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 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 Feb 2008 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 hour 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 kpc^2 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.

💡 Research Summary

The paper reports a targeted search for radio pulsations from the 6.85‑second X‑ray pulsar XTE J0103‑728, a member of a Be/X‑ray binary system located in the Small Magellanic Cloud (SMC). The motivation is clear: detecting radio pulses would make this the second known system—after PSR B1259‑63—to exhibit both Be‑star companionship and a radio‑emitting neutron star, thereby offering a rare laboratory for studying the interplay between high‑energy and radio emission mechanisms in long‑period pulsars.

Observations were carried out in February 2008 with the Parkes 64‑m radio telescope using the centre beam of the multibeam receiver. A continuous 6.4‑hour integration was recorded at a centre frequency of 1390 MHz with a 256 MHz bandwidth. The data were processed over a wide dispersion‑measure (DM) range of 0–800 pc cm⁻³, comfortably encompassing the expected DM for sources within the SMC. Four complementary search techniques were employed: (1) a folding search using the known 6.85 s period, (2) a standard Fourier transform search, (3) a Fast‑Folding Algorithm (FFA) optimized for long periods, and (4) a single‑pulse search to capture sporadic bright bursts.

No statistically significant periodic or single‑pulse signals were detected in any of the analyses. Using the system temperature, bandwidth, integration time, and assuming a distance of ~60 kpc, the authors derive a 1‑σ upper limit on the periodic radio luminosity of ≈45 mJy kpc². This limit is compared to the known radio luminosities of PSR B1259‑63, which are strongly modulated by orbital phase and become undetectable near periastron due to eclipses by the Be‑star’s circumstellar disc. The authors argue that a similar eclipsing geometry could be at work in XTE J0103‑728, potentially hiding radio emission during certain orbital phases.

The discussion also places the derived luminosity limit in the context of other long‑period (≥5 s) radio pulsars, noting that many such objects have low radio efficiencies and may fall below the current detection threshold. Consequently, a single observation cannot rule out the presence of radio emission; repeated observations sampling different orbital phases are recommended. The paper concludes that while no radio pulsations were found, the established upper limit provides a valuable benchmark for future, more sensitive searches (e.g., with the SKA) and underscores the importance of phase‑resolved monitoring for systems where circumstellar material may intermittently obscure radio beams.