Transient radio emisison from SAX J1808.4-3658

Abstract

We report on the detection of radio emission from the accretion-powered X-ray millisecond pulsar SAX J1808.4-3658, using the Australia Telescope Compact Array. We detected a ~0.8 mJy source at the position of SAX J1808.4-3658 on 1998 April 27, approximately one day after the onset of a rapid decline in the X-ray flux; no such source was seen on the previous day. We consider this emission to be related to the radio emission from other X-ray binaries, and is most likely associated with an ejection of material from the system. No radio emission was detected at later epochs, indicating that if SAX J1808.4-3658 is a radio pulsar during X-ray quiescence then its monochromatic luminosity must be less than L(1.4 GHz) ~6 mJy/kpc^2.