[vsnet-grb-info 26205] Zwicky Transient Facility discovery of ZTF20abtxwfx/AT2020sev, a fast optical transient and candidate afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28305
SUBJECT: Zwicky Transient Facility discovery of ZTF20abtxwfx/AT2020sev, a fast optical transient and candidate afterglow
DATE:    20/08/29 02:48:17 GMT
FROM:    Igor Andreoni at Caltech  <igor.andreoni at gmail.com>

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)
on behalf of the Zwicky Transient Facility (ZTF) and Global Relay of
Observatories Watching Transients Happen (GROWTH) collaborations

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 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 at UW
(Patterson et al. 2019). Alert database searches are done with
Kowalski (Duev et al. 2019).