TITLE:   GCN CIRCULAR
NUMBER:  6448
SUBJECT: GRB 070521 Milagro GeV/TeV Observations
DATE:    07/05/22 01:49:29 GMT
FROM:    Pablo Saz Parkinson at UCSC/Milagro  <pablo@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 GRB
070521 detected by Swift (GCN Circ 6431, C. Guidorzi et al.), during the
main period of emission lasting 60s (GCN Circ 6440, D. Palmer et al.).

No evidence for prompt GeV/TeV emission was found. TeV photons are
attenuated by pair production with infrared photons in intergalactic
space. We calculate an upper limit assuming two possible values for the
redshift: z=0.03 (GCN Circ 6433, E. O. Ofek et al.), and z=0.55 (GCN Circ
6444, T. Hattori et al.) using the extragalactic infrared background light
(EBL) absorption model of Primack et al. 2005 (AIP Conf. Proc. 745, p.
23).

A preliminary analysis, assuming a differential photon spectral
index of -2.4, gives upper limits on E^2dN/dE at 99% confidence of:

E^2 dN/dE at 250 GeV < 3.5 * 10^(-7) erg cm^(-2) (Assuming z=0.03)

and

E^2 dN/dE at 200 GeV < 5.5 * 10^(-6) erg cm^(-2) (Assuming z=0.55)

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.