Fermi-LAT constraints on the pulsar wind nebula nature of HESS J1857+026

¹ Centre d’Études Nucléaires de Bordeaux Gradignan, Université Bordeaux 1, CNRS/IN2p3, 33175 Gradignan, France
e-mail: rousseau@cenbg.in2p3.fr; lemoine@cenbg.in2p3.fr
² Max-Planck-Institut für Kernphysik, 69029 Heidelberg, Germany
e-mail: Marie-Helene.Grondin@mpi-hd.mpg.de
³ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
⁴ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
e-mail: ave@stanford.edu
⁵ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁶ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
⁷ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁸ Department of Physics, McGill University, Montreal, PQ H3A 2T8, Canada
⁹ X-ray Astrophysics Laboratory and CRESST, NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
¹⁰ Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
¹¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, M13 9PL, UK
¹² CSIRO Astronomy and Space Science, Australia Telescope National Facility, Epping NSW 1710, Australia
¹³ Institut de Ciències de l’Espai (IEEE-CSIC), Campus UAB, 08193 Barcelona, Spain

Received: 20 December 2011
Accepted: 23 May 2012

Abstract

Context. Since its launch, the Fermi satellite has firmly identified 5 pulsar wind nebulae plus a large number of candidates, all powered by young and energetic pulsars. HESS J1857 + 026 is a spatially extended γ-ray source detected by H.E.S.S. and classified as a possible pulsar wind nebula candidate powered by PSR J1856 + 0245.

Aims. We search for γ-ray pulsations from PSR J1856+0245 and explore the characteristics of its associated pulsar wind nebula.

Methods. Using a rotational ephemeris obtained from the Lovell telescope at Jodrell Bank Observatory at 1.5 GHz, we phase-fold 36 months of γ-ray data acquired by the Large Area Telescope (LAT) aboard Fermi. We also perform a complete γ-ray spectral and morphological analysis.

Results. No γ-ray pulsations were detected from PSR J1856+0245. However, significant emission is detected at a position coincident with the TeV source HESS J1857 + 026. The γ-ray spectrum is well described by a simple power-law with a spectral index of Γ = 1.53  ±  0.11_stat  ±  0.55_syst and an energy flux of G(0.1–100 GeV) = (2.71 ± 0.52_stat ± 1.51_syst) × 10^-11 erg cm^-2 s^-1. The γ-ray luminosity is L_PWN^γ (0.1–100 GeV)=(2.5 ± 0.5_stat ± 1.5_syst) × 10³⁵ (d/9 kpc)² erg s^-1, assuming a distance of 9 kpc. This implies a γ-ray efficiency of  ~5% for Ė = 4.6 × 10³⁶ erg s^-1, in the range expected for pulsar wind nebulae. Detailed multi-wavelength modeling provides new constraints on its pulsar wind nebula nature.