A search for pulsations from the compact object of GRB 060218

¹ Ramón y Cajal Fellow; Dpto. de Física Atómica, Molecular y Nuclear, Universidad Complutense de Madrid, Spain e-mail: mirabal@gae.ucm.es
² Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027–6601, USA

Received: 29 June 2009
Accepted: 19 December 2009

Abstract

Aims. A fraction of massive stars are expected to collapse into compact objects (accreting black holes or rapidly rotating neutron stars) that successfully produce gamma-ray bursts (GRBs). We examine the possibility of directly observing these gamma-ray burst compact objects (GCOs) using post-explosion observations of past and future GRB sites.

Methods. We present a search for early pulsations from the nearby (z=0.0335) gamma-ray burst GRB 060218, which exhibited features possibly consistent with a rapidly spinning neutron star as its underlying GCO. We also consider alternative techniques that could potentially achieve a detection of GCOs either in the Local Volume or near the plane of our own Galaxy.

Results. We report the non-detection of pulsations from the GCO of GRB 060218. In particular, fast fourier transform analysis applied to the light curve shows no significant power over the range of frequencies with an upper limit on the pulsed fraction of ~2%. In addition, we present detection limits of current high-resolution archival X-ray images of galaxies within the Local Volume. The existing data could be harnessed to rule out the presence of any background contaminants at the GRB position of future nearby events.

Conclusions. The null detection of pulsations from the GCO of GRB 060218 is most likely explained by the fact that the afterglow emission occurs near the head of the jet and should be far removed from the compact object. We also find that the comparison of pre- and post-explosion explosion images of future GRBs within the Local Volume, as well as the firm identification of a GCO within an ancient GRB remnant near the Galactic plane are extremely challenging with current GeV/TeV capabilities. Finally, we conclude that only under some very exceptional circumstances will it be possible to directly detect the compact object responsible for gamma-ray bursts.