//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29662 SUBJECT: GRB 210318A: Swift detection of a burst DATE: 21/03/18 04:09:40 GMT FROM: Caryl Gronwall at PSU/Swift-UVOT E. Troja (NASA/GSFC/UMCP), C. Gronwall (PSU), J.D. Gropp (PSU), N. J. Klingler (PSU), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 03:54:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210318A (trigger=1037339). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 33.930, -37.789 which is RA(J2000) = 02h 15m 43s Dec(J2000) = -37d 47' 21" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 30 sec. The peak count rate was ~700 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 03:56:35.2 UT, 123.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 33.9263, -37.7772 which is equivalent to: RA(J2000) = 02h 15m 42.32s Dec(J2000) = -37d 46' 38.1" with an uncertainty of 3.2 arcseconds (radius, 90% containment). This location is 43 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.88 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 126 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.016. Burst Advocate for this burst is E. Troja (eleonora.troja AT nasa.gov). 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: 29663 SUBJECT: GRB210318B: Swift detection of a burst DATE: 21/03/18 16:04:20 GMT FROM: Eleonora Troja at NASA/GSFC/UMD E. Troja (NASA/GSFC, UMCP), K. L. Page (U. Leicester), A. Lien (GSFC/UMBC), M. Siegel (PSU), and P. Evans (U. Leicester) report on behalf of the Neil Gehrels Swift Observatory Team: At 03:08:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210318B (trigger=1037335). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 264.227, -35.819 which is RA(J2000) = 17h 36m 55s Dec(J2000) = -35d 49' 08" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed several overlapping pulses with a duration of about 20 sec. The peak count rate was ~22000 counts/sec (15-350 keV), at ~3 sec after the trigger. We have analysed 1.4 ks of XRT data for GRB 210318B, from 52 s to 1.5 ks after the BAT trigger. The data comprise 584 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 264.2328, -35.8220 which is equivalent to: RA (J2000): 17 36 55.88 Dec(J2000): -35 49 19.2 with an uncertainty of 3.5 arcsec (radius, 90% confidence). The light curve can be modelled with an initial power-law decay with an index of alpha=0.71 (+0.03, -0.04), followed by a break at T+564 s to an alpha of 1.23 (+0.18, -0.24). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.78 (+/-0.09). The best-fitting absorption column is 1.43 (+/-0.13) x 10^22 cm^-2, in excess of the Galactic value of 7.1 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.69 (+0.27, -0.25) and a best-fitting absorption column of 1.6 (+/-0.4) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.5 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.6 (+/-0.4) x 10^22 cm^-2 Galactic foreground: 7.1 x 10^21 cm^-2 Excess significance: 4.0 sigma Photon index: 1.69 (+0.27, -0.25) If the light curve continues to decay with a power-law decay index of 1.23, the count rate at T+24 hours will be 0.011 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.9 x 10^-13 (1.1 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01037335. UVOT took a finding chart exposure of 150 seconds with the White filter starting 57 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle with a 3-sigma upper limit of 20.1 mag. No correction has been made for the large, but uncertain, extinction expected. Due to a temporary TDRSS outage, no automatic GCN notices or alerts were promptly distributed. Burst Advocate for this burst is E. Troja (eleonora@umd.edu). 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: 29664 SUBJECT: Konus-Wind detection of GRB 210318B DATE: 21/03/18 17:27:52 GMT FROM: Dmitry Svinkin at Ioffe Institute D. Svinkin, S. Golenetskii, D. Frederiks, M. Ulanov, A. Tsvetkova, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, report: The long-duration GRB 210318B (Swift-BAT detection: Troja et al., GCN Circ. 29663) triggered Konus-Wind at T0=11309.57 s UT (03:08:29.570). The burst light curve shows a multipeaked structure which starts at ~T0-0.5 s and has a total duration of ~14 s. The emission is seen up to ~3 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB210318_T11309/ As observed by Konus-Wind, the burst had a fluence of 3.83(-0.44,+0.50)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+1.872 s, of 1.57(-0.28,+0.29)x10^-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+13.824 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.81(-0.12,+0.15), the high energy photon index beta = -2.51(-0.39,+0.23), the peak energy Ep = 264(-32,+33) keV (chi2 = 70/82 dof). The spectrum near the maximum count rate (measured from T0+0.256 to T0+5.632 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.64(-0.13,+0.14), the high energy photon index beta = -2.59(-0.45,+0.23), the peak energy Ep = 297(-32,+36) keV (chi2 = 67/81 dof). All the quoted errors are at the 90% confidence level. All the quoted values are preliminary. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29665 SUBJECT: Swift GRB 210318A: Global MASTER-Net observations report DATE: 21/03/18 18:04:28 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 210318A ( E. Troja et al., GCN 29662) errorbox 50452 sec after notice time and 50501 sec after trigger time at 2021-03-18 17:56:12 UT, with upper limit up to 19.7 mag. Observations started at twilight. The observations began at zenith distance = 58 deg. The sun altitude is -14.2 deg. The galactic latitude b = -69 deg., longitude l = 248 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1570922 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 50591 | MASTER-SAAO | C | 180 | 19.7 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29666 SUBJECT: GRB 210318B: Swift-BAT refined analysis DATE: 21/03/18 20:57:27 GMT FROM: Hans Krimm at NSF/NASA-GSFC M. Stamatikos (OSU), S. D. Barthelmy (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), E. Troja (NASA/GSFC, UMCP), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-61 to T+242 sec from the recent telemetry downlink, we report further analysis of BAT GRB 210318B (trigger #1037335) (Troja, et al., GCN Circ. 29663). The BAT ground-calculated position is RA, Dec = 264.228, -35.823 deg which is RA(J2000) = 17h 36m 54.7s Dec(J2000) = -35d 49' 21.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 95%. The mask-weighted light curve shows a multi-peak structure running from ~T-4 sec to T+15 sec. T90 (15-350 keV) is 14.95 +- 0.50 sec (estimated error including systematics). The time-averaged spectrum from T-11.22 to T+19.42 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.10 +- 0.13, and Epeak of 252.1 +- 191.6 keV (chi squared 41.33 for 0 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.0 +- 0.02 x 10^-5 erg/cm2 and the 1-sec peak flux measured from T+2.70 sec in the 15-150 keV band is 14.5 +- 0.4 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.31 +- 0.03 (chi squared 48.69 for 0 d.o.f.). All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/1037335/BA/ //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29668 SUBJECT: GRB 210318B: Fermi GBM detection DATE: 21/03/18 21:59:41 GMT FROM: Christian Malacaria at NASA-MSFC/USRA C. Malacaria (NASA-MSFC/USRA) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: At 03:08:46.50 UT on 18 March 2021, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 210318B (trigger 637729731 / 210318131), which was also detected by the Swift/BAT (Troja et al. 2019, GCN 29663) and by Konus-Wind (Svinkin et al. 2021, GCN 29664). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time is 91 degrees. The GBM light curve shows a peak with a duration (T90) of about 24.3 s (50-300 keV). The time-averaged spectrum from T0-4.1 s to T0+0.0 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is 2.72 +/- 1.53 and the cutoff energy, parameterized as Epeak, is 84.5 +/- 8.7 keV. The event fluence (10-1000 keV) in this time interval is (2.537 +/- 0.393)E-07 erg/cm^2. The 1-sec peak photon flux measured starting from T0-1.9 s in the 10-1000 keV band is 1.4 +/- 0.2 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/ //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29669 SUBJECT: Swift GRB210318.13: Global MASTER-Net observations report DATE: 21/03/18 22:06:18 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB210318.13 (trigger No 1037335,17h 36m 55.88s , -35d 49m 19.2s, R=0.01) errorbox 65903 sec after trigger time at 2021-03-18 21:26:56 UT, with upper limit up to 17.3 mag. The observations began at zenith distance = 83 deg. The sun altitude is -52.2 deg. The galactic latitude b = -3 deg., longitude l = 354 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1571373 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 65934 | MASTER-SAAO | C | 60 | 14.1 | 66013 | MASTER-SAAO | C | 60 | 14.5 | 66093 | MASTER-SAAO | C | 60 | 14.9 | 67868 | MASTER-SAAO | C | 60 | 17.3 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29670 SUBJECT: GRB 210318A: Enhanced Swift-XRT position DATE: 21/03/19 01:16:18 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 1075 s of XRT Photon Counting mode data and 2 UVOT images for GRB 210318A, 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 = 33.92621, -37.77693 which is equivalent to: RA (J2000): 02h 15m 42.29s Dec (J2000): -37d 46' 36.9" with an uncertainty of 2.4 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: 29671 SUBJECT: GRB 210318A: Swift-XRT refined Analysis DATE: 21/03/19 04:20:47 GMT FROM: Phil Evans at U of Leicester B. Sbarufatti (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , D.N. Burrows (PSU), J. D. Gropp (PSU) and E. Troja report on behalf of the Swift-XRT team: We have analysed 3.1 ks of XRT data for GRB 210318A (Troja et al. GCN Circ. 29662), from 107 s to 45.6 ks after the BAT trigger. The data comprise 9 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 29670). The light curve can be modelled with a power-law decay with a decay index of alpha=1.09 (+0.27, -0.29). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.6, -0.5). The best-fitting absorption column is 1.4 (+1.9, -1.2) x 10^21 cm^-2, consistent with the Galactic value of 1.9 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.4 (+1.9, -1.2) x 10^21 cm^-2 Galactic foreground: 1.9 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.8 (+0.6, -0.5) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01037339. This circular is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29672 SUBJECT: GRB 210318A: Swift-BAT refined analysis DATE: 21/03/19 12:04:17 GMT FROM: Hans Krimm at NSF/NASA-GSFC T. Sakamoto (AGU), S. D. Barthelmy (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), M. Stamatikos (OSU), E. Troja (NASA/GSFC, UMCP), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-60 to T+243 sec from the recent telemetry downlink, we report further analysis of BAT GRB 210318A (trigger #1037339) (Troja, et al., GCN Circ. 29662). The BAT ground-calculated position is RA, Dec = 33.892, -37.755 deg which is RA(J2000) = 02h 15m 34.0s Dec(J2000) = -37d 45' 17.7" with an uncertainty of 2.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%. The mask-weighted light curves shows a single broad FRED-shaped peak with some structure. The emission starts at ~T10 sec, peaks at T+5 sec, and falls to background by T+50 sec. T90 (15-350 keV) is 48.00 +- 22.63 sec (estimated error including systematics). The time-averaged spectrum from T-12.37 to T+35.63 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.48 +- 0.33. The fluence in the 15-150 keV band is 6.4 +- 1.3 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from T+11.13 sec in the 15-150 keV band is 0.3 +- 0.3 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/1037339/BA/ //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29675 SUBJECT: GRB 210318B: MITSuME Okayama optical upper limits DATE: 21/03/20 02:11:32 GMT FROM: Ryohei Hosokawa at Tokyo Institute of Technology R. Hosokawa, K. L. Murata, R. Adachi, M. Niwano, F. Ogawa, N.Nakamura, N. Ito, S. Ogata, H. Takamatsu, H. Hara, Y. Yatsu, and N.Kawai (TokyoTech) report on behalf of the MITSuME collaboration: We observed the field of GRB 210318B (Troja et al. GCN Circular #29663, Svinkin et al. GCN Circular #29664, Stamatikos et al. GCN Circular #29666, Lipunov et al. GCN Circular #29667, Malacaria et al. GCN Circular #29668) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm telescope of Okayama Astrophysical Observatory, Okayama, Japan. The observation with a series of 60 sec exposures started at 2021-03-19 19:11:03 UT.(40.4 hours after Swift BAT trigger) We stacked the images with good conditions. We did not find any new point sources within the Swift XRT circle (Troja et al. GCN Circular #29663) in all three bands. We obtained the 5-sigma limits of the stacked images as follows. T0+[hour] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 40.4 19:50:32 2040 g'>14.3, Rc>14.2, Ic>13.4 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the burst T-EXP: Total Exposure time We used the UCAC4 catalog for flux calibration. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ, Vol.73, Issue 1, Pages 4-24; https://github.com/MNiwano/Eclaire). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29695 SUBJECT: GRB 210318B: Swift/UVOT Upper Limits DATE: 21/03/23 03:30:27 GMT FROM: Sam LaPorte at PSU GRB 210318B: Swift/UVOT Upper Limits M. H. Siegel (PSU) and E. Troja (NASA/GSFC, UMCP) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210318B 70 s after the BAT trigger (Troja et al., GCN Circ. 29663). No optical afterglow consistent with the XRT position (Troja et al. GCN Circular 29663) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 70 219 147 >20.0 u_FC 282 532 246 >20.1 white 70 1357 373 >20.9 v 612 1406 97 >18.2 b 538 1500 92 >19.1 u 282 1479 324 >20.1 w1 661 1455 97 >18.8 m2 1066 1430 58 >19.3 w2 588 1382 97 >19.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.718 in the direction of the burst (Schlegel et al. 1998). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29706 SUBJECT: GRB 210318A: Swift/UVOT Upper Limits DATE: 21/03/24 11:05:42 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210318A 127 s after the BAT trigger (Troja et al., GCN Circ. 29662). No optical afterglow consistent with the XRT position (Troja et al. GCN Circ. 29662) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 127 277 147 >20.5 u_FC 285 535 246 >19.9 white 127 1188 353 >21.0 v 614 1064 58 >18.6 b 541 1163 58 >19.3 u 285 535 246 >19.9 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.017 in the direction of the burst (Schlegel et al. 1998).