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Astron. Astrophys. 362, L5-L8 (2000)

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Letter to the Editor

Radio observations of the 33.8 ms X-ray pulsar SAX J0635+0533

L. Nicastro 1, B.M. Gaensler  *  2 and M.A. McLaughlin 3

1 Istituto di Fisica Cosmica con Applicazioni all'Informatica, CNR, Via U. La Malfa 153, 90146 Palermo, Italy
2 Massachusetts Institute of Technology, Center for Space Research, Cambridge, MA 02139, USA
3 Cornell University, Astronomy Department, Ithaca, NY 14850, USA

Received 27 July 2000 / Accepted 18 September 2000


The hard X-ray source SAX J0635+0533 shows pulsations at a period of 33.8 ms and is thought to be associated with a bright Be star. We here report on radio observations towards this source with the VLA and Arecibo radiotelescopes. With the VLA we do not detect any radio flux at the Be-star position, but one 4.7 mJy (at a radio frequency of 1.4 GHz) source lies just on the border of the [FORMULA] X-ray error circle. Periodicity searches at 430 MHz performed with the Arecibo telescope failed to detect any pulsation. We argue that the 4.7 mJy source is not the radio counterpart of SAX J0635+0533. Most likely SAX J0635+0533 is a rather "non-standard" Be-star/neutron-star X-ray binary pulsar, probably with a relatively short orbital period. It could still be a radio emitting pulsar but with an emission beam not pointing toward the Earth.

Key words: stars: individual: SAX J0635+0533 – stars: neutron – stars: pulsars: general – radio continuum: stars – X-rays: stars

* Hubble Fellow

Send offprint requests to: nicastro@ifcai.pa.cnr.it

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© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000