Noyes, D

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29th International Cosmic Ray Conference Pune (2005) 4, 463466A Search for Short Duration VHE Emission from GRBs with MilagroDavid Noyes for the Milagro CollaborationPresenter: David Noyes (dnoyes@umdgrb.umd.edu), usa-noyes-D-abs1-og24-oralMilagro is a water-Cherenkov detector capable of observing air showers produced by gamma rays with primaryenergies of approximately 100 GeV and higher. The wide field of view (   2 sr) and high duty cycle (  90%)of Milagro make it ideal for searching for transient emission from gamma-ray bursts (GRBs). In the absenceof a GRB localization provided by another instrument, the Milagro data is searched independently for VHEemission from GRBs. In 2.3 years of searching for bursts with durations ranging from 	 to  , nosignificant evidence was observed for VHE emission from GRBs. Models for different GRB parameters (suchas redshift and isotropic energy distributions) are used to constrain the VHE spectrum of GRBs.1. IntroductionMilagro [1, 2] is a water-Cherenkov detector located in the Jemez Mountains near Los Alamos, New Mex-ico, and is capable of detecting air showers induced by cosmic rays and gamma rays with primary energiesabove   100 GeV. The primary detector consists of a 6-million gallon artificial pond instrumented with 723photomultiplier tubes (PMTs), arranged in two layers. In addition to this, the pond is surrounded by an arrayof 175 individual Cherenkov detectors (“outriggers”). Each outrigger consists of a 1500 gallon tank of water,measuring 2.4 m in diameter and 1 m high with a single PMT facing the bottom of the tank.A number of models predict a Very High Energy (VHE) component to the GRB spectrum [4, 5, 6]. It iswidely believed that the prompt keV/MeV emission detected by previous and current instruments is due tosynchrotron emission by relativistic electrons. If this is the case, then it is possible for these synchrotron pho-tons to Compton scatter off of the relativistic electrons to higher energies. This is only one possible mechanismfor producing VHE emission from GRBs, other possible mechanisms include proton synchrotron emission,photo-meson cascade emission, and others [7].Milagro’s large field of view (   2 sr) and high duty cycle (  90%), allow it to observe a large portion of thesky, nearly all the time, making it well suited to searching for the transient emission produced by GRBs orother sources of short duration VHE gamma-ray flares. The median energy of the events detected by Milagrois a few TeV, however the effective area is still large at hundreds of GeV (    at 100 GeV). This resultsin a gamma-ray fluence sensitivity at VHE energies comparable to previous satellite detectors at keV energies.Photons above a few hundred GeV are highly attenuated by infrared (IR) background photons which are acomponent of the extra-galactic background light (EBL). The attenuation increases with redshift, and limitsMilagro to relatively nearby bursts. However, the large effective area at a few hundred GeV makes Milagrosensitive to large sample of the burst population.2. The Short Duration Burst SearchIn the absence of a burst localization provided by another instrument, it is possible to search the Milagro datafor VHE emission from GRBs. Due to the large cosmic-ray flux at the earth, the bulk of the events detectedby Milagro are air showers induced by cosmic rays. It is therefore necessary to search for emission from aGRB on top of this large cosmic-ray background. In contrast to doing a search for a known source, the starttime, duration, and coordinates of a potential GRB are not known a priori, making it necessary to search overthe entire sky, at all times, and over multiple durations. The first step in this process is to create a background464 David Noyesmap using a long period of time (compared to the duration). The background map is normalized to containthe number of events expected from any location on the sky in a time window specified by the start time andthe duration. Next, a signal map is created and contains the number of events observed in the time windowspecified by the start time and the duration. Given these maps, the probability that the number of observedsignal events was due to a random fluctuation of the background is calculated for each point on the sky. Thesignature of a GRB would be a very low probability, inconsistent with being due to a random fluctuation of thebackground.The search is done in real time over 27 durations ranging from 	 to 40 s. Fig. 1 shows the probabilitydistribution for the 1 s duration search for an entire day of data. Given the fact that the start time, duration, andcoordinates of a potential GRB are unknown, a large number of overlapping spatial and time bins are used inthe search. An increased number of spatial bins is used whenever a bin is found with a probability less  .This is the origin of the feature in the probability distribution at this probability value. A GRB candidate wouldappear on this figure as a low probability value, away from the main distribution.The Milagro data was searched for VHE emission from GRBs from MJD 52353 to MJD 53372 (1020 days).After accounting for the dead time of the detector (   10%) and various data quality cuts, the total numberof live days searched was 836.585 (2.3 years), giving a post-cuts duty factor of 82%. In this time period, noevidence for VHE emission from a GRB was observed.log10(probability)-20 -18 -16 -14 -12 -10 -8 -6 -4 -2110210310410510610710810910Probabilities for the 1.0 s duration search (MJD 2390)Figure 1. Probability distribution for the 1 s duration search for an entire day of data.3. Constraining the VHE Spectrum of GRBsGiven the fact that no evidence for VHE from GRBs was observed, it is possible to constrain the VHE compo-nent of the GRB spectrum. Little is known about the VHE component of the burst spectrum. EGRET measureda number of bursts up to a few GeV, and were found to be consistent with an extension of the Band spectrumto these energies. However GRB941017 showed a distinct high energy component, which evolved indepen-dently of the lower energy component [3]. Furthermore, the redshift and luminosity distributions are not wellA Search for Short Duration VHE Emission from GRBs with Milagro 465constrained by observations for the long duration bursts, and have not been observed at all for short durationbursts (except perhaps GRB050509b).In order to constrain the VHE component of the burst spectrum, simulations are done of burst populations. Forthe purpose of these simulations, it is assumed that the VHE spectrum is a simple  power law between 100GeV and 10 TeV. Attenuation due to the IR background is accounted for using models for the amount of IR asa function of redshift and photon energy [8, 9]. Then models must be taken for the burst redshift and isotropicenergy distributions. The measured BATSE duration distribution is used for the burst durations.To simulate a burst population, a redshift, duration, isotropic energy, and zenith angle are randomly drawn fromtheir respective distributions. Then the number of photons detected from a GRB at a redshift  , with isotropicenergy ﬁﬀﬃﬂ (radiated in the form of 100 GeV to 10 TeV photons), and at a zenith angle  for Milagro, is givenby "!#%$& 'ﬀﬃﬂ $&(*),+ ﬂ#-$. 'ﬀﬃﬂ (/10ﬃ2.3465072.2.89465;:4ﬃ<=<#>$&(ﬃ?@ACBED FHGJI>$ (1)where +ﬂ#%$&ﬁﬀﬃﬂK( is the normalization to the spectrum,:46<K<#>$6( is the effective area of Milagro as a functionof zenith angle and primary particle energy, and ? @ACBLD FHG is the survival probability for absorption on the IRbackground. The number of photons expected ( !) is then compared to the measured Milagro background( NMPOHQ) for the same zenith angle and a fixed duration. If the Poisson probability of observing ;MPOHQR; !eventswhen expecting MPOHQis 5 S or greater after accounting for the large number of trials, then it is considered thatthe burst would be detectable by Milagro.A large number of bursts are simulated in this manner in order to determine what fraction of this populationwould be detectable by Milagro. The models for the Tﬁﬀﬃﬂ distribution apply to the keV/MeV portion of thespectrum. It is then assumed that all bursts have a VHE component to their spectrum, and that the totalenergy radiated in the form of 100 GeV to 10 TeV photons is a constant factor times that radiated in the formof lower energy photons. For each simulated GRB, this energy ratio (  'ﬀﬃﬂ#VU"WX(&Yﬁﬀ6ﬂ#JZ?U( ) is varied inorder to determine the fraction of the simulated burst population that would be detectable as a function ofﬁﬀﬃﬂ#VUXWX(.Y['ﬀﬃﬂ#VZ?U( . This allows an upper limit to be set on the energy ratio. Since no evidence forVHE emission was observed by Milagro in the 2.3 years that were searched, the 90% confidence level (CL)upper limit on the number of GRBs/year in this data set is 2.3/2.3 =1 GRB/year. The value of the energy ratiothat gives this many events expected per year is the 90% CL upper limit on the ratio. Given a model of theredshift and isotropic energy distribution of GRBs, a weaker or stronger limit will be set on the energy ratiodepending on how many bursts in the model are bright enough and nearby enough for Milagro to detect. Forexample, Fig. 2 shows the results from a simulation which used a simple fit to the measured GRB redshiftand isotropic energy distributions. The top three plots in the figure show the model distributions used in thesimulation. The bottom left plot shows the redshift distribution of the simulated bursts that were detectable.The bottom right plot shows the total number of GRBs/year expected to be observed by Milagro as a functionof the energy ratio given the model described above. Drawn on this plot is the 90% CL upper limit on theenergy ratio, which for this model is 0.21. This means that the total energy released in the form 100 GeV to10 TeV photons is constrained to be less than 0.21 times that released by keV/MeV photons. This was donefor a number of different models of the redshift and isotropic energy distributions. These included theoreticalredshift and isotropic energy distributions based on the lag-luminosity relation, redshift distributions basedon theoretical distributions of binary mergers and the star formation rate, and isotropic energy distributionsderived from the measured BATSE fluence distribution. The different models result in a range of limits on theenergy ratio depending on the number of nearby GRBs predicted and how much energy they release. For themodels mentioned above, the limit on the energy ratio varies from around 0.2 to 90. Future work will includemodels for \\ absorption at the burst source and taking into account any improved knowledge of the GRBredshift and isotropic energy distributions.466 David NoyesRedshift Model: Fit to Measured Distribution: Total GRB Rate: 700/yearDuration Model: BATSE T90 DistributionIsotropic Energy Model: Fit to Measured DistributionRedshift0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 201000200030004000500060007000Simulated Redshift Distributionlog10(duration/sec)-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 302000400060008000Simulated Duration Distributionlog10(Eiso/erg)47 48 49 50 51 52 53 54 55 56 5705000Simulated Isotropic Energy Distributionredshift0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2050100150200250Redshift Distribution for those Observed in the SimulationEiso(TeV)/Eiso(keV)-410 -310 -210 -110 1 10 210 310# GRB/year-410-310-210-11011021090% CL on ratio: 0.12Number of GRB/year expectedFigure 2. Results of a simulation using ts to the measured redshift and ]_^C`Ja distributions. The top three plots show themodel distributions. On the bottom left is the redshift distribution of GRBs that would be detectable in this model for anenergy ratio of 1. On the bottom right is the number of GRBs/yr detectable by Milagro, as function of the energy ratio.4. AcknowledgementsWe thank Scott Delay and Michael Schneider for their dedicated efforts on the Milagro experiment. Wealso gratefully acknowledge the financial support of the National Science Foundation (under grants PHY-0075326,-0096256,-0097315,-206656,-0245143,- 0245234,-0302000, and ATM-0002744, the Department ofEnergy (Office of High Energy Physics), Los Alamos National Laboratory, the University of California, andthe Institute for Geophysics and Planetary Physics at Los Alamos National Laboratory.References[1] Atkins, R. et al., Nucl. Instr. And Meth., A449, 478 (2000).[2] Atkins, R. et al, Astrophys. J., 595, 803 (2003).[3] Gonzales, M. M., et al., Nature 424, 847 (2003).[4] Zhang, B. & Me´sza´ros, P. 2001, ApJ, 559, 110.[5] Pe’er, A. & Waxman, E. Astrophys. J. 613 (2004) 448-459.[6] Razzaque, S., Me´sza´ros, P., & Zhang, B. Astrophys. J. 613 (2004) 1072-1078.[7] Zhang, B. & Me´sza´ros, P. Int. J. Mod. Phys. A19 (2004) 2385-2472.[8] Primack, J. R., Bullock, J. S., Somerville, R. S. & Maxminn, D. 1999 Astroparticle Phys., 11, 93.[9] de Jager, O. C. & Stecker, Astrophys. J. 566 (2002) 738-743.

A Search for Short Duration VHE Emission from GRBs with Milagro

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