VLBA detection of Swift J1818.0-1607

ATel #14005; Hao Ding (Swinburne University of Technology ''SUT''), Adam T. Deller (SUT), Marcus E. Lower (SUT), Ryan M. Shannon (SUT)
on 11 Sep 2020; 12:13 UT
Credential Certification: Hao Ding (haoding@swin.edu.au)

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The magnetar Swift J1818.0-1607 (J1818) was discovered in March 2020 (ATel #13551), and found to emit radio pulses (ATel #13553). The radio spectrum of the magnetar was reported to have flattened in July 2020 at cm- to sub-mm-wavelengths (e.g., ATel #13898; ATel #14001).

Two 3.5-hr observations of J1818 with the Very Long Baseline Array (VLBA) were carried out in April and August. The first observation was made from UT 09:56 to 13:26 on 20 April, operating at 1.6 GHz (see ATel #13737 for more information). In response to the recent spectral flattening, the second observation was conducted at 8.8 GHz from UT 02:31 to 06:00 on 19 August (around MJD 59080.2). For each observation, pulsar gating was applied at correlation using a spin period derived from on-going monitoring by the Parkes radio telescope (e.g. Lower et al., 2020). J1818 was correlated at the position obtained with XRT (Evans, et al., GCN Circ. 27373).

Data from both observations were reduced using the procedure described in Ding et al. (2020). We detected a radio candidate in the ungated data of the second observation with an integrated flux density of 1.29(5) mJy (all uncertainties quoted to 68% confidence level in this telegram) at the J2000 position 18:18:00.19340(1), -16:07:53.0049(2), where the position uncertainties are statistical only. This position is only 0.8 arcsecond away from the XRT position (Evans, et al., GCN Circ. 27373). We found this radio candidate was detected in <12.5% of the J1818 (spin) phase, and undetected in the rest of the phase, thus identifying this candidate as J1818. Subsequently, we re-visited the data from the first observation and placed a 5-sigma upper limit of 0.29 mJy/beam on the flux density of J1818 at its VLBA position. We thank the VLBA team for approving and coordinating the high-quality VLBA+Y1 observations, especially during the ongoing pandemic.

References:

Lower, M. E., et al., 2020, ApJL 896, L37.

Ding, H., et al., 2020, MNRAS, in press. arXiv:2008.06438