GCN Circular 5061

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 060427B (GCN Circ 5030
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 , K. Hurley et al.), during the burst
duration (0.2 s) reported by the IPN team.
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 TeV < 4.8 * 10^(-8) erg cm^(-2) (No EBL absorption)

TeV photons are attenuated by pair production with infrared photons in
intergalactic space. We calculate an upper limit assuming a
redshift of 0.5 (a value representative of short/hard bursts) using the
extragalactic infrared background light (EBL) absorption model of
Primack et al. 2005 (AIP Conf. Proc. 745, p. 23).
We find 99% confidence level upper limits on E^2dN/dE of:

E^2dN/dE at 150 GeV < 5.1 * 10^(-6) erg cm^(-2) (Primack et al. EBL model)

The energies quoted represent the approximate median energy of the
events that would be detected assuming a power law spectrum with
differential index -2.4 convolved with the absorption model.
These upper limits are preliminary and will be refined with further
analysis.