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GCN Circular 24191

Subject
LIGO/Virgo S190425z: Candidates from the Zwicky Transient Facility
Date
2019-04-25T17:46:37Z (5 years ago)
From
Mansi M. Kasliwal at Caltech/Carnegie <mansikasliwal@gmail.com>
Mansi M. Kasliwal (Caltech), Michael W. Coughlin (Caltech),  Eric C. Bellm
(UW), Leo P. Singer (NASA GSFC), K. De (Caltech),  Igor Andreoni (Caltech),
Dmitry Duev (Caltech), Shreya Anand (Caltech), Tomas Ahumada (UMD), S.
Bradley Cenko (NASA GSFC), D. Goldstein (Caltech), A. Ho (Caltech), D. A.
Perley (LJMU), V. Bhalerao (IITB),  H. Kumar (IITB), Y. Sharma (IITB), C.
Copperwheat (LJMU), Virginia Cunningham (UMD), Shaon Ghosh (UWM), Ariel
Goobar (OKC), David Kaplan (UWM), Jesper Sollerman (OKC), Joshua S. Bloom
(UCB), M. Bulla (OKC), Nobuyuki Kawai (Tokyo Tech), Yoichi Yatsu (Tokyo
Tech), Katsuhiro Murata (Tokyo Tech), Hidekazu Hanayama (IAO), Takashi
Horiuchi (IAO), G. C. Anupama (IIA),  M. Rigault (CNRS/IN2P3), C.
Barbarino(OKC), R. Biswas (OKC), D. Cook (Caltech), G. Helou (IPAC)

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
S190425z (GCN 24168) with the Palomar 48-inch telescope equipped with the
47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We
were already observing the GW localization region as part of routine survey
operations. 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 2019-04-25
09:19:07.161. A total of 4327 square degrees covering 41% of the enclosed
probability were observed before 12-deg twilight and analyzed in real-time.
Each exposure was 30s with a typical depth of 20.4 mag.

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.

Our most promising candidates thus far are:
------------------------------------------------------------------------
 ZTF Name     | RA (deg)    | DEC (deg)  | Filter | Mag   | Magerr
--------------+-------------+------------+--------+-------+-------------
 ZTF19aarykkb |  258.341454 | -9.964466  | r      | 18.63 |    0.10
 ZTF19aarzaod |  262.791487 | -8.450722  | r      | 20.11 |    0.18
--------------+-------------+------------+--------+-------+-------------
ZTF19aarykkb is near 2MASX J17132113-0957536 at z=0.024. ZTF19aarzaod is
near 2MASX J17311017-0827103 at z=0.028. Both have no ZTF detections prior
to the reported binary neutron star merger time (although our most recent
upper limits are only from 2019 April 19). Both host galaxies are in the
Census of the Local Universe (CLU) galaxy catalog (Cook et al. 2017). Thus,
with the data in-hand, the position, redshift of putative host galaxy ,
age, color and luminosity of both events are consistent with the GW
trigger. The positions are also consistent with being in the region that
Fermi-GBM could not constrain (GCN 24185). Both candidates have not yet
been reported to the Transient Name Server.  We caution that we cannot rule
out these are young supernovae and spectroscopic follow-up is urgently
needed. Additional analysis and follow-up is ongoing.

Also, just as a reference, we also provide a list of relatively young
candidates whose first ZTF detection was within the past three days below,
including a magnitude measurement from tonight. We emphasize that all the
events listed below have recent previous detections with ZTF prior to the
gravitational wave event trigger time and are unlikely to be related to
S190425z (e.g., background supernovae, active galactic nuclei, foreground
CV).

------------------------------------------------------------------------
 ZTF Name     | RA (deg)    | DEC (deg)  | Filter | Mag   | Magerr
--------------+-------------+------------+--------+-------+-------------
 ZTF19aarzfoz |  235.600249 | 19.5876421 | g      | 19.90 |    0.21
 ZTF19aarpptk | 254.2036677 |  9.6049366 | g      | 18.10 |    0.05
 ZTF19aarywve | 250.9039956 | 17.5801296 | r      | 20.45 |    0.19
 ZTF19aarpfjs | 252.0464742 |  1.0668639 | r      | 18.93 |    0.10
 ZTF19aarkmsl |  242.401716 | 25.7497917 | r      | 19.40 |    0.18
 ZTF19aarppnl | 258.9096792 | 15.7457655 | g      | 19.80 |    0.11
 ZTF19aarpplo | 262.9779752 | 18.7380718 | g      | 19.74 |    0.17
 ZTF19aarypbv | 248.5874816 | 30.5050049 | g      | 20.48 |    0.29
 ZTF19aartzok | 238.9634555 | 36.3591878 | g      | 20.27 |    0.20
 ZTF19aarxvub | 265.2782205 |   8.470448 | r      | 19.53 |    0.13
 ZTF19aaryvld |  250.075357 |  9.1071601 | r      | 20.01 |    0.18
 ZTF19aarpzuh | 252.1452493 | 15.7402238 | r      | 20.19 |    0.16
 ZTF19aarprog | 248.5427404 | 19.6347848 | r      | 19.00 |    0.11
 ZTF19aarycpi | 245.0910206 |  18.348845 | r      | 19.20 |    0.10
 ZTF19aailtzw | 256.934132  |   2.948056 | r      | 18.50 |    0.08
------------------------------------------------------------------------

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 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).
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