EVN limits to compact milliarcsecond-scale radio emission from the proposed galaxy counterpart to FRB 150418

ATel #8865; B. Marcote (JIVE), M. Giroletti (INAF), M. Garrett (ASTRON), J. Yang (OSO), Z. Paragi (JIVE), K. Hada (NAOJ), C. C. Cheung (NRL)
on 24 Mar 2016; 22:29 UT
Credential Certification: Benito Marcote (marcote@jive.eu)

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The fast radio burst FRB 150418 reported by Keane et al. (2016, Nature 530 453) was associated with a radio transient coincident with the galaxy WISE J071634.59-190039.2 at z ~ 0.49. However, Williams & Berger (2016, arXiv:1602.08434; see also ATel #8752) and Vedantham et al. (2016, arXiv:1603.04421) argue that the claimed transient is actually a variable radio source from an active galactic nucleus (AGN) and unrelated to the FRB 150418. The measured flux densities for this source range between 105 and 225 μJy up to 328 days after the observed FRB. Since a radio-variable AGN is naturally very compact, it would be detectable with the very long baseline interferometry (VLBI) technique at milliarcsecond resolution.

We have conducted an EVN observation on 2016 March 16 from 16:30 to 22:00 UTC at 5.0 GHz centered on the WISE J071634.59-190039.2 galaxy. The observation was conducted at a bitrate of 1024 Mbps, with a total bandwidth of 128 MHz divided in 8 IFs of 16 channels each. The total on-source time was 2.18 hr.

We do not detect any radio emission above five times the rms noise level of 28 μJy/beam in a region of 2 x 2 arcsec^2 around the provided coordinates. A natural weighting was used, obtaining a synthesized beam of 5.6 x 10.1 mas, PA = 9.8 degrees. Therefore, we exclude the presence of a compact radio source brighter than 170 μJy/beam at a 6-sigma rms noise level in the field. This non-detection threshold is in agreement with the mean flux density value of ~150 μJy observed in WISE J071634.59-190039.2 during the previous weeks with the VLA (Williams & Berger 2016; Vedantham et al. 2016). Deeper observations are planned.

We thank the EVN PC for approving the 9-hr e-EVN ToO observation during the e-EVN session. The participating telescopes were Effelsberg (Germany), Westerbork Single Telescope (Netherlands), Jodrell Bank Mk2 Telescope (UK), Medicina (Italy), Noto (Italy), Onsala (Sweden), Torun (Poland), Yebes (Spain), and Hartebeesthoek (South Africa). The European VLBI Network (EVN) is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. The observations presented here were obtained under the project code RG008A.