GRB 241228A

V. D'Elia (SSDC & INAF-OAR), A. D'Ai (INAF-IASFPA), S. Dichiara (PSU),
R. Gupta (NASA GSFC) and N. P. M. Kuin (UCL-MSSL) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 03:57:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 241228A (trigger=1277277).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 344.824, +18.604 which is 
   RA(J2000) = 22h 59m 18s
   Dec(J2000) = +18d 36' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multipeak
structure with a duration of about 20 sec.  The peak count rate
was ~11085 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 03:58:52.3 UT, 97.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 344.80173, 18.61005
which is equivalent to:
   RA(J2000)  = 22h 59m 12.42s
   Dec(J2000) = +18d 36' 36.2"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 79 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (6.67 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.2
(+4.07/-3.30) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 100 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
0.091.

Burst Advocate for this burst is V. D'Elia (valerio.delia AT ssdc.asi.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/)

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

Using 1737 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 241228A, 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 = 344.80186, +18.61065 which is equivalent
to:

RA (J2000): 22h 59m 12.45s
Dec (J2000): +18d 36' 38.3"

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.

N. P. M. Kuin (UCL-MSSL) and V. D'Elia (SSDC & INAF-OAR)

report on behalf of the Swift/UVOT team:


The Swift/UVOT began settled observations of the field of
GRB 241228A  101 s after the BAT trigger (D'Elia et al.,
GCN Circ. 38681
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 ).  No optical afterglow consistent with
the XRT position in that circular was 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           101          251          147         >21.0
u_FC               314          564          246         >19.8
white              101         1538          392         >21.3
v                  644         1416           97         >19.1
b                  570         1514           97         >19.8
u                  314         1489          324         >20.0
w1                 694         1465           78         >19.2

The magnitudes in the table are not corrected for the
Galactic extinction due to the reddening of E(B-V) = 0.091
in the direction of the burst (Schlegel et al. 1998).

C. Salvaggio (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A.
Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 9.1 ks of XRT data for GRB 241228A, from 82 s to 28.8
ks after the  BAT trigger. The data comprise 83 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.8 ks) can be modelled with a
power-law decay with a decay index of alpha=1.54 (+0.13, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.00 (+0.18, -0.17). The
best-fitting absorption column is  2.2 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 6.7 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.86 (+0.15, -0.14)
and a best-fitting absorption column of 3.6 (+0.8, -0.7) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.2 x 10^-11 (6.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 6.7 x 10^20 cm^-2
Excess significance: 6.9 sigma
Photon index:	     1.86 (+0.15, -0.14)

If the light curve continues to decay with a power-law decay index of
1.54, the count rate at T+24 hours will be 4.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-13 (2.7 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/01277277.

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

T. Mohan, V. Swain, A.P. Saikia, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:

We observed the field of the GRB 241228A (D'Elia et al., GCN 38681
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 ) with the 0.7m GROWTH-India Telescope (GIT). Observation started at 2024-12-28T13:13:58 UT, about 9.27 hours after the trigger. We responded to the enhanced Swift-XRT coordinates (Evans et al., GCN 38683
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 ) and obtained multiple images of 360s in r' filter.

The obtained upper limit in our stacked image is:

| MJD (mid)   | Filter | Exposure (s) | Limiting Magnitude (AB) |
| ----------- | ------ | ------------ | ----------------------- |
| 60672.56537 | r'     | 4x360        | 20.8


The upper limit is consistent with Kuin et al., GCN 38685
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 .

The magnitude is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

The GROWTH India Telescope (GIT, Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,
V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

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 241228A ( V. D'Elia et al., GCN 38681
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 ) errorbox  53242 sec after notice time and 53275 sec after trigger time at 2024-12-28 18:45:10 UT, with upper limit up to  16.7 mag. Observations started at twilight.  The observations began at zenith distance = 72 deg. The sun  altitude  is -11.2 deg. 

The galactic latitude b = -37 deg., longitude l = 90 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2724378

We obtain a following upper limits.  

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

   53366 |         MASTER-SAAO |   C |   180 | 12.2 |        
   53565 |         MASTER-SAAO |   C |   180 | 16.4 |        
   53764 |         MASTER-SAAO |   C |   180 | 16.5 |        
   53964 |         MASTER-SAAO |   C |   180 | 16.6 |        
   54163 |         MASTER-SAAO |   C |   180 | 16.7 |        
   54364 |         MASTER-SAAO |   C |   180 | 16.5 |        
   54563 |         MASTER-SAAO |   C |   180 | 16.6 |        
   54763 |         MASTER-SAAO |   C |   180 | 16.5 |        
Filter C is a clear (unfiltred) band. 


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

L. Scotton (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 03:57:10.17 UT on 28 December 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 241228A (trigger 757051035/241228165),
which was also detected by Swift BAT (V. D'Elia et al. 2024, GCN 38681
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 ).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 55 degrees.

The GBM light curve consists of a minor triggering episode 
followed by a two-peak episode, with a duration (T90)
of about 8.3 s (50-300 keV). The time-averaged spectrum
from T0-0.5 to T0+10.9 s is best fit by
a Band function with Epeak = 66 +/- 3 keV,
alpha = -0.68 +/- 0.08, and beta = -2.8 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is
(5.6 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+7.0 s in the 10-1000 keV band is 19.9 +/- 0.4 ph/s/cm^2.

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/"

D. Sadaula (GSFC/UMBC), S. D. Barthelmy (GSFC), D’Elia (SSDC & INAF-OAR),
R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U
Tampa), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), T.
Parsotan (GSFC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+1101 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 241228A (trigger #1277277)
(D'Elia et al., GCN Circ. 38681
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 ).  The BAT ground-calculated position is
RA, Dec = 344.810, 18.608 deg which is
   RA(J2000)  =  22h 59m 14.4s
   Dec(J2000) = +18d 36' 28.6"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 39%.

The mask-weighted light curve shows several overlapping pulses that start
at ~T-5 s and end at ~T+5 s. T90 (15-350 keV) is 8.06 +- 0.53 sec
(estimated error including systematics).

The time-averaged spectrum from T-5.16 to T+4.71 sec is best fit by a power
law with an exponential cutoff.  This fit gives a photon index 0.90 +-
0.24, and Epeak of 61.0 +- 5.2 keV (chi squared 82.23 for 56 d.o.f.).  For
this model the total fluence in the 15-150 keV band is 3.0 +- 0.1 x 10^-06
erg/cm2 and the 1-sec peak flux measured from T+1.94 sec in the 15-150 keV
band is 13.1 +- 0.6 ph/cm2/sec.  All the quoted errors are at the 90%
confidence level.

The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1277277

Z.P. Zhu (NAOC), J. T. Palmerio (CEA/Irfu), J. An, D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud Univ.), T. Pursimo (NOT) report on behalf of a large collaboration:

We observed the field of GRB 241228A detected by Swift (D'Elia et al., GCN 38681
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 ), using the ALFOSC instruments mounted on the Nordic Optical Telescope (NOT), with 3 x 300 s in the SDSS-r band and 5 x 200 s photometry in the SDSS-z band. Observations started at 21:10:46 UT on 2024-12-28, i.e. 0.72 day after the trigger.

No new source is detected within the Swift/XRT enhanced error circle (Evans et al., GCN 38683
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 ), being consistent with the previous reports (Kuin et al., GCN 38685
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 ; Mohan et al., GCN 38689
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 ; Lipunov et al., GCN 38693
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 ), down to the 5-sigma upper limits of r > 22.0 (AB) and z > 20.5 (AB), calibrated with nearby Pan-STARRS stars without Galactic extinction correction.

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

We observed the field of GRB 241228B triggered by Fermi (Fermi GBM team,
GCN 38682
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 ) with AZT-33IK telescope of Mondy observatory in R-filter
starting on (UT) 2024-12-28 19:42:31. We clearly detect optical afterglow
(Kumar et al., GCN 38684
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  and GCN 38691
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 ; An et al. GCN 38687
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 ; Ortega-Casas
et al., GCN 38692
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 ; Ghosh et al., GCN 38702
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 ; An et al., GCN 38704
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 ).
Preliminary photometry of the afterglow is the following

Date UT start t-T0 Exp. Filter OT Err. UL(3sigma)
(mid, days) (s)
2024-12-28 19:42:31 0.66628 30x120 R 20.28 0.04 23.0

The photometry is based on nearby SDSS DR16 stars (R mags by Lupton
transformation).

A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 241228A
(Swift-BAT detection: D'Elia et al., GCN 38681
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 ; 
Sadaula et al., GCN 38701
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 ;
Fermi-GBM detection: Scotton & Meegan, GCN 38695
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 )
triggered Konus-Wind at T0=14233.924 s UT (03:57:13.924).

The burst light curve shows a two-peak structure
which starts at ~T0-0.2 s and has a total duration of ~5.1 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB241228_T14233/

As observed by Konus-Wind, the burst
had a fluence of 3.38(-0.18,+0.18)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+0.004 s,
of 2.32(-0.26,+0.26)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.88(-0.24,+0.26)
and Ep = 75(-5,+5) keV (chi2 = 96/81 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 96/80 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

Amit K. Ror, Anshika Gupta, Pranshu, Shashi B. Pandey, Kuntal Mishra
(ARIES) report:


We observed the field of GRB 241228A detected by Swift (Swift Observatory
Team, D'Elia et al. 2024, GCN 38681
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 ) with the 1.3m Devasthal Fast Optical
Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta
Research Institute of Observational Sciences (ARIES), India. The
observations were started on 2024-12-30 at 13:16:07 UT, i.e., ~ 2.38 days
after the Swift-BAT trigger. We have taken multiple frames with an exposure
time of 300 s in the R filter. We stacked the images after the alignment.
We could not detect the optical emission reported by Volnova et al. (2024,
GCN 38709
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 ) in our stacked image within the error box enhanced Swift-XRT
position (GCN 38683
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 ). We obtain the following 3-sigma upper limit in the
stacked image:


Date Start_UT T_start-T0 (days) Filter  Exp time (s)  Magnitude

=========================================================

2024-12-30 13:16:07    ~ 2.38     R     300s*12     >21.2


The non-detection of the burst is consistent with the upper limits reported
by Kuin et al. 2024, GCN 38685
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 ; Mohan et al. 2024, GCN 38689
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 ; Lipunov et
al. 2024, GCN 38693
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 ; Zhu et al. 2024, GCN 38703
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 .


The magnitude is not corrected for the Galactic extinction in the direction
of the burst. Photometric calibration is performed using the standard stars
from the USNO-B1.0 catalogue. This circular may be cited.

R. Querrard (UVI), P. Gokuldass (ERAU), N. Orange (OrangeWave Innovative Science, LLC), B. Gendre (UVI), D. Morris (NASA), T. Lombardi (Eckerd College), F. George (ERAU), D. Smith (UVI), K. Smith (UVI), C. Watson (UVI) report:

We observed the field of GRB241228A (D'Elia et al., GCN 38681
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 ) with the 0.5m Virgin Island Robotic Telescope (VIRT) at the University of the Virgin Islands' Etelman Observatory on 2024-12-28 starting at 03:57:15 UT (T-mid ~ T0 + 43.91 hrs). We performed a series of exposures in an R filter with a total exposure of 2060s. The weather conditions were partly cloudy during the hours of observation with an average airmass of 1.68. 

We do not detect any source within the enhanced XRT position (Evans et al. GCN 38683
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 ). This non-detection is consistent with reported upper limits  (Kuin  et al., GCN 38685
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 ; Mohan et al., GCN 38689
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 ; Lipunov et al., GCN 38693
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 ; Zhu et al., GCN 38703
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 ; Volnova et al., GCN 38709
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 ; and Ror et al., GCN 38815
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 ). We report the following 3-sigma upper limit:

T_mid                ||Exposure   ||Filter   ||Limit 
T+43.91 h            || 2060 s    || R       || > 20.4

The limit is estimated from comparison to nearby USNO B1 stars and is not corrected for Galactic extinction. 

We acknowledge financial support from NASA EPSCoR award 80NNSC22M0063, NSF PAARE award 2319415, and NASA EPSCoR award 80NSSC24M0112. This message can be cited.