{
  "bibcode": "2021GCN.29446....1A",
  "body": "Igor Andreoni (Caltech), Michael Coughlin (UMN), Tomas Ahumada (UMD), Harsh\nKumar (IITB), Mansi Kasliwal (Caltech), Erik Kool (OKC), Leo Singer\n(NASA/GSFC), Shreya Anand (Caltech), Ana Sagues Carracedo (OKC) on\nbehalf of the Zwicky Transient Facility and GROWTH collaborations\n\nWe report the discovery of the fast optical transient\nZTF21aahifke/AT2021clk with the Zwicky Transient Facility (ZTF, Bellm et\nal. 2019, Graham et al. 2019) at coordinates (J2000, <0.5''):\n\nRA = 02:54:27.54 (43.61475d)\nDec = +36:31:56.80 (+36.53244d)\n\nZTF21aahifke was first detected on 2021-02-06 03:14 UT, hereafter labelled\nT_det. It faded by ~0.9 mag in r-band in the first 24 hours since T_det.\nThe transient was last detected by ZTF on 2021-02-07 03:58 UT at r = 20.9\n+- 0.2 mag. Upper limits constrain the transient onset time to be within ~1\nday from T_det. The color of the transient appears to be red, with g-r~0.3\n(un-corrected) at T_det. The Galactic extinction on the line of sight is\nE(B-V)=0.12 mag (Planck Collaboration et al., 2015). ZTF21aahifke is\nlocated at Galactic latitude b_Gal = -20.1 deg.\n\n\nIn the table below, we report photometry obtained using ForcePhotZTF (Yao\net al., 2019) on images processed in real-time through the ZTF reduction\nand image subtraction pipelines at IPAC (Masci et al. 2019).\n-------------------------------------\nDate (UT) | mag | emag | band\n-------------------------------------\n2021-02-05 04:53 | > 20.6 | - | g\n2021-02-06 03:14 | 20.2 | 0.1 | g\n2021-02-06 04:16 | 19.9 | 0.1 | r\n2021-02-07 03:58 | 20.9 | 0.2 | r\n2021-02-09 03:59 | > 21.0 | - | r\n-------------------------------------\n\nForced photometry on 1109 ZTF images taken before T_det did not reveal any\nprevious activity. Previous activity was also not found in the Pan-STARRS\n(Chambers et al., 2016) DR2 catalog and in ATLAS images, explored via the\npublic forced photometry server (Tonry et al., 2018; Smith et al., 2020).\n\nA possible host galaxy, SDSS J025427.89+363151.5 (r=21.4), is located ~6.6\narcsec away from ZTF21aahifke. The source has SDSS photometric redshift of\nzphot = 0.433 +- 0.1735.\n\nWe do not find any Fermi-identified gamma-ray bursts consistent with the\ndetection location and explosion time of this object, assuming that a GRB\nhappened between the last non-detection and T_det.\n\nThe nature of ZTF21aahifke remains uncertain and we cannot exclude the\npossibility that it is the afterglow of an un-triggered (or off-axis) GRB,\nor Galactic in origin.\n\nFollow-up observations are encouraged.\n\n\nZTF21aahifke was found by the ''ZTF Realtime Search and Triggering''\n(ZTFReST) project, which aims at near real-time identification of\ncompelling kilonova candidates in ZTF data using the methods described in\nAndreoni et al. (2020, ApJ, 904, 2), independently of gravitational-wave or\ngamma-ray triggers.\n\nBased on observations obtained with the Samuel Oschin Telescope 48-inch and\nthe 60-inch Telescope at the Palomar Observatory as part of the Zwicky\nTransient Facility project. ZTF is supported by the National Science\nFoundation under Grant No. AST-2034437 and a collaboration including\nCaltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center\nat Stockholm University, the University of Maryland, Deutsches\nElektronen-Synchrotron and Humboldt University, the TANGO Consortium of\nTaiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin,\nLawrence Livermore National Laboratories, and IN2P3, France. Operations are\nconducted by COO, IPAC, and UW.",
  "circularId": 29446,
  "createdOn": 1612939525000,
  "email": "igor.andreoni@gmail.com",
  "subject": "ZTF21aahifke/AT2021clk: ZTF discovery of an optical fast transient (possible afterglow)",
  "submitter": "Igor Andreoni at Caltech  <igor.andreoni@gmail.com>",
  "eventId": "ZTF21aahifke"
}