TITLE:   GCN CIRCULAR
NUMBER:  24313
SUBJECT: LIGO/Virgo S190425z: No Optical Detection of the Swift/UVOT Transient in ASAS-SN observations hours before and after discovery
DATE:    19/04/28 00:26:54 GMT
FROM:    Benjamin Shappee at U. of Hawaii  <shappee@hawaii.edu>

B. J. Shappee (IfA-Hawaii), C. S. Kochanek (OSU), K. Z. Stanek (OSU), S.
Holmbo (Aarhus), A. Franckowiak (DESY), T. W.-S. Holoien (Carnegie
Observatories), J. L. Prieto (Diego Portales; MAS), Subo Dong (KIAA-PKU),
 T. A. Thompson (OSU), J. F. Beacom (OSU)


Swift reported a possible counterpart (Breeveld et al., GCN #24296) to
LIGO/Virgo
Alert GCN #24168 of a BNS merger.  The reported detection was acquired
on 2019-04-26.784282
UT Swift UVOT u=17.74 +/- 0.18 mag (Vega). ZTF (Andreoni et al., GCN #24302
) reported non-detections 9 hours before the reported Swift observations.


ASAS-SN observations cover both 7 hours before and 12 hours after the
reported Swift transient with 3-sigma g-band limits deeper than the
reported u-band detection:

2019-04-25.4180376   >18.755

2019-04-26.4875420   >18.749

2019-04-27.3040667   >18.421



The transient Swift observed must have evolved on a short timescale or have
been very blue (u-g)~-1.



We would like to thank Las Cumbres Observatory and its staff for their
continued support of ASAS-SN. ASAS-SN is funded in part by the Gordon and
Betty Moore Foundation through grant GBMF5490 to the Ohio State University,
NSF grant AST-1515927, the Mt. Cuba Astronomical Foundation, the Center for
Cosmology and AstroParticle Physics (CCAPP) at OSU, the Chinese Academy of
Sciences South America Center for Astronomy (CASSACA), and the Villum
Fonden (Denmark). For more information about the ASAS-SN project, see
http://www.astronomy.ohio-state.edu/asassn/index.shtml.