GRB 210901A

S. Laha (GSFC/UMBC/CRESST), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), P. A. Evans (U Leicester),
C. Gronwall (PSU), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 15:04:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210901A (trigger=1071488).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 295.292, -76.932 which is 
   RA(J2000) = 19h 41m 10s
   Dec(J2000) = -76d 55' 55"
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 60 sec. As this is an image trigger a more 
complete description of the lightcurve will require the ground pass data. 
The peak count rate was ~500 counts/sec (15-350 keV), at ~0 sec after 
the trigger. 

The XRT began observing the field at 15:07:07.7 UT, 151.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
295.2774, -76.9263 which is equivalent to:
   RA(J2000)  = 19h 41m 06.57s
   Dec(J2000) = -76d 55' 34.8"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 23 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 consistent with the Galactic value of 1.02
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.86e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 157 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
0.200. 

Burst Advocate for this burst is S. Laha (sib.laha AT gmail.com). 
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/)

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 210901A ( S. Laha et al., GCN 30746
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 ) errorbox  6860 sec after notice time and 6950 sec after trigger time at 2021-09-01 17:00:26 UT, with upper limit up to  18.6 mag. Observations started at twilight.  The observations began at zenith distance = 49 deg. The sun  altitude  is -8.9 deg. 

The galactic latitude b = -30 deg., longitude l = 318 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=1706835

We obtain a following upper limits.  

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

    7040 |         MASTER-SAAO |   C |   180 | 17.4 |        
    7220 |         MASTER-SAAO |   C |   540 | 17.8 |  Coadd 
    7240 |         MASTER-SAAO |   C |   180 | 17.9 |        
    7439 |         MASTER-SAAO |   C |   180 | 18.3 |        
    7639 |         MASTER-SAAO |   C |   180 | 18.6 |        
Filter C is a clear (unfiltred) band. 


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

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 210901A (Laha, et al., GCN 30746
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 ) with the LCO 1-m Sinistro instrument at the South African Astronomical Observatory site, on September 1, from 17:14 to 17:36 UT (corresponding to 2.17 to 2.53 hours from the GRB trigger time) with the Bessel R and I filters.

We performed a series of 3x300s exposures in R and I. In the XRT detection region (Laha, et al., GCN 30746
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 ) no optical counterpart could be detected down to the following upper limits, calculated using the USNO-B.1 catalog as reference:

R>21.55

I>21.16

These magnitudes are not corrected for galactic extinction.

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

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

Using 302 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 210901A, 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 = 295.27805, -76.92664 which is equivalent
to:

RA (J2000): 19h 41m 6.73s
Dec (J2000): -76d 55' 35.9"

with an uncertainty of 2.5 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.

D. M. Palmer (LANL), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
T. Sakamoto (AGU), M. Stamatikos (OSU)
(i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 210901A (trigger #1071488)
(Laha et al., GCN Circ. 30746
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 ).  The BAT ground-calculated position is
RA, Dec = 295.224, -76.942 deg which is
   RA(J2000)  =  19h 40m 53.8s
   Dec(J2000) = -76d 56' 31.9"
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 12%.

The mask-weighted light curve shows a multi-peaked structure. The main
pulse structure start at ~T+30 s and ends at ~T+85 s, with the highest
peak occurring at ~T+60 s. In addition, there might be some weak emission
prior to the main pulses and starts at ~T-50 s. T90 (15-350 keV)
is 46.77 +- 7.87 sec (estimated error including systematics).

The time-averaged spectrum from T+29.77 to T+84.70 sec is best fit by
a simple power-law model.  The power law index of the time-averaged
spectrum is 2.08 +- 0.17.  The fluence in the 15-150 keV band
is 3.1 +- 0.3 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from
T+60.64 sec in the 15-150 keV band is 2.4 +- 0.6 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/1071488/BA/

J.A. Kennea (PSU), P.A. Evans (U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), M.G.
Bernardini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) , A. Tohuvavohu (U.
Toronto), B. Sbarufatti (PSU) and S. Laha report on behalf of the
Swift-XRT team:

We have analysed 2.5 ks of XRT data for GRB 210901A (Laha et al. GCN
Circ. 30746
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 ), from 139 s to 79.9 ks after the  BAT trigger. The data
comprise 126 s in Windowed Timing (WT) mode (the first 7 s were 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. 30749
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.8 (+0.3, -0.4). At T+207 s  the decay
steepens to an alpha of 5.1 (+/-0.4) before breaking again at T+517 s
to a final decay with index alpha=0.60 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.55 (+0.14, -0.13). The
best-fitting absorption column is  2.8 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.0 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.0 (+0.4, -0.3) and
a best-fitting absorption column of 1.7 (+1.2, -0.6) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (4.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+1.2, -0.6) x 10^21 cm^-2
Galactic foreground: 1.0 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.0 (+0.4, -0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01071488.

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

A. Nicuesa Guelbenzu, S. Klose (both TLS Tautenburg), and
A. Rau (MPE Garching) report:


We observed the field of GRB 210901A (Swift trigger #1071488; Laha et al.,
GCN 30746
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 ) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m
MPG telescope at La Silla Observatory (Chile).

Observations started at 23:43 UT on September 1, about 8.5 hr after the
GRB trigger. They were performed at an average seeing of 2.3 arcsec and at
an average airmass of 1.5.

Inside the 2.5 arcsec enhanced XRT error circle (Osborne et al., GCN
30749
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 ), we do not detect any source. Based on an observation with 23.4
minutes integration time in g'r'i'z' and 20 minutes in JHK, at a midtime
of 23:43 UT on September 1, we measure the following preliminary 3-sigma
upper limits (AB mags):

g' > 23.5,
r' > 23.6,
i' > 22.6,
J  > 20.1,
H  > 19.7, and
K  > 16.3.

We note the presence of an object at the north-western border of the XRT
error circle at coordinates RA, DEC (J2000) = 19:41:05.98, -76:55:33.0 for
which we measure r' = 21.33 +/- 0.05 mag (AB). Due to the relatively bad
seeing at present we cannot decide with certainty if this is a galaxy or a
star.

Given magnitudes are calibrated against the SkyMapper catalog (Wolf et
al., 2018, PASA 35, e010) in g'r'i'z' and 2MASS field stars in JHK
(Skrutskie et al., 2006, AJ 131, 1163).

We thank Maren Hempel for excellent support and for performing the
observations.

M. H. Siegel (PSU) and S. L. Laha (GSFC/UMBC/CRESST)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210901A
158 s after the BAT trigger (Laha et al., GCN Circ. 30746
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 ).
No optical afterglow consistent with the XRT position
(Osborne et al. GCN Circ. 30749
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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           158          308          147         >20.9
u_FC               316          566          246         >19.6
white              158          974          289         >20.9
v                  646          839           39         >17.8
b                  572          764           39         >18.8
u                  316          740          265         >19.8
w1                 695          715           19         >17.8

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.200 in the direction of the burst
(Schlegel et al. 1998).