[vsnet-grb-info 2428] GRB 051109A Milagro GeV/TeV Observations

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4265
SUBJECT: GRB 051109A Milagro GeV/TeV Observations
DATE:    05/11/11 22:18:05 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 GRB
051109A (GCN Circ 4213, G. Tagliaferri et al.), during the burst duration
(36 s) reported by the Swift team (GCN Circ 4217, E. Fenimore 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 < 8.4 * 10^(-8) erg cm^(-2) (No EBL absorption assumed)

The spectrum of the host galaxy of the proposed afterglow of GRB 051109A
implies a redshift of 2.346 (GCN Circ 4221, R. Quimby et al.). TeV photons
are 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, one by Kneiske et al.
2004 (A&A 413, 807) and one by 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 55 GeV < 1.7 * 10^(-3) erg cm^(-2) (Primack et al. EBL model)
E^2dN/dE at 60 GeV < 2.3 * 10^(-4) erg cm^(-2) (Kneiske 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 each of the absorption models. These upper
limits are preliminary and will be refined with further analysis.