[vsnet-grb-info 23557] LIGO/Virgo S190901ap: Additional candidates from the Zwicky Transient Facility

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25656
SUBJECT: LIGO/Virgo S190901ap: Additional candidates from the Zwicky Transient Facility
DATE:    19/09/04 15:35:38 GMT
FROM:    Robert Stein at DESY  <robert.stein at desy.de>

Robert Stein (DESY), Erik Kool (OKC), Harsh Kumar (IITB), Michael Coughlin (Caltech), Mansi Kasliwal (Caltech), Daniel Perley (LJMU), Shreya Anand (Caltech), Yashvi Sharma (Caltech), Viraj Karambelkar (Caltech), Leo P. Singer (NASA GSFC), Igor Andreoni (Caltech), Gaurav Waratkar (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 again observed the localization region of the gravitational wave trigger S190901ap (LVC et al. GCN 25606, GCN 25614) 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 our new target-of-opportunity observations in the g-band and r-band beginning at UT 2019-09-04 10:18 UT. Each exposure was 30s, with a typical median depth of 20.8 mag in g band and 20.6 mag in r band. Across the three nights of observations (Kool et al. GCN 25616, Stein et al. GCN 25634), we have covered 73% of the enclosed probability at least once, and 67% of the enclosed probability at least twice. This estimate does not account for chip gaps.

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. We rejected stellar sources (Tachibana and Miller 2018) and moving objects, applied machine learning algorithms (Mahabal et al. 2019), and removed candidates with history of variability prior to the merger time. Two additional candidates were found by our pipeline.
 
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ZTF Name     | RA (deg)   | DEC (deg)  | Filter | Mag   | Magerr
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ZTF19abwsmmd | 22.666409  | -19.712405 | r      | 20.08 | 0.22
ZTF19abwvals | 73.250555  | +12.693030 | r      | 19.83 | 0.19
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ZTF19abwsmmd has a blue color (g - r ~ -0.25) and is located in a compact host galaxy with a SDSS photo z of ~0.05, consistent with the merger distance. It was not detected in g band on Aug 30 UT to a depth of 20.4 mag. ZTF19abwvals is red (g - r ~ 0.5), located in a host galaxy with an SDSS photo-z of ~0.13. It was not detected in g band on Sep 1 UT to a depth of 20.64. We encourage spectroscopic and photometric observations to discern the nature of these candidates.
 
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 at 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).