MAGIC reveals a complex morphology within the unidentified gamma-ray source HESS J1857+026

¹ IFAE, Campus UAB, 08193 Bellaterra, Spain
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁵ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
⁶ Max-Planck-Institut für Physik, 80805 München, Germany
⁷ Universidad Complutense, 28040 Madrid, Spain
⁸ Inst. de Astrofísica de Canarias, 38200, La Laguna, Tenerife, Spain
⁹ University of Łódź, 90236 Lodz, Poland
¹⁰ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹¹ ETH Zurich, 8093 Zurich, Switzerland
¹² Universität Würzburg, 97074 Würzburg, Germany
¹³ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁴ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁵ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
¹⁶ Università di Padova and INFN, 35131 Padova, Italy
¹⁷ Università dell’Insubria, Como, 22100 Como, Italy
¹⁸ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁹ Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
²⁰ Japanese MAGIC Consortium, Division of Physics and Astronomy, Kyoto University, Japan
²¹ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, Finland
²² Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²³ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
²⁴ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁵ Now at: NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA and Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁶ Now at Ecole polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
²⁷ Now at Department of Physics & Astronomy, UC Riverside, CA 92521, USA
²⁸ Now at Finnish Centre for Astronomy with ESO (FINCA), Turku, Finland
²⁹ Also at INAF-Trieste  
³⁰ Also at Instituto de Fisica Teorica, UAM/CSIC, 28049 Madrid, Spain
³¹ Now at School of Chemistry & Physics, University of Adelaide, 5005 Adelaide, Australia
³² Now at: Stockholm University, Oskar Klein Centre for Cosmoparticle Physics, 106 91 Stockholm, Sweden
³³ Now at GRAPPA Institute, University of Amsterdam, 1098XH Amsterdam, The Netherlands

Received: 27 January 2014
Accepted: 11 August 2014

Abstract

Aims. HESS J1857+026 is an extended TeV gamma-ray source that was discovered by H.E.S.S. as part of its Galactic plane survey. Given its broadband spectral energy distribution and its spatial coincidence with the young energetic pulsar PSR J1856+0245, the source has been put forward as a pulsar wind nebula (PWN) candidate. MAGIC has performed follow-up observations aimed at mapping the source down to energies approaching 100 GeV in order to better understand its complex morphology.

Methods. HESS J1857+026 was observed by MAGIC in 2010, yielding 29 h of good quality stereoscopic data that allowed us to map the source region in two separate ranges of energy.

Results. We detected very-high-energy gamma-ray emission from HESS J1857+026 with a significance of 12σ above 150 GeV. The differential energy spectrum between 100 GeV and 13 TeV is described well by a power law function dN/dE = N₀(E/1TeV)^−Γ with N₀ = (5.37 ± 0.44_stat ± 1.5_sys) × 10^-12 (TeV^-1 cm^-2 s^-1) and Γ = 2.16 ± 0.07_stat ± 0.15_sys, which bridges the gap between the GeV emission measured by Fermi-LAT and the multi-TeV emission measured by H.E.S.S.. In addition, we present a detailed analysis of the energy-dependent morphology of this region. We couple these results with archival multiwavelength data and outline evidence in favor of a two-source scenario, whereby one source is associated with a PWN, while the other could be linked with a molecular cloud complex containing an Hii region and a possible gas cavity.