[vsnet-grb-info 3693] GRB 060904A Milagro GeV/TeV Observations

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5527
SUBJECT: GRB 060904A Milagro GeV/TeV Observations
DATE:    06/09/05 17:57:46 GMT
FROM:    Pablo Saz Parkinson at UCSC/Milagro  <pablo at scipp.ucsc.edu>

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 the
bright GRB 060904A detected by Swift (GCN Circ 5502, D. Rupe et al.),
during the main period of emission lasting 80s (GCN Circ 5516, H. Krimm 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 TeV < 2.9 * 10^(-7) 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 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 200 GeV < 5.5 * 10^(-5) 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.