Tev Gamma-Ray Observations of the Perseus and Abell 2029 Galaxy Clusters J
Total Page：16
File Type：pdf, Size：1020Kb
Physics Physics Research Publications Purdue University Year 2006 TeV gamma-ray observations of the Perseus and Abell 2029 galaxy clusters J. S. Perkins, H. M. Badran, G. Blaylock, S. M. Bradbury, P. Cogan, Y. C. K. Chow, W. Cui, M. K. Daniel, A. D. Falcone, S. J. Fegan, J. P. Finley, P. Fortin, L. F. Fortson, G. H. Gillanders, K. J. Gutierrez, J. Grube, J. Hall, D. Hanna, J. Holder, D. Horan, S. B. Hughes, T. B. Humensky, G. E. Kenny, M. Kertzman, D. B. Kieda, J. Kildea, K. Kosack, H. Krawczynski, F. Krennrich, M. J. Lang, S. LeBohec, G. Maier, P. Moriarty, R. A. Ong, M. Pohl, K. Ragan, P. F. Rebillot, G. H. Sembroski, D. Steele, S. P. Swordy, L. Valcarcel, V. V. Vassiliev, S. P. Wakely, T. C. Weekes, and D. A. Williams This paper is posted at Purdue e-Pubs. http://docs.lib.purdue.edu/physics articles/208 The Astrophysical Journal, 644:148–154, 2006 June 10 # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A. TeV GAMMA-RAY OBSERVATIONS OF THE PERSEUS AND ABELL 2029 GALAXY CLUSTERS J. S. Perkins,1 H. M. Badran,2 G. Blaylock,3 S. M. Bradbury,4 P. Cogan,5 Y. C. K. Chow,6 W. Cui,7 M. K. Daniel,5 A. D. Falcone,8 S. J. Fegan,6 J. P. Finley,7 P. Fortin,9 L. F. Fortson,10 G. H. Gillanders,11 K. J. Gutierrez,1 J. Grube,4 J. Hall,12 D. Hanna,13 J. Holder,4 D. Horan,14 S. B. Hughes,1 T. B. Humensky,15 G. E. Kenny,11 M. Kertzman,16 D. B. Kieda,12 J. Kildea,13 K. Kosack,1 H. Krawczynski,1 F. Krennrich,17 M. J. Lang,11 S. LeBohec,12 G. Maier,4 P. Moriarty,18 R. A. Ong,6 M. Pohl,17 K. Ragan,13 P. F. Rebillot,1 G. H. Sembroski,7 D. Steele,10 S. P Swordy,19 L. Valcarcel,13 V. V. Vassiliev,6 S. P. Wakely,19 T. C. Weekes,14 and D. A. Williams20 (The VERITAS Collaboration) Received 2005 November 9; accepted 2006 February 10 ABSTRACT Galaxy clusters might be sources of TeV gamma rays emitted by high-energy protons and electrons accelerated by large-scale structure formation shocks, galactic winds, or active galactic nuclei. Furthermore, gamma rays may be produced in dark matter particle annihilation processes at the cluster cores. We report on observations of the galaxy clusters Perseus and A2029 using the 10 m Whipple Cerenkov telescope during the 2003–2004 and 2004–2005 observing seasons. We apply a two-dimensional analysis technique to scrutinize the clusters for TeVemission. In this paper we first determine flux upper limits on TeV gamma-ray emission from point sources within the clusters. Second, we derive upper limits on the extended cluster emission. We subsequently compare the flux upper limits with EGRET upper limits at 100 MeVand theoretical models. Assuming that the gamma-ray surface brightness profile mimics that of the thermal X-ray emission and that the spectrum of cluster cosmic rays extends all the way from thermal energies to multi-TeV energies with a differential spectral index of À2.1, our results imply that the cosmic-ray proton energy density is less than 7.9% of the thermal energy density for the Perseus Cluster. Subject headings: galaxies: clusters: general — galaxies: individual (NGC 1275) — galaxies: clusters: individual (Abell 426, Perseus, Abell 2029) — gamma rays: observations 1. INTRODUCTION netic field. CRPs can diffusively escape clusters only on timescales much longer than the Hubble time. Therefore, they accumulate As our universe evolves and structure forms on increasingly larger scales, the gravitational energy of matter is constantly con- over the entire formation history (Vo¨lk & Atoyan 1999). CREs lose their energy by emitting synchrotron, bremsstrahlung, and verted into random kinetic energy of cosmic gas. In galaxy clus- inverse Compton emission on much shorter timescales. For ICM ters, collisionless structure formation shocks are thought to be the main agents responsible for heating the intercluster medium magnetic fields on the order of B ’ 1 G, synchrotron and in- verse Compton emission losses alone cool CREs of energy E ¼ (ICM) to temperatures of k T ’ 10 keV.Through this and other B 1 TeV on a timescale processes, gravitational energy is converted into the random ki- netic energy of nonthermal baryons (protons) and leptons (elec- À1 4 B02 trons). Galactic winds (Vo¨lk & Atoyan 1999) and reacceleration s ¼ Tc 2 e ; ð1Þ of mildly relativistic particles injected into the ICM by powerful 3 8mec ÀÁ cluster members (Enlin & Biermann 1998) may accelerate ad- 0 2 2 1/2 where T is the Thomson cross section, B ¼ B þ BCMB , ditional particles to nonthermal energies. Using Galactic cosmic 2 and BCMB ¼ 3:25(1 þ z) G; for the clusters considered here, rays (CRs) as a yard stick, one expects that the energy density T 6 of cosmic-ray protons (CRPs) dominates over that of cosmic-ray z 1 and s 10 yr. electrons (CREs) by approximately 2 orders of magnitude and There is good observational evidence of nonthermal electrons may be comparable to that of thermal particles and the ICM mag- in galaxy clusters. For a number of clusters, diffuse synchrotron radio halos and/or radio relic sources have been detected (Giovannini et al. 1993, 1999; Giovannini & Feretti 2000; 1 Department of Physics, Washington University in St. Louis, St. Louis, MO 63130. 11 Physics Department, National University of Ireland, Galway, Ireland. 2 Physics Department, Tanta University, Tanta, Egypt. 12 Physics Department, University of Utah, Salt Lake City, UT 84112. 3 Department of Physics, University of Massachusetts, Amherst, MA 01003- 13 Physics Department, McGill University, Montreal, QC H3A 2T8, Canada. 4525. 14 Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for 4 School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK. Astrophysics, Amado, AZ 85645. 5 School of Physics, University College Dublin, Belfield, Dublin 4, Ireland. 15 Enrico Fermi Institute, University of Chicago, Chicago, IL 60637. 6 Department of Physics and Astronomy, University of California, Los Angeles, 16 Department of Physics and Astronomy, DePauw University, Greencastle, CA 90095. IN 46135-0037. 7 Department of Physics, Purdue University, West Lafayette, IN 47907. 17 Department of Physics and Astronomy, Iowa State University, Ames, IA 8 Department of Astronomy and Astrophysics, Penn State University, Uni- 50011. versity Park, PA 16802. 18 Department of Physical and Life Sciences, Galway-Mayo Institute of 9 Department of Physics and Astronomy, Barnard College, Columbia Uni- Technology, Dublin Road, Galway, Ireland. versity, NY 10027. 19 Enrico Fermi Institute, University of Chicago, Chicago, IL 60637. 10 Astronomy Department, Adler Planetarium and Astronomy Museum, 20 Santa Cruz Institute for Particle Physics and Department of Physics, Chicago, IL 60605. University of California, Santa Cruz, CA 95064. 148 TeV OBSERVATIONS OF A426 AND A2029 CLUSTERS 149 Kempner & Sarazin 2001; Feretti 2003). For some clusters, an The search described below assumes that the high-energy (HE) excess of extreme ultraviolet (EUV) and/or hard X-ray radiation surface brightness mimics the X-ray surface brightness and fo- over that expected from the thermal X-ray–emitting ICM has cuses on the detection of gamma rays from within 0N8fromthe been observed (Bowyer & Bergho¨fer 1998; Lieu et al. 1999; cluster center. There are several possibilities relating the thermal Rephaeli et al. 1999; Fusco-Femiano et al. 2004). The excess and nonthermal particles within clusters. From general consid- radiation originates most likely as inverse Compton emission erations, Vo¨lk & Atoyan (1999) assume that the nonthermal par- from CRE scattering cosmic microwave background photons (Lieu ticles carry a certain fraction of the energy density of the ICM. et al. 1996; Enlin & Biermann 1998; Blasi & Colafrancesco 1999; One of the aims of very high energy (VHE) astronomy is to con- Fusco-Femiano et al. 1999). strain this fraction. Indeed, we do know the CRP energy density in The detection of gamma-ray emission from galaxy clusters the interstellar medium (ISM) of the Milky Way. In this case it would make it possible to measure the energy density of nonther- turns out that the CRP energy density is comparable to the energy mal particles. The density and energy density of the thermal ICM density of the thermal ISM, the energy density of the interstellar can be derived from imaging-spectroscopy observations made with magnetic field, and the energy density of starlight. If nonthermal such satellites as Chandra and XMM-Newton (Krawczynski 2002; particles in clusters indeed carry a certain fraction of the energy Markevitch et al. 1998; Donahue et al. 2004). The density and density of the ICM, the HE surface brightness would mimic that energy spectra of the nonthermal protons could be computed from of the thermal X-ray emission. In another line of argument, one the detected gamma-ray emission once the density of the thermal may assume that powerful cluster members (i.e., radio sources) ICM is known (Pfrommer & Enlin 2004). Gamma rays can are the dominant source of nonthermal particles in the ICM; also originate as inverse Compton and bremsstrahlung emission from in this case we would expect that CRPs accumulate at the cluster CREs and as 0 ! emission from hadronic interactions of cores where usually the most powerful radio galaxies are found CRPs with thermal target material. If successful measurements of (Pfrommer & Enlin 2004). Ryu et al. (2003) and Kang & Jones the gamma-ray fluxes from several galaxy clusters were obtained, (2005) performed numerical calculations to estimate the energy one could explore the correlation of the CRP luminosity with clus- density of CRPs by large-scale structure formation shocks.