[vsnet-grb-info 27343] ZTF21aahifke/AT2021clk: ZTF discovery of an optical fast transient (possible afterglow)

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29446
SUBJECT: ZTF21aahifke/AT2021clk: ZTF discovery of an optical fast transient (possible afterglow)
DATE:    21/02/10 06:45:25 GMT
FROM:    Igor Andreoni at Caltech  <igor.andreoni at gmail.com>

Igor Andreoni (Caltech), Michael Coughlin (UMN), Tomas Ahumada (UMD), Harsh
Kumar (IITB), Mansi Kasliwal (Caltech), Erik Kool (OKC), Leo Singer
(NASA/GSFC), Shreya Anand (Caltech), Ana Sagues Carracedo (OKC) on
behalf of the Zwicky Transient Facility and GROWTH collaborations

We report the discovery of the fast optical transient
ZTF21aahifke/AT2021clk with the Zwicky Transient Facility (ZTF, Bellm et
al. 2019, Graham et al. 2019) at coordinates (J2000, <0.5''):

RA = 02:54:27.54 (43.61475d)
Dec = +36:31:56.80 (+36.53244d)

ZTF21aahifke was first detected on 2021-02-06 03:14 UT, hereafter labelled
T_det. It faded by ~0.9 mag in r-band in the first 24 hours since T_det.
The transient was last detected by ZTF on 2021-02-07 03:58 UT at r = 20.9
+- 0.2 mag. 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
(un-corrected) at T_det. The Galactic extinction on the line of sight is
E(B-V)=0.12 mag (Planck Collaboration et al., 2015). ZTF21aahifke is
located at Galactic latitude b_Gal = -20.1 deg.


In the table below, we report photometry obtained using 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
-------------------------------------
2021-02-05 04:53 | > 20.6 | - | g
2021-02-06 03:14 | 20.2 | 0.1 | g
2021-02-06 04:16 | 19.9 | 0.1 | r
2021-02-07 03:58 | 20.9 | 0.2 | r
2021-02-09 03:59 | > 21.0 | - | r
-------------------------------------

Forced photometry on 1109 ZTF images taken before T_det did not reveal any
previous activity. Previous activity was also not found in the Pan-STARRS
(Chambers et al., 2016) DR2 catalog and in ATLAS images, explored via the
public forced photometry server (Tonry et al., 2018; Smith et al., 2020).

A possible host galaxy, SDSS J025427.89+363151.5 (r=21.4), is located ~6.6
arcsec away from ZTF21aahifke. The source has SDSS photometric redshift of
zphot = 0.433 +- 0.1735.

We do not find any Fermi-identified gamma-ray bursts consistent with the
detection location and explosion time of this object, assuming that a GRB
happened between the last non-detection and T_det.

The nature of ZTF21aahifke remains uncertain and we cannot exclude the
possibility that it is the afterglow of an un-triggered (or off-axis) GRB,
or Galactic in origin.

Follow-up observations are encouraged.


ZTF21aahifke was found by the ''ZTF Realtime Search and Triggering''
(ZTFReST) project, which aims at near real-time identification of
compelling kilonova candidates in ZTF data using the methods described in
Andreoni et al. (2020, ApJ, 904, 2), independently of gravitational-wave or
gamma-ray triggers.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and
the 60-inch Telescope at the Palomar Observatory as part of the Zwicky
Transient Facility project. ZTF is supported by the National Science
Foundation under Grant No. AST-2034437 and a collaboration including
Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center
at Stockholm University, the University of Maryland, Deutsches
Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of
Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin,
Lawrence Livermore National Laboratories, and IN2P3, France. Operations are
conducted by COO, IPAC, and UW.