TITLE: GCN CIRCULAR NUMBER: 25801 SUBJECT: LIGO/Virgo S190814bv: HST and ALMA observations of the host galaxy of ASKAP J005547-270433 / AT 2019osy DATE: 19/09/21 18:03:22 GMT FROM: Daniele B Malesani at DTU Space F. E. Bauer (PUC), A. S. Fruchter (STScI), J. Gonzalez Lopez (UDP), J. Hjorth (DARK/NBI), T. Kangas (STScI), S. Kim (PUC), A. J. Levan (Radboud Univ.), D. B. Malesani (DTU Space), M. J. Michalowski (AMU), B. Milvang-Jensen (DAWN/NBI), R. Paladino (INAF/IRA), S. Schulze (WIS), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), report on behalf of the ENGRAVE collaboration: We observed the host galaxy of the radio transient ASKAP 005547-270433 / AT 2019osy (Stewart et al., GCN 25487) on 2019 Sep 17.9 UT using the F140W (1.4 micron) broadband filter of the Wide-Field Camera 3 on the Hubble Space Telescope and at 109 and 242 GHz in the configuration C43-7 (beam size of 0.21'' at 100 GHz) with ALMA on 2019 Sep 5.2 UT. A point source is detected by ALMA at 109 GHz consistent with the location reported by ASKAP (Stewart et al., GCN 25496), for which we measure these refined coordinates (~0.05" error): RA(J2000) = 00:55:47.417 Dec(J2000) = -27:04:33.14 We aligned the HST data to the Gaia catalog (to be consistent with the ALMA astrometry) using four stars in common inside the HST field of view. While we resolve a small elongated nucleus at the center of the host, we see no clear evidence of a point source superposed on the apparently normal structure of an edge-on spiral. Nonetheless, the position of the ALMA source aligns with the peak of the light in the nucleus to within its positional error of ~0.05". This result agrees with the finding of Mooley et al. (GCN 25539), who reported an alignment between the optical centroid of the host obtained from ground-based imaging and a VLA position at the 0.1" level. While the galaxy nucleus is not detected by ALMA at 242 GHz, a separate source is marginally detected approximately 1.5" away (not visible at 109 GHz), at a position still overlaying the stellar extent of the host galaxy and consistent with the ASKAP position of AT 2019osy (Stewart et al., GCNs 25487, 25496). The lack of detections by both ALMA at 109 GHz and by the VLA at 1-10 GHz (on Aug 28; Mooley et al., GCN 25539) makes it unlikely to be associated with AT 2019osy. It could be a noise peak, an unrelated background source, or a star-forming region inside the galaxy. Cutouts from the HST image, both wide-field and zoomed-in, can be seen at http://www.engrave-eso.org/AT2019osy_host_images . This circular is based on data collected under ALMA program 2018.1.01652.T, and on HST GO program #15664. We thank the staffs of STScI (in particular Alison Vick) and of the ALMA observatory for their assistance with these time-constrained observations. For further information on ENGRAVE, see http://www.engrave-eso.org/.