GCN Circular 25801

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
Not found
 
 
 (Stewart et al., GCN 25487
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) 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
Not found
 
 
), 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
Not found
 
 
), 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
Not found
 
 
 (Stewart et 
al., GCNs 25487
Not found
 
 
, 25496
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). 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
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) makes 
it unlikely to be associated with AT 2019osy
Not found
 
 
. 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
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_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/.