[vsnet-grb-info 20871] GRB180715B: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 553369644)

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22969
SUBJECT: GRB180715B: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 553369644)
DATE:    18/07/20 01:53:36 GMT
FROM:    S. Bradley Cenko at NASA/GSFC  <brad.cenko at nasa.gov>

S. Bradley Cenko (NASA GSFC), Michael W. Coughlin (Caltech), Shaon Ghosh (UWM), Tomas Ahumada (UMD), Virginia Cunningham (UMD), Pradip Gatkine (UMD), Eric C. Bellm (UW), Mansi M. Kasliwal (Caltech), Leo P. Singer (NASA GSFC), V. Zach Golkhou (UW), and Lin Yan (Caltech) on behalf of the ZTF and GROWTH collaborations and the KPED team

We observed the localization region of the short GRB 180715B (trigger 553369644) detected by the Gamma-Ray Burst Monitor (GBM) on the Fermi satellite with the Palomar 48-inch telescope equipped with the 47 square degree Zwicky Transient Facility (ZTF) camera. We obtained a series of r- and g-band images covering 254.31 square degrees beginning at 04:18 UT on 2018 July 16 (10:22 hours after the burst trigger time), corresponding to ~ 36% of the probability enclosed in the localization region.  

The images were processed through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts. 45 high-significance transient and variable candidates were identified by our pipeline in the area observed, most (31) of which had previous detections with ZTF in the days and weeks prior to the GRB trigger time (e.g., supernovae, active galactic nuclei). The 14 transients discovered the night of July 16 are listed in the table below. 

 ZTF ID           RA           DEC     discovery magnitude    last non-detection    
----------------------------------------------------------------------------------
ZTF18aamwzlv  13:06:44.59  +68:59:52.9  2018 Jul 16 g=21.26  2018 Jul 08 g>20.34    
ZTF18abhbevp  14:21:00.83  +72:11:43.8  2018 Jul 16 g=20.64  2018 Jul 07 g>20.08  
ZTF18abhbpkm  16:02:36.78  +70:47:05.1  2018 Jul 16 g=21.24  2018 Jul 13 g>20.52   
ZTF18abhhjyd  13:02:32.07  +75:16:49.4  2018 Jul 16 g=21.24  2018 Jul 05 g>20.56  
ZTF18abhbgan  15:43:18.86  +72:05:24.8  2018 Jul 16 g=21.22  2018 Jul 13 g>20.17 
ZTF18abhbfoi  13:24:34.01  +70:56:47.5  2018 Jul 16 g=21.12  2018 Jul 08 g>20.55   
ZTF18abhbcjy  14:20:50.39  +73:25:40.5  2018 Jul 16 g=20.65  2018 Jul 17 g>20.51   
ZTF18abhaogg  13:42:45.47  +74:19:38.3  2018 Jul 16 r=20.40  2018 Jul 05 r>20.35   
ZTF18abhbamj  15:26:58.78  +72:02:17.8  2018 Jul 16 g=21.22  2018 Jul 13 g>20.01   
ZTF18abhawjn  13:31:27.33  +66:46:45.4  2018 Jul 16 r=21.19  2018 Jul 08 r>20.31   
ZTF18abharzk  13:41:09.05  +70:43:06.8  2018 Jul 16 r=21.30  2018 Jul 08 r>20.28   
ZTF18abhbckn  12:49:53.85  +73:02:00.5  2018 Jul 16 r=20.93  2018 Jul 08 r>19.96   
ZTF18abhbfqf  13:16:00.24  +69:37:24.1  2018 Jul 16 g=19.59  2018 Jul 08 g>20.50 
ZTF18aauhpyb  13:21:45.49  +70:55:59.8  2018 Jul 16 r=18.99  2018 Jul 08 r>20.28 

The median 5-sigma upper limit for an isolated point source in our images was r > 21.4 and g > 21.6 mag for the observations made on July 16 and r > 21.4 and g > 21.3 mag for the observations made on July 17.

We obtained follow-up spectroscopy of ZTF18aauhpyb with the Spectral Energy Distribution Machine (SEDM; Blagorodnova et al., 2018, PASP) on the Palomar 60 inch telescope on the night of July 18.  The spectrum displays a blue continuum with emission lines from Balmer H at a redshift of 0.  We thus classify the source as a variable star.

We also obtained follow-up spectroscopy of ZTF18abhbfqf with the Double Spectrograph (DBSP) on the 200 inch Hale Telescope at Palomar Observatory on the night of July 17.  Cross-correlation with template spectra using the Superfit package (Howell et al., 2005, ApJ) indicates the source is a type Ia supernova before maximum light at a redshift of z = 0.110 (from host galaxy features).

Further observations are planned.

ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266), and includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW, USA; DESY, Germany; NRC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. Alert distribution service provided by DIRAC at UW. Alert filtering is being undertaken by the GROWTH marshal system, supported by NSF PIRE grant 1545949.