TITLE:   GCN CIRCULAR
NUMBER:  28841
SUBJECT: ZTF20acozryr/AT2020yxz: Zwicky Transient Facility discovery of a fast optical transient with no associated GRB
DATE:    20/11/05 21:01:37 GMT
FROM:    Michael Coughlin at U of Minnesota  <cough052@umn.edu>

Michael Coughlin (UMN), Igor Andreoni (Caltech), Shreya Anand (Caltech),
Erik Kool (OKC), Ana Sagues Carracedo (OKC), Anna Ho (UCB), Mansi Kasliwal
(Caltech), Daniel Perley (LJMU) 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 ZTF20acozryr/AT2020yxz
with the Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al.
2019) at coordinates (J2000, <0.5”):

RA = 02:48:44.31 (42.1846d)

Dec = +12:08:14.08 (+12.1372d)

ZTF20acozryr was found by the new “ZTF Realtime Search and Triggering”
(ZTF-ReST) project, which aims 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.

ZTF20acozryr was first detected on 2020-11-03 09:44 UT, hereafter labelled
T_det. It faded by ~0.7 mag in g-band in the first 0.9 days since T_det.
The transient was last detected on 2020-11-05 08:13:20 UT at r = 19.9 ± 0.2
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~0.3 at T_det. The Galactic extinction on the line of sight is low, with
E(B-V)=0.10 mag (Planck Collaboration et al., 2015).

In the table below, we report photometry obtained 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

-----------------±-------±-----±-----

2020-11-03 09:44 | > 19.4 |  -  | g

2020-11-04 08:47 | 19.5 | 0.2 | g

2020-11-04 10:28 | 19.2 | 0.1 | r

2020-11-05 06:43 | 20.2 | 0.3 | g

2020-11-05 08:13 | 19.9 | 0.2 | r

-----------------±-------±-----±-----

ZTF20acozryr is located off the Galactic plane, with Galactic latitude
b_Gal = -41.5 deg. Deep Legacy Survey and Pan-STARRS1 images of the field
do not reveal any permanent source at the transient location. There are two
nearby sources reported in the Legacy Survey DR8 catalog. The first source
(RA, Dec = 42.1845d, 12.1358d) has a separation of 5.24 arcsec (with a
galaxy classification from DR8) from ZTF20acozryr and has reported
magnitudes of g=24.38 mag, r=23.45 mag, and z=23.04 mag. The second source
(RA, Dec = 42.1861d, 12.1395d) has a separation of 9.60 arcsec from
ZTF20acozryr (with a point source classification from DR8) and has reported
magnitudes of g=24.84 mag, r=24.07 mag, and z=23.49 mag.

There do not appear to be any publicly reported GRBs consistent with the
sky location between the last non-detection and the first detection.

We encourage spectroscopic classification and multi-wavelength follow-up to
discern the nature of ZTF20acozryr, a potential orphan afterglow candidate.

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).

[GCN OPS NOTE(05nove2020): Per author's request, in the 2nd paragraph
the sentence   "ZTF20acozryr was first detected on 2020-11-03 09:44 UT,..."
was changed to "ZTF20acozryr was first detected on 2020-11-04 08:47 UT,...".]