GRB 240101A

S. B. Cenko (GSFC), C. Gronwall (PSU), J.D. Gropp (PSU) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:

The Swift Burst Alert Telescope (BAT) triggered and located GRB 240101A
(trigger=1205708), and Swift has slewed to this burst. The trigger time is
believed to be no earlier than 12:52 UT. Given the loss of some of the TDRSS
notices, we will not have values for T0 or the BAT Position until the
full data have been downloaded, in approximately 1.25 hours (by around 16:05
UT). 

Using promptly downlinked data we find an uncatalogued X-ray source
with an enhanced position: RA, Dec 10.34819, 8.51706 which is
equivalent to:
   RA(J2000)  = 00h 41m 23.57s
   Dec(J2000) = +08d 31' 01.4"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). 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 (5.84 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.1
(+0.67/-0.52) x 10^22 cm^-2 (90% confidence). 

No afterglow is detected in the initial UVOT data products. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). 
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/)

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

Using 885 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 240101A, 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 = 10.34841, +8.51582 which is equivalent
to:

RA (J2000): 00h 41m 23.62s
Dec (J2000): +08d 30' 57.0"

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.

WeiKang Zheng and Alexei V. Filippenko (UC Berkeley) report on

behalf of the KAIT GRB team:


The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, responded to Swift GRB 240101A (Cenko et al.,

GCN 35448
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 ) starting at 15.57 hours after the burst. A total of

31x60s images were obtained in the clear (roughly R) filters.

We do not detect any optical afterglow candidate within the Enhanced

XRT position error circle (Beardmore et al., GCN 35452
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 ), neither in

single image, nor in the co-add images. The typical limiting magnitude

of our single clear image is about 18.5 mag calibrated to the PS1 catalog.

D.N. Burrows (PSU), J. D. Gropp (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), S. Dichiara (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 240101A, from 142 s to 57.6
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.06 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.1 (+/-0.3). The
best-fitting absorption column is  4.3 (+1.8, -1.5) x 10^21 cm^-2, in
excess of the Galactic value of 5.8 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 3.9 x 10^-11 (6.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.3 (+1.8, -1.5) x 10^21 cm^-2
Galactic foreground: 5.8 x 10^20 cm^-2
Excess significance: 4.1 sigma
Photon index:	     2.1 (+/-0.3)

If the light curve continues to decay with a power-law decay index of
1.06, the count rate at T+24 hours will be 1.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.6 x
10^-14 (9.3 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/01205708.

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

N. P. M. Kuin (UCL-MSSL) and S. B. Cenko (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 240101A
168 s after the BAT trigger (Cenko et al., GCN Circ. 35448
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 ).
No optical afterglow consistent with the BAT position
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           168          318          147         >20.4
white              168         1053          334         >20.8
v                  656         4661          236         >18.7
b                  582          773           39         >18.9
u                  729          749           19         >18.3
w1                 705         5071          236         >19.5
w2                 804          824           19         >18.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.059 in the direction of the burst
(Schlegel et al. 1998).

A. Volnova (IKI), P. Phorunzij (INASAN), A. Pozanenko (IKI),  N. Pankov (IKI, HSE), S. Belkin (IKI, HSE)  report on behalf of GRB IKI FuN:

We observed the GRB 240101A  (Cenko et al., GCN 35448
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 ) with Zeiss-2000   telescope of Terskol observatory starting on 2024-01-01 (UT) 16:10:24.   Within enhanced Enhanced Swift-XRT position (Beardmore et al., GCN 35452
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 )  we do not detect any optical objects. However we marginally detected the object at coordinates (J2000) 00:41:23.50 +8:30:58.48, which is 3.4 arcsec from XRT position (Beardmore et al., GCN 35452
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 ). Preliminary photometry of the marginally detected object is following.

Date       UT start  t-T0     Filter Exp.    OT   Err. UL.    
                     (mid, days)      (s)         S/N 


2024-01-01 16:10:24  0.1823   R      19*60   23.7 2.0 23.0  


The photometry is based on the nearby PS1 stars photometry (Lupton transformations)
RA           Dec  c      R(Lupton)
00:41:22.24 +08:31:56.6  16.277  +/-  0.012
00:41:17.90 +08:31:41.5  18.013  +/-  0.014

A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC),  S. B. Cenko (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), 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-10 to T+242 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 240101A (trigger #1205708)
(Cenko, et al., GCN Circ. 35448
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 ).  The trigger time, which was unavailable in the
first circular was 2024-01-01 at 14:20:17.331 UTC.  The BAT ground-calculated position is
RA, Dec = 10.289, 8.540 deg which is
   RA(J2000)  =  00h 41m 09.5s
   Dec(J2000) = +08d 32' 25.3"
with an uncertainty of 3.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 66%.

The mask-weighted light curve shows a weak, complex structure lasting from
About T-5 to T+10 seconds.  T90 (15-350 keV) is 15.8 +- 6.1 sec (estimated error
including systematics).

The time-averaged spectrum from T-1.94 to T+20.28 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.03 +- 0.34.  The fluence in the 15-150 keV band is 3.3 +- 0.7 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+2.11 sec in the 15-150 keV band
is 0.9 +- 0.2 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/1205708/BA/

Rahul Gupta, Amit K. Ror, Amar Aryan, and Shashi B. Pandey (ARIES) report
on behalf of a larger collaboration:

We performed the optical follow-up observations of the field of new year
burst GRB 240101A detected by Swift (Cenko et al. 2024, GCN 35448
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 ),
utilizing the 1.3m Devasthal Fast Optical Telescope (DFOT) situated at the
Devasthal Observatory of the Aryabhatta Research Institute of Observational
Sciences (ARIES), India. Our observations started on 2024-01-01 at 15:37:15
UT, approximately 1.28 hours post-detection. Multiple frames were captured in
the R filter with an exposure time of 300 seconds each. Subsequently, we
aligned and stacked the images. In our stacked image, no optical afterglow
was detected within the enhanced Swift-XRT error box (Beardmore et al.
2024, GCN 35452
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 ), resulting in a preliminary 3-sigma upper limit of ~ 22.4
mag. This non-detection is consistent with the observations of Zheng et al.
2024, GCN 35453
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 ; Kuin et al. 2024, GCN 35457
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 ; and Volnova et al. 2024, GCN
35460
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 .

Additionally, we observed a potential signature of very faint optical
emission at the position of the optical source reported by Volnova et al.
2024, GCN 35460
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 . The detected faint optical source (if related) may
potentially represent a candidate afterglow or the associated host galaxy
of GRB 240101A. Further deep follow-up observations using larger telescopes
will help to unveil the true nature of the optical source.

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 catalog. This circular may be cited.