GRB 210411C

T. Sbarrato (INAF-OAB), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 15:05:51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210411C (trigger=1042398).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 296.559, -39.373 which is 
   RA(J2000) = 19h 46m 14s
   Dec(J2000) = -39d 22' 21"
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 20 sec.  The peak count rate
was ~5500 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 15:06:54.6 UT, 63.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 296.60899, -39.39786 which is
equivalent to:
   RA(J2000)  = 19h 46m 26.16s
   Dec(J2000) = -39d 23' 52.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 165 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 (8.10 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.2
(+2.66/-2.35) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 66 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
  RA(J2000)  =	19:46:26.83 = 296.61178
  DEC(J2000) = -39:23:51.9  = -39.39774
with a 90%-confidence error radius of about 1.10 arc sec. This position is 7.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.99. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.080. 

Burst Advocate for this burst is T. Sbarrato (tullia.sbarrato AT inaf.it). 
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/)

M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 318 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 210411C, 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 = 296.61122, -39.39793 which is equivalent
to:

RA (J2000): 19h 46m 26.69s
Dec (J2000): -39d 23' 52.6"

with an uncertainty of 2.0 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.

R. Strausbaugh (U. of the Virgin Islands), A. Cucchiara (U. of the Virgin Islands/College of Marin) report on behalf of a larger collaboration:

We observed Swift GRB 210411C (Sbarrato, GCN 29794
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 ) with the LCO 1-m Sinistro instrument at the Siding Spring Observatory, Australia site, on April 11, from 16:18 to 16:42 UT (corresponding to 1.22 to 1.62 hours from the GRB trigger time) with the Bessel R and I filters.

We performed a series of 4x180s exposures in R and I. We detect an optical source in R and I band images at a location consistent the initial UVOT detection (Sbarrato, GCN 29794
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 ) and enhanced XRT position (Goad, GCN 29800
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 ), that is not present in either USNO or 2MASS catalogs.  Using the USNO-B.1 catalog as reference, we calculate the following magnitudes:

R = 18.51 +/- 0.03

I = 18.01 +/- 0.03

These magnitudes are not corrected for galactic extinction.  These magnitudes may also be contaminated by a bright foreground star.

R.S. is funded by NSF AST grant #1831682

Y.-D. Hu, E. Fernandez-Garcia, A. J. Castro-Tirado, M. D. Caballero-Garcia, M. A. Castro Tirado (IAA-CSIC), C. Perez del Pulgar (Univ. de Malaga), 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 210411C by Swift (Sbarrato et al. GCNC 29794), follow-up observations at the 0.6m BOOTES-4/MET robotic telescope in Lijiang Astronomical Observatory (China) were performed starting on Apr 11, 20:52 UT (~5.8 hr after trigger) at high airmass. No new source is detected down to 19.6 mag at the optical afterglow position reported by UVOT (Sbarrato et al. GCNC 29794) and the 1.0m LCO in Siding Spring (Strausbaugh et al. GCNC 29802) in our co-added image (25x60 s, clear filter) confirming the fading in the optical range.

We thank the staff at Lijiang observatory for their excellent support.

[GCN OPS NOTE(12apr21):  Per author's request, the GRB name in the first sentence 
has been changed from "210406A" to 210411C".]

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 210411C ( T. Sbarrato et al., GCN 29794
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 ) errorbox  60730 sec after notice time and 60747 sec after trigger time at 2021-04-12 07:58:18 UT, with upper limit up to  19.0 mag. The observations began at zenith distance = 37 deg. The sun  altitude  is -38.0 deg. 

The galactic latitude b = -27 deg., longitude l =  1 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=1589515

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   60837 |         MASTER-OAFA |   C |   180 | 19.0 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

A. de Ugarte Postigo (HETH/IAA-CSIC and DARK/NBI), B. Sbarufatti (PSU), 
D. Xu (NAO/CAS), J.-B. Vielfaure (APC, Paris University), D. B. Malesani 
(DTU Space), D. A. Kann (HETH/IAA-CSIC) V. D'Elia (ASI/SSDC, INAF/OAR), 
P. J. Pessi (FCAG/ESO), report on behalf of the Stargate consortium:

We observed the optical afterglow of GRB 210411C (Sbarrato et al., GCN 
29794
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 ) using the ESO VLT UT3 (Melipal) equipped with the X-shooter 
spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and 
consist of 4 exposures by 1200 s each (8x600 s in the NIR). The 
observation mid time was 2021 Apr 12.33 UT (16.9 hr after the GRB).

In a 60 s image taken with the acquisition camera on Apr 12.294 UT, we 
detect the optical afterglow, for which we measure an AB magnitude r = 
21.69 +- 0.04 (calibrated against nearby stars from the SkyMapper 
catalog; Wolf et al. 2018, PASA, 35, 010; 
https:doi.org/10.4225/41/593620ad5b574).

We clearly detect continuum over the wavelength range 3520 - 17500 AA. A 
trough is visible around 4650 AA, which we identify as due to H I. From 
the detection of several narrow absorption features, which we interpret 
as due to, among others, Si II, Si II*, O I, C II, Si IV, C IV, Fe II, 
Al II, we infer a redshift z = 2.826. We also identify the FeII* 2396 
fine-structure transition, though at low S/N.

We acknowledge excellent support from the observing staff at Paranal, in 
particular Eleonora Sani and Bin Yang.

[GCN OPS NOTE(12apr2021):  Per author's request, GRB in the SUBJECT line
was changed from 210411A" to the correct name "210411C".]

J. D. Gropp (PSU), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), P.A.
Evans (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and T.
Sbarrato report on behalf of the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 210411C (Sbarrato et al.
GCN Circ. 29794
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 ), from 66 s to 42.0 ks after the  BAT trigger. The data
comprise 136 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN Circ. 29800
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 ).

The late-time light curve (from T0+5.4 ks) can be modelled with a
power-law decay with a decay index of alpha=1.49 (+/-0.17).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.66 (+0.19, -0.18). The
best-fitting absorption column is  1.2 (+0.4, -0.3) x 10^21 cm^-2, in
excess of the Galactic value of 8.1 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.84 (+0.14, -0.11)
and a best-fitting absorption column of 9.4 (+4.0, -1.3) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (4.3 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.4 (+4.0, -1.3) x 10^20 cm^-2
Galactic foreground: 8.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.84 (+0.14, -0.11)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01042398.

This circular is an official product of the Swift-XRT team.

P. Pessi (ESO), G. Csoernyei, A. Holas, S. Taubenberger, C. Vogl (MPA 
Garching), A. Floers (GSI), G. Pignata (UNAB), J. Teffs (Liverpool), M. 
Dennefeld (IAP), D. B. Malesani (DTU Space), P. D'Avanzo (INAF/Brera), 
J. Anderson (ESO), T. M��ller Bravo (Southampton), T.-W. Chen 
(Stockholm), M. Gromadzki (Warsaw), C. Inserra (Cardiff), E. Kankare 
(Turku), M. Nicholl (Birmingham), O. Yaron (Weizmann), D. Young (QUB), 
E. Zimmerman (Weizmann) report:

We observed the optical afterglow of GRB 210411C (Sbarrato et al., GCN 
29794
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 ; Strausbaugh & Cucchiara, GCN 29802
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 ; de Ugarte Postigo et al., GCN 
29806
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 ) under the the advanced Public ESO Spectroscopic Survey for 
Transient Objects (ePESSTO+; see Smartt et al. 2015, A&A, 579, 40 and 
http://www.pessto.org ). The observations were performed on the ESO New 
Technology Telescope (NTT) at La Silla with the EFOSC2 instrument. A 
single 5-min exposure was acquired in the Gunn r filter, with mean epoch 
2021 Apr 12.234 UT (14.55 hr after the GRB).

The optical afterglow is well detected and we report the following 
coordinates (calibrated against the Gaia catalog):

RA = 19:46:26.83
Dec = -39:23:52.7

For the afterglow, we measure a magnitude R = 21.28 +- 0.04 (Vega), 
assuming R = 17.31 for the USNO star at RA = 19:46:25.68, Dec = 
-39:23:48.9 . An extra uncertainty of ~0.3 mag should be added to 
account for the photometric calibration error of the USNO catalog.

S. Belkin (IKI), A. Pozanenko (IKI), M. Krugov (AFIF), N. Pankov (HSE) 
report on behalf of IKI-FuN follow-up collaboration:

We observed the Swift GRB 210411C (Sbarrato et al., GCN 29794
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 ) with 
Chilescope RC-1000 started on April 12 (UT) 07:43:39 in r'-filter. We 
clearly detected optical afterglow (Sbarrato et al., GCN 29794
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 ; 
Strausbaugh et al., GCN 29802
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 ; Hu et al., GCN 29803
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 ; Lipunov et al., GCN 
29804
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 ; de Ugarte Postigo et al., GCN 29806
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 ; Pessi et al., GCN 29808
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 ) in 
a stacked image with a total exposure of 3*600 seconds.

Preliminary photometry is following.

Date       UT start   t-T0       Filter Exp.   OT    Err.  UL(3sigma)
                      (mid, days)        (s)
2021-04-12 07:43:39   0.70333    r'     3*600  21.76 0.16  22.4

The photometry is based on nearby stars from the SkyMapper catalog
SMSS-DR1.1_id r
194633.88-392221.5 17.428
194648.73-392400.2 16.180

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL), T. Sakamoto (AGU),
T. Sbarrato (INAF-OAB), 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 210411C (trigger #1042398)
(Sbarrato et al., GCN Circ. 29794
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 ).  The BAT ground-calculated position is
RA, Dec = 296.601, -39.399 deg which is
   RA(J2000)  =  19h 46m 24.2s
   Dec(J2000) = -39d 23' 57.8"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 79%.

The mask-weighted light curve shows a multi-peaked structure that
starts at ~T0 and ends at ~T+17 s. The two major peaks occur at ~T+4 s
and ~T+11 s, respectively. T90 (15-350 keV) is 12.80 +- 0.60 sec
(estimated error including systematics).

The time-averaged spectrum from T-0.45 to T+14.44 sec is best fit by a
power law
with an exponential cutoff.  This fit gives a photon index 1.60 +- 0.40,
and Epeak of 14.8 +- 10.8 keV (chi squared 51.24 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.2 +- 0.1 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+3.87 sec in the 15-150 keV band is
4.8 +- 0.3 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.62 +- 0.09 (chi squared 68.87 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/1042398/BA/

N. P. M. Kuin (UCL-MSSL) and Sbarrato (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210411C
66 s after the BAT trigger (Sbarrato et al., GCN Circ. 29794
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 ).
A source consistent with the XRT position (Goad et al. GCN Circ. 29800
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 )
is detected in the initial UVOT exposures. Further detections were
reported by Strausbaugh et al. (GCN Circ 29802
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 )

The preliminary UVOT position is:
    RA  (J2000) =  19:46:26.81 = 296.61170 (deg.)
    Dec (J2000) = -39:23:52.2  = -39.39784 (deg.)
with an estimated uncertainty of 0.43 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT photometric
system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures
are:

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white               66          216          147         17.16 +/- 0.03
v                  609          801           39         17.43 +/- 0.18
b                  535          555           20         17.82 +/- 0.17
u                  279          528          246         17.82 +/- 0.07
w1                 658         1280           78        >19.7
m2                 633         5836           83        >19.6
w2                 584         5600          255        >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.081 in the direction of the burst
(Schlegel et al. 1998).

Y. Aimuratov, C.L. Bianco, L. Li, R. Moradi, F. Rastegar Nia, J.A. Rueda,
R. Ruffini,  N. Sahakyan, Y. Wang, S.S. Xue on behalf of the ICRANet team,
report:

GRB 210411C (trigger=1042398) with a redshift z=2.826 (A. de Ugarte Postigo
et al, 2021, GCN29806
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 ) was triggered and located by Swift Burst Alert
Telescope (BAT) (T. Sbarrato et al, 2021, GCN 29794
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 ) with T90=12.8 sec in
15-150 keV (C. B. Markwardt et al, 2021, GCN29810
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 ).

The corresponding luminosity light curves of Swift-BAT and Swift-XRT (J.D.
Gropp et al, 2021, GCN29807
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 ) show this source is a twin of GRB 190114C
(Ruffini et al, 2021, https://doi.org/10.1093/mnras/stab724
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 ). A new
additional feature is identified in the early luminosity of Swift-XRT data
of GRB 210411C (see enclosed Fig.1).

We recommend to check for the possible existence of a yet unobserved GeV
radiation and to follow up the late keV and possible GeV temporal power-law
luminosities.

Fig. 1:
http://www.icranet.org/documents/Swift_GRB210411C-Fermi_GRB190114C.pdf