TITLE: GCN CIRCULAR NUMBER: 27153 SUBJECT: LIGO/Virgo S200213t: Swift UVOT and XRT observations of ZTF20aamvmzj (AT2020cja) DATE: 20/02/20 13:50:37 GMT FROM: Samantha Oates at MSSL S. R. Oates (U. of Birmingham), K. L. Page (U.Leicester), A. A. Breeveld (MSSL-UCL), P. Brown (TAMU), M. De Pasquale (Istanbul U.), C. Gronwall (PSU), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), M. J. Page (UCL-MSSL), M. H. Siegel (PSU), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), S. D. Barthelmy (NASA/GSFC), A. P. Beardmore (U. Leicester), D. N. Burrows (PSU), S. Campana (INAF-OAB), S. B. Cenko (NASA/GSFC), G. Cusumano (INAF-IASF PA), V. D’Elia (ASDC), P. A. Evans (U. Leicester),P. Giommi (ASI), D. Hartmann (Clemson U.), J. A. Kennea (PSU), N. J. Klingler (PSU), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), D. B. Malesani (DTU Space), A. Melandri (INAF-OAB), J. A. Nousek (PSU), P. T. O'Brien (U. Leicester), J. P. Osborne (U. Leicester), C. Pagani (U. Leicester), D. M. Palmer (LANL), M. Perri (ASDC), J. L. Racusin (NASA/GSFC), T. Sakamoto (AGU), B. Sbarufatti (INAF-OAB/PSU), G. Tagliaferri (INAF-OAB), A. Tohuvavohu (U Toronto), and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift team: We report Swift/UVOT and XRT follow-up observations of ZTF20aamvmzj (AT2020cja; Kasliwal et al., GCN Circ. 27051) found by Zwicky Transient Facility during the search for the EM counterpart of the LVC event S200213t (LIGO/VIRGO Collaboration GCN Circ. 27042). This object was also observed by Lulin (Li et al., GCN Circ. 27082) and spectroscopically by DBSP (Ho et al., GCN Circ. 27074). Swift/UVOT and XRT observations began 6.7 days after the LIGO/VIRGO trigger (LIGO/VIRGO Collaboration GCN Circ. 27042). We detect ZTF20aamvmzj in the u and UV filters only. In the following we give preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373): Filter T_start(s) T_stop(s) Exp(s) Mag v 577293 588978 228 >19.2 b 576882 588576 236 >19.92 u 576797 588492 236 19.05 +/- 0.27 w1 576633 588408 472 19.46 +/- 0.30 m2 577377 583756 753 19.82 +/- 0.29 w2 576967 588902 944 19.64 +/- 0.23 The magnitudes in the table are not corrected for the Galactic extinction due to the significant reddening of E(B-V) = 0.23 in the direction of the burst (Schlegel et al. 1998). In XRT, no source was found at the location of ZTF20aamvmzj, down to a 3-sigma upper limit of 3.1e-3 count s^-1 in 3 ks of exposure. Assuming a power-law spectrum with NH=3x10^20 cm^-2, and photon index (Gamma)=1.7, this equates to a upper limit of F < 1.2x10^-13 erg cm^-2 s^-1 on the 0.3-10 keV observed flux. This circular is an official product of the Swift team.