Gamma-ray Burst 241209B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #38478
SVOM/ECLAIRs Commissioning Team: Wenjin Xie, Donghua Zhao, Chao Wu (NAOC), Nicolas Dagoneau, Stéphane Schanne, Frédéric Chateau, Hervé Le Provost (CEA), Jean-Luc Atteia, Laurent Bouchet, Marius Brunet, Sebastien Guillot, Juliette Alaux, Hui Yang (IRAP), Alexis Coleiro (APC), Tais Maiolino (LUPM), Karine Mercier, Marie-Claire Charmeau, Stefano Crepaldi (CNES)
SVOM/GRM team: Chen-Wei Wang, Yue Wang, Wang-Chen Xue, Chao Zheng, Shi-Jie Zheng, Yong-Wei Dong, Jiang-Tao Liu, Jian-Chao Sun, Yue Huang, Jiang He, Min Gao, Hao-Xuan Guo, Lu Li, Yong-Ye Li, Hong-Wei Liu, Xin Liu, Hao-Li Shi, Li-Ming Song, You-Li Tuo, Wen-Long Zhang, Wen-Jun Tan, Hao-Xi Wang, Jin Wang, Jin-Zhou Wang, Ping Wang, Rui-Jie Wang, Yu-Xi Wang, Bo-Bing Wu, Shao-Lin Xiong, Jian-Ying Ye, Yi-Tao Yin, Wen-Hui Yu, Fan Zhang, Li Zhang, Peng Zhang, Shuang-Nan Zhang, Yan-Qiu Zhang, Yan-Ting Zhang, Shu-Min Zhao, Xiao-Yun Zhao (IHEP), Maria-Grazia Bernardini (LUPM/INAF-OAB), Laurent Bouchet (IRAP), David Corre (CEA), Tais Maiolino (LUPM), Frédéric Piron (LUPM), Stéphane Schanne (CEA), Jingwei Wang (IAP), JeanLuc Attéia (IRAP)
SVOM/MXT Commissioning Team: P. Ferrando, H. Goto, D. Götz, A. Meuris, M. Moita, C. Plasse, A. Sauvageon (CEA), F. Robinet, N. Leroy, C. van Hove (IJCLab), P. Maggi, L. Michel (ObAS), A. Fort, J. Joubert, K. Mercier, S. Crepaldi (CNES)
SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Olivier Godet (IRAP), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV) Report on behalf of the SVOM team:
During the commissioning phase, the SVOM/ECLAIRs telescope triggered and located the long duration GRB 241209B (sb24120901) at 2024-12-09 03:56:20 UT (Tb) which was also detected by SVOM/GRM.
The following ECLAIRs trigger information was received on the ground with low-latency by the SVOM VHF Alert Network. The burst was detected by both the on-board Count-Rate Trigger (CRT) and Image Trigger (IMT) and 13 alerts were received. The best detection is obtained by CRT with a signal-to-noise ratio of 20.01 in the 8-120 keV energy band over a time window of 1.2 s starting at Tb. The light curve shows one main pulse. The burst duration is 3.7 -0.2 / +1.1 s (5-120 keV).
The localization of the best alert is RA, Dec = 194.983, 76.222 (J2000). The statistical uncertainty on this position is 4.2 arcminutes, to which we recommend adding 2 arcminutes of systematic uncertainty in quadrature.
SVOM/GRM was also triggered in-flight by this burst (SVOM trigger reference: sb24120901) at 2024-12-09T03:56:20.000 UTC (T0). With the VHF data, the GRM light curve shows that this burst consists of one main pulse with a duration of 3.7 +/- 0.7 s (15-550 keV).
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb241209B.png
SVOM immediately slew to the ECLAIRs error box and MXT started to observe the field of GRB 241209A at 2024-12-09T04:43 UT in an automatic way, about 50 minutes after ECLAIRs detection. The observation start delay is due to Earth occultation. The analysis of the prompt VHF data shows no clear evidence of an afterglow candidate in the MXT data. Further analysis will be done once the X-band telemetry data are received.
Observations by other telescopes are encouraged.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by APC, CEA, CNES, and IRAP. GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. MXT was developed jointly by CNES, CEA, IJCLab, University of Leicester and MPE.
The SVOM point of contact for this burst is: xiewj@bao.ac.cn
GCN Circular #38494
P. A. Evans (U. Leicester) reports on behalf of the Swift team:
Swift has initiated a ToO observation of the SVOM/ECLAIRs GRB 241209B. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021739
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 SVOM/ECLAIRs 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 #38499
I. Perez-Garcia, E. J. Fernandez-Garcia, G. Garcia-Segura, M. D. Caballero-Garcia, S.-Y. Wu, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar, A. Castellon (Univ. de Malaga), Y.-D. Hu (INAF-OAB, Brera), S. Jeong (ADD, Daejeon) and D. Hiriart and W. H. Lee (UNAM), on behalf of a larger collaboration, report:
Following the detection of GRB 241209B by SVOM/ECLAIRs (Xie et al. GCNC [38478](https://gcn.nasa.gov/circulars/38478)), the 0.6m BOOTES-5/JG robotic telescope at Observatorio Astronomico Nacional in San Pedro Martir (Mexico) observed the GRB location starting on Dec 09, 07:01 UT (3.1 hours after trigger and 30 minutes after notification) in different optical bands. No new optical source is detected on the co-added images (15x60s, clear-filter) within the SVOM/ECLAIRs error box down to 20.3 mag.
We thank the staff at Observatorio Astronomico Nacional in San Pedro Martir for their excellent support.
GCN Circular #38516
SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, L. P. Xin, C. Wu, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, D. H. Zhao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM), H. Zhou (PMO), C. Plasse (CEA)
SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)
report on behalf of the SVOM team:
The GRB 241209B (Xie et al., GCN 38478) was observed by on-board VT after the automatic slew of the platform. The VT conducted observations in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously. Revisit observation (TOO-EX mode) was also operated ~ 10 hr after the trigger.
An uncatalogued optical source was detected in VT_B and VT_R band images within the errorbox of source #1 reported by Swift team (GCN 38494), which is 5.7 arcmin to the center of ECLAIRs localization, and further confirmed with the data from revisit observation. The source is located at RA, Dec = 194.63406, +76.17617 which is equivalent to:
```
RA (J2000) :  12h 58m 32.17s
Dec (J2000): +76d 10' 34.2" 
```
with an uncertainty of 0.5 arcsec.
Its brightness was fading for ~1 magnitude in both of VT_B and VT_R images in the initial 10 hours. The magnitude was VT_B= 23.20 +/-0.20 mag and VT_R= 21.75 +/-0.05 mag in AB magnitude at 2024-12-09T05:04:49 UT, about 1.14 hr post the trigger.
We proposed that this source is the optical counterpart of the burst.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.
GCN Circular #38525
M. A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), S. Dichiara (PSU), J.A. Kennea (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected burst GRB 241209B. We searched for X-ray sources in 3.1 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 3.1 ks, obtained between T0+50.8 ks and T0+57.9 ks.
An uncatalogued X-ray source is detected within the estimated 3-sigma SVOM/ECLAIRs error region (459 arcsec) and is believed to be the afterglow, given its positional coincidence with the SVOM/VT optical transient (GCN 38516). Using 1725 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 194.63185, +76.17637 which is equivalent to:
```
RA (J2000): 12h 58m 31.64s
Dec(J2000): +76d 10' 34.9"
```
with an uncertainty of 4.5 arcsec (radius, 90% confidence). This position is 5.7 arcmin from the SVOM/ECLAIRs position. We cannot determine at the present time whether the source is fading.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021739. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021739.
This circular is an official product of the Swift-XRT team.
GCN Circular #38528
James DeLaunay (PSU), Samuele Ronchini (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT did not localize GRB 241209B onboard (T0: 2024-12-09T03:56:20 UTC, SVOM/ECLAIRs and SVOM/GRM trig sb24120901 GCN 38478)
The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 12.3 in a 0.512 s analysis time bin, starting at T0 - 0.0 s.
Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2024. in prep)
The 90% credible area is 9,116 deg2 and the 50% credible area is 1881 deg2. The integrated probability inside the coded field of view is 7%.
The NITRATES skymap is consistent with the position from SVOM/ECLAIRs (GCN 38478) and the afterglow (SVOM/VT GCN 38516, Swift-XRT GCN 38525), with the position lying on the 20% credible region contour of the NITRATES skymap.
A plot of the probability skymap can be viewed here:
[skymap_plot](https://guano.swift.psu.edu/trigger_report?id=755409414/#:~:text=Probability%20Skymap)
The probability skymap file can be downloaded from the link here
[skymap_fits_file](https://guano.swift.psu.edu/files/755409414/0_n_PROBMAP)
Instructions on how to read and manipulate this map can be found here:
https://guano.swift.psu.edu/documentation
More details about this burst can be found on the trigger report page here:
https://guano.swift.psu.edu/trigger_report?id=755409414
GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/
GCN Circular #38534
M. Dafcikova, J. Ripa, M. Kolar (Masaryk U.), A. Pal (Konkoly Observatory), N. Werner (Masaryk U.), M. Ohno, H. Takahashi (Hiroshima U.), L. Meszaros, B. Csak (Konkoly Observatory), N. Husarikova, F. Munz , M. Topinka, M. Duriskova, L. Szakszonova, J.-P. Breuer, F. Hroch (Masaryk U.), T. Urbanec, M. Kasal, A. Povalac (Brno U. of Technology), J. Hudec, J. Kapus, M. Frajt (Spacemanic s.r.o), R. Laszlo, M. Koleda (Needronix s.r.o), M. Smelko, P. Hanak, P. Lipovsky (Technical U. of Kosice), G. Galgoczi (Wigner Research Center/Eotvos U.), Y. Uchida, H. Poon, H. Matake (Hiroshima U.), N. Uchida (ISAS/JAXA), T. Bozoki (Eotvos U.), G. Dalya (Eotvos U.), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), G. Friss (Eotvos U.), Y. Fukazawa, K. Hirose (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), K. Kapas (Eotvos U.), L. L. Kiss (Konkoly Observatory), T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), J. Takatsy (Eotvos U.), K. Torigoe (Hiroshima U.), N. Kogiso, M. Yoneyama (Osaka Metropolitan U.), M. Moritaki (U. Tokyo), T. Kano (U. Michigan) -- the GRBAlpha collaboration.
The long-duration GRB 241209B (SVOM/GRM detection: GCN 38478; Swift/BAT-GUANO detection: GCN 38528) was observed by the GRBAlpha 1U CubeSat (Pal et al. 2023, A&A, 677, 40; https://ui.adsabs.harvard.edu/abs/2023A%26A...677A..40P/abstract).
The detection was confirmed at the peak time 2024-12-09 03:56:19.7 UTC. The T90 duration measured by GRBAlpha is 1.5 s and the overall significance during T90 reaches 6.7 sigma.
The light curve obtained by GRBAlpha is available here: https://grbalpha.konkoly.hu/static/share/GRB241209B_GCN.pdf
All GRBAlpha detections are listed at: https://monoceros.physics.muni.cz/hea/GRBAlpha/ GRBAlpha, launched on 2021 March 22, is a demonstration mission for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). The detector of GRBAlpha consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~50 keV to ~1000 keV. To increase the duty cycle and the downlink rate, the upgrade of the on-board data acquisition software stack is in progress. The ground segment is also supported by the radio amateur community and it takes advantage of the SatNOGS network for increased data downlink volume.
GCN Circular #38537
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 241209B (SVOM/ECLAIRs and GRM detection: Xie et al., GCN 38478; Swift/BAT-GUANO localization: DeLaunay et al., GCN 38528; GRBAlpha detection: Dafcikova et al., GCN 38534) triggered Konus-Wind at T0=14182.105 s UT (03:56:22.105).
The burst light curve shows a single pulse, which starts at ~T0-0.480 s and has a total duration of ~0.7 s.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB241209_T14182/
As observed by Konus-Wind, the burst had a fluence of 4.45(-0.41,+1.38)x10^-7 erg/cm2, and a 16-ms peak flux, measured from T0-0.318 s, of 1.66(-0.55,+0.74)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
Since the brightest peak of the burst light curve was detected before the trigger, the spectral analysis was performed using the KW 3-channel light curve data.
Modelling the KW 3-channel time-integrated spectrum (measured from T0-0.480 s to T0+0.174 s) by a power law with exponential cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep), yields alpha = -0.21(-0.40,+1.27) and Ep = 163(-48,+45) keV.
All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.
GCN Circular #38539
N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP), R. Ya. Inasaridze (AbAO) report on behalf of IKI-GRB-FuN:
We performed optical observations of GRB 241209B (Xie et. al, GCN 38478; Evans, GCN 38494; Perez-Garcia et. al, GCN 38499; Qiu et. al, GCN 38516; Williams et. al, GCN 38525; DeLaunay et. al, GCN 38528; Dafcikova et. al, GCN 38534; Ridnaia et. al, GCN 38537) in the R-filter with 1.5-meter AZT-33IK telescope of Sayan observatory (Mondy) and 0.7-meter AS-32 telescope of Abastumani observatory (AbAO). The observations began at Mondy on 2024-12-10 18:11:43 UT, i.e. ~1.6 days since trigger. The optical counterpart (Xie et. al, GCN 38478) is not detected in the stacked images from both telescopes. The preliminary upper limits are given below:
```
Date       UTstart   t-T0         Exp.    Filter   OT  Err.  UL         Telescope
                     (mid, days)  (s)                        (3sigma)
2024-12-10 18:11:43  1.621802     40*120  R        n/d n/d   22.1       AZT-33IK
2024-12-10 21:27:04  1.759885     87*60   R        n/d n/d   20.9       AS-32
2024-12-11 20:35:13  2.722143     41*120  R        n/d n/d   21.8       AZT-33IK
```
The magnitudes were calibrated using nearby stars from USNO-B1.0 (R2 magnitudes) and are not corrected for the Galactic extinction towards the GRB 241209B.
GCN Circular #38543
M. Pillas (ULiege), M. Tanasan, K. Noysena (NARIT), S. Antier (OCA), O. Pyshna (Caltech), N. Guessoum (AUS), A. Klotz (IRAP), C. Andrade (UMN) S. Karpov (FZU), M. Coughlin (UMN), , P. Hello (IJCLAB), P-A Duverne (APC), T. Pradier (Unistra/IPHC), D.Turpin (CEA-Saclay/Irfu), on behalf of the GRANDMA collaboration:
We observed the field of GRB 241209B, detected by SVOM/ECLAIRs and GRM (GCN 38478) using TRT-SRO. Observations began 0.209 days after ECLAIRs T0.
We didn't detect any optical afterglow candidate with an upper limit of 22 mag in Johnson-R (3 sigma, Vega Mag)
Further analysis is required to check consistency with the afterglow candidate of SVOM/VT (GCN 38516).
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular #38568
SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, L. P. Xin, C. Wu, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, D. H. Zhao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM), H. Zhou (PMO), C. Plasse (CEA)
SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)
report on behalf of the SVOM team:
SVOM/VT revisited GRB 241209B（Xie et al., GCN 38478）since 2024-12-10T15:07:15 UT with a total exposure time of 8750 seconds. The optical counterpart (Qiu et al. GCN 38516) was continuously fading, reaching a magnitude of 23.70 +/-0.30 in VT_R. Nothing was seen down to limiting magnitude of 23.80 (3 sigma) in simultaneous channel VT_B.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.
GCN Circular #38632
D. Turpin (CEA), Y. L. Wang, T. Zhao, M. J. Liu (NAOC, CAS), Z. Y. Liu, M. Q. Huang (USTC), H. W. Pan, W. Yuan (NAOC, CAS), D. Adrien, C. Plasse (CEA/irfu), J. Guan, C. K. Li, Y . Chen, S. M. Jia, W. W. Cui, D. W. Han, W. Li, C. Z. Liu, F . J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F . Zhao (IHEP , CAS), Y . Liu, C. C. Jin, C. Zhang, Z. X. Ling, J. Wang, L. P . Xin (NAOC,CAS), E. Kuulkers, A. Santovincenzo (ESA), P . O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the SVOM and Einstein Probe teams
We performed a follow-up observation of GRB 241209B (SVOM/ECLAIRs and GRM, Xie et al., GCN 38478) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation started at 2024-12-09T16:01:19 (T-TGRB ~ 12.1hr) for about 3ks of exposure in total and a second epoch was also performed at 2024-12-12T22:35:16 (T-TGRB ~ 3.8 days) for again about 3ks of exposure in total.
In the first epoch, an uncatalogued X-ray source is detected by both FXT-A and FXT-B at the position (J2000) RA, DEC = 194.6388, 76.1747 (error=10", 90% C.L.), 5.75 arcminute away from the SVOM/ECLAIRs position (Xie et al., GCN 38478). This position is also consistent with the optical afterglow detected by SVOM/VT (Qiu et al., GCN 38516, GCN 38568) and Swift/XRT source 1 (Williams et al., GCN 38525).
This source is no longer detected in the second epoch 3.8 days post GRB trigger time confirming it has significantly faded and is indeed the x-ray afterglow of GRB 241209B.
The above observation was made with the EP-FXT instrument. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
GCN Circular #38646
A. de Ugarte Postigo (CNRS, OCA and LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), N. R. Tanvir (Univ. Leicester), L. Izzo (INAF/OACn and DARK/NBI), J. F. Agui Fernandez (CAHA), J. P. U. Fynbo (DAWN/NBI), S. Geier (GTC), G. Lombardi (GTC), A. Martin-Carrillo (UCD), N. A. Rakotondrainibe (LAM), C. C. Thoene (ASU-CAS, AbAO), N. Castro-Rodriguez (GTC), report:
At the location of the optical afterglow (Qiu et al., GCN 38516, GCN 38568) of the SVOM GRB 241209B (Xie et al., GCN 38478; see also DeLaunay et al., GCN 38428; Dafcikova et al., GCN 38534; Ridnaia et al., GCN 38537), a faint object is visible in the Legacy Survey r-band image (its detection in the g and i bands is marginal at best). We measure r = 23.52 +/- 0.12 mag (AB) calibrated against nearby Pan-STARRS objects. This is likely the GRB host galaxy.
A spectrum of this object was secured using OSIRIS+ on the 10.4 m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain) on the 11th December 2024 (2.06 days after the burst). Blind offsets were used to perform acquisition. The observation consisted of 3 spectra of 1200 s each, using grism R1000R which provides coverage over the spectral range 5100-10200 AA at a resolution R ~ 700.
On top of faint continuum, a single emission line is detected at the observed wavelength of 5872 AA. We discuss possible interpretations of this feature.
(A) The line is unlikely to be [O III] 5008 at z = 0.173, as the galaxy would be very faint and we see no hint of [O III] 4959 nor Halpha.
(B) The line could be the [O II] doublet at z = 0.575. At this redshift, Halpha falls out of the covered range, Hbeta lands in the telluric A band and [O II] 5008 in a region affected by strong sky line residuals. We believe this to be the most favoured solution, but the lack of [OIII] detection, could imply that it is fainter than [O II], which is uncommon for both short and long GRB host galaxies (Kruehler et al. 2014, A&A, 581, A125; Fong et al. 2022, ApJ, 940, 56).
(C) The feature could be Ly-alpha at z = 3.831. At this redshift, the galaxy would have an unprecedented luminosity with an absolute UV magnitude M = -22.5 (AB), significantly brighter than other GRB hosts at comparable redshift (e.g., Hjorth et al. 2012, ApJ, 756, 187; Sears et al. 2024, ApJ, 966:133). however, this interpretation would however be consistent with a mild break in the continuum blueward of the line, which could be the onset of the Lyman forest, as well as with the blue color of the VT afterglow (VT R-B = 1.45 +/- 0.20 mag: Qiu et al., GCN 38516).
With the available data, it is difficult to provide a secure redshift. Imaging of the host is planned, pending favorable weather, in order to constrain the redshift photometrically and discriminate between the two possibilities.

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Jochen Greiner, last update: 21-Dec-2024 [Disclaimer]