Search for Very High Energy Emission from Gamma-Ray Bursts using Milagro

TL;DR

This study used the Milagro detector to search for very high energy gamma-ray emission from 106 GRBs detected since 2000, but found no evidence and set upper limits on such emissions.

Contribution

First comprehensive search for VHE emission from a large sample of GRBs using Milagro, providing upper limits in the 100 GeV to 100 TeV range.

Findings

No VHE emission detected from any GRB in the sample.

Set upper limits on VHE gamma-ray flux from GRBs.

Demonstrated the sensitivity of Milagro for future VHE GRB searches.

Abstract

Gamma-Ray Bursts (GRBs) have been detected at GeV energies by EGRET and models predict emission at > 100 GeV. Milagro is a wide field (2 sr) high duty cycle (> 90 %) ground based water Cherenkov detector that records extensive air showers in the energy range 100 GeV to 100 TeV. We have searched for very high energy emission from a sample of 106 gamma-ray bursts (GRB) detected since the beginning of 2000 by BATSE, BeppoSax, HETE-2, INTEGRAL, Swift or the IPN. No evidence for emission from any of the bursts has been found and we present upper limits from these bursts.

Tables

Table 1. Table 1: GRBs in the Milagro field of view (2000-2006). Column 1 is the GRB name. A superscript refers to the number of IPN error regions in the Milagro field of view. A superscript of one implies only one of two error regions fell in the Milagro field of view, while a two implies that both did, and they are listed one after the other. Column 2 gives the duration of the burst (in seconds), column 3 the zenith angle (in degrees), column 4 the measured redshift, column 5 the satellite(s) detecting the GRB, and column 6 gives the Milagro 99% confidence upper limit on the 0.2–20 TeV fluence in erg cm -2 . Numbers in bold (also labelled with a # ) take into account absorption by the EBL (using the Primack 05 model) for a redshift given in column 4. Those with three dots imply the redshifts are so high that all the emission is expected to be absorbed.

GRB	Dur.	$\theta$	z	Inst.	99% UL
000113	370	21	…	BATSE	5.5e-6
0001311	12	41	…	IPN	6.5e-7
000205	23	25	…	BSAX	6.9e-7
000206	10	39	…	BSAX	9.3e-7
000212	8	2.2	…	BATSE	1.1e-6
000220	2.4	49	…	BATSE	1.1e-5
000226	10	32	…	BATSE	3.4e-6
000226b1	94.5	32	…	IPN	7.8e-7
000301C	14	38	2.03	BATSE	…
000302	120	32	…	BATSE	6.8e-6
000314	12.8	45	…	BSAX	3.6e-5
000317	550	6.4	…	BATSE	7.9e-6
000330	${0.2}^{\ast }$	30	…	BATSE	1.0e-6
000331	55	38	…	BATSE	1.2e-5
000402	120	48	…	BSAX	4.5e-5
000408	2.5	31	…	BATSE	1.0e-6
000424	5	36	…	BATSE	7.6e-7
000508	30	34	…	BATSE	3.7e-6
0006071	0.12	42	…	IPN	4.6e-7
000615	10	39	…	BSAX	1.6e-6
000630	20	32	…	IPN	2.2e-6
0007072	18	43	…	IPN	1.9e-6
0007072	18	41	…	IPN	1.0e-6
000727	10	41	…	IPN	2.6e-6
000730	7	19	…	IPN	4.2e-7
0008211	8	27	…	IPN	6.9e-7
0008301	8	35	…	IPN	9.1e-7
000926	25	16	2.04	IPN	…
001017	10	42	…	IPN	2.2e-6
001018	31	32	…	IPN	2.1e-6
001019	10	20	…	IPN	1.1e-6
001105	30	8.5	…	IPN	1.4e-6
001204	0.44	48	…	BSAX	1.2e-5
010104	2	20	…	IPN	4.0e-7
010220	150	27	…	BSAX	2.1e-6
010613	152	25	…	IPN	2.9e-6
010706	48	37	…	IPN	2.6e-6
010903	41	49	…	IPN	2.9e-5
010921	24.6	10	0.45	HETE	2.9e-5#
011130	83.2	34	…	HETE	3.4e-6
011212	84.4	33	…	HETE	6.7e-6
020311	11.5	27	…	IPN	1.7e-7
0204292	16	39	…	IPN	4.6e-7
0204292	16	30	…	IPN	3.0e-7
020625b	125	38	…	HETE	5.7e-6
020702	26	34	…	IPN	1.4e-6
0209081	17	19	…	IPN	7.3e-7
020914	9	5.7	…	IPN	4.2e-7
021104	19.7	13	…	HETE	7.5e-7
021112	7.1	34	…	HETE	9.4e-7
021113	20	18	…	HETE	6.4e-7
021211	6	35	1.01	HETE	…
030413	15	27	…	IPN	1.0e-6
030823	56	33	…	HETE	2.8e-6

Full text

Some of the most important contributions to understanding gamma-ray bursts (GRBs) have come from observations of afterglows over a wide spectral range (van Paradijs et al. , 2000). Many GRB models predict very high energy (VHE, ¿ 100 GeV) emission from GRBs at a level comparable to that at MeV energies (e.g. (Dermer, Chiang & Mitman , 2000; Zhang & Mészáros , 2001)). EGRET detected several GRBs at energies above 100 MeV, indicating that the spectrum of GRBs extends at least out to 1 GeV, with no evidence for a spectral cut-off (Dingus , 2001). A second component was also found in one burst which extended up to at least 200 MeV and had a much slower temporal decay than the main burst (Gonzalez et al., 2003). At very high energies, there has been no conclusive emission detected for any single GRB, though a search for counterparts to 54 BATSE bursts with Milagrito, a prototype of Milagro, found evidence for emission from one burst, with an after trials significance slightly greater than 3$\sigma$ (Atkins et al., 2000a). At these high energies, gamma rays are attenuated by the redshift-dependent extra-galactic background light (EBL) (Primack et al., 2005), making the detection of VHE emission from GRBs very challenging.

A search for an excess of events above those due to the background was carried out for each of the 106 satellite-detected GRBs in our sample (see Table 1). These represent all the GRBs known to have occurred within the field of view of Milagro during its first seven years of operations (2000-2006)222GRB 060218, due to its long duration of more than 2000 s moved out of Milagro’s field of view after the start of the burst. The limit presented here is for the initial 10 s hard spike reported by the instrument team.. Milagro detected no significant emission from any of these bursts, and fluence upper limits are given in Table 1.

We acknowledge Scott Delay and Michael Schneider for their dedicated efforts in the construction and maintenance of the Milagro experiment. This work has been supported by the National Science Foundation (under grants PHY-0245234, -0302000, -0400424, -0504201, -0601080, and ATM-0002744) the US Department of Energy (Office of High-Energy Physics and Office of Nuclear Physics), Los Alamos National Laboratory, the University of California, and the Institute of Geophysics and Planetary Physics.