Gamma-ray bursts (GRBs) are among the potential very-high-energy (VHE) gamma-ray sources. VHE emission from GRBs is predicted by most GRB models. Despite its generally fast-fading nature in many wavebands, the time evolution of any VHE radiation is still not clear. The highest energy radiation from GRBs ever detected firmly by any instrument was a 18 GeV photon coming from GRB 940217 detected with EGRET about 1.5 hour after the onset of the GRB. There is also a tentative detection using MILAGRITO of TeV excess events from GRB 970417a. In order to probe the largely unexplored VHE spectra of GRBs, a GRB observing program has been set up by the H.E.S.S. collaboration. With the high sensitivity of the H.E.S.S. array, VHE flux levels predicted by GRB models are well within reach. Extra-galactic background light absorption is taken into account in cases where redshifts are known. We will present the H.E.S.S. observations of and results from some of the reported GRB positions during the past few years.Pak-Hin Tam, Paula Chadwick, Yves Gallant, Dieter Horns, Gerd Puhlhofer, Gavin Rowell, Stefan Wagner for the H.E.S.S. COLLABORATIO

Chadwick, P.

Gallant, Y.

Horns, D.

Puhlhofer, G.

Rowell, G.

Tam, P.

Wagner, S.

Adelaide Research &amp; Scholarship

 PUBLISHED VERSION  Tam, P. H.; Chadwick, P. M.; Gallant, Y. A.; Horns, D.; Puhlhofer, G.; Rowell, Gavin Peter; Wagner, S. J. for the H.E.S.S. COLLABORATION  Gamma-ray burst observations with the H.E.S.S. air Cherenkov array  Proceedings of 30th International Cosmic Ray Conference (ICRC 2007), 3-11 July, 2007  © ESO 2007               http://hdl.handle.net/2440/44651           PERMISSIONS www.edpsciences.org/alr   Authors can make their article, published by EDP Sciences, available on their personal site, their institution’s web site and Open Archive Initiative sites, provided the source of the published article is cited and the ownership of the copyright clearly mentioned. These must be not for profit sites. Reprint and postprint may be used (with the publisher’s PDF). Authors are requested to create a link to the publisher’s internet service. The link must be accompanied by the following text “The original publication is available at www.edpsciences.org/alr”.   10th May, 2011                30TH INTERNATIONAL COSMIC RAY CONFERENCEGamma-ray burst observations with the H.E.S.S. Air Cherenkov arrayPAK-HIN TAM1, PAULA CHADWICK2 , YVES GALLANT3, DIETER HORNS4 , GERD PU¨HLHOFER1 ,GAVIN ROWELL5, STEFAN WAGNER1 FOR THE H.E.S.S. COLLABORATION1Landessternwarte, Universita¨t Heidelberg, Ko¨nigstuhl, D 69117 Heidelberg, Germany2University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.3Laboratoire de Physique The´orique et Astroparticules, IN2P3/CNRS, Universite´ Montpellier II, CC 70,Place Euge`ne Bataillon, F-34095 Montpellier Cedex 5, France4Institut fu¨r Astronomie und Astrophysik, Universita¨t Tu¨bingen, Sand 1, D 72076 Tu¨bingen, Germany5School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australiaphtam@lsw.uni-heidelberg.deAbstract: Gamma-ray bursts (GRBs) are among the potential very-high-energy (VHE) gamma-rayand cosmic-ray sources. Particles are accelerated to highly-relativistic speeds. This might give rise toemission of VHE gamma-ray and/or cosmic-ray particles with ultra-high energy > 1019 eV. Despite itsgenerally fast-fading behavior seen in many longer wavebands, the time evolution of any VHE radiationis still not clear. In order to probe the largely unexplored VHE spectra of GRBs, a GRB observingprogram has been set up by the H.E.S.S. collaboration. With the high sensitivity of the H.E.S.S. array andgiven favorable observational conditions, VHE flux levels predicted by GRB models are within reach.Extra-galactic background light (EBL) absorption is considered in cases where redshifts of the GRBs arereported. We present the H.E.S.S. VHE gamma-ray observations of and results from some of the reportedGRB positions during the past few years, including recent observations in early 2007.IntroductionGamma-ray bursts (GRBs), being established asoriginating from highly-relativistic ejecta, areamong potential VHE gamma-ray and cosmic-raysources. First detected in late 1960s [1], the ori-gin of GRBs is still not well understood, espe-cially compared with other objects such as pul-sars and quasars which were also detected in thesame era but the origins of which are much moreunderstood nowadays than GRBs. The main rea-son is that to obtain multi-wavelength informationother than gamma-rays proves to be operationallyvery difficult. Two breakthroughs in understandingGRBs include the establishment of the isotropicspatial distribution by the BATSE experiment onboard CGRO in the 1990s [2] and the discovery oflonger-wavelength counterparts after the launch ofBeppoSAX in Feb 1997 [3]. Since then not onlyGRBs are confirmed to be of cosmological origin,but the theories of GRBs and their modeling havedrawn a lot of attentions from the astrophysicalcommunity.After the highly variable radiation seen in keV-MeV gamma-ray energies (known as the promptGRB phase), fast-fading behavior is generally seenin counterparts in longer wavebands (known asthe afterglow phase). It is believed that dur-ing the GRBs, particles are accelerated to highly-relativistic speeds. Highly-relativistic particlesmight give rise to emission of VHE gamma-rayand/or cosmic-ray particles with ultra-high energy> 1019eV (see e.g. [4]). Although VHE emis-sion from GRBs during the prompt GRB phase orthe afterglow phase is predicted, its flux level andtemporal behavior is not clear, given that no emis-sion in this energy range has ever been detectedunambiguously by now. With the high sensitiv-ity of the H.E.S.S. array and given favorable ob-servational conditions, VHE flux levels predictedby GRB models are within reach.Proceedings of the 30th International Cosmic Ray ConferenceRogelio Caballero, Juan Carlos D’Olivo, Gustavo Medina-Tanco,Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.)Universidad Nacional Autónoma de México,Mexico City, Mexico, 2008Vol. 3 (OG part 2), pages 1119–1122ID 4661119GRB OBSERVATIONS WITH H.E.S.S.VHE emission from GRBsThe highest energy radiation from GRBs ever de-tected firmly by any instrument was a ∼ 18 GeVphoton coming from GRB 940217, detected us-ing EGRET about 1.5 hour after the onset of theGRB [5]. From the theoretical point of view, pho-tons with energies up to ∼ 10 TeV from GRBsare expected (for review, see e.g. [6] and refer-ences therein). In one case considered by [7] whereelectron IC emission dominates, an energy flux ofabout 5 × 10−12erg cm−2s−1 at 1 TeV one dayafter GRB onset is predicted1, if one assumes aredshift of 0.15. This is within H.E.S.S. detec-tion limit. The detection of VHE photons (and itsquantity) or upper limits (in cases of null detec-tions) could be used to constrain GRB properties,eg. bulk Lorentz factor and ambient density [8, 9].Currently, the most sensitive detectors in the VHEgamma-ray regime are air Cherenkov systems, in-cluding H.E.S.S., MAGIC, VERITAS, and Whip-ple. While still no detection using any of the instru-ments is established, results on upper limits havebeen reported by the MAGIC collaboration [10]and the Whipple collaboration [11]. In general,their results are consistent with power-law extrap-olation of the keV spectra obtained with satellitedata.At cosmological distances, one has to take into ac-count the absorption of VHE gamma-ray by extra-galactic background light (EBL). For low-redshiftGRBs and sub-TeV energies, the attenuation isless significant. Moreover, there is evidence fromdistant blazar spectra that the Universe is moretransparent for VHE gamma-ray than previouslythought [12]. Thus, current air Cherenkov systemsare able to observe out to z ∼ 1 at ∼ 100 GeV.H.E.S.S. GRB observing programThe H.E.S.S. array is a system of four 13m-diameter Imaging Atmospheric Cherenkov Tele-scopes (IACTs) located in the Khomas Highland ofNamibia [13]. Since the completion of the wholearray in late 2003, H.E.S.S. has proven to be verysuccessful in VHE gamma-ray astronomy, thusopening a new era in astronomy in this observa-tional window. The array is one of the most sensi-tive VHE gamma-ray detectors. For a point sourcewith integral flux ∼ 1.4 × 10−11 ph cm−2s−1above 1 TeV and spectral index 2.6, only a 2-hourH.E.S.S. observation is required for a 5-σ detec-tion.We have been observing GRBs since early 2003.At the beginning of 2005, a GRB coordinationteam was set up and since then our GRB observa-tion program has been fully established. An auto-mated program is running on site to keep the shiftcrew alerted of any new detected GRBs in real-time.We have followed on-board GRB triggers dis-tributed by the Swift satellite, as well as trig-gers from INTEGRAL and HETE II confirmedby ground-based analysis. Upon the reception ofa GRB Coordinates Network (GCN [14]) noticefrom one of these satellites (with good indicationsthat the source is a genuine GRB), we will ob-serve the burst position as soon as possible, lim-ited to ZA < 45◦ (to ensure a reasonably low en-ergy threshold) and HESS dark time2. We startobserving the burst position up to 24 hours afterthe burst time. The nominal observation time is 2hours (within the above observational constraints).If there are tentative positive signals indicated by aquick analysis, further observations will be carriedout accordingly.In Table 1, we show 17 GRBs which we haveobserved using H.E.S.S. during the period fromMarch 2003 to April 2007. For each burst, the startobservation time, live-time of the observation andthe mean zenith angle (ZA) are presented.Data Analysis and ResultsCalibration of data, the event reconstructionand rejection of the cosmic-ray background (i.e.gamma-ray event selection criteria) were per-formed as described in [15]. Except for the caseof GRB 030329, where a different analysis cut wasused because only two telescopes were operating,standard analysis procedures as described in [16]1. The attenuation factor due to EBL absorption isabout 0.1 at 1 TeV for a GRB with z = 0.15 [12].2. H.E.S.S. observations are taken in darkness andwhen the moon is below the horizon. The fraction ofH.E.S.S. dark time is about 0.2.112030TH INTERNATIONAL COSMIC RAY CONFERENCEwere applied to each GRB to search for any possi-ble signal. The background was then estimated us-ing the reflected-region model as described in [17].The energy threshold (Eth) after analysis cuts ofeach GRB observation (mainly depending on thezenith angle) is shown in Table 1.No evidence of excess events for any GRB ob-served using H.E.S.S. was seen. The 99.9% confi-dence level (c.l.) integral photon flux upper limitsabove Eth using the method of [18] for each GRBare included in Table 1. For GRBs with reportedredshifts < 1, the EBL-corrected values using theP0.45 model as in [12] are also shown. For GRBswith redshifts> 1, the attenuation of VHE gamma-ray by EBL is large, thus the EBL-corrected valuesare much higher (which are not shown here).AcknowledgementsThe support of the Namibian authorities and ofthe University of Namibia in facilitating the con-struction and operation of H.E.S.S. is gratefully ac-knowledged, as is the support by the German Min-istry for Education and Research (BMBF), the MaxPlanck Society, the French Ministry for Research,the CNRS-IN2P3 and the Astroparticle Interdisci-plinary Programme of the CNRS, the U.K. Sci-ence and Technology Facilities Council (STFC),the IPNP of the Charles University, the Polish Min-istry of Science and Higher Education, the SouthAfrican Department of Science and Technologyand National Research Foundation, and by theUniversity of Namibia. We appreciate the excel-lent work of the technical support staff in Berlin,Durham, Hamburg, Heidelberg, Palaiseau, Paris,Saclay, and in Namibia in the construction and op-eration of the equipment.References[1] Klebesadel, R. W., Strong, I. B. & Olson, R.A. 1973, ApJ, 182, L85.[2] Fishman, G. J. & Meegan, C. A. 1995,ARA&A, 33, 415.[3] van Paradijs, J., Kouveliotou, C., & Wijers, R.A. M. J., 2000, ARA&A, 38, 379.[4] Waxman, E. 2006, Nuclear Physics B (Proc.Suppl.), 151, 46.[5] Hurley, K. et al. 1994, Nature, 372, 652.[6] Zhang, B. & Me´sza´ros, P. 2004, Int. J. Mod.Phys. A., 19, 2385.[7] Zhang, B. & Me´sza´ros, P. 2001, ApJ, 559, 110.[8] Pe’er, A. & Waxman, E. 2005, ApJ, 633, 1018.[9] Wang X. Y. et al. 2005, A&A, 439, 957.[10] Albert, J. et al. 2007, submitted to ApJ[11] Horan, D. et al. 2007, ApJ, 655, 396.[12] Aharonian, F. A. et al. (HESS collaboration)2006a, Nature, 440, 1018.[13] Hinton, J. A. 2004, New Astronomy Review,48, 331.[14] The Gamma ray bursts Coordinates Network,http://gcn.gsfc.nasa.gov/[15] Aharonian, F. A. et al. (HESS collaboration)2006b, A&A, 457, 899.[16] Benbow, W. 2005, Proc. of Towards a Net-work of Atmospheric Cherenkov Detectors VII(Palaiseau), 163[17] Berge, D., Funk, S. & Hinton, J. A. 2007,A&A, 466, 1219.[18] Feldman, G. J. & Cousins, R. D. 1998, Phys.Rev. D., 57, 3873.[19] Berger, E. & Gladders, M. 2006, GCN Circu-lar, 5170.[20] Ofek, E. O. et al. 2007, ApJ, 662, 1129.[21] D’Elia, V. et al. 2005, GCN Circular 4044.[22] Soderberg, A. M. et al. 2006, ApJ, 636, 391.[23] Stanek, K. Z. et al. 2003, ApJ, 591, L17.[24] de Pasquale, M. et al. 2007, MNRAS, 377,1638.1121GRB OBSERVATIONS WITH H.E.S.S.Table 1: GRBs observed with H.E.S.S. from March 2003 to April 2007. For each burst, start observationtime after GRB onset (tstart), live-time, mean zenith angle (ZA), energy threshold (Eth) and 99.9% c.l.upper limit (UL) of the observation are presented.GRB tstart live time mean ZA Eth Flux ULs⋆ redshift(hrs) (deg) (GeV) (photon cm−2 s−1)070429A 64 min 0.4 23 290 1.06× 10−12 –070419B 15.1 h 0.9 47 700 3.89× 10−12 –070209 15.4 h 1.0 41 480 7.48× 10−12 –060526 4.7 h 1.9 25 200 5.90× 10−12 3.21 [19]060505 19.4 h 2.0 42 450 6.29× 10−12(1.55× 10−11) 0.089 [20]060403 13.6 h 0.9 39 310 9.37× 10−12 –050922C 52 min 0.7 23 200 1.22× 10−11 2.199 [21]050801 16 min 0.5 43 370 3.40× 10−12 1.56†050726 10.8 h 2.0 40 400 4.22× 10−12 –050607 14.8 h 1.5 37 290 5.39× 10−12 –050509C 21 h 1.0 22 220 1.08× 10−11 –050209 20.2 h 2.5 48 520 3.32× 10−12 –041211 9.5 h 2.0 46 420 4.00× 10−12 –041006 10.4 h 1.4 27 220 1.01× 10−11(2.69× 10−8) 0.716 [22]040425 26 h 0.4 28 230 2.37× 10−11 –030821 18 h 1.0 28 290 1.52× 10−11 –030329 11.5 d 0.5 60 1360 2.58× 10−12(5.59× 10−11) 0.169 [23]⋆ Flux upper limits (> Eth) are at the 99.9% c.l. using the method given in [18]. For GRBs with red-shifts < 0.2, the EBL-corrected values in the energy range [Eth,10 TeV] are also given in the brackets. ForGRB041006, the EBL-corrected value in the energy range [Eth,0.7 TeV] is given.† Photometric redshift according to [24].1122

Gamma-ray burst observations with the H.E.S.S. air Cherenkov array

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Gamma-ray burst observations with the H.E.S.S. air Cherenkov array

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Gamma-ray burst observations with the H.E.S.S. air Cherenkov array

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