AIP Conf. Proc. 745, p. 23). We find 99% confidence level upper limits on E^2dN/dE of: E^2dN/dE at 150 GeV < 5.5 * 10^(-7) erg cm^(-2) (Stecker et al. EBL model) E^2dN/dE at 300 GeV < 2.0 * 10^(-7) erg cm^(-2) (Primack et al. EBL model) The energies quoted represent the median energy of the events that would be detected assuming a power law spectrum with differential index -2.4 convolved with each of the absorption models. These upper limits are preliminary and will be refined with further analysis. ___________________________________________________________ Santa Cruz Institute for Particle Physics (SCIPP) University of California, Santa Cruz Natural Sciences II, Room 313 1156 High Street, Santa Cruz, CA 95064 Phone: (831) 459-4870 Fax: (831) 459-5777 ___________________________________________________________