A new type of very high energy (\u3e a few 100 GeV) gamma-ray observatory, Milagro, has been built with a large field of view of \u3e1 steradian and nearly 24 hours/day operation. Milagrito, a prototype for Milagro, was operated from February 1997 to May 1998. During the summer of 1998, Milagrito was dismantled and Milagro was built. Both detectors use a 80 m×60 m×8 mpond of water in which a 3 m×3 m grid of photomultiplier tubes detects the Cherenkov light produced in the water by the relativistic particles in extensive air showers. Milagrito was smaller and had only one layer of photomultipliers, but allowed the technique to be tested. Milagrito observations of the Moon’s shadow and Mrk 501 are consistent with the Monte Carlo prediction of the telescopes parameters, such as effective area and angular resolution. Milagro is larger and consists of two layers of photomultiplier tubes. The bottom layer detects penetrating particles that are used to reject the background of cosmic-ray initiated showers

Atkins, R

Benbow, W

Berley, D

Chen, M L

Coyne, D G

Dingus, B L

Dorfan, D E

Ellsworth, R W

Evans, D

Falcone, A

Fleysher, L

Fleysher, R

Gisler, G

Goodman, J A

Haines, T J

Hoffman, C M

Hugenberger, S

Kelley, L A

Leonor, I

McConnell, Mark L

McCullough, J F

McEnery, J E

Miller, R S

Mincer, A I

Morales, M F

Nemethy, P

Ryan, James M.

Shen, B

Shoup, A

Sinnis, C

Smith, A J

Sullivan, G W

Tumer, O T

Wang, K

Wascko, M O

Westerhoff, S

Williams, D A

Yang, T

Yodh, G B

English

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositorySpace Science Center Institute for the Study of Earth, Oceans, and Space(EOS)2000Milagro: A TeV gamma-ray monitor of theNorthern Hemisphere SkyB L. DingusUniversity of UtahR AtkinsUniversity of UtahW BenbowUniversity of California - Santa CruzD BerleyUniversity of Maryland - College ParkM L. ChenUniversity of Maryland - College ParkSee next page for additional authorsFollow this and additional works at: https://scholars.unh.edu/sscPart of the Astrophysics and Astronomy CommonsThis Conference Proceeding is brought to you for free and open access by the Institute for the Study of Earth, Oceans, and Space (EOS) at University ofNew Hampshire Scholars' Repository. It has been accepted for inclusion in Space Science Center by an authorized administrator of University of NewHampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.Recommended CitationMilagro: A TeV gamma-ray monitor of the Northern Hemisphere Sky Dingus, B. L. and Atkins, R. and Benbow, W. and Berley, D. andChen, M. L. and Coyne, D. G. and Dorfan, D. E. and Ellsworth, R. W. and Evans, D. and Falcone, A. and Fleysher, L. and Fleysher, R.and Gisler, G. and Goodman, J. A. and Haines, T. J. and Hoffman, C. M. and Hugenberger, S. and Kelley, L. A. and Leonor, I. andMcConnell, M. and McCullough, J. F. and McEnery, J. E. and Miller, R. S. and Mincer, A. I. and Morales, M. F. and Nemethy, P. andRyan, J. M. and Shen, B. and Shoup, A. and Sinnis, C. and Smith, A. J. and Sullivan, G. W. and Tumer, T. and Wang, K. and Wascko, M.O. and Westerhoff, S. and Williams, D. A. and Yang, T. and Yodh, G. B., AIP Conference Proceedings, 510, 642-645 (2000),DOI:http://dx.doi.org/10.1063/1.1303280AuthorsB L. Dingus, R Atkins, W Benbow, D Berley, M L. Chen, D G. Coyne, D E. Dorfan, R W. Ellsworth, D Evans,A Falcone, L Fleysher, R Fleysher, G Gisler, J A. Goodman, T J. Haines, C M. Hoffman, S Hugenberger, L A.Kelley, I Leonor, Mark L. McConnell, J F. McCullough, J E. McEnery, R S. Miller, A I. Mincer, M F. Morales, PNemethy, James M. Ryan, B Shen, A Shoup, C Sinnis, A J. Smith, G W. Sullivan, O T. Tumer, K Wang, M O.Wascko, S Westerhoff, D A. Williams, T Yang, and G B. YodhThis conference proceeding is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ssc/140Milagro: A TeV gamma-ray monitor of the Northern Hemisphere SkyB. L. Dingus, R. Atkins, W. Benbow, D. Berley, M. L. Chen, D. G. Coyne, D. E. Dorfan, R. W. Ellsworth, D. Evans, A. Falcone, L. Fleysher, R. Fleysher, G. Gisler, J. A. Goodman, T. J. Haines, C. M. Hoffman, S. Hugenberger,L. A. Kelley, I. Leonor, M. McConnell, J. F. McCullough, J. E. McEnery, R. S. Miller, A. I. Mincer, M. F. Morales,P. Nemethy, J. M. Ryan, B. Shen, A. Shoup, C. Sinnis, A. J. Smith, G. W. Sullivan, T. Tumer, K. Wang, M. O.Wascko, S. Westerhoff, D. A. Williams, T. Yang, and G. B. Yodh  Citation: AIP Conference Proceedings 510, 642 (2000); doi: 10.1063/1.1303280 View online: http://dx.doi.org/10.1063/1.1303280 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/510?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in A “Snapshot” survey of the gamma-ray sky at GeV energies AIP Conf. Proc. 510, 611 (2000); 10.1063/1.1303275  First results of a study of TeV emission from GRBs in Milagrito AIP Conf. Proc. 510, 543 (2000); 10.1063/1.1303263  A search for gamma-ray burst optical emission with the automated patrol telescope AIP Conf. Proc. 428, 909 (1998); 10.1063/1.55461  Searches for TeV counterparts to classical gamma-ray bursts AIP Conf. Proc. 384, 603 (1996); 10.1063/1.51577  MILAGROA TeV air shower array AIP Conf. Proc. 220, 275 (1991); 10.1063/1.40309   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 15:33:45Milagro: A TeV Gamma-Ray Monitor ofthe Northern Hemisphere SkyB.L. Dingus1, R. Atkins1, W. Benbow2, D. Berley3'10,M.L. Chen3'11, D.G. Coyne2, D.E. Dorfan2, R.W. Ellsworth5,D. Evans3, A. Falcone6, L. Fleysher7, R. Fleysher7, G. Gisler8,J.A. Goodman3, T.J. Haines8, C.M. Hoffman8, S. Hugenberger4,L.A. Kelley2, I. Leonor4, M. McConnell6, J.F. McCullough2,J.E. McEnery1, R.S. Miller8'6, A.L Mincer7, M.F. Morales2,P. Nemethy7, J.M. Ryan6, B. Shen9, A. Shoup4, C. Sinnis8,A.J. Smith9, G.W. Sullivan3, T. Turner9, K. Wang9, M.O. Wascko9,S. Westerhoff2, D.A. Williams2, T. Yang2, G.B. Yodh4(1) University of Utah, Salt Lake City, UT 84112, USA(2) University of California, Santa Cruz, CA 95064, USA(3) University of Maryland, College Park, MD 20742, USA(4) University of California, Irvine, CA 92697, USA(5) George Mason University, Fairfax, VA 22030, USA(6) University of New Hampshire, Durham, NH03824, USA(7) New York University, New York, NY 10003, USA(8) Los Alamos National Laboratory, Los Alamos, NM87545, USA(9) University of California, Riverside, CA 92521, USA(10) Permanent Address: National Science Foundation, Arlington, VA ,22230, USA(11) Now at Brookhaven National Laboratory, Upton, NY 11973, USAAbstract. A new type of very high energy (> a few 100 GeV) gamma-ray observatory,Milagro, has been built with a large field of view of > 1 steradian and nearly 24hours/day operation. Milagrito, a prototype for Milagro, was operated from February1997 to May 1998. During the summer of 1998, Milagrito was dismantled and Milagrowas built. Both detectors use a 8 0 m x 6 0 m x 8 m pond of water in which a 3 m x3 m grid of photomultiplier tubes detects the Cherenkov light produced in the waterby the relativistic particles in extensive air showers. Milagrito was smaller and hadonly one layer of photomultipliers, but allowed the technique to be tested. Milagritoobservations of the Moon's shadow and Mrk 501 are consistent with the Monte Carloprediction of the telescopes parameters, such as effective area and angular resolution.Milagro is larger and consists of two layers of photomultiplier tubes. The bottomlayer detects penetrating particles that are used to reject the background of cosmic-rayinitiated showers.CP510, The Fifth Compton Symposium, edited by M. L. McConnell and J. M. Ryan© 2000 American Institute of Physics l-56396-932-7/00/$17.00642 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 15:33:45I A NEW TYPE OF TEV 7-RAY OBSERVATORYSeveral active galactic nuclei and supernova remnants have now been observed toemit TeV gamma-rays. [1], [2], [3] These TeV observations were made by detectingthe atmospheric Cherenkov light produced by the extensive air shower of particlescreated when a high energy gamma ray interacts in the atmosphere. Due to thelimitations of this detection technique, these observatories only operate on clear,moonless nights and must point at the source using a large mirror. Therefore,observations of unpredictable, short duration transients, such as gamma-ray bursts,and all sky surveys are difficult.A new type of TeV 7-ray observatory with a large field of view and continuousoperation has recently been built in the Jemez Mountains near Los Alamos, NM(106.7° W, 35.9° N, 2650 m above sea level). The observatory is called Milagro,and the prototype which operated at the same site from February 1997 to May1998 is called Milagrito [4].Both detectors used the large pond of water 80 m x 60 m x 8 m which can beseen in the photograph of Figure 1. The pond has a light-tight cover. Milagrocontains 723 photomultiplier tubes (PMTs) which are placed on a 3 m x 3 m gridin 2 layers at 1.5 m and 7 m below the surface. The prototype Milagrito had onlyone layer of 228 PMTs on a 3 m x 3 m grid spread over the smaller area of 30 m x50 m.FIGURE 1. Aerial photograph of the 60 m x 80 m x 8 m pond and cover used by Milagro andMilagrito. The pond as instrumented for Milagro contains 5 million gallons of water.An extensive air shower is detected when the relativistic particles radiateCherenkov light in the water causing several tens of PMTs to observe the lightwithin a few 100 nsec of each other. From the relative timing of the photomultipli-er tube signals, the direction of the particle or gamma-ray initiating the shower canbe determined to ~1 degree depending on the number of PMTs hit. The field ofview is such that 50% of the showers detected are within 20 degrees of zenith and643 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 15:33:4590% are within 50 degrees. Almost all of these triggers are due to the backgroundof showers that are initiated by charged cosmic rays. Monte Carlo simulations cor-rectly predict the observed rate and zenith angle distribution of cosmic-ray initiatedshowers. Simulations of 7-ray initiated showers show sensitivity to 7-rays as low as~100 GeV with the effective area increasing as E ~2, where E is the 7-ray energy,and flattening near 10 TeV.II MILAGRITO OBSERVATIONSThe performance of Milagri-to has been confirmed by obser-vations. The Moon blocks cos-mic rays and a deficit of show-ers has been detected, which isa 10 sigma deviation from thebackground (Figure 2) [5]. Theshape and size of the deficit isconsistent with the Moon's an-gular size and Milagrito's angu-lar resolution, and the deflec-tion in Right Ascension is con-sistent with the Earth's mag-netic field and the energy ofthe cosmic rays detected by Mi-lagrito.The simulations of gamma-ray initiated showers were ver-ified by Milagrito observation-s of Mrk501, an x-ray select-ed BL Lac, which was a brightTeV source during 1997. The Milagrito detection of Mrk501 shown in Figure 3 wasa 3.7 sigma deviation from the cosmic-ray background [6]. The flux and spectrumwere well measured by several atmospheric Cherenkov telescopes and the signifi-cance of the Milagrito detection agrees with this spectrum folded with the Milagritoeffective area. Simulations also indicate that the sensitivity of Milagrito was toolow to detect the Crab nebula, the standard candle of TeV astronomy.New observations have been performed by Milagrito that atmospheric Cherenkovtelescopes have not been able to do because of their low duty cycle (5-10%) andsmall field of view (2-4 degrees in radius). Specifically, an all sky survey of theNorthern Hemisphere was performed and no sources brighter than 5 times the Crabnebular flux were detected [7]. An all-sky search for 10 second duration transientswas also done and none were found [8], which can place limits on the local densityof evaporating primordial black holes. However, the most interesting results have~ 3 ~2 - 1 0 1 2(»W - WW^eosCPee^FIGURE 2. The shadow of the Moon due to the block-age of cosmic rays. The scale is in standard deviations ofthe Gaussian distributed background. The deficit is notlocated at the direction to the Moon, but is deflected inR.A. because the trajectories of the charged cosmic raysare bent by the Earth's magnetic field.644 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 15:33:45FIGURE 3. The number of showers in the vicinity of Mrk501 plotted in standard deviations ofthe background. For each position the number of showers is determined for directions within onedegree of that position; thus neighboring bins are highly correlated. The position of Mrk 501 isat the center of the plot. The excess at Mrk 501 is 3624±990 events, or 3.7 a.been from the search for TeV emission correlated with BATSE detected bursts [9]and the detection of a coronal mass ejection from the Sun [10]. These analyses wereboth reported separately at this conference.Ill MILAGRO EXPECTATIONSSimulations and preliminary data indicate that Milagro will be more than 5times more sensitive than Milagrito. The improvement comes from a combinationof factors - larger area, improved angular resolution, and cosmic-ray backgroundrejection. Milagro will be fully operational Fall 1999 with a data taking rate of2000 showers per second resulting in more than 100 GBytes of data per day.REFERENCES1. M. Catanese & T.C. Weekes, Astronomical Society of the Pacific (1999) (in press)2. C.M. Hoffman, C. Sinnis, P. Fleury, M. Punch Rev. of Mod. Phys. 71(4), 897 (1999)3. R. Ong Physics Reports 305, 93 (1998)4. R. Atkins et al., submitted to Nucl. Inst. and Methods (2000)5. M. Wascko et al., 26th ICRC Proceedings (Salt Lake City,UT) (1999)6. R. Atkins et al., Ap. J. Lett. 525, L25 (1999).7. A.J. Smith et al. AIP Proc. GeV-TeV Gamma-Ray Astrophysics- Toward a MajorAtmospheric Cherenkov Telescope VI (Snowbird, UT) (2000)8. C. Sinnis et al., 26th ICRC Proceedings (Salt Lake City,UT) (1999)9. R. S. Miller et al., AIP Proc. 5th Compton Symp. (Portsmouth,NH) (2000)10. A. Falcone et al., AIP Proc. 5th Compton Symp. (Portsmouth,NH) (2000)645 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 15:33:45

Milagro: A TeV gamma-ray monitor of the Northern Hemisphere Sky

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Milagro: A TeV gamma-ray monitor of the Northern Hemisphere Sky

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