Gamma-ray Burst 240225B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #35796
  M. Nakajima, H. Negoro (Nihon U.), Y. Kawakubo (LSU), K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto, E.Goto (Chuo U.), M. Shidatsu (Ehime U.), I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA), Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani, Y. Okada (Kyoto U.), M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (NAOC), W. Iwakiri (Chiba U.) report on behalf of the MAXI team:
  The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 20:15:46 UT on February 25, 2024. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (128.458 deg, 27.487 deg) = (08 33 49, +27 29 13) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.13 deg and 0.11 deg, respectively. The roll angle of long axis from the north direction is 67.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 874 +- 86 mCrab (4.0-10.0keV, 1 sigma error). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners:

  (R.A., Dec) = (127.906, 26.756) deg = (08 31 37, +26 45 21) (J2000)
  (R.A., Dec) = (128.157, 26.654) deg = (08 32 37, +26 39 14) (J2000)
  (R.A., Dec) = (128.895, 28.075) deg = (08 35 34, +28 04 29) (J2000)
  (R.A., Dec) = (128.642, 28.179) deg = (08 34 34, +28 10 44) (J2000)
  

  There was no significant excess flux in the previous transit at 18:43 UT and in the next transit at 21:48 UT with an upper limit of 20 mCrab for each.
• GCN Circular #35797
  P. A. Evans (U. Leicester) reports on behalf of the Swift team:
  Swift has initiated a series of observations, tiled on the sky, of the MAXI GRB 240225B. Automated analysis of the XRT data will be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00124
  Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. The probability of finding serendipitous sources, unrelated to the MAXI event is high: any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration.
  Details of the XRT automated analysis methods are detailed in Evans et al. (2007, AA, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
  This circular is an official product of the Swift-XRT team.
• GCN Circular #35798
  J. Joshi (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
  Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 240225B which was also detected by MAXI/GSC (M. Nakajima et al., GCN Circ. 35796). Inspection of INTEGRAL/SPI-ACS data also showed the detection of the burst. We note that the INTEGRAL/SPI-ACS detection time is consistent with our time which is about 3 min offset from the time reported in the MAXI/GSC detection (GCN 35796).
  The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2024-02-25 20:12:08.50 UTC. The measured peak count rate associated with the burst is 185 (+40, -22) counts/s above the background in the combined data of all quadrants, with a total of 1617 (+331, -224) counts. The local mean background count rate was 346 (+1, -5) counts/s. Using cumulative rates, we measure a T90 of 19 (+10, -5) s.
  The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2024-02-25 20:12:01.80 UTC. The measured peak count rate associated with the burst is 570 (+72, -69) counts/s above the background in the combined data of all quadrants, with a total of 5119 (+490, -666) counts. The local mean background count rate was 1539 (+7, -6) counts/s. We measure a T90 of 23 (+3, -4) s from the cumulative Veto light curve.
  CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
  CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb
• GCN Circular #35805
  B. P. Gompertz, R. L. C. Starling, M. Kennedy, G. Ramsay, D. B. Malesani, B= . Godson, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, A= . Kumar, D. O'Neill, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K.= Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Pall'e and D. Pollacco report on behalf of the GOTO collaboration:
  We report on observations of the MAXI-discovered GRB 240225B (Nakajima et a= l, GCN 35796) with the Gravitational-wave Optical Transient Observer (GOTO, Steeghs et al. 2022). GOTO-North serendipitously tiled the MAXI localisati= on at 21:45:51 UT on 2024-02-25, 1.5 hours after trigger. The observation c= onsisted of 4x45 s exposures in the GOTO L-band (400-700 nm).
  Images were processed immediately after acquisition using the GOTO pipeline= . Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifie= r (Killestein et al. 2021) and cross-matched against a variety of contextua= l and minor planet catalogues. Human vetting was carried out in real time o= n any candidates that passed the above checks.
  We identify a new optical source (GOTO24tz/AT2024dgu) with a magnitude of L= =3D 17.12 +/- 0.04 at RA 08:33:26.67, Dec +27:04:32.71 (J2000). The optica= l source is close to the probable X-ray afterglow identified in tiled Swift observations (Evans, GCN 35797), offset by 7.2=E2=80=9D, whilst formally o= utside the 4.4=E2=80=9D 90% confidence contour of the X-ray localisation. T= his source was not present in the previous GOTO epoch, taken one day prior at 22:05:36 UT on 2024-02-24, to a limiting magnitude of L > 18.7. Given th= e tight pre-burst limit and close proximity with a new X-ray source, we pro= pose this as a strong candidate afterglow of GRB 240225B.
  Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.
  GOTO (https://goto-observatory.org) is a network of telescopes that is prin= cipally funded by the STFC and operated at the Roque de los Muchachos Obser= vatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash Univ= ersity, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Th= ailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC)= .
• GCN Circular #35810
  A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans (U. Leicester) and A. A. Breeveld (UCL-MSSL) reports on behalf of the Swift-XRT and the Swift-UVOT team:
  Swift-XRT has performed follow-up observations of the MAXI-detected burst GRB 240225B in a series of observations tiled on the sky. The total exposure time is 1.8 ks, distributed over 7 tiles; the maximum exposure at a single sky location was 693 s. The data were collected between T0+29.1 ks and T0+30.9 ks, and are entirely in Photon Counting (PC) mode.
  Two uncatalogued X-ray sources are detected, of which one ("Source 1") is above the RASS 3-sigma upper limit at this position. The position of this source is RA, Dec=128.3629, +27.0746 which is equivalent to:

  RA (J2000): 08:33:27.11
  Dec(J2000): +27:04:28.5
  

  with an uncertainty of 4.4 arcsec (radius, 90% confidence). This position is 25.3 arcmin from the MAXI position. At the present time the light curve of Source 1 shows no signs of fading and we can not confirm this source as the GRB afterglow.
  The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00124.
  Source 1 was out of the field of view for UVOT so we can't say anything about optical properties.
  This circular is an official product of the Swift team.
• GCN Circular #35811
  Y. Kawakubo (LSU), A. Yoshida, T. Sakamoto, S. Sugita (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration:
  The long GRB 240225B (MAXI/GSC detection: Nakajima et al., GCN Circ. 35796; AstroSat CZTI detection: Joshi et al., 35798) was detected in the ground analysis of the CALET Gamma-ray Burst Monitor (CGBM) data around 20:12:08.50 on 25 February 2024 (referenced to the AstroSat detection: GCN Circ. 35798) (http://cgbm.calet.jp/cgbm_trigger/flight/1392926875/index.html). The main episode that AstroSat CZTI detected was seen by all CGBM detectors. Also, a hint of emission that MAXI/GSC detected was seen in the HXM data around T+230 s.
  The burst light curve of the main episode shows a multi-peaked structure that starts at T-7.6 sec, and ends at T+31.1 sec. The T90 and T50 durations measured by the SGM data are 33.0 +/- 0.9 sec and 17.1 +/- 1.3 sec (40-1000 keV), respectively.
  The ground-processed light curve is available at
  http://cgbm.calet.jp/cgbm_trigger/ground/1392926875/index.html
  The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University.
• GCN Circular #35812
  X. Liu,, J. An, S.Y. Fu, S.Q. Jiang, Z.P. Zhu, T.H. Lu, D. Xu (NAOC), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:
  We observed the field of GRB 240225B detected by MAXI (Nakajima et al., GCN 35796) and AstroSat (Joshi et al., GCN 35798) using the HMT-0.5m telescope with big FOV (~1 deg^2) located at Nanshan, Xinjiang, China. Photometry was carried out twice in the same night without any filter..
  A slowly decaying uncatalogued optical transient is detected at coordinates

  R.A. = 08:33:26.80 (J2000)
  Dec. = +27:04:32.77 (J2000)
  

  with an uncertainty of radius ~ 0.3 arcsec, with r ~ 18.9 +/- 0.1 mag (AB) at a median time of 18.90 hr post-burst, calibrated with the nearby PanSTAR field. This OT is positionally at the border of the error circle of Swift/XRT "Source 1" (D'Ai et al., GCN 35810), and is also consistent with and thus confirming the GOTO candidate (Gompertz et al., GCN 35805).
  We thus think the OT is the optical afterglow of GRB 240225B.
• GCN Circular #35819
  D. B. Malesani (DAWN/NBI and Radboud Univ.), B. P. Gompertz (Birmingham), Z.-P. Zhu, D. Xu (NAOC), L. Izzo (INAF/OACN and DARK/NBI), on behalf of a larger collaboration, and R. Forsberg (Lund), S. Bijavara Seshashayana (Malmo univ.), report:
  We observed the optical afterglow (Gompertz et al., GCN 35805; D'Ai et al., GCN 35810) of GRB 240225B (Nakajima et al., GCN 35796; Joshi et al., GCN 35798; Kawakubo et al., GCN 35811). We used the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started on 2024 Feb 26.934 UT (1.09 days after the MAXI trigger time).
  In a single 300-s exposure in the r band, we measure r = 19.53 +- 0.03 (AB), calibrated against nearby Pan-STARRS stars.
  This confirms the decay of the GOTO candidate noted by Liu et al. (GCN 35812). Assuming an unbroken power law decay (F propto t^-alpha) between their observation and ours, the inferred decay slope is alpha = 1.8 +- 0.3, suggesting a faster decay then indicated in Liu et al. (GCN 35812).
• GCN Circular #35820
  J. Wise, A. Bochenek, D. A. Perley (LJMU) report:
  We report observations the optical afterglow GOTO24tz/AT2024dgu (Gompertz e= t al, GCN 35805; D'Ai et al, GCN 35810; Xu, D., GCN 35812; Malesani et al, = GCN 35819) associated with GRB 240225B (Nakajima et al, GCN 35796; Joshi et= al, GCN 35798; Kawakubo et al, GCN 35811) using the IO:O optical camera on= the 2-meter Liverpool Telescope. Observations commenced at 2024-02-27 00:2= 0:06.433 UTC, approximately 28.1 hours after the burst. 2x75s exposures wer= e taken using the SDSS g, r, i, and z filters, with Pan-STARRS foreground s= tars used for calibration. Conditions were generally favourable throughout.= =20
  The transient was well detected in all 4 filters, with magnitudes listed in= the following table. Times since burst for each filter were calculated usi= ng the strongest peak found with the AstroSat CsI anticoincidence (Veto) de= tector as reference (Joshi et al, GCN 35798).

  MJD=09        Time since burst (hours)=09Filter=09Mag. (AB)
  60367.01396=0928.135=09                        g=0919.97 =C2=B1 0.12
  60367.01624=0928.189=09                        r=0919.51 =C2=B1 0.15
  60367.01848=0928.243=09                        i=0919.44 =C2=B1 0.10=C2=A0
  60367.02073=0928.297=09                        z=0919.28 =C2=B1 0.14
  

  Magnitudes are not corrected for foreground extinction.
• GCN Circular #35824
  B. P. Gompertz (U. Birmingham), D. B. Malesani (DAWN/NBI and Radboud Univ.) report:
  We observed the position of the optical afterglow (Gompertz et al., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et al., GCN 35820) of GRB 240225B (Nakajima et al., GCN 35796; Joshi et al., GCN 35798; Kawakubo et al., GCN 35811) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 21:37:18 on 2024-02-27, 2.06 days after trigger, and consisted of 15x120 s exposures in the SDSS r filter.
  We detect the afterglow with an AB magnitude of r = 20.04 +/- 0.05, calibrated against nearby SDSS stars and not corrected for foreground extinction. Between this epoch and the initial detection by GOTO (Gompertz et al., GCN 35805), the afterglow is well described by a power-law decay with an index of 0.8, which is consistent with the magnitudes reported by NOT (Malesani et al., GCN 35819) and LT (Wise et al., GCN 35820), though under-predicts those reported by Nanshan/HMT (Liu et al., GCN 35812), potentially suggestive of flaring behaviour at the time of their observation. The steepening in the light curve noted in GCN 35819 may be thus due to local variability, rather than a break in the power-law evolution.
  We note the presence of a faint underlying source at the afterglow position in Legacy Survey archival imaging, which may be the host galaxy of GRB 240225B.
• GCN Circular #35826
  N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP), S. Belkin (IKI, HSE) report on behalf of IKI-GRB-FuN:
  We observed the field of GRB 240225B (Nakajima et al., GCN 35796; Evans, GCN 35797, Joshi et al., GCN 35798; D'Ai et al., GCN 35810; Kawakubo et al., GCN 35811) with AZT-33IK telescope of Sayan observatory (Mondy) equipped with CMOS-photometer ANDOR NEO. We took 60x120 sec frames in the R-filter starting on 2024-02-26 (UT) 12:30:03. The optical afterglow (Gompertz et al., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et al., GCN 35820; Gompertz & Malesani, GCN 35824) is well detected in the stacked frame with a total exposure of 59x120 sec. Photometry of the optical afterglow is following:

  Date       UT-start t-T0    Exp    Filter OT    Err   UL         FWHM
  (UT)          (mid.)        (nxT)                   (3sigma)   (arcsec)
  2024-02-27 13:01:23 1.71919 59x120 R      19.16 0.09  22.8       2.1
  

  The magnitudes were calibrated using nearby USNO-B1.0 stars (R2 magnitudes).
• GCN Circular #35828
  A. S. Moskvitin, O. I. Spiridonova (SAO RAS) report on behalf of the GRB follow-up team.
  We observed the field of GRB 240225B (Nakajima et al., GCN 35796; Evans, GCN 35797; Joshi et al., GCN 35798; D'Ai et al., GCN 35810; Kawakubo et al., GCN 35811) with the SAO RAS 1m telescope Zeiss-1000 equipped with CCD-photometer. We obtained 8 x 300 sec frames in the Rc band on Feb. 28, 18:20:36--19:11:22 UT (t_mid - T0 = 2.9307 days).
  The OT (Gompertz et al., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et al., GCN 35820; Gompertz & Malesani, GCN 35824; Pankov et al., GCN 35826) is clearly detected in our stacked frame with the brightness of R = 20.51 +/- 0.02.
  The photometry is based on nearby SDSS stars (magnitudes converted with Lupton 2005 equations).
• GCN Circular #35829
  Geoffrey Mo (MIT), Viraj Karambelkar (Caltech), Danielle Frostig (MIT), Robert Stein (Caltech), Nathan Lourie (MIT), Tomas Ahumada (Caltech), Robert Simcoe (MIT), Mansi Kasliwal (Caltech) report:
  We observed the field of GRB 240225B (Nakajima et al., GCN 35796; Evans, GCN 35797; Joshi et al., GCN 35798; D'Ai et al., GCN 35810; Kawakubo et al., GCN 35811) using the Palomar 1-m telescope equipped with the 1-square degree infrared WINTER camera (Lourie et al., 2020) in the Y and J bands. Observations began at 2024-02-28T05:55:09 UTC (~2.4 days after the GRB) and consisted of 8 x 120 s exposures in each filter.
  The images were processed using the WINTER data reduction pipeline (https://github.com/winter-telescope/mirar), with J-band image subtraction performed against reference images from the UKIRT Hemisphere Survey (Dye et al., 2017).
  We do not detect the GOTO-discovered counterpart (Gompertz et al., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et al., GCN 35820; Gompertz & Malesani, GCN 35824; Pankov et al., GCN 35826; Moskvitin et al., GCN 35828), setting the following upper limits (AB): Y = 18.3 mag and J = 18.8 mag.
• GCN Circular #35830
  Amit K. Ror, Shashi B. Pandey, Rahul Gupta, and Amar Aryan, Shivangi Pandey (ARIES) report:
  We observed the field of GRB 240225B detected by MAXI (Nakajima et al. 2024, GCN 35796), Swift (Evans 2024, GCN 35797), and AstroSat (Joshi et al. 2024, GCN 35798) using the 3.6m Devasthal Optical Telescope of ARIES Nainital. The observations were started on 2024-02-28 at 01:04:01.085 UT, i.e., 2.2 days after the MAXI trigger. We have taken multiple frames with a 60-second exposure time in the r, i, and z filters. We detected the OT at the position of the GOTO afterglow candidate reported by Gompertz et al. (2024, GCN 35805) in each observed image in the i and z bands and the stacked image of the r band, indicating a redder nature. The preliminary magnitude measurements are as follows:
  Date Start_UT T_start-T0 (days) Filter Exp time (s) Limiting magnitude
  =========================================================
  2024-02-28 01:04:01.085 2.2 i 60s 19.96 +/- 0.04
  Our i-band observation, combined with the observation of Wise et al. (2024, GCN 35820), yielded a decay index of ~ 0.8 in the i-band.
  The magnitude quoted is not corrected for the host and Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the Pan-STARRS catalog.
  Our detection is consistent with Gompertz et al. 2024, GCN 35805; Liu et al. 2024, GCN 35812; Malesani et al. 2024, GCN 35819; Wise et al. 2024, GCN 35820; Gompertz et al. 2024, GCN 35824; Pankov et al. 2024, GCN 35826; Moskvitin et al. 2024, GCN 35828; and Geoffrey et al. 2024, GCN 35829.
  This circular may be cited. 3.6m Devasthal Optical Telescope (DOT) is a 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.
• GCN Circular #35831
  M. Sasada, I. Takahashi, M. Niwano, S. Sato, N. Higuchi, S. Hayatsu, H. Takei, H. Seki, Y. Yatsu and N. Kawai (Tokyo Tech) report on behalf of the MIT SuME collaboration:
  We observed the field of GRB 240225B (Nakajima et al. GCN 35796) with the o= ptical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 = cm telescope Akeno.=20
  The observation with a series of 60 sec exposures started at 2024-02-27 09:= 49:24 UT (1.565 days after the MAXI trigger). We stacked the images in good= conditions. Here we report the g=E2=80=99- and Rc-band magnitudes of the o= ptical candidate and upper limits by the aperture photometry.

  T0+[days] | MID-UT | T-EXP[sec] | candidate magnitude and 5-sigma limits
  ---------------------------------------------------------------------------=
  -------------------------------------
  1.638 | 2024-02-27 11:35:11 | 9480 | g=E2=80=99=3D19.78+/-0.13, Rc=3D19.53+=
  /-0.10, Ic>19.7
  ---------------------------------------------------------------------------=
  -------------------------------------
  T0+ : Elapsed time after the trigger
  T-EXP: Total Exposure time
  

  The position of the optical candidate is consistent with Gompertz et al., G= CN 35805. The obtained magnitudes are broadly consistent with Gompertz et a= l., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et a= l., GCN 35820; Gompertz et al., GCN 35824; Pankov et al., GCN 35826; Moskvi= tin et al., GCN 35828; and Geoffrey et al., GCN 35829; and Ror et al., GCN = 35830.
  We used the PS1 catalog for flux calibration. The catalog magnitudes in PS1= g, r and i bands were converted to our g'-, Rc- and Ic-band magnitudes fol= lowing Tonry et al. (2012), Table 6. The magnitudes are expressed in the AB= system. The images were processed in real-time through the MITSuME GPU red= uction pipeline (Niwano et al. 2021, PASJ, Vol.73, Issue 1, Pages 4-24; htt= ps://github.com/MNiwano/Eclaire).
• GCN Circular #35832
  B. Schneider (MIT), G. Pugliese (API), A. Rossi (INAF/OAS), J. Palmerio (GEPI/Obs. de Paris & IAP), D. B. Malesani (DAWN/NBI and Radboud Univ.), Z. Zhu (NAOC), D. Xu (NAOC), B. P. Gompertz (Birmingham), A. Saccardi (GEPI/Obs. de Paris) report on behalf of the Stargate collaboration:
  We observed the field of the MAXI/GSC GRB 240225B (Nakajima et al., GCN 35796; see also Joshi et al. 2024, GCN 35798; Kawakubo et al., GCN 35811) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 1200 s each. The observation mid-time is 02:29:56 UT on 2024 Feb 29 (~3.3 days after the MAXI trigger).
  In grz images taken with the acquisition camera on Feb 29 01:37:53 UT, we clearly detect the optical afterglow (Gompertz et al., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et al., GCN 35820; Gompertz et al., GCN 35824; Pankov et al., GCN 35826; Moskvitin et al., GCN 35828; Mo et al., GCN 35829; Ror et al., GCN 35830; Sasada et al., GCN 35831), for which we measure a preliminary AB magnitude r ~ 21 (the absolute calibration is uncertain due to the low number of Pan-STARRS calibrators in our small FoV).
  In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range. From detection of multiple absorption features, which we interpret as being due to Fe II, Mg II, AlIII, Ca II, and Fe II*, we infer a common redshift of z = 0.946. We conclude this is the redshift of the burst. We also detect emission lines ([O II] and [O III] doublets, and H_alpha) at a consistent redshift, which we interpret as being due to the GRB host galaxy. As noted by Gompertz & Malesani (GCN 35824), the host galaxy is also detected in the Legacy Survey with r ~ 24.2 and a photometric redshift z ~ 0.9, which is consistent with the value we measured.
  We acknowledge expert support from the ESO staff in Paranal, in particular Francesca Lucertini, Israel Blanchard and Thomas Rivinius.
• GCN Circular #35835
  D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report:
  GRB 240225B (MAXI/GCS detection: Nakajima et al., GCN 35796; AstroSat CZTI detection: Joshi al., GCN 35798) triggered Konus-Wind (KW) at T0=3D72727.251 s UT (20:12:07.251), i.e., ~3.5 min before the MAXI trigger.
  The burst light curve shows a multi-peaked emission pulse, which starts at ~T0 - 2 s, peaks around ~T0 + 6 s, and has a total duration of ~35 s. The emission is seen up to ~3 MeV.
  The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB240225_T72727/
  As observed by Konus-Wind, the burst had a fluence of (2.16 =C2=B1 0.35)x10^-5 erg/cm^2 and a 64-ms peak energy flux, measured from T0 + 6.144 s, of (2.87 =C2=B1 0.41)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy = range).
  A time-averaged spectrum (measured from T0 to T0+41.216 s) is best fit in the 20 keV - 15 MeV range by a GRB (Band) function with the following model parameters: the low-energy photon index alpha =3D -1.37 (-0.14,+0.15), the high energy photon index beta =3D -2.74 (-7.26,+0.43), the peak energy Ep =3D 270 (-43,+64) keV, chi2 =3D 107/97 dof.
  A spectrum near the peak count rate (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 15 MeV range by a GRB (Band) function with the following model parameters: the low-energy photon index alpha =3D -0.85 (-0.110,+0.10), the high energy photon index beta =3D -2.43 (-0.55,+0.24), the peak energy Ep =3D 287 (-47,+59) keV, chi2 =3D 75/79 dof.
  Assuming the redshift z=3D0.946 (Schneider et al., GCN 35832) and a standard cosmology with H_0 =3D 67.3 km/s/Mpc, Omega_M =3D 0.315, and Omega_Lambda =3D 0.685 (Planck Collaboration, 2014), we estimate the burst isotropic energy release E_iso to (5.38 =C2=B1 0.95)x= 10^52 erg, the isotropic peak luminosity L_iso to (1.87 =C2=B1 0.27)x10^52 erg/s, the rest-frame peak energy of the time-integrated spectrum Ep,i,z to ~525 k= eV, and the rest-frame peak energy at the peak of the emission Ep,p,z to ~558 k= eV. With the obtained estimates, GRB 240225B is inside 68% prediction bands for both 'Amati' and 'Yonetoku' relations derived for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 202= 1), see http://www.ioffe.ru/LEA/GRBs/GRB240225_T72727/GRB240225B_rest_frame.pdf
  All the quoted errors are estimated at the 68% confidence level. All the presented results are preliminary.
• GCN Circular #35836
  P. A. Evans (U. Leicester) reports on behalf of the Swift team:
  Swift has initiated a ToO observation of the MAXI GRB 240225B. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021682
  Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. These are not necessarily related to the MAXI event. Any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration.
  Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
  This circular is an official product of the Swift-XRT team.
• GCN Circular #35839
  A. S. Moskvitin, O. I. Spiridonova (SAO RAS) report on behalf of the GRB follow-up team.
  We observed the field of GRB 240225B (Nakajima et al., GCN 35796; Evans, GCN 35797; Joshi et al., GCN 35798; D'Ai et al., GCN 35810; Kawakubo et al., GCN 35811; Frederiks et al., GCN 35835) with the SAO RAS 1m telescope Zeiss-1000 equipped with CCD-photometer. We obtained 8 x 300 sec frames in the Rc band on Feb. 29, 17:11:52--18:03:46 UT (t_mid - T0 = 3.8903 days).
  The OT (Gompertz et al., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et al., GCN 35820; Gompertz & Malesani, GCN 35824; Pankov et al., GCN 35826; Ror et al., GCN 35830; Sasada et al., GCN 35831; Schneider et al., GCN 35832) is clearly detected in our stacked frame with the brightness of R = 21.11 +/- 0.04.
  The photometry is based on nearby SDSS stars (magnitudes converted with Lupton 2005 equations).
• GCN Circular #35848
  C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
  The Glowbug gamma-ray telescope [1,2], operating on the International Space= Station, reports the detection of GRB 240225B, which was also detected by = MAXI/GSC (GCN 35796), AstroSat/CZTI (GCN 35798), CALET (GCN 35811), and Kon= us/Wind (GCN 35835).
  Using an adaptive window with a resolution of 32-ms, the burst onset is det= ermined to be 2024-02-25 20:11:58.416 with a duration of 38.9 s and a total= significance of ~89 sigma. The light curve comprises a multi-peaked struc= ture with two primary peaks at ~T0+3s and ~T0+8s, and a fainter peak at ~T0= +34s.
  Using a standard power-law function with an exponential high-energy cutoff = [3] to model the emission in two defined intervals from T0 to T0+22.5s and = T0+22.5s to +38.9s resulted respectively in photon indices dN/dE~E^x of x= =3D0.6 and x=3D0.7, and cutoff energies ("Epeak") of 324 keV and 262 keV. = The respective modeled 10-10000 keV fluences are 9.7e-06 erg/cm^2 and 2.0e-= 06 erg/cm^2.
  The analysis results presented here are preliminary and use a response func= tion that lacks a detailed characterization of the surrounding passive stru= cture of the ISS.
  Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost ga= mma-ray transient telescopes developed, built, and operated by the U.S. Nav= al Research Laboratory (NRL) with support from the University of Alabama in= Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard = the Department of Defense Space Test Program=E2=80=99s STP-H9 to the ISS. = The detector comprises 12 large-area (15 cm x 15 cm) CsI:Tl panels covering= the surface of a half cube, and two hexagonal (5-cm diameter, 10-cm length= ) CLLB scintillators, giving it a large field of view (instantaneous FoV ~2= /3 sky) over a wide energy band of 50 keV to >2 MeV.

  [1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
  [2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
  [3] Goldstein, A. et al. 2020, ApJ 895, 40, arXiv :1909.03006
  

  Distribution Statement A: Approved for public release. Distribution is unl= imited.
• GCN Circular #35859
  C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), A. A. Breeveld (UCL-MSSL), P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team:
  Swift-XRT has performed further follow-up observations of the MAXI-detected burst GRB 240225B (Nakajima et al., GCN Circ. 35796; Joshi et al., GCN Circ. 35798; Kawakubo et al., GCN Circ. 35811; Frederiks et al., GCN Circ. 35835; Cheung et al., GCN Circ. 35848). The data were collected between T0+461.7 ks and T0+508.9 ks, and are entirely in Photon Counting (PC) mode for a total exposure time of 1.4 ks.
  The uncatalogued X-ray source reported by D'Ai et al. ("Source 1"; GCN Circ. 35810), is still detected at an average count rate of ~ 1.3e-2 ct/s and shows signs of fading with >3-sigma significance, and is therefore likely the GRB afterglow. Using 1113 s of XRT Photon Counting mode data and 1 UVOT image, 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 = 128.36159, 27.07601 which is equivalent to:

  RA (J2000):  08 33 26.78
  Dec (J2000): +27 04 33.6
  

  with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position is consistent with the reported optical afterglow position (Gompertz et el., GCN Circ. 35805; Liu et al., GCN Circ. 35812; Malesani et al., GCN Circ. 35819; Wise et al., GCN Circ. 35820; Gompertz et el., GCN Circ. 35824; Pankov et al., GCN Circ. 35826; Moskvitin et al., GCN Circ. 35828; Amit et al., GCN Circ. 35030; Sasada et al., GCN Circ. 35831; Moskvitin et al., GCN Circ. 35839).
  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).
  The X-ray afterglow light curve can be modelled with a power-law with no breaks and decay index alpha =3D 1.09 (+0.16, -0.13).
  A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.1 (+0.8, -0.6). The best-fitting absorption column is 1.0 (+2.0, -1.0) =C3=97 10^21 cm^-2, at a redshift of 0.946 (Schneider et al., GCN Circ. 35832), in addition to the Galactic value of 4.0 =C3=97 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 2.72 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1.
  The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/00021675.
  This circular is an official product of the Swift-XRT team.