GRB 210402A

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/)

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
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 ) 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
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 ), 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.

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
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 ) 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.

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
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 ), 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.

Nat Butler (ASU), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Simone Dichiara
(GSFC/UMD), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Oc��lotl 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
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 ) with the DDOTI wide-field imager at the Observatorio Astron��mico 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
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 ) and Leonini et al. (GCN Circ. 29747
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 ).

We thank the staff of the Observatorio Astronomico Nacional in San Pedro Martir.

A Rossi (INAF-OAS) report on behalf of the CIBO collaboration:

We observed the field of GRB 210402A (Page et al., GCN 29743
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 ) 
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
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 ) 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.

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.

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
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 ), 
given that the position of the Swift/XRT error circle has significantly 
changed (Page et al., GCN 29743
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 ; Evans et al., GCN 29750
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 ).

An optical source, S1, is detected at the western edge of the refined 
XRT error circle (Evans et al., GCN 29750
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 ) 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
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 ), 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.

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
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 ).  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.]

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
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 ) 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
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 , see also 
Rossi et al., GCN 29753
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 ) 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.

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
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 ). 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/"

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
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 ).
No optical afterglow consistent with the XRT position
(Evans et al. GCN Circ. 29750
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 )
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).

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
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 ,
Evans et al. GCN 29750
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 , 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
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 )
at MASTER-IAC, -Tavrida, -SAAO, -Kislovodsk, -OAFA, -Tunka (Lipunov et al. GCN 29741
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 )
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.