Simultaneous absolute timing of the Crab pulsar at radio and optical wavelengths

¹ Advanced Studies and Technology Preparation Division, Science Directorate of the European Space Agency, ESTEC-ESA, PO Box 299, 2200 AG, Noordwijk, The Netherlands e-mail: toosterb@rssd.esa.int
² Laboratoire de Physique et Chimie de l'Environnement, CNRS, 3A avenue de la Recherche Scientifique, 45071 Orleans, France
³ Computational Astrophysics Laboratory, I.T. Building, National University of Ireland, Galway, Ireland
⁴ Research and Scientific Support Department of ESA, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁵ Sterrewacht Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands

Received: 10 March 2008
Accepted: 17 June 2008

Abstract

Context. The Crab pulsar emits across a large part of the electromagnetic spectrum. Determining the time delay between the emission at different wavelengths will allow to better constrain the site and mechanism of the emission. We have simultaneously observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with μs time resolution and at 2 GHz using the Nançay radio telescope with an instrument doing coherent dedispersion and able to record giant pulses data.

Aims. We have studied the delay between the radio and optical pulse using simultaneously obtained data therefore reducing possible uncertainties present in previous observations.

Methods. We determined the arrival times of the (mean) optical and radio pulse and compared them using the tempo2 software package.

Results. We present the most accurate value for the optical-radio lag of  μs and suggest the likelihood of a spectral dependence to the excess optical emission asociated with giant radio pulses.