GRB210402A

This page lists all entries on GRB210402A in GRBweb

Summary Fermi GBM IPN GCN 29743 GCN 29745 GCN 29747 GCN 29750 GCN 29752 GCN 29753 GCN 29755 GCN 29757 GCN 29761 GCN 29762 GCN 29763 GCN 29764 GCN 29765

Summary table
Variable Value Source
GRB_name_Fermi GRB210402920
T0 22:03:17.331 UTC Fermi_GBM
ra 198.2583° IPN
decl 44.5167° IPN
pos_error 5.00e-02° IPN
T90 76.032 s Fermi_GBM
T90_error 0.923 s Fermi_GBM
T90_start 22:03:17.331 UTC Fermi_GBM
fluence 1.70e-06 erg/cm² Fermi_GBM
fluence_error 7.77e-09 erg/cm² Fermi_GBM
T100 76.032 s
GBM_located False
mjd 59306.918950590276 Fermi_GBM
Fermi GBM table
GRB_name_Fermi GRB210402920
trigger_name bn210402920
ra 198.2592°
decl 44.5128°
datum 2021-04-02
t_trigger 22:04:16.979 UTC
T90 76.032 s
T90_error 0.923 s
T90_start 22:03:17.331 UTC
fluence 1.70e-06 erg/cm²
fluence_error 7.77e-09 erg/cm²
flux_1024 1.46e+00 erg/cm²/s
flux_1024_error 1.81e-01 erg/cm²/s
flux_1024_time -5.27e+01 erg/cm²/s
flux_64 3.96e+00 erg/cm²/s
flux_64_error 1.01e+00 erg/cm²/s
IPN table
GRB_name GRB210402A
ra 198.2583°
decl 44.5167°
pos_error 5.00e-02°
GCN 29743 table
GRB_name GRB210402A
GCN_number 29743
Detection_method Swift Det
ra 198.2587°
decl 44.5129°
Circular_text TITLE: GCN CIRCULAR NUMBER: 29743 SUBJECT: GRB 210402A: Swift detection of a burst DATE: 21/04/02 22:30:12 GMT FROM: Scott Barthelmy at NASA/GSFC K. L. Page (U Leicester), S. D. Barthelmy (GSFC), F. E. Marshall (NASA/GSFC) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: BAT has triggered on a new GRB 210402A (Trigger #1040123) causing Swift to slew to the location and make follow-up observations. Due to a ground pass at the time of the GRB, no precise trigger time, location, or light curve data is immediately available. These details will be given in a later circular. We note that Fermi triggered on a GRB at around the same time at a consistent location (Fermi Trigger #639093861) confirming its GRB status. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 198.25866, 44.51291 which is equivalent to: RA(J2000) = 13h 13m 02.08s Dec(J2000) = +44d 30' 46.5" with an uncertainty of 3.5 arcseconds (radius, 90% containment). 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 consistent with the Galactic value of 1.51 x 10^20 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 1.31e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 150 seconds with the White filter. 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. No correction has been made for the extinction in the Milky Way. Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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/)
GCN 29745 table
GRB_name GRB210402A
GCN_number 29745
Detection_method Optical
Circular_text TITLE: GCN CIRCULAR NUMBER: 29745 SUBJECT: GRB 210402A: NOT optical upper limits DATE: 21/04/03 02:55:03 GMT FROM: Dong Xu at NAOC/CAS Z.P. Zhu (NAOC, HUST), D. Xu (NAOC), L. Izzo (DARK/NBI), J. Martikainen (NOT) report on behalf of a larger collaboration: We observed the field of GRB 210402A (Page et al., GCN 29743) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 00:19:22 UT on 2021-04-03, i.e., 2.23 hr after the burst, and we obtained 3x300 s and 5x200 s frames in the SDSS r- and z- filters, respectively. Preliminary analysis reveals that no optical transient is detected in our stacked images within or at the edge of the current XRT error circle (3.5 arcsec; Page et al., GCN 29743), down to limiting magnitudes of r ~ 24.6 and z ~ 23.2, calibrated with nearby PanSTARRS stars. Compared with the bright X-ray afterglow and no much X-ray column density excess, it may indicate that the burst is a high-redshift one. NIR follow-ups are strongly encouraged.
GCN 29747 table
GRB_name GRB210402A
GCN_number 29747
Detection_method Optical
Circular_text TITLE: GCN CIRCULAR NUMBER: 29747 SUBJECT: GRB 210402A: Montarrenti Observatory upper limit DATE: 21/04/03 07:17:41 GMT FROM: Simone Leonini at Monarrenti Obs Simone Leonini, M. Conti, P. Rosi and L.M. Tinjaca Ramirez (Montarrenti Observatory, Siena, Italy) report: We observed the field of GRB210402A (Swift trigger 1040123, Fermi trigger 639093861, K.L. Page et al., GCN Circ. 29743) with the automatic 0.53m RC telescope + U47 detector at Montarrenti Observatory (Siena, Italy, IAU code C88). The observations were started at 2021-04-02 22:48:18 UT (18 minutes post-burst) stacking 4 unfiltered CCD exposures of 40s each. We have not found optical transient within the error-box given. Mag. upper limit R = 20.10+/-0.19 Magnitudes were obtained using Astrometrica software using USNO-B1 catalogue and are not corrected for galactic extinction.
GCN 29750 table
GRB_name GRB210402A
GCN_number 29750
Detection_method Swift-XRT Other
ra 198.2593°
decl 44.5145°
Circular_text TITLE: GCN CIRCULAR NUMBER: 29750 SUBJECT: GRB 210402A: Swift-XRT refined Analysis DATE: 21/04/03 12:53:41 GMT FROM: Phil Evans at U of Leicester P.A. Evans (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), J.P. Osborne (U. Leicester) and K.L. Page report on behalf of the Swift-XRT team: We have analysed 6.0 ks of XRT data for GRB 210402A (Page et al. GCN Circ. 29743), from 160 s to 46.0 ks after the BAT trigger. The data comprise 1.1 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. Using 1629 s of PC mode data and 3 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 198.25927, +44.51454 which is equivalent to: RA (J2000): 13h 13m 02.22s Dec(J2000): +44d 30' 52.3" with an uncertainty of 2.0 arcsec (radius, 90% confidence). The late-time light curve (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of alpha=1.03 (+0.21, -0.27). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.811 (+0.027, -0.026). The best-fitting absorption column is 3.25 (+/-0.13) x 10^21 cm^-2, in excess of the Galactic value of 1.5 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.09 (+0.20, -0.19) and a best-fitting absorption column of 3.2 (+/-0.8) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (5.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.2 (+/-0.8) x 10^21 cm^-2 Galactic foreground: 1.5 x 10^20 cm^-2 Excess significance: 6.5 sigma Photon index: 2.09 (+0.20, -0.19) If the light curve continues to decay with a power-law decay index of 1.03, the count rate at T+24 hours will be 0.017 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.0 x 10^-13 (9.4 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/01040123. This circular is an official product of the Swift-XRT team.
GCN 29752 table
GRB_name GRB210402A
GCN_number 29752
Detection_method Swift-BAT Det
Circular_text TITLE: GCN CIRCULAR NUMBER: 29752 SUBJECT: GRB 210402A: DDOTI Upper Limits DATE: 21/04/03 15:04:05 GMT FROM: Alan M Watson at UNAM Nat Butler (ASU), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Simone Dichiara (GSFC/UMD), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl Lopez (UNAM), Diego Gonzalez (UNAM), Margarita Pereyra (UNAM), Srihari Ravi (ASU), and Eleonora Troja (GSFC/UMD) report: We observed the field of the Swift/BAT GRB 210402A (Page et al., GCN Circ. 29743) with the DDOTI wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-04-03 02:52 UTC to 2021-04-03 12:18 UTC (from 4.8 to 14.2 hours after the trigger). We observed a region covering aproximately 7 degrees in RA and 10 degrees in declination (about 70 square degrees), including the XRT position in the w filter, and obtained 23ks of exposure. We calibrated our images against the APASS catalog. Compared to the USNO-B1, Pan-STARRS DR1, and APASS DR10 catalogs, we do not detect a source at the X-ray position to a 3-sigma limiting magnitude of w = 23.2. Our non-detection is consistent with the non-detections reported by Zhu et al. (GCN Circ. 29745) and Leonini et al. (GCN Circ. 29747). We thank the staff of the Observatorio Astronomico Nacional in San Pedro Martir.
GCN 29753 table
GRB_name GRB210402A
GCN_number 29753
Detection_method Other
Circular_text TITLE: GCN CIRCULAR NUMBER: 29753 SUBJECT: GRB 210402A: LBT observations DATE: 21/04/03 18:51:46 GMT FROM: Andrea Rossi at INAF A Rossi (INAF-OAS) report on behalf of the CIBO collaboration: We observed the field of GRB 210402A (Page et al., GCN 29743) simultaneously in the r' and z' bands with the LBC imager mounted on LBT (Mt Graham, AZ, USA). We obtained 20min of imaging on 2021-04-03 at mid-time 05:33:00 UT, 0.305 days days after the swift GCN. Observations were performed under an average seeing of ~1.4". Within the refined XRT position (Evans etal., GCN 29750) we detect an extended source in both bands at position (J2000) RA =13:13:02.04 DEC=+44:30:52.7 and preliminary magnitudes (AB system), r=23.9+-0.2 z=23.1+-0.3 calibrated against SDSS field stars. We note that a second source with r=23.4+-0.1 lies at position (J2000) RA =13:13:02.46 DEC=+44:30:50.3 which is at south-east and outside the XRT error circle, and thus unlikely associated eith the GRB. Moreover, this second source is already present (but it is very faint) in archival Pan-STARRS images, while the first one is beyond the survey limits. We note that, assuming that both are galaxies, the first one is more likely to be the host galaxy because of its brightness and distance which result in a smaller probability of chance association. In both cases we cannot exclude that the afterglow is still contributing to the brightness of this sources. Further observations are necessary to assess their variability. We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly B. Rothberg, J. Williams, and F. Cusano, in obtaining these observations.
GCN 29755 table
GRB_name GRB210402A
GCN_number 29755
Detection_method Optical
Circular_text TITLE: GCN CIRCULAR NUMBER: 29755 SUBJECT: GRB 210402A: BOOTES-network early optical upper limit DATE: 21/04/03 21:43:03 GMT FROM: Youdong HU at IAA-CSIC Y.-D. Hu, E. Fernandez-Garcia, A. J. Castro-Tirado, M. D. Caballero-Garcia, M. A. Castro Tirado (IAA-CSIC), C. Perez del Pulgar, A. Castellon, I. Carrasco (Univ. de Malaga), R. Fernandez-Munoz (IHSM/UMA-CSIC), M. Jelinek (ASU-CAS), S. Guziy (Univ. of Nikolaev) and D. R. Xiong, Y. F. Fan, J. M. Bai, C. J. Wang, Y. X. Xin, X. H. Zhao (Yunnan Observatories of CAS), on behalf of a larger collaboration, report: Following the detection of GRB 210402A by Swift (Page et al. GCNC 29743), the 0.6m BOOTES-4/MET robotic telescope at Lijiang Astronomical Observatory (China) automatically followed up this event at 22:10:15 UT (~ 19 s after notice) gathering images (60 s exposure time, clear filter). Furthermore, the 0.6 m BOOTES-2/TELMA robotic telescope at IHSM La Mayora (UMA-CSIC) in Algarrobo Costa (Malaga, Spain) also responded to this event, with the first image (60 s exposure, clear filter) obtained at 22:12:04 UT on April 2 (~ 2 min after notice). In any case, no optical transient is detected down to 19.2 and 18.9 mag respectively within the Swift/XRT error region (Evans et al, GCNC 29750). These two non-detections are consistent with the limits reported by MASTER (Lipunov et al, GCNC 29741), Swift/UVOT (Page et al. GCNC 29743), Montarrenti (Leonini et al. GCNC 29747), NOT (Zhu et al, GCNC 29745) and DDOTI (Butler et al, GCNC 29752). We thank the staff at both La Mayora and Lijiang Astronomical Observatory for their excellent support.
GCN 29757 table
GRB_name GRB210402A
GCN_number 29757
Detection_method Optical
ra 198.2585°
decl 44.5146°
Circular_text TITLE: GCN CIRCULAR NUMBER: 29757 SUBJECT: GRB 210402A: NOT optical afterglow candidate DATE: 21/04/04 08:23:51 GMT FROM: Dong Xu at NAOC/CAS D. Xu (NAOC), Z.P. Zhu (NAOC,HUST), D. B. Malesani (DTU Space) report on behalf of a larger collaboration: We reanalysed our previously reported NOT data (Zhu et al., GCN 29745), given that the position of the Swift/XRT error circle has significantly changed (Page et al., GCN 29743; Evans et al., GCN 29750). An optical source, S1, is detected at the western edge of the refined XRT error circle (Evans et al., GCN 29750) at coordinates: R.A.(J2000) =13:13:02.04 and Dec.(J2000)=+44:30:52.69, with r = 24.0 +/- 0.2 and z = 22.6 +/- 0.3, calibrated with nearby PanSTAR stars. S1 is also detected by the LBT observations (Rossi, GCN 29753), and the magnitudes are consistent with each other with 1sigma error. In addition, the GRB field is covered by the Legacy Survey, and forced aperture photometry of the S1 position gives r ~ 24.5 and z ~ 23.3, both fainter than the NOT measurements. Thus, S1 is the optical afterglow candidate of the burst. A second source, S2, is detected at the south-east and outside of the refined XRT error circle at coordinates: R.A.(J2000) =13:13:02.45 and Dec.(J2000)=+44:30:50.31, as pointed by the LBT observations. It has r = 23.5 +/- 0.1 and z = 23.0 +/- 0.2 from the NOT images, and again no r-band variability between the NOT and LBT epochs. Moreover, forced aperture photometry of the S2 position from the Legacy Survey gives r ~ 23.6 and z ~ 23.0, being consistent with the NOT measurements. Thus, we're inclined to conclude that S2 is unlikely the optical afterglow of the burst. Further observations are planned to asses the brightness variability.
GCN 29761 table
GRB_name GRB210402A
GCN_number 29761
Detection_method Swift-BAT Det
ra 198.2650°
decl 44.5100°
Circular_text TITLE: GCN CIRCULAR NUMBER: 29761 SUBJECT: GRB 210402A: Swift-BAT refined analysis DATE: 21/04/04 20:51:19 GMT FROM: Amy Lien at GSFC S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), K. L. Page (U Leicester), 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-60 to T+243 sec from the recent telemetry downlink, we report further analysis of BAT GRB 210402A (trigger #1040123) (Page et al., GCN Circ. 29743). The BAT ground-calculated position is RA, Dec = 198.265, 44.510 deg which is RA(J2000) = 13h 13m 03.7s Dec(J2000) = +44d 30' 36.1" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 69%. The mask-weighted light curve shows a multi-peaked structure that starts at ~T-50 s and ends at ~T+310 s. The main peak occurs at ~T+174 s. T90 (15-350 keV) is 238.7 +- 18.8 sec (estimated error including systematics). The time-averaged spectrum from T-51.60 to T+310.20 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.49 +- 0.27, and Epeak of 63.6 +- 22.9 keV (chi squared 58.82 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 8.6 +- 0.4 x 10^-6 erg/cm2 and the 1-sec peak flux measured from T+173.53 sec in the 15-150 keV band is 1.6 +- 0.2 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.87 +- 0.06 (chi squared 65.10 for 57 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/1040123/BA/ [GCN OPS NOTE(04apr21): The word "DRAFT:" was removed fron the SUBJECT line. The body of the Circular does contain the author's final text.]
GCN 29762 table
GRB_name GRB210402A
GCN_number 29762
Detection_method Optical
Circular_text TITLE: GCN CIRCULAR NUMBER: 29762 SUBJECT: GRB 210402A: Further NOT optical observations DATE: 21/04/05 15:18:50 GMT FROM: Dong Xu at NAOC/CAS Z.P. Zhu (NAOC,HUST), D. Xu (NAOC), D. B. Malesani (DTU Space) report on behalf of a larger collaboration: We continued to observe the field of GRB 210402A (Page et al., GCN 29743) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We obtained 9x360 s frames in the Sloan r-filter, starting at 23:04:56 UT on 2021-04-04, i.e., 2.04 days after the burst. The optical afterglow candidate, S1, (Xu et al. GCN 29757, see also Rossi et al., GCN 29753) is still detected in the stacked image. S1 now has r = 24.3 +/- 0.1, calibrated with nearby Pan-STARRS stars, while it had r = 24.0 +/- 0.2 in the last NOT epoch. The magnitudes are basically consistent with each other within 1\sigma error, although there might be slight decay, which indicates that S1 has been dominated by the potential host galaxy since the first NOT epoch. Meanwhile, the unlikely optical afterglow source, S2, is well detected in the stacked image with r= 23.5 +/- 0.1, the same as the last NOT epoch. We thus conclude that S2 is not associated to the GRB.
GCN 29763 table
GRB_name GRB210402A
GCN_number 29763
Detection_method Fermi GBM Det
t_trigger 22:04:16.980 UTC
Circular_text TITLE: GCN CIRCULAR NUMBER: 29763 SUBJECT: GRB 210402A: Fermi GBM detection DATE: 21/04/05 20:19:05 GMT FROM: Oliver J Roberts at USRA/NASA O.J. Roberts (USRA/MSFC) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 22:04:16.98 UT on the 2nd April 2021, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 210402A (trigger 639093861 / 210402920), which was also detected by the Swift/BAT (Page et al. 2021, GCN 29743). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time is 44 degrees. The GBM light curve consists of several broad, faint peaks, including emission about 60 s prior to the trigger time. The T90 duration of the event is about 76 s (50-300 keV). The time-averaged spectrum from T0-60 s to T0+16 s is adequately fit by a simple power law function with index -1.34 +/- 0.03. The event fluence (10-1000 keV) in this time interval is (1.699 +/- 0.008)E-6 erg/cm^2. The 1-sec peak photon flux measured starting from T0-52.7 s in the 10-1000 keV band is 1.5 +/- 0.2 ph/s/cm^2. Due to the powering down of Fermi-GBM detectors because of SAA entry about 19 s after the trigger time (note T90), the T90 duration, peak fluxes and fluences are lower limits only. 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/"
GCN 29764 table
GRB_name GRB210402A
GCN_number 29764
Detection_method Swift-UVOT Other
Circular_text TITLE: GCN CIRCULAR NUMBER: 29764 SUBJECT: GRB 210402A: Swift/UVOT Upper Limits DATE: 21/04/05 23:52:02 GMT FROM: Sam LaPorte at PSU GRB 210402A: Swift/UVOT Upper Limits S. J. LaPorte (PSU) and K. L. Page (U Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210402A 73 s after the BAT trigger (Page et al., GCN Circ. 29743). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 29750) 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 163 313 147 >20.8 u_FC 321 571 246 >20.3 white 163 1718 411 >21.4 v 653 5581 229 >19.6 b 578 1694 117 >20.2 u 321 1669 304 >20.2 w1 1108 1644 78 >19.2 w2 1207 1227 19 >18.4 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.018 in the direction of the burst (Schlegel et al. 1998).
GCN 29765 table
GRB_name GRB210402A
GCN_number 29765
Detection_method Optical
Circular_text TITLE: GCN CIRCULAR NUMBER: 29765 SUBJECT: GRB 210402A: MASTER prompt and follow up optical observation DATE: 21/04/06 08:33:46 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs K.Zhirkov, V. Lipunov, V.Kornilov, E.Gorbovskoy,P.Balanutsa,V.Vladimirov, A.Kuznetsov, D. Vlasenko, N.Tiurina, I.Gorbunov, A.Chasovnikov, D.Zimnukhov, V.Senik, V.Topolev, F.Balakin, D.Cheryasov (Lomonosov Moscow State University,SAI, Physics Department), D. Buckley (South African Astronomical Observatory), O.A. Gres, N.M. Budnev (Irkutsk State University, API), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar OAFA), R. Rebolo, M. Serra(The Instituto de Astrofisica de Canarias), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity) MASTER Global robotic net (MASTER-Net:http://observ.pereplet.ru Lipunov et al.,2010,Advances in Astronomy,2010,30L) observed GRB 210402A (Swift trigger 1040123 Page et al. GCN 29743, Evans et al. GCN 29750, Laha et al. 29761, Ttrigger=2021-04-02T22:03:17.22, also Fermi trigger #639093861 T_trigger(Fermi) = 22 04 17 Roberts et al. GCN 29763) at MASTER-IAC, -Tavrida, -SAAO, -Kislovodsk, -OAFA, -Tunka (Lipunov et al. GCN 29741) by MASTER-VWF cameras (two 24.3x16.2sq.deg.) with mlim=13 and by MASTER-II (two 4sq.deg.) with mlims at MASTER cover map https://master.sai.msu.ru/site/master2/observ.php?id=1582249 We observed this field with the MASTER-VWFC of MASTER-IAC, covering the Swift/XRT position from 2021-04-02 22:04:21 to 2021-04-02 22:07:17, i.e. during the prompt emission. Frames were taken with the exposure time of 5s and with no filter . Calibrated with NOMAD V-filter, we marginally see optical source inside T90 at 2 images: started at 2021-04-02 22:05:36(79 from Fermi trigger time) and at 2021-04-02 22:05:46(89 from Fermi trigger) with m_OT~11 at XRT position, but the pixel of this camera is 21". Comparing to the GAIA EDR3, we see no optical counterpart down to the magnitude of 10.5 at other images. It puts the upper limit on the flux of the prompt optical emission at ~5e-10 erg cm^-2 s^-1. Considering that the x-ray flux was ~1e-08 erg cm^-2 s^-1, we constrain the relation between optical flux and x-ray flux at ~10^-2.