MeerKAT detection of SAXJ1808.4-3658 at 1.3 GHz

ATel #13026; David Williams, Sara Motta (Oxford), Rob Fender (Oxford, UCT), Patrick Woudt (UCT), James Miller-Jones (Curtin) on behalf of the ThunderKAT collaboration
on 14 Aug 2019; 10:40 UT
Credential Certification: David Williams (david.williams@physics.ox.ac.uk)

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Following the first report of activity from the accreting millisecond X-ray pulsar SAXJ1808.4-3658 (ATEL#12964), we observed it at 1.28 GHz in the radio band on 2019-07-31 (ATEL#12982) with the MeerKAT telescope, and we reported a non detection. However, subsequent observations in the optical, UV and X-ray bands (ATEL#12993, ATEL#13000, ATEL#13001, ATEL#13006, ATEL#13022) have confirmed the multi-band brightening of this source. Therefore, as part of the ThunderKAT Large Survey Programme ( http://adsabs.harvard.edu/abs/2017arXiv171104132F ), we continued to monitor SAXJ1808.4-3658 and we now report our findings from two additional epochs.

We observed SAXJ1808.4-3658 on 2019-08-04 for 15 minutes, starting at 20:00:25.3 UTC. In this observation we do not detect the source. The rms noise level in the centre of the image is ~30uJy/beam, yielding a 3 sigma upper limit of 90uJy/beam. We performed a further observation of SAXJ1808.4-3658 on 2019-08-10, starting at 19:09:32.0 UTC for 15 minutes. In this image, we detect SAXJ1808.4-3658 at a peak flux density of 700+/-40uJy/beam, where the uncertainty on the flux density includes a 5% flux calibration error. The noise in a region close to the phase center of the image is 34uJy/beam, therefore we detect SAXJ1808.4-3658 at a ~20 sigma level.

Our detection on 2019-08-10 aligns well with the peak in the i'-band from the Optical Faulkes/LCO light curves of SAX J1808.4-3658, shown by Russell et al. in (ATEL#12964) and further reported in Goodwin et al. (ATEL#12993). We will continue to monitor this source with MeerKAT in the coming weeks.

ThunderKAT will run for 5 years and aims to monitor all bright, active, southern hemisphere X-ray binaries in the radio band. For further information on this programme please contact Rob Fender and/or Patrick Woudt.

We thank the staff at the South African Radio Astronomy Observatory (SARAO) for their rapid scheduling of these observations. The MeerKAT telescope is operated by SARAO, which is a facility of the National Research Foundation, an agency of the Department of Science and Technology.