AstroNote 2020-167

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DRAFT
2020-08-29 02:59:25
Type: Object/s-Discovery/Classification
Zwicky Transient Facility discovery of ZTF20abtxwfx/AT2020sev, a fast optical transient and candidate afterglow
Authors: Igor Andreoni (Caltech), Anna Ho (UCB), Michael Coughlin (UMN), Mansi Kasliwal (Caltech), Eric Burns (LSU), Dmitry Svinkin (Ioffe Institute), Tomas Ahumada (UM), Yuhan Yao (Caltech), Daniel Perley (LJMU), Shreya Anand (Caltech), Erik Kool (OKC), Ana Sagues Carracedo (OKC)
Source Group: ZTF
Abstract:
We report the discovery of the fast optical transient ZTF20abtxwfx/AT2020sev with the Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al. 2019). ZTF20abtxwfx was found during the science validation of the new "ZTF Realtime Search and Triggering" (ZTF-ReST) project. ZTF-ReST will aim at near real-time identification of compelling kilonova candidates in ZTF data using the methods described in Andreoni et al. (2020d), independently of gravitational-wave or gamma-ray triggers. ZTF20abtxwfx was first detected on 2020-08-18 05:20 UT, heareafter labelled T_det. ZTF20abtxwfx faded by ~1.3 mag in r-band in the first 2 days since T_det. The transient was last detected on 2020-08-23 04:51 UT at r = 20.98 +- 0.24 mag. Stringent upper limits constrain the transient onset time to be within ~1 day from T_det. The color of the transient appears to be red, with g-r~0.1 and g-i~0.3 one day after T_det. An association between ZTF20abtxwfx and GRB 200817A is possible.

We report the discovery of the fast optical transient ZTF20abtxwfx/AT2020sev with the Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al. 2019) at coordinates:

RAJ2000 = 16:41:21.21 (250.338384d)
DecJ2000 = +57:08:20.5 (+57.139027d)

ZTF20abtxwfx was found during the science validation of the new "ZTF Realtime Search and Triggering" (ZTF-ReST) project. ZTF-ReST will aim at near real-time identification of compelling kilonova candidates in ZTF data using the methods described in Andreoni et al. (2020d), independently of gravitational-wave or gamma-ray triggers.

ZTF20abtxwfx was first detected on 2020-08-18 05:20 UT, heareafter labelled T_det. ZTF20abtxwfx faded by ~1.3 mag in r-band in the first 2 days since T_det. The transient was last detected on 2020-08-23 04:51 UT at r = 20.98 +- 0.24 mag. Stringent upper limits constrain the transient onset time to be within ~1 day from T_det. The color of the transient appears to be red, with g-r~0.1 and g-i~0.3 one day after T_det. The Galactic extinction on the line of sight is low, with E(B-V)=0.015 mag (Planck Collaboration et al., 2015).

In the table below, we report forced photometry obtained with ForcePhotZTF (Yao et al., 2019) on images processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC (Masci et al. 2019). 

-----------------+--------+------+------
    Date (UT)    |  mag   | emag | band 
-----------------+--------+------+------
2020-08-17 06:45 | > 21.0 |      |  r 
2020-08-18 05:20 | 19.22  | 0.04 |  r 
2020-08-19 04:19 | 20.03  | 0.11 |  r
2020-08-19 05:23 | 20.16  | 0.09 |  g
2020-08-19 05:53 | 19.83  | 0.14 |  i
2020-08-20 04:47 | 20.54  | 0.15 |  r
2020-08-20 05:46 | 20.66  | 0.16 |  g
2020-08-21 04:25 | 20.71  | 0.17 |  r
2020-08-21 06:54 | > 20.8 |      |  g
2020-08-23 04:12 | > 21.0 |      |  g
2020-08-23 04:51 | 20.98  | 0.24 |  r
2020-08-24 04:26 | > 20.5 |      |  r
-----------------+--------+------+------


ZTF20abtxwfx is located at high Galactic latitude b_Gal = 40.0 deg. Deep Legacy Survey, Pan-STARRS1, and CFHT images of the field do not reveal any permanent source at the transient location. The spatially closest source reported in the Legacy Survey DR8 catalog (RA, Dec = 250.3368d, 57.1377d) has a separation of 5.7 arcsec from ZTF20abtxwfx, is extended (not point-like), has photometric redshift of z = 0.73 +- 0.2, and has reported magnitudes of g=23.37 mag, r=22.63 mag, and z=21.86 mag.

We searched the GCN archives, the Fermi GBM catalog, the Fermi GBM subthreshold catalog, and the Konus-Wind triggered and waiting mode data for any possible gamma-ray burst (GRB) counterpart. We found one possible counterpart, GRB 200817A (Fermi GBM Team, GCN #28254). The trigger time of GRB 200817A was on 2020-08-17 09:25:20 UT, which lies in the time interval between our last optical non-detection and T_det. ZTF20abtxwfx is located in the 93rd percentile of the localization region, at an angular distance of about 32 degrees from the central GRB position. We confirm that the position of ZTF20abtxwfx was within the GBM field-of-view at the time of the GRB.

Multi-wavelength follow-up of ZTF20abtxwfx is encouraged. 


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;
IITB, 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 with Kowalski (Duev et al. 2019).

Show current TNS values
CatalogNameReported RAReported DECReported Obj-TypeReported RedshiftHost NameHost RedshiftRemarksTNS RATNS DECTNS Obj-TypeTNS Redshift
TNS2020sev [ZTF20abtxwfx]16:41:21.212+57:08:20.5016:41:21.212+57:08:20.50