GRB 241218A

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

We performed optical observations of the field of GRB 241218A (Williams et. al, GCN 38601
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 ; Beardmore et. al, GCN 38604
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 ; Strausbaugh et. al, GCN 38605
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 ; Maksut et. al, GCN 38611
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 ; D'Elia  et. al, GCN 38614
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 ; Brivio et. al, GCN 38620
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 ; Gupta et. al, GCN 38629
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 ; Fu et. al, GCN 38630
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 ; Siegel et. al, GCN 38634
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 ) in the R filter with AZT-33IK of Sayan observatory (Mondy). The observations began on 2024-12-18 20:52 UT, i.e. ~0.88 days since Swift trigger. We do not detect an optical counterpart inside the XRT enhanced localization (Beardmore et. al, GCN 38604
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 ) in the stacked image of 58x60 sec. The preliminary photometry is given below:

Date       UT start  t-T0         Exp.    Filter  UL(3sigma)
                     (mid, days)  (s)
2024-12-18 20:53:01  0.88415     58*60   R       20.8

The magnitudes were calibrated using nearby stars from USNO-B1.0 catalog (R2 magnitudes) and are not corrected for the Galactic extinction. The results are consistent with non-detections reported by other groups.

M. H. Siegel (PSU) and M. Williams (PSU) report on behalf of the
Swift/UVOTteam:

The Swift/UVOT began settled observations of the field of GRB 241218A
152 s after the BAT trigger (Williams et al., GCN Circ. 38601
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 ).
No optical afterglow consistent with the XRT position
(D'Elia et al., GCN Circ. 38614
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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           152          301          147         >20.7
u_FC               310          559          246         >19.8
white              152         1014          334         >21.0
v                  640         1411           97         >18.5
b                  565         1337           78         >18.7
u                  310         1312          304         >19.8
w1                 689          709           19         >21.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.024 in the direction of the burst
(Schlegel et al. 1998).

S.Y. Fu (NAOC), D. B. Malesani (DAWN/NBI and Radboud Univ.), Z.P. Zhu, X. Liu, D. Xu (NAOC), L. Cotter (UCD), A. Amanda Djupvik (NOT) report on behalf of a large collaboration:

We observed the field of GRB 241218A detected by Swift (Williams et al., GCN 38601
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 ) using the StanCam instrument mounted on the Nordic Optical Telescope (NOT), 3 x 300 s in the Bessel R band and 5x200 s in the Sloan z band frames were obtained. Observations started at 04:37:19 UT on 2024-12-18, i.e., 4.5 hr after the trigger.

No new source is detected in the stacked frames within the enhanced Swift/XRT error circle (Beardmore et al. GCN 38604
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 ), with the limiting magnitudes of R > ~ 21.7 (Vega, converted from r-band, Lupton 2005) and z > ~ 21.0 (AB), calibrated with nearby Pan-STARRS stars and without Galactic extinction correction.

R. Gupta (GSFC), S. D. Barthelmy (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), D. Sadaula (GSFC/UMBC),
T. Sakamoto (AGU), M. A. Williams (PSU)
(i.e. the Swift-BAT team):

Using the data set from T-520 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 241218A (trigger #1275005)
(M. A. Williams, et al., GCN Circ. 38601
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 ). The BAT ground-calculated position is
RA, Dec = 185.687, 29.984 deg which is
   RA(J2000)  =  12h 22m 44.8s
   Dec(J2000) = +29d 59' 04.2"
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 52%.

The mask-weighted light curve shows a single pulse of emission lasting 40 seconds
with a possible second pulse at T+70 seconds.
The T90 (15-350 keV) is 23.31 +- 4.19 sec (estimated error including systematics).

The time-averaged spectrum from T-1.82 to T+26.16 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.31 +- 0.26.  The fluence in the 15-150 keV band is 5.5 +- 1.0 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+16.64 sec in the 15-150 keV band
is 0.6 +- 0.2 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/1275005
 

R. Brivio, M. Ferro, P. D’Avanzo (INAF-OAB), D. B. Malesani (DAWN/NBI and Radboud Univ.) S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of GRB 241218A detected by Swift (Williams et al., GCN 38601
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 ) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z bands, starting on 2024 December 18 at 08:05:53 UT (i.e. 8.0 hours after the Swift trigger), and lasting for about 1 hour.

From preliminary photometry we do not detect any optical counterpart at the XRT enhanced position (Beardmore et al., GCN 38604
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 ) down to the following 3sigma magnitude upper limits:

r > 19.2 (AB; calibrated against the Pan-STARRS catalogue)
at a mid-time of t - t0 = 8.4 hr after the trigger.

A. Bochenek and D. A. Perley (LJMU) report:

We observed the Gamma-Ray Burst GRB 241218a (Williams et al., GCN 38601
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 ) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 3x200s exposures with the SDSS r’ and SDSS i’ filters starting at 2024-12-18 03:33:56 UT, approximately 3.4 hours after the trigger.

We do not detect any new sources at the Swift/XRT enhanced position (Beardmore et al., GCN 38604
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 ). The 3-sigma limiting magnitudes on the stacked images are r > 20.9 mag and i > 21.0 mag, calculated using the nearby PanSTARRS secondary standards as reference. The photometry was not corrected for extinction.

V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.1 ks of XRT data for GRB 241218A, from 131 s to 23.9
ks after the  BAT trigger. The data comprise 19 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.7 (+0.4, -0.3). At T+539 s  the decay
flattens to an alpha of -1.5 (+0.0, -0.7) before breaking again at
T+972 s to a final decay with index alpha=1.52 (+0.19, -0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.72 (+0.22, -0.12). The
best-fitting absorption column is  consistent with the Galactic value
of 2.2 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.8 x 10^-11 (3.9 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+/-4.5) x 10^20 cm^-2
Galactic foreground: 2.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.72 (+0.22, -0.12)

If the light curve continues to decay with a power-law decay index of
1.52, the count rate at T+24 hours will be 4.1 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-14 (1.6 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01275005.

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

Z. Maksut (NU), B. Grossan (UCB, NU), T. Komesh (NU), Z. Abdullayev (NU), M. Krugov (FAI) and E. Abdikamalov (NU) report on behalf of the Energetic Cosmos Laboratory:


The Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO) observed the field of GRB 241218A, 12 seconds after receipt of an automated GCN / BAT position alert, in Sloan g' and r' bands, with the Burst Simultaneous Three-Channel Imager (Grossan, Kumar & Smoot 2019, JHEA, 32, 14).


We started observations of GRB 241218A (Williams et al., GCN 38601
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 ; Strausbaugh et al., GCN 38605
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 ) at UT 00:13:43 on 2024-12-18, 292 seconds after the BAT trigger. Observations were made in clear conditions.  We report the following upper limit results:


start time  t_m-t0(s)  end time    UL g'   UL r'   exposure_time (image x s)
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00:26:30     1191     00:29:30     19.51     19.57    3 x 60


The start and end time is in UT. t_m is the middle time, t0 is the BAT trigger time, t_m-t0 is given in seconds. UL gives the 5 sigma upper limit sensitivity in magnitudes, for images co-added to the given exposure time. Calibration was done with 4 Pan-STARRS catalog stars on our images. No color corrections were applied to the values above.

----------------------------------
NU = Nazarbayev University, Astana, Kazakhstan
UCB = University of California, Berkeley, USA
FAI = Fesenkov Astrophysical Institute, Kazakhstan

R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration:

We observed the Swift GRB 241218A field (Williams et al., GCN 38601
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 )  with the LCOGT 1-meter Sinistro instrument at the Teide Tenerife site, on December 18, from 03:45 to 04:17 UT (corresponding to 3.62 to 4.15 hours after the GRB trigger time) with the sdss r and i filters.

We performed a series of 3x300s exposures in i-band and r-band.  We do not detect a source within the Swift XRT enhanced error region (Beardmore et al., GCN 38604
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 ) in either band.

The following 5-sigma upper limits are calculated using the PanSTARRS catalog as reference:

r > 21.2
i > 20.9

These magnitudes are not corrected for galactic extinction.

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

Using 1225 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 241218A, 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 = 185.68168, +29.97318 which is equivalent
to:

RA (J2000): 12h 22m 43.60s
Dec (J2000): +29d 58' 23.5"

with an uncertainty of 2.4 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.

M. A. Williams (PSU), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL),
B. Sbarufatti (INAF-OAB) and A. Tohuvavohu (Caltech) report on behalf
of the Neil Gehrels Swift Observatory Team:

At 00:08:51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 241218A (trigger=1275005).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 185.682, +29.991 which is 
   RA(J2000) = 12h 22m 44s
   Dec(J2000) = +29d 59' 27"
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 25 sec.  The peak count rate
was ~896 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 00:11:15.6 UT, 144.5 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find a fading, uncatalogued X-ray source with an enhanced position:
RA, Dec 185.68146, 29.97303 which is equivalent to:
   RA(J2000)  = 12h 22m 43.55s
   Dec(J2000) = +29d 58' 22.9"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 64 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 2.23
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 151 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
0.023.

Burst Advocate for this burst is M. A. Williams (mjw6837 AT psu.edu).
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/)