The search for VHE emission from satellite-triggered GRBs with Milagro Pablo Saz Parkinson University of California, Santa Cruz LANL Collaboration Meeting, 2 June 2005

What we would like in a GRB: ● Low z (< 0.5) [1] ● Good zenith angle (< 45 degrees) [2] ● Bright (Fluence > 1 x erg cm -2 ) [3]

What we get: – 25 in (10 from BATSE alone) – 17 of these well localised – 3 with redshifts (2.03, 2.04, 0.45) – 1 GRB (010921) at good zenith angle (~ 10 degrees), relatively low z (0.45), but relatively faint (10 -6 erg cm -2 between 8-85 keV) Milagro upper limits 2.9 (5.8) x erg cm -2 Paper submitted to ApJ

GRBs in Milagro FOV since (19) in (6) in (8) in (21) so far (5/31) in 2005 ● GRBs since 2002: – New trigger – Outriggers

The Swift Era: 2005 (up to 5/31) ● In 5 months we have a sample of 21 GRBs (~ 50 GRBs/year), compared to 33 GRBs for the previous 3 years (~ 11 GRBs/year) ● Only 5 with redshift: 3.24, 3.793, 4.3 at 45 deg ● From we have 6 or 7 more with redshift: 1.01 and at 45 deg.

A few interesting candidates ● GRB a -> Brightest ● Very long and bright (10 -4 erg cm keV) ● Good zenith angle (26.9 degrees) ● No redshift ● 520 s 816 events (766 Bckgd) -> 1.7 sigma ● GRB > Closest z=0.859 ● -> sigma

GRB b (GCN Circular 3411) ● Very good zenith angle (10 degrees) but very faint (2.3x10 -8 erg cm keV), ● redshift ~ 0.2 (some controversy) ● E TeV < 5.4 x erg cm -2 (No EBL absorption assumed) ● E GeV < 5.5 x erg cm -2 (Stecker et al. EBL model) ● E GeV < 2.0 x erg cm -2 (Primack et al. EBL model)

A different kind of burst: 27 Dec 2004 SGR Giant flare ● Very close (z=0) and Very bright( erg cm -2 ), but very bad zenith angle (68 degrees) ● Brightest transient event ever recorded ● Effective area of Milagro is ~ 0.5 m 2 ● Our upper limit is ~ 6 x erg cm -2 ● Preparing draft of paper

The 27 Dec 2004 SGR Flare

Conclusions ● No VHE emission detected from GRBs ● Need to run for 2 more years to get another ~ 100 GRBs in our FOV, more than doubling our current sample. ● Hopefully in that time we’ll get a bright, nearby GRB at a good zenith angle.

To do: ● Finish computing effective area at angles greater than 45 degrees. ● Search for emission for GRBs at high zenith angles. ● Write ICRC paper by June 30 including preliminary analysis of all GRBs through June ● Submit SGR draft.