GRB 230325A

M. J. Moss (GWU), N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 03:15:35 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230325A (trigger=1161390).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 296.820, -46.094 which is 
   RA(J2000) = 19h 47m 17s
   Dec(J2000) = -46d 05' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex structure
structure with a duration of about 40 sec.  The peak count rate
was ~2200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 03:17:09.9 UT, 94.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 296.84910,
-46.07455 which is equivalent to:
   RA(J2000)  = 19h 47m 23.78s
   Dec(J2000) = -46d 04' 28.4"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 100 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 (4.88 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.9
(+3.24/-2.71) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 2.06e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 101 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
0.046. 

Burst Advocate for this burst is M. J. Moss (mikejmoss3 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/)

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 230325A, we find an
enhanced XRT position of the afterglow: RA, Dec: 296.84935, -46.07561
which is equivalent to:
   RA (J2000)  = 19 47 23.85
   Dec (J2000) = -46 04 32.2
with an uncertainty of 1.9 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1161390.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

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

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

Using 290 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 230325A, 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 = 296.84984, -46.07613 which is equivalent
to:

RA (J2000): 19h 47m 23.96s
Dec (J2000): -46d 04' 34.1"

with an uncertainty of 2.3 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.

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), A. Tohuvavohu (U.
Toronto) and M.J. Moss report on behalf of the Swift-XRT team:

We have analysed 7.1 ks of XRT data for GRB 230325A (Moss et al. GCN
Circ. 33516
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 ), from 83 s to 45.2 ks after the  BAT trigger. The data
comprise 139 s in Windowed Timing (WT) mode (the first 10 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 Goad et al.
(GCN Circ. 33517
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.1 (+/-0.5). At T+123 s  the decay
steepens to an alpha of 3.24 (+0.27, -0.16) before breaking again at
T+598 s to a final decay with index alpha=0.42 (+/-0.14).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.16 (+0.11, -0.10). The
best-fitting absorption column is  3.2 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 4.9 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.13 (+0.25, -0.23)
and a best-fitting absorption column of 2.5 (+0.9, -0.8) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.4 x 10^-11 (5.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.5 (+0.9, -0.8) x 10^21 cm^-2
Galactic foreground: 4.9 x 10^20 cm^-2
Excess significance: 4.2 sigma
Photon index:	     2.13 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
0.42, the count rate at T+24 hours will be 0.028 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.3 x
10^-13 (1.4 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/01161390.

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

Danielle Pieterse (Radboud Univ.), Johan P. U. Fynbo (DAWN/NBI), Luca 
Izzo (DARK/NBI), Jesse Palmerio (GEPI, Obs. Paris and IAP), Andrea 
Saccardi (GEPI, Obs. Paris), Nial R. Tanvir (Leicester), Dong Xu 
(NAO/CAS), Zi-Pei Zhu (NAOC, HUST), Daniele B. Malesani (Radboud univ. 
and DAWN/NBI) and Pall Jakobsson (Univ. Iceland), report on behalf of 
the Stargate collabaration:

We observed the field of GRB 230325A (Moss et al., GCN 33516
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 ) using the 
ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Images 
were also taken with the acquisition camera. An object is visible 
consistent with the UVOT-enhanced XRT position (Goad et al., GCN 33518
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 ), 
at coordinates (0.5" error):

RA = 19:47:23.98�� (J2000)
Dec = -46:04:33.3�� (J2000)

For this object, we measure the following magnitudes (calibrated against 
nearby objects from the Legacy Survey; Dey et al. 2019, AJ, 157, 168):

Filter Mean date�������������� Time since GRB�� Exptime�� Magnitude
 �������������� (UT)������������������������ (days)������������������ (s)���������� (AB)
--------------------------------------------------------------
r���������� 2023 Mar 25.371�� 0.235�������������������� 45������������ 22.70 +- 0.08
z���������� 2023 Mar 25.372�� 0.236�������������������� 60������������ 22.35 +- 0.08
g���������� 2023 Mar 25.374�� 0.237�������������������� 45������������ 22.16 +- 0.23

An object is present at a consistent location in the Legacy Survey, but 
significantly fainter, and it is the likely GRB host galaxy. The 
consistency with the XRT position and the flux increase compared to the 
Legacy Survey confirm that the object we see in the X-shooter images is 
the optical afterglow of GRB 230325A.


Spectroscopy was carried out, covering the wavelength range 3000-21000 
AA, and consisting of 2 exposures by 600 s each. The observation mid 
time was 2023 Mar 25.383 UT (0.247 days after the GRB).

Only a faint continuum is detected from this source, which allows us to 
set a redshift upper limit z <~ 3. In addition, two emission lines are 
visible, which are consistent with Halpha and [O III] 5008 at a common 
redshift z = 1.664, which we suggest to be the GRB redshift.

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Fuyan Bian and Konrad Tristram.

N Paul Kuin (UCL/MSSL) and M. J. Moss (GWU)report on behalf
of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of
GRB 230325A 102 s after the BAT trigger (Moss et al., GCN
Circ. 33516
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 ).   No optical afterglow consistent with the
XRT position (Goad et al. GCN Circ. 33518
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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           102          252          147         >20.1
u_FC               314          540          222         >19.2
white              102         4222          344         >20.8
v                 4434         4634          197         >18.9
b                 3818         4018          197         >20.0
u                  314         5106          277         >19.3
w1                4845         5045          197         >19.9
m2                4640         4839          197         >19.8
w2                4230         4429          197         >20.2
The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.046 in the
direction of the burst (Schlegel et al. 1998).

A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), C. B. Markwardt (GSFC), M. J. Moss (GWU),
D. M. Palmer (LANL), 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 230325A (trigger #1161390)
(Moss, et al., GCN Circ. 33516
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 ).  The BAT ground-calculated position is
RA, Dec = 296.854, -46.067 deg which is
   RA(J2000)  =  19h 47m 24.9s
   Dec(J2000) = -46d 03' 59.8"
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 48%.

The mask-weighted light curve shows a main pulse with a FRED shape, from
approximately T+0 to T+8 seconds.  This is preceded by some precursor emission
starting around T-10 sec, and some low-level emission out to about T+40 sec.
The spacecraft slewed away from the burst location around T+620 seconds.
T90 (15-350 keV) is 38.05 +- 11.56 sec (estimated error including systematics).

The time-averaged spectrum from T-8.55 to T+41.70 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.82 +- 0.18.  The fluence in the 15-150 keV band is 1.3 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T-0.07 sec in the 15-150 keV band
is 3.3 +- 0.4 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/1161390/BA/