GRB 230409B

Rahul Gupta, Amit K. Ror, S. B. Pandey, A. Aryan, K. Misra (ARIES), A. J.
Castro-Tirado (IAA-CSIC), D. Bhattacharya (Ashoka University/IUCAA), and V.
Bhalerao (IITB) report on behalf of a larger collaboration:


GRB 230409B was detected by the Swift Burst Alert Telescope (BAT) at
04:56:57 UT on 9th April 2023 (Beardmore et al., GCN 33592
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 ). The prompt
emission mask-weighted BAT light curve consists of a single-peaked
structure with a T90 duration of 9.79 +- 2.59 sec in 15-350 keV energy
range (Palmer et al., GCN 33598
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 ). We compare the reported value of BAT
fluence and peak photon flux for this GRB (Palmer et al., GCN 33598
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 ) with
all the BAT-detected GRBs sample; this burst is positioned at the
middle-near top of this distribution. We also determine the peak energy of
the burst using BAT energy fluence and peak energy correlation. We
calculated the peak energy of the burst is 61.49 (+23.39, -16.96) KeV; the
softer value of peak energy is consistent with those of long GRBs (Type II).


The Swift XRT detected an X-ray afterglow ~ 101.2 sec after the BAT trigger
(Beardmore et al., GCN 33592
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 ). The XRT count-rate light curve could be best
described with a broken power-law model with temporal indices of 0.62
(+0.12, -0.12) and 1.15 (+0.19, -0.18) before and after break time
(~4600-sec post-BAT detection), respectively. As no redshift has been
reported for this source, we modeled the late time time-averaged XRT
spectrum (T0 + 18528 to 113195 sec) considering redshift = 2, roughly
average redshift value for long GRBs (Type II). The spectrum could be
modeled using an absorption power-law with the following spectral
parameters: NH_host= 4.85 (-4.85,+8.76) X 10^{22} cm^{-2} and \beta_x= 0.73
(-0.36,+0.40). Considering the adiabatic deceleration without energy
injection, closure relations indicate that the late time X-ray afterglow
could be best described with \nu_m < \nu < \nu_c spectral regime for ISM
medium for the electron energy index p ~ 2.46.


The Swift UVOT detected a weak fading source within the XRT error circle
(Kuin et al., GCN 33595
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 ). We performed the follow-up observations of this
fading optical source using the TANSPEC mounted at the axial port of the
3.6m Devasthal Optical Telescope (DOT) of ARIES Nainital at multiple epochs
in J filter (near-infrared). We report the preliminary brightness of the
afterglow to be J = 20.1 +/- 0.3 mag ~ 0.75 days after the BAT trigger. We
obtained the limiting mag of 20.5 mag ~ 2.7 days post-burst at successive
epochs. Our observations confirm this fading source to be the afterglow of
GRB 230409B.


The magnitude values reported are calibrated against 2MASS nearby stars.


This circular may be cited. 3.6m Devasthal Optical Telescope (DOT) is the
recently commissioned facility in the Northern Himalayan region of India
(long:79 41 04E, lat:29 21 40N, alt:2540m) owned and operated by the
Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital (
https://www.aries.res.in). Authors of this GCN circular thankfully
acknowledge consistent support from the staff members to run and maintain
the 3.6m DOT. We also thank Director ARIES for approving the DDT slot for
the ToO observations.

S. Belkin (IKI), E. Klunko (ISTP), A. Pozanenko (IKI), N. Pankov (HSE) 
report on behalf of GRB IKI FuN:

We observed the field of Swift GRB 230409B (Beardmore et al., GCN 33592
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 ) 
with AZT-33IK telescope of Mondy observatory in R-filter on 2023-04-09 
starting (UT) 14:52:05. We do not detect the optical counterpart (Kuin 
et al., GCN 33595
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 ) within the enhanced Swift/XRT error box (Evans et 
al., GCN 33594
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 ). Preliminary photometry of a stacked image is following

Date       UT start  t-T0     Exp.  Filter   OT   Err.  UL(3sigma)
                     (mid, days)  (s)

2023-04-09  14:52:05 0.42231  26*60 R        n/d  n/d   20.5

The photometry is based on nearby USNO-B1.0 stars.
USNO-B1.0
RA DEC R2
21:58:01.9195200 +52:49:21.982800 15.15
21:58:13.7054400 +52:48:57.002400 15.56
21:57:54.5028000 +52:50:57.080400 15.95

The upper limit does not contradict result reported by Adami et al. (GCN 
33607
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 ).

C. Adami (LAM), B. Schneider (MIT), S. Basa (LAM), 
E. Le Floc'h, D. Turpin, D. G��tz (CEA Paris-Saclay), 
S. D. Vergani (GEPI, Obs. de Paris), report on behalf 
of a larger collaboration:

We observed the field of GRB 230409B (Beardmore et al., GCN 33592
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 ) 
using the T193cm telescope at Observatoire de Haute-Provence (France) 
equipped with the MISTRAL spectro-imager. Four exposures were obtained 
in the i-band (2x300s + 2x600s) from 2023 10 April 03:03:03 UT to 
2023 10 April 03:44:17 UT (mid time ~22.3h after trigger). In the combined 
frame, we do not detect any source at the position reported by Evans et al., 
GCN 33594
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  and Kuin et al., GCN 33595
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  down to the following 3-sigma limit:

i > 20.6 mag (AB)

The photometric calibration was performed using nearby stars from 
the PanSTARRS catalog and the magnitude is not corrected for Galactic 
extinction.

We acknowledge the excellent support from Observatoire de Haute-Provence, 
in particular Jean-Pierre Troncin.

P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi 
(INAF-IASFPA) , D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara
(PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and A.P.
Beardmore report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 230409B (Beardmore et al.
GCN Circ. 33592
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 ), from 338 s to 52.1 ks after the  BAT trigger. The
data comprise 19 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 33594
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.87 (+0.04, -0.03).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+0.22, -0.21). The
best-fitting absorption column is  1.23 (+0.27, -0.24) x 10^22 cm^-2,
in excess of the Galactic value of 7.6 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.2 x 10^-11 (9.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.23 (+0.27, -0.24) x 10^22 cm^-2
Galactic foreground: 7.6 x 10^21 cm^-2
Excess significance: 3.3 sigma
Photon index:	     1.97 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
0.87, the count rate at T+24 hours will be 0.020 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (2.0 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/01163401.

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

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

Using the data set from T-240 to T+962 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 230409B (trigger #1163401)
(Beardmore et al., GCN Circ. 33592
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 ).  The BAT ground-calculated position
is RA, Dec = 329.516, 52.832 deg which is
   RA(J2000)  =  21h 58m 03.8s
   Dec(J2000) = +52d 49' 55.1"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 32%.

The mask-weighted light curve shows a single-pulse structure
that starts at ~T-1 s, peaks at ~T+1 s, and ends at ~T+12 s.
The hump at ~50 s to 100 s mentioned in the previous circular
(Beardmore et al., GCN Circ. 33592
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 ) is due to background fluctuation
and is not part of the burst. T90 (15-350 keV) is 9.79 +- 2.59 sec
(estimated error including systematics).

The time-averaged spectrum from T-0.58 to T+11.78 sec is best fit by a
simple
power-law model.  The power law index of the time-averaged spectrum is
1.86 +- 0.19.  The fluence in the 15-150 keV band is 9.9 +- 1.2 x 10^-7
erg/cm2.
The 1-sec peak photon flux measured from T+0.43 sec in the 15-150 keV band
is 3.7 +- 0.5 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/1163401/BA/

V. Lipunov, V.Kornilov, E.Gorbovskoy, K.Zhirkov, N.Tyurina, P.Balanutsa, A.Kuznetsov, D. Vlasenko, 
G.Antipov, D.Zimnukhov, V.Senik, E.Minkina, A.Chasovnikov, V.Topolev, D.Kuvshinov,  D.Cheryasov, Ya.Kechin
(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
(Irkutsk State University, API),

L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez 
(INAOE, Guillermo Haro Astrophysics Observatory),

A. Tlatov, D. Dormidontov 
(Kislovodsk Solar Station of the Pulkovo Observatory),

A. Gabovich, V.Yurkov 
(Blagoveschensk Educational State University)


MASTER-Kislovodsk robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB 230409B ( A. P. Beardmore et al., GCN 33592
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 ) errorbox  42781 sec after notice time and 42806 sec after trigger time at 2023-04-09 16:50:23 UT, with upper limit up to  15.5 mag. Observations started at twilight.  The observations began at zenith distance = 83 deg. The sun  altitude  is -12.2 deg. 

The galactic latitude b = -2 deg., longitude l = 99 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2209455

We obtain a following upper limits.  

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

   42897 |   MASTER-Kislovodsk |   C |   180 | 15.5 |        
Filter C is a clear (unfiltred) band. 


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

GRB 230409B: Swift/UVOT Detection

N. Paul Kuin (UCL/MSSL) and A. P. Beardmore (U. Leicester) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of
GRB 230409B  386 s after the BAT trigger (Beardmore et al.,
GCN Circ. 33592
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 ).  A weak fading source consistent with the
XRT position (Beardmore et al. GCN Circ. 33592
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 ) is detected
in the initial UVOT exposures at position RA=329.5158,
Dec=52.8321 deg.

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              386          536          147         19.35 +/- 0.20
v                  542         1931          156        >18.1
b                  642         2029          156        >19.4
u                  616         2004          156        >19.0
w1                 592         1980          156        >18.8
m2                 567         1955          156        >19.2
w2                 692         1906          136        >20.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.226 in the direction of the burst
(Schlegel et al. 1998).

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

Using 1709 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 230409B, 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 = 329.51474, +52.83212 which is equivalent
to:

RA (J2000): 21h 58m 3.54s
Dec (J2000): +52d 49' 55.6"

with an uncertainty of 2.2 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.

A. P. Beardmore (U Leicester), C. Gronwall (PSU),
A. Y. Lien (U Tampa), K. L. Page (U Leicester), T. Sakamoto (AGU),
M. H. Siegel (PSU) and M. A. Williams (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 04:56:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230409B (trigger=1163401). Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 329.530, +52.854 which is 
   RA(J2000) = 21h 58m 07s
   Dec(J2000) = +52d 51' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The immediately available BAT light curve
starts at ~ a few seconds after the BAT trigger time. The light 
curve shows a small hump from ~50 to 100 s, however, further ground 
data will be required to determine whether the hump is due to 
background fluctuation. 

The XRT began observing the field at 04:58:38.0 UT, 101.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 329.51486, 52.83228
which is equivalent to:
   RA(J2000)  = 21h 58m 03.57s
   Dec(J2000) = +52d 49' 56.2"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 84 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 7.56
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 386 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. No correction has been made for
the large, but uncertain, extinction expected. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.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/)