GCN Circular 24283

Michael W. Coughlin (Caltech), Mansi M. Kasliwal (Caltech), Daniel A.
Perley (LJMU), Ariel Goobar (OKC), Leo P. Singer (NASA GSFC), Shreya Anand
(Caltech), Tomas Ahumada (UMD), Igor Andreoni (Caltech), Eric C. Bellm
(UW), K. De (Caltech), R. Biswas (OKC), S. Nissanke (UvA), Dmitry Duev
(Caltech), S. Bradley Cenko (NASA GSFC), D. Goldstein (Caltech), A. Ho
(Caltech), V. Bhalerao (IITB), H. Kumar (IITB), V. Karambelkar (IITB), K.
Deshmukh (IITB), D. Saraogi (IITB), G. C. Anupama (IIA), C. Copperwheat
(LJMU), Virginia Cunningham (UMD), Shaon Ghosh (UWM), David Kaplan (UWM),
Jesper Sollerman (OKC), Joshua S. Bloom (UCB), M. Bulla (OKC), Matthew
Graham (Caltech), L. Yan (Caltech), C. Fremling (Caltech), Pradip Gatkine
(UMD), A. Miller (Northwestern)

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of
Observatories Watching Transients Happen (GROWTH) collaborations

We observed the localization region of the gravitational wave trigger
S190426c (GCN 24237
Not found
 
 
) with the Palomar 48-inch telescope equipped with the
47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). A new
tiling was automatically optimally determined and triggered using the
GROWTH Target of Opportunity marshal (Coughlin et al. 2019a, Kasliwal et
al. 2019b). We started obtaining target-of-opportunity observations in the
g-band and r-band filters beginning at UT 2019-04-27 05:45. The projected
enclosed probability with the original sky map was 75%. However, with the
new sky map (GCN 24277
Not found
 
 
, GCN 24279
Not found
 
 
) and taking account into chip gaps and
processing, a total of 4340 square degrees covering 55% of the enclosed
probability were observed before 12-deg twilight and analyzed in real-time.
Exposure length varied between 120s, 180s and 300s. We note that the area
around the north celestial pole covered by our partner GROWTH-India
telescope covers an additional 6% of the updated probability map,
complementary to ZTF (see GCN 24258
Not found
 
 
).

The images were processed through the ZTF reduction and image subtraction
pipelines at IPAC to search for potential counterparts (Masci et al. 2019).
After rejecting stellar sources (Tachibana and Miller 2018) and moving
objects and applying machine learning algorithms (Mahabal et al. 2019),
several high-significance transient candidates were identified by our
pipeline in the area observed. Thanks to the overlap in sky maps between
the two GW triggers S190426c and S190425z, we have very good constraints on
past history of variability in the last few days.

The only candidate with the first detection after the merger time is:

-----------------------------------------------------------------------------------------------
 ZTF Name     | RA (deg)    | DEC (deg)   | Filter | Mag   |  Magerr |
Filter| Mag   | Magerr
--------------+-------------+-------------+--------+-------+---------+-------+-------+---------
ZTF19aaslzfk  | 308.968271  | 72.3536353  | r      | 20.91 |  0.17   | g
  | 21.38 |  0.18
-----------------------------------------------------------------------------------------------

We caution that our upper limits in the last few days for ZTF19aaslzfk are
shallower than the detection. So we cannot rule out an old, unrelated
transient. The line-of-sight extinction is Ar of 1.4 mag (Schlafly et al.
2011). We note that the source is detected in all four WISE filters in the
AllWISE catalog (Wright et al. 2010).  Its W1-W3 colors are intermediate
between galaxies and AGN relative to the color loci of Assef et al. (2018),
but the clear W4 detection suggests contribution from an active galactic
nucleus.

Additional analysis and continued follow-up is in progress.

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC,
USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,
Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan;
IIT-B, India; IIA, India; LJMU, UK; TTU, USA; and USyd, Australia. ZTF
acknowledges the generous support of the NSF under AST MSIP Grant No
1440341. GROWTH acknowledges generous support of the NSF under PIRE Grant
No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et
al. 2019). Alert filtering and follow-up co-ordination is being undertaken
by the GROWTH marshal system (Kasliwal et al. 2019).