GCN Circular 25616

Erik Kool (OKC), Robert Stein (DESY), Yashvi Sharma (Caltech), Viraj
Karambelkar (Caltech), Mansi Kasliwal (Caltech), Daniel Perley (LJMU),
 Valery Brinnel (HU Berlin), Jakob Nordin (HU Berlin), Shreya Anand
(Caltech), Michael Coughlin (Caltech), Leo P. Singer (NASA GSFC), Igor
Andreoni (Caltech), Gaurav Waratkar (IITB), Harsh Kumar (IITB), Maitreya
Khandagale (IITB), Kunal Deshmukh (IITB), Varun Bhalerao (IITB), G. C.
Anupama (IIA), Dougal Dobie (USyd/CSIRO), Brad Cenko (NASA GSFC), Tomas
Ahmuda (UMD), Eric Bellm (UW), Albert Kong (NTHU), Anna Franckowiak (DESY),
Pradip Gatkine (UMD)

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
S190901ap (LVC et al. GCN 25606
Not found
 
 
, GCN 25614
Not found
 
 
) with the Palomar 48-inch
telescope equipped with the 47 square degree ZTF camera (Bellm et al. 2019,
Graham et al. 2019). The tiling was 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 beginning at UT 2019-09-02 03:08 UT.
We covered 44% of the enclosed probability based on the new lalinference
map (38% of the enclosed probability based on the initial bayestar map) in
6500 sq deg mapped before we had to close due to clouds. Each exposure was
30s with a typical depth of 20.7 mag.

The images were processed in real-time through the ZTF reduction and image
subtraction pipelines at IPAC to search for potential counterparts (Masci
et al. 2019). AMPEL (Nordin et al. 2019) was used to search the alerts
database for candidates. After rejecting stellar sources (Tachibana and
Miller 2018) and moving objects and applying machine learning algorithms
(Mahabal et al. 2019), and after removing candidates with history of
variability prior to the merger time, the following high-significance
transient candidates were identified by our pipeline in the 95%
localization of the new lalinference map (LVC et al. GCN 25614
Not found
 
 
).

------------------------------------------------------------------------
 ZTF Name     | RA (deg)   | DEC (deg)  | Filter | Mag   | Magerr
------------------------------------------------------------------------
 ZTF19abvizsw | 279.472820 | +61.497984 | r      | 19.45 | 0.11
 ZTF19abvixoy | 279.552972 | +27.420935 | r      | 19.22 | 0.10
 ZTF19abvjnsm | 267.202697 | +44.693203 | r      | 20.23 | 0.20
 ZTF19abvionh | 253.750924 | +14.051330 | g      | 20.73 | 0.31
------------------------------------------------------------------------

ZTF19abvizsw has a red color (g-r ~ 0.5 mag), no underlying host in the
reference image and is on the outer periphery of the new LVC sky
localization. ZTF has observed this field every night for the past month as
part of routine survey operations and the first detections of this
transient are only after the binary neutron star merger time. ZTF19abvixoy
has an upper limit from Aug 30 UT and possibly a faint counterpart in PS1.
ZTF19abvjnsm has an upper limit from Sep 1 UT but its host galaxy has too
high a phot-z estimate from SDSS of 0.51 +/- 0.11. The host galaxy of
ZTF19abvionh has a consistent SDSS phot-z (0.064 +/- 0.016) but the two
detections last night are separated by only a short baseline of 7 minutes
(a moving object in our solar system cannot be ruled out for this
candidate). We encourage spectroscopic and photometric follow-up to discern
the nature of these transients.

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; SDSU, 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 database searches are done by AMPEL (Nordin
et al. 2019). Alert filtering and follow-up co-ordination is being
undertaken by the GROWTH marshal system (Kasliwal et al. 2019).