Search for radio pulsations in LS I +61 303

¹ Departament d’Astronomia i MeteorologiaInstitut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona Spain
e-mail: acanyelles@am.ub.es; jmoldon@am.ub.es; vzabalza@am.ub.es; mribo@am.ub.es; jmparedes@ub.edu
² National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, 411007 Pune, India
e-mail: bcj@ncra.tifr.res.in; ishwar@ncra.tifr.res.in
³ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
e-mail: Victor.Zabalza@mpi-hd.mpg
⁴ Departamento de Física (EPSJ), Universidad de Jaén, Campus Las Lagunillas s/n, 23071 Jaén, Spain
e-mail: jmarti@ujaen.es

Received: 1 July 2011
Accepted: 23 May 2012

Abstract

Context. LS I +61 303 is a member of the select group of gamma-ray binaries: galactic binary systems that contain a massive star and a compact object, show a changing milliarcsecond morphology and a similar broad spectral energy distribution (SED) that peaks at MeV–TeV energies and is modulated by the orbital motion. The nature of the compact object is unclear in LS I +61 303, LS 5039 and HESS J0632+057, whereas PSR B1259–63 harbours a 47.74 ms radio pulsar.

Aims. A scenario in which a young pulsar wind interacts with the stellar wind has been proposed to explain the very high energy (VHE, E > 100 GeV) gamma-ray emission detected from LS I +61 303, although no pulses have been reported from this system at any wavelength. We aim to find evidence of the pulsar nature of the compact object.

Methods. We performed phased array observations with the Giant Metrewave Radio Telescope at 1280 MHz centred at phase 0.54. Simultaneous data from the multi-bit phased array back-end with a sampling time of t_samp = 128 μs and from the polarimeter back-end with t_samp = 256 μs where taken.

Results. No pulses have been found in the data set, with a minimum detectable mean flux density of ~0.38 mJy at 8-σ level for the pulsed emission from a putative pulsar with period P > 2 ms and duty cycle D = 10% in the direction of LS I +61 303.

Conclusions. The detection of posible radio pulsations will require deep and sensitive observations at frequencies ~0.5–5 GHz and orbital phases 0.6−0.7. However, it may be unfeasible to detect pulses if the putative pulsar is not beamed at the Earth or if there is a strong absorption within the binary system.