UPPER LIMITS ON PULSED RADIO EMISSION FROM THE 6.85 s X-RAY PULSAR XTE J0103−728 IN THE SMALL MAGELLANIC CLOUD

X-ray pulsations with a 6.85 s period were recently detected in the Small Magellanic Cloud (SMC) and were subsequently identified as originating from the Be/X-ray binary system XTE J0103–728. The recent localization of the source of the X-ray emission has made a targeted search for radio pulsations from this source possible. The detection of pulsed radio emission from XTE J0103–728 would make it only the second system after PSR B1259–63 that is both a Be/X-ray binary and a radio pulsar. We observed XTE J0103–728 in 2008 February with the Parkes 64 m radio telescope soon after the identification of the source of X-ray pulsations was reported in order to search for corresponding radio pulsations. We used a continuous 6.4 hr observation with a 256 MHz bandwidth centered at 1390 MHz using the center beam of the Parkes multibeam receiver. In the subsequent data analysis, which included a folding search, a Fourier search, a fast-folding algorithm search, and a single pulse search, no pulsed signals were found for trial dispersion measures (DMs) between 0 and 800 pc cm–3. This DM range easily encompasses the expected values for sources in the SMC. We place an upper limit of ~45 mJy kpc2 on the luminosity of periodic radio emission from XTE J0103–728 at the epoch of our observation, and we compare this limit to a range of luminosities measured for PSR B1259–63, the only Be/X-ray binary currently known to emit radio pulses. We also compare our limit to the radio luminosities of neutron stars having similarly long spin periods to XTE J0103–728. Since the radio pulses from PSR B1259–63 are eclipsed and undetectable during the portion of the orbit near periastron, repeated additional radio search observations of XTE J0103–728 may be valuable if it is undergoing similar eclipsing and if such observations are able to sample the orbital phase of this system well.