{
  "bibcode": "2020GCN.28305....1A",
  "body": "Igor Andreoni (Caltech), Anna Ho (UCB), Michael Coughlin (UMN), Mansi\nKasliwal (Caltech), Eric Burns (LSU), Dmitry Svinkin (Ioffe\nInstitute), Tomas Ahumada (UM), Yuhan Yao (Caltech), Daniel Perley\n(LJMU), Shreya Anand (Caltech), Erik Kool (OKC), Ana Sagues Carracedo\n(OKC)\non behalf of the Zwicky Transient Facility (ZTF) and Global Relay of\nObservatories Watching Transients Happen (GROWTH) collaborations\n\nWe report the discovery of the fast optical transient\nZTF20abtxwfx/AT2020sev with the Zwicky Transient Facility (ZTF, Bellm\net al. 2019, Graham et al. 2019) at coordinates:\n\nRAJ2000 = 16:41:21.21 (250.338384d)\nDecJ2000 = +57:08:20.5 (+57.139027d)\n\nZTF20abtxwfx was found during the science validation of the new \"ZTF\nRealtime Search and Triggering\" (ZTF-ReST) project. ZTF-ReST will aim\nat near real-time identification of kilonova candidates in ZTF data\nusing the methods described in Andreoni et al. (2020d), independently\nof gravitational-wave or gamma-ray triggers.\n\nZTF20abtxwfx was first detected on 2020-08-18 05:20 UT, heareafter\nlabelled T_det. ZTF20abtxwfx faded by ~1.3 mag in r-band in the first\n2 days since T_det. The transient was last detected on 2020-08-23\n04:51 UT at r = 20.98 +- 0.24 mag. Stringent upper limits constrain\nthe transient onset time to be within ~1 day from T_det. The color of\nthe transient appears to be red, with g-r~0.1 and g-i~0.3 one day\nafter T_det. The Galactic extinction on the line of sight is low, with\nE(B-V)=0.015 mag (Planck Collaboration et al., 2015).\n\nIn the table below, we report forced photometry obtained with\nForcePhotZTF (Yao et al., 2019) on images processed in real-time\nthrough the ZTF reduction and image subtraction pipelines at IPAC\n(Masci et al. 2019).\n\n-----------------+--------+------+------\n    Date (UT)    |  mag   | emag | band\n-----------------+--------+------+------\n2020-08-17 06:45 | > 21.0 |      |  r\n2020-08-18 05:20 | 19.22  | 0.04 |  r\n2020-08-19 04:19 | 20.03  | 0.11 |  r\n2020-08-19 05:23 | 20.16  | 0.09 |  g\n2020-08-19 05:53 | 19.83  | 0.14 |  i\n2020-08-20 04:47 | 20.54  | 0.15 |  r\n2020-08-20 05:46 | 20.66  | 0.16 |  g\n2020-08-21 04:25 | 20.71  | 0.17 |  r\n2020-08-21 06:54 | > 20.8 |      |  g\n2020-08-23 04:12 | > 21.0 |      |  g\n2020-08-23 04:51 | 20.98  | 0.24 |  r\n2020-08-24 04:26 | > 20.5 |      |  r\n-----------------+--------+------+------\n\n\nZTF20abtxwfx is located at high Galactic latitude b_Gal = 40.0 deg.\nDeep Legacy Survey, Pan-STARRS1, and CFHT images of the field do not\nreveal any permanent source at the transient location. The spatially\nclosest source reported in the Legacy Survey DR8 catalog (RA, Dec =\n250.3368d, 57.1377d) has a separation of 5.7 arcsec from ZTF20abtxwfx,\nis extended (not point-like), has photometric redshift of z = 0.73 +-\n0.2, and has reported magnitudes of g=23.37 mag, r=22.63 mag, and\nz=21.86 mag.\n\nWe searched the GCN archives, the Fermi GBM catalog, the Fermi GBM\nsubthreshold catalog, and the Konus-Wind triggered and waiting mode\ndata for any possible gamma-ray burst (GRB) counterpart. We found one\npossible counterpart, GRB 200817A (Fermi GBM Team, GCN #28254). The\ntrigger time of GRB 200817A was on 2020-08-17 09:25:20 UT, which lies\nin the time interval between our last optical non-detection and T_det.\nZTF20abtxwfx is located in the 93rd percentile of the localization\nregion, at an angular distance of about 32 degrees from the central\nGRB position. We confirm that the position of ZTF20abtxwfx was within\nthe GBM field-of-view at the time of the GRB.\n\nMulti-wavelength follow-up of ZTF20abtxwfx is encouraged.\n\n\nZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC,\nUSA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,\nGermany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan;\nIITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia.\nZTF acknowledges the generous support of the NSF under AST MSIP Grant No\n1440341. GROWTH acknowledges generous support of the NSF under PIRE\nGrant No 1545949. Alert distribution service provided by DIRAC@UW\n(Patterson et al. 2019). Alert database searches are done with\nKowalski (Duev et al. 2019).",
  "circularId": 28305,
  "createdOn": 1598669297000,
  "email": "igor.andreoni@gmail.com",
  "subject": "Zwicky Transient Facility discovery of ZTF20abtxwfx/AT2020sev, a fast optical transient and candidate afterglow",
  "submitter": "Igor Andreoni at Caltech  <igor.andreoni@gmail.com>",
  "eventId": "ZTF20abtxwfx"
}