The optical spectrum of the Vela pulsar ^*

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK e-mail: rm2@mssl.ucl.ac.uk
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 877, 22830, Ensenada, Baja California, México e-mail: zhar@astrosen.unam.mx
³ INAF, Istitituto di Astrofisica Spaziale, via Bassini 15, Milan 20133, Italy e-mail: pat@iasf-milano.inaf.it

Received: 2 May 2007
Accepted: 8 July 2007

Abstract

Context.Our knowledge of the optical spectra of Isolated Neutron Stars (INSs) is limited by their intrinsic faintness. Among the fourteen optically identified INSs, medium resolution spectra have been obtained only for a handful of objects. No spectrum has been published yet for the Vela pulsar (PSR B0833–45), the third brightest () INS with an optical counterpart. Optical multi-band photometry underlines a flat continuum.

Aims.In this work we present the first optical spectroscopy observations of the Vela pulsar, performed in the 4000-11 000 Å spectral range.

Methods.Our observations have been performed at the ESO Very Large Telescope  (VLT) using the FORS2 instrument.

Results.The spectrum of the Vela pulsar is characterized by a flat power-law with (4000-8000 Å), which compares well with the values obtained from broad-band photometry. This confirms, once more, that the optical emission of Vela is entirely of magnetospheric origin.

Conclusions.The comparison between the optical spectral indeces of rotation-powered INSs does not show evidence for a spectral evolution, suggesting that, as in the X-rays, the INS aging does not affect the spectral properties of the magnetospheric emission. At the same time, the optical spectral indeces are found to be nearly always flatter then the X-rays ones, clearly suggesting a general spectral turnover at lower energies.