GCN Circular 3411

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.


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