AstroNote 2021-58

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DRAFT
2021-02-10 06:30:36
Type: Object/s-Discovery/Classification
ZTF21aahifke/AT2021clk: ZTF discovery of an optical fast transient
Authors: Igor Andreoni (Caltech), Michael Coughlin (UMN), Tomas Ahumada (UMD), Harsh Kumar (IITB), Mansi Kasliwal (Caltech), Erik Kool (OKC), Shreya Anand, Ana Sagues Carracedo (OKC)
Source Group: ZTF
Abstract:
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), as part of the "ZTFReST" project aiming at real-time kilonova and fast transient discovery. 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. ZTF21aahifke is located at Galactic latitude b_Gal = -20.1 deg. 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.

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).
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Date (UT) | mag | emag | band
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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.

Show current TNS values
CatalogNameReported RAReported DECReported Obj-TypeReported RedshiftHost NameHost RedshiftRemarksTNS RATNS DECTNS Obj-TypeTNS Redshift
TNS2021clk [ZTF21aahifke]02:54:27.540+36:31:56.7402:54:27.540+36:31:56.74