//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30652 SUBJECT: GRB 210818A: Swift detection of a burst with an optical counterpart DATE: 21/08/18 01:26:40 GMT FROM: Kim Page at U.of Leicester A. P. Beardmore (U Leicester), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:02:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210818A (trigger=1069169). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 149.131, -87.031 which is RA(J2000) = 09h 56m 32s Dec(J2000) = -87d 01' 52" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 60 sec. The peak count rate was ~28000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 01:03:06.9 UT, 59.0 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 148.8666, -87.0253 which is equivalent to: RA(J2000) = 09h 55m 27.99s Dec(J2000) = -87d 01' 31.0" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 53 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.74 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.7 (+2.38/-2.11) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 1.40e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 66 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 09:55:28.15 = 148.86729 DEC(J2000) = -87:01:28.8 = -87.02467 with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.93 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.213. Although this was originally reported in the GCN Notices as a known source we believe that this is a GRB. Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30653 SUBJECT: GRB 210818A: Enhanced Swift-XRT position DATE: 21/08/18 05:20:55 GMT FROM: Phil Evans at U of Leicester J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team. Using 1521 s of XRT Photon Counting mode data and 1 UVOT images for GRB 210818A, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 148.86765, -87.02526 which is equivalent to: RA (J2000): 09h 55m 28.23s Dec (J2000): -87d 01' 30.9" with an uncertainty of 2.2 arcsec (radius, 90% confidence). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular was automatically generated, and is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30654 SUBJECT: GRB 210818A: AGILE detection DATE: 21/08/18 08:27:40 GMT FROM: Alessandro Ursi at INAF/IAPS A. Ursi (INAF/IAPS), F. Verrecchia (SSDC, and INAF/OAR), C. Casentini (INAF/IAPS), P. Tempesta (TeleSpazio), M. Tavani (INAF/IAPS, and Univ. Roma Tor Vergata), A. Argan, M. Cardillo, Y. Evangelista, G. Piano (INAF/IAPS), F. Lucarelli, C. Pittori, (SSDC, and INAF/OAR), A. Bulgarelli, V. Fioretti, F. Fuschino, N. Parmiggiani (INAF/OAS-Bologna), M. Marisaldi (INAF/OAS-Bologna, and Bergen University), M. Pilia, A. Trois (INAF/OA-Cagliari), I. Donnarumma (ASI), F. Longo (Univ. Trieste and INFN Trieste), A. Giuliani (INAF/IASF-Mi), report on behalf of the AGILE Team: The AGILE satellite detected the long GRB 210818A at T0 = 2021-08-18 01:02:08.19 s (UTC), reported by Swift (GCNs #30652, #30653). The burst is clearly visible in the AGILE scientific ratemeters of the SuperAGILE (SA; 20-60 keV), MiniCALorimeter (MCAL; 0.4-100 MeV), and AntiCoincidence (AC; 50-200 keV) detectors. The event lasted about 35 s and it released a total number of 3020 counts in the SA detector (above a background rate of 70 Hz), 48170 counts in the MCAL detector (above a background rate of 1250 Hz), and 130730 counts in the AC detector (above a background rate of 3650 Hz). The AGILE ratemeter light curves can be found at http://www.agilescienceapp.it/notices/GRB210818A_AGILE_RM.png . The event also triggered a partial high time resolution MCAL data acquisition, from T1 = 2021-08-18 01:02:07.81 s +/- 0.01 (UTC) to T2 =2021-08-18 01:02:21.18 +/- 0.01 s (UTC), and released 21640 counts in the detector, above a background rate of 625 Hz. The MCAL light curve can be found at: http://www.agilescienceapp.it/notices/074408_GRB_556333328.188243.png . Additional analysis of AGILE data is in progress. Automatic MCAL GRB alert Notices can be found at: https://gcn.gsfc.nasa.gov/agile_mcal.html. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30655 SUBJECT: GRB 210818A: AstroSat CZTI detection DATE: 21/08/18 12:01:28 GMT FROM: Gaurav Waratkar at IIT,Bombay V. Prasad (IUCAA), P. Sawant (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al, 2020, arxiv:2011.07067) showed detection of a bright long GRB 210818A, which was also detected by Swift-BAT (Beardmore et al., GCN #30652), Swift-XRT (Osborne et al., GCN #30653) and AGILE (Ursi et al., GCN #30654). The source was clearly detected in the 20-200 keV energy range, with the strongest peak at 2021-08-18 01:02:10.5 UT in the single-peak light curve. The measured peak count rate associated with the burst is 1018 (+64, -64) cts/s above the background in the combined data of four quadrants, with a total of 13993 (+552, -512) cts. The local mean background count rate was 526 (+3, -3) cts/s. Using cumulative rates, we measure a T90 of 33 (+4, -3) s. It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range, with the strongest peak at 2021-08-18 01:02:09.9 UT in the single-peak light curve. The measured peak count rate is 3748 (+116, -126) cts/s above the background in the combined Veto data of four quadrants, with a total of 52829 (+844, -1097) cts. The local mean background count rate was 1616 (+5, -5) cts/s. We measure a T90 of 37 (+3, -2) s from the cumulative Veto light curve. CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb [1]. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. Links: ------ [1] http://astrosat.iucaa.in/czti/?q=grb //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30657 SUBJECT: Konus-Wind detection of GRB 210818A DATE: 21/08/18 18:23:06 GMT FROM: Dmitry Frederiks at Ioffe Institute D. Frederiks, S. Golenetskii, A.Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long GRB 210818A (Swift detection: Beardmore et al., GCN 30652; AGILE detection: Ursi et al., GCN 30654) triggered Konus-Wind (KW) at T0=3729.710 s UT (01:02:09.710). The burst light curve started at ~T0 with a bright, short (<0.5s), hard-spectrum initial pulse, which was followed, after a ~0.5 s interval, by a similarly bright and hard, multi-peaked emission, which gradually decays until ~T0+90 s. We also note a QPO-like behavior of the light curve in the interval from ~T0+1.6 s to ~T0+3.2 s, with the quasi period of ~0.21 s. The emission is seen up to ~7 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB210818_T03729/ As observed by Konus-Wind, the burst had a fluence of (2.67 ± 0.12)x10^-4 erg/cm^2 and a 64-ms peak energy flux, measured from T0 + 0.128 s, of (5.12 ± 0.54)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+80.640 s) is best fit in the 20 keV - 20 MeV range by a power law with exponential cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.99(-0.03,+0.03) and Ep = 1107(-71,+78) keV (chi2 = 117/98 dof). Fitting this spectrum by a Band function yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.2 (chi2 = 117/97 dof). The spectrum near the peak count rate (measured from T0 to T0+0.256 s, that corresponds to the short initial pulse) is best fit in the 20 keV - 20 MeV range by the CPL model with alpha = -0.28(-0.13,+0.14) and Ep = 1247(-133,+47) keV (chi2 = 38/58 dof). Fitting this spectrum by a Band function yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.4 (chi2 = 38/57 dof). All the quoted errors are estimated at the 90% confidence level. All the presented results are preliminary. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30659 SUBJECT: GRB 210818A: Swift-BAT refined analysis DATE: 21/08/18 21:36:00 GMT FROM: Amy Lien at GSFC H. A. Krimm (NSF), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-240 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 210818A (trigger #1069169) (Beardmore et al., GCN Circ. 30652). The BAT ground-calculated position is RA, Dec = 148.949, -87.029 deg which is RA(J2000) = 09h 55m 47.7s Dec(J2000) = -87d 01' 45.3" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 97%. The mask-weighted light curve shows many overlapping pulses on top of a general FRED-like structure. The burst emission starts at ~T0, peaks at ~T+2 s, and ends at ~T+130 s. T90 (15-350 keV) is 73.56 +- 1.55 sec (estimated error including systematics). The time-averaged spectrum from T-0.69 to T+128.97 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.22 +- 0.03. The fluence in the 15-150 keV band is 2.6 +- 0.03 x 10^-5 erg/cm2. The 1-sec peak photon flux measured from T+1.20 sec in the 15-150 keV band is 11.2 +- 0.4 ph/cm2/sec. All the quoted errors are at the 90% confidence level. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30660 SUBJECT: GRB 210818A: Swift-XRT refined Analysis DATE: 21/08/18 22:39:25 GMT FROM: Phil Evans at U of Leicester P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), J. D. Gropp (PSU), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), K.L. Page (U. Leicester) and A.P. Beardmore report on behalf of the Swift-XRT team: We have analysed 7.1 ks of XRT data for GRB 210818A (Beardmore et al. GCN Circ. 30652), from 65 s to 69.5 ks after the BAT trigger. The data comprise 188 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 30653). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=1.04 (+/-0.20). At T+111 s the decay steepens to an alpha of 6.8 (+0.7, -0.6). The light curve breaks again at T+157 s to a decay with alpha=0.67 (+0.38, -0.29), before a final break at T+479 s s after which the decay index is 1.50 (+/-0.06). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 0.86 (+/-0.06). The best-fitting absorption column is 2.5 (+0.5, -0.4) x 10^21 cm^-2, in excess of the Galactic value of 1.7 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.64 (+0.14, -0.11) and a best-fitting absorption column of 1.94 (+0.56, -0.20) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.4 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.94 (+0.56, -0.20) x 10^21 cm^-2 Galactic foreground: 1.7 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.64 (+0.14, -0.11) If the light curve continues to decay with a power-law decay index of 1.50, the count rate at T+24 hours will be 3.1 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x 10^-13 (1.6 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01069169. This circular is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30661 SUBJECT: Swift GRB 210818A: Global MASTER-Net observations report DATE: 21/08/19 01:06:55 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, P.Balanutsa, A.Kuznetsov, F.Balakin, V.Vladimirov, D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, T.Pogrosheva, D.Kuvshinov, D. Cheryasov (Lomonosov Moscow State University, SAI, Physics Department), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar OAFA), R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias), D. Buckley (South African Astronomical Observatory), O.A. Gres, N.M. Budnev, O.Ershova (Irkutsk State University, API), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), V. Yurkov, A. Gabovich, Yu. Sergienko (Blagoveschensk Educational State University) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 210818A ( A. P. Beardmore et al., GCN 30652) errorbox 22995 sec after notice time and 23077 sec after trigger time at 2021-08-18 07:27:44 UT, with upper limit up to 18.9 mag. The observations began at zenith distance = 62 deg. The sun altitude is -47.3 deg. The galactic latitude b = -25 deg., longitude l = 301 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1693376 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 23167 | MASTER-OAFA | C | 180 | 13.8 | 23347 | MASTER-OAFA | C | 540 | 14.9 | Coadd 23379 | MASTER-OAFA | C | 180 | 14.9 | 23559 | MASTER-OAFA | C | 540 | 15.3 | Coadd 23592 | MASTER-OAFA | C | 180 | 14.6 | 23812 | MASTER-OAFA | C | 180 | 14.3 | 24026 | MASTER-OAFA | C | 180 | 14.8 | 24242 | MASTER-OAFA | C | 180 | 15.8 | 24456 | MASTER-OAFA | C | 180 | 15.0 | 24636 | MASTER-OAFA | C | 540 | 17.0 | Coadd 24668 | MASTER-OAFA | C | 180 | 14.9 | 24883 | MASTER-OAFA | C | 180 | 16.4 | 25099 | MASTER-OAFA | C | 180 | 17.3 | 25279 | MASTER-OAFA | C | 540 | 18.0 | Coadd 25329 | MASTER-OAFA | C | 180 | 17.4 | 25546 | MASTER-OAFA | C | 180 | 17.4 | 25762 | MASTER-OAFA | C | 180 | 16.9 | 25942 | MASTER-OAFA | C | 540 | 18.0 | Coadd 25975 | MASTER-OAFA | C | 180 | 18.0 | 26190 | MASTER-OAFA | C | 180 | 17.3 | 26405 | MASTER-OAFA | C | 180 | 17.3 | 26585 | MASTER-OAFA | C | 540 | 17.6 | Coadd 26621 | MASTER-OAFA | C | 180 | 17.7 | 26833 | MASTER-OAFA | C | 180 | 17.5 | 27049 | MASTER-OAFA | C | 180 | 17.7 | 27229 | MASTER-OAFA | C | 540 | 18.3 | Coadd 27264 | MASTER-OAFA | C | 180 | 17.5 | 27479 | MASTER-OAFA | C | 180 | 17.7 | 27691 | MASTER-OAFA | C | 180 | 17.5 | 27871 | MASTER-OAFA | C | 540 | 18.5 | Coadd 27906 | MASTER-OAFA | C | 180 | 17.6 | 28123 | MASTER-OAFA | C | 180 | 18.2 | 28337 | MASTER-OAFA | C | 180 | 17.7 | 28517 | MASTER-OAFA | C | 540 | 18.1 | Coadd 28551 | MASTER-OAFA | C | 180 | 17.3 | 28765 | MASTER-OAFA | C | 180 | 18.0 | 28983 | MASTER-OAFA | C | 180 | 18.0 | 29199 | MASTER-OAFA | C | 180 | 17.9 | 29415 | MASTER-OAFA | C | 180 | 17.7 | 29595 | MASTER-OAFA | C | 540 | 18.0 | Coadd 29630 | MASTER-OAFA | C | 180 | 17.5 | 29810 | MASTER-OAFA | C | 540 | 18.6 | Coadd 29842 | MASTER-OAFA | C | 180 | 18.2 | 30058 | MASTER-OAFA | C | 180 | 18.4 | 30272 | MASTER-OAFA | C | 180 | 18.2 | 30488 | MASTER-OAFA | C | 180 | 17.8 | 30701 | MASTER-OAFA | C | 180 | 18.0 | 30881 | MASTER-OAFA | C | 540 | 18.9 | Coadd 30917 | MASTER-OAFA | C | 180 | 18.1 | 31130 | MASTER-OAFA | C | 180 | 18.3 | 31329 | MASTER-OAFA | C | 180 | 18.3 | 31729 | MASTER-OAFA | C | 180 | 18.4 | 31909 | MASTER-OAFA | C | 540 | 18.5 | Coadd 31928 | MASTER-OAFA | C | 180 | 18.4 | 32127 | MASTER-OAFA | C | 180 | 18.0 | 32327 | MASTER-OAFA | C | 180 | 17.4 | 32507 | MASTER-OAFA | C | 540 | 18.0 | Coadd 32527 | MASTER-OAFA | C | 180 | 17.3 | 32726 | MASTER-OAFA | C | 180 | 17.1 | 32926 | MASTER-OAFA | C | 180 | 17.3 | 33106 | MASTER-OAFA | C | 540 | 17.9 | Coadd 33125 | MASTER-OAFA | C | 180 | 17.5 | 33325 | MASTER-OAFA | C | 180 | 17.1 | 33524 | MASTER-OAFA | C | 180 | 16.7 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30662 SUBJECT: Swift GRB210818.68: Global MASTER-Net observations report DATE: 21/08/19 16:21:54 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, P.Balanutsa, A.Kuznetsov, F.Balakin, V.Vladimirov, D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, T.Pogrosheva, D.Kuvshinov, D. Cheryasov (Lomonosov Moscow State University, SAI, Physics Department), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar OAFA), R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias), D. Buckley (South African Astronomical Observatory), O.A. Gres, N.M. Budnev, O.Ershova (Irkutsk State University, API), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), V. Yurkov, A. Gabovich, Yu. Sergienko (Blagoveschensk Educational State University) MASTER-Tavrida robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, SAI Crimea astronomical station) was pointed to the Swift GRB210818.68 (trigger No 1069242,19h 58m 22.32s , +35d 11m 39.5s, R=0.0015) errorbox 5096 sec after notice time and 5059 sec after trigger time at 2021-08-18 17:43:40 UT, with upper limit up to 18.0 mag. Observations started at twilight. The observations began at zenith distance = 26 deg. The sun altitude is -10.8 deg. MASTER-Kislovodsk robotic telescope located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB210818.68 errorbox 6531 sec after notice time and 6495 sec after trigger time at 2021-08-18 18:07:35 UT, with upper limit up to 17.5 mag. The observations began at zenith distance = 16 deg. The sun altitude is -19.6 deg. MASTER-IAC robotic telescope located in Spain (IAC Teide Observatory) was pointed to the Swift GRB210818.68 errorbox 22873 sec after notice time and 22836 sec after trigger time at 2021-08-18 22:39:57 UT, with upper limit up to 17.5 mag. The observations began at zenith distance = 10 deg. The sun altitude is -35.9 deg. MASTER-OAFA robotic telescope located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB210818.68 errorbox 26839 sec after notice time and 26802 sec after trigger time at 2021-08-18 23:46:02 UT, with upper limit up to 15.5 mag. The observations began at zenith distance = 78 deg. The sun altitude is -20.9 deg. The galactic latitude b = 3 deg., longitude l = 72 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1693885 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 5150 | MASTER-Tavrida | C | 180 | 16.7 | 5330 | MASTER-Tavrida | C | 540 | 17.1 | Coadd 5349 | MASTER-Tavrida | C | 180 | 16.9 | 5548 | MASTER-Tavrida | C | 180 | 17.0 | 5747 | MASTER-Tavrida | C | 180 | 17.1 | 5927 | MASTER-Tavrida | C | 540 | 17.4 | Coadd 5946 | MASTER-Tavrida | C | 180 | 17.1 | 6145 | MASTER-Tavrida | C | 180 | 17.2 | 6345 | MASTER-Tavrida | C | 180 | 17.1 | 6525 | MASTER-Tavrida | C | 540 | 17.0 | Coadd 6544 | MASTER-Tavrida | C | 180 | 16.8 | 6585 | MASTER-Kislovodsk | C | 180 | 13.3 | 6765 | MASTER-Kislovodsk | C | 540 | 15.9 | Coadd 6585 | MASTER-Kislovodsk | C | 180 | 13.8 | 6765 | MASTER-Kislovodsk | C | 540 | 16.1 | Coadd 6743 | MASTER-Tavrida | C | 180 | 15.6 | 6786 | MASTER-Kislovodsk | C | 180 | 14.7 | 6786 | MASTER-Kislovodsk | C | 180 | 14.8 | 6942 | MASTER-Tavrida | C | 180 | 17.1 | 7122 | MASTER-Tavrida | C | 540 | 17.5 | Coadd 6986 | MASTER-Kislovodsk | C | 180 | 16.3 | 6986 | MASTER-Kislovodsk | C | 180 | 16.3 | 7141 | MASTER-Tavrida | C | 180 | 17.3 | 7187 | MASTER-Kislovodsk | C | 180 | 16.8 | 7187 | MASTER-Kislovodsk | C | 180 | 16.7 | 7340 | MASTER-Tavrida | C | 180 | 17.3 | 7962 | MASTER-Kislovodsk | C | 180 | 17.0 | 7962 | MASTER-Kislovodsk | C | 180 | 17.0 | 8448 | MASTER-Kislovodsk | C | 180 | 17.5 | 8448 | MASTER-Kislovodsk | C | 180 | 17.4 | 8731 | MASTER-Kislovodsk | C | 180 | 17.3 | 8731 | MASTER-Kislovodsk | C | 180 | 17.3 | 9014 | MASTER-Kislovodsk | C | 180 | 16.4 | 9014 | MASTER-Kislovodsk | C | 180 | 16.5 | 10273 | MASTER-Tavrida | C | 180 | 16.9 | 10865 | MASTER-Kislovodsk | C | 180 | 16.6 | 10865 | MASTER-Kislovodsk | C | 180 | 16.7 | 12671 | MASTER-Kislovodsk | C | 180 | 16.3 | 12671 | MASTER-Kislovodsk | C | 180 | 16.3 | 14474 | MASTER-Kislovodsk | C | 180 | 17.0 | 14474 | MASTER-Kislovodsk | C | 180 | 17.0 | 16278 | MASTER-Kislovodsk | C | 180 | 16.1 | 16278 | MASTER-Kislovodsk | C | 180 | 15.9 | 16891 | MASTER-Tavrida | C | 180 | 17.0 | 17790 | MASTER-Tavrida | C | 180 | 17.8 | 18446 | MASTER-Tavrida | C | 180 | 17.8 | 18735 | MASTER-Tavrida | C | 180 | 14.9 | 19022 | MASTER-Tavrida | C | 180 | 15.1 | 20563 | MASTER-Tavrida | C | 180 | 17.7 | 22927 | MASTER-IAC | C | 180 | 17.3 | 22927 | MASTER-IAC | C | 180 | 17.0 | 23127 | MASTER-IAC | C | 180 | 17.4 | 23127 | MASTER-IAC | C | 180 | 17.1 | 23527 | MASTER-IAC | C | 180 | 17.3 | 23527 | MASTER-IAC | C | 180 | 17.2 | 23932 | MASTER-IAC | C | 180 | 17.4 | 23932 | MASTER-IAC | C | 180 | 17.1 | 24400 | MASTER-Tavrida | C | 180 | 18.0 | 24934 | MASTER-IAC | C | 180 | 17.3 | 24934 | MASTER-IAC | C | 180 | 17.1 | 26735 | MASTER-IAC | C | 180 | 17.4 | 26735 | MASTER-IAC | C | 180 | 17.2 | 26892 | MASTER-OAFA | C | 180 | 15.0 | 27128 | MASTER-OAFA | C | 180 | 15.3 | 27718 | MASTER-OAFA | C | 180 | 15.5 | 28166 | MASTER-Tavrida | C | 180 | 18.0 | 28739 | MASTER-IAC | C | 180 | 17.3 | 28739 | MASTER-IAC | C | 180 | 17.2 | 30740 | MASTER-IAC | C | 180 | 17.3 | 30740 | MASTER-IAC | C | 180 | 17.1 | 32543 | MASTER-IAC | C | 180 | 17.3 | 32543 | MASTER-IAC | C | 180 | 17.0 | 34348 | MASTER-IAC | C | 180 | 17.3 | 34348 | MASTER-IAC | C | 180 | 17.0 | 36161 | MASTER-IAC | C | 180 | 17.3 | 36161 | MASTER-IAC | C | 180 | 17.2 | 37985 | MASTER-IAC | C | 180 | 17.4 | 37985 | MASTER-IAC | C | 180 | 17.2 | 39809 | MASTER-IAC | C | 180 | 17.5 | 39809 | MASTER-IAC | C | 180 | 17.5 | 41613 | MASTER-IAC | C | 180 | 17.4 | 41613 | MASTER-IAC | C | 180 | 17.3 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.