GRB 240819A

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the SVOM/Eclairs GRB 240819A. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021702

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.

A.Li (BNU), H. Sun, H. Q. Cheng, Z. X. Ling (NAOC, CAS), J. Q. Peng, J. Guan, C. K. Li, Y. Chen, S. M. Jia, W. W. Cui, H. Feng, 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), J. Wang, L. P. Xin, Y. Liu, C. C. Jin, W. Yuan, C. Zhang (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 Einstein Probe and SVOM teams


We performed an observation of SVOM/Eclairs-triggered GRB 240819A with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation began at 2024-08-19T09:29:55 (UTC), and the exposure time is about 5.7 ks. FXT-A detected 3 uncatalogued sources in the 90% localization error circle (with a radius of 10.4 arcmin centered at RA, DEC = 310.909 deg, 49.589 deg), and two of them are also detected by FXT-B.

The following table lists the detailed information of the three X-ray sources detected by FXT-A. The positions are given with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The 0.5 - 10 keV flux is derived from FXT-A data; The uncertainties of flux are at the 90% confidence level.

Source name         | RA       | DEC     | Estimated Flux      |
                    | deg      | deg     | (erg/s/cm^2)        |

-----------------------------------------------------------------------------------

EP J204334.6+493448 | 310.8943 | 49.5800 | 7.5(+/-4.0) x 10^-14
EP J204347.6+493054 | 310.9483 | 49.5149 | 2.1(+/-0.6) x 10^-13 
EP J204244.0+494023 | 310.6835 | 49.6730 | 1.7(+/-0.5) x 10^-13 
-----------------------------------------------------------------------------------

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.

S.Q. Jiang, J. An (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), X. Liu, S.Y. Fu, Z.P. Zhu, S.Y. Gao, D. Xu, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang (NAOC) report on behalf of a large collaboration:

We observed the field of GRB 240819A detected by SVOM/Eclairs using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Fresno, California, US. Observations started at 09:41:45.015 UTC on 2024-08-19, i.e., 7.107 hr after the T0 of this GRB, and 16 x 240 s  frames were obtained in R-band aiming to cover the whole SVOM/Eclairs error circle.

No new optical source is detected within the X-ray error circles provided by Swift/XRT (Evans et al., GCN37203
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 ) and EP/FXT (Li et al., GCN 37206
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 ), down to the following 3-sigma optical upper limits:

Source | R.A. | Dec. | Upper Limit (mag)
--------------------------------------------------------
XRT Source1          | 311.0531 | 49.6821 | R>19.0
XRT Source2*         | 310.9468 | 49.5141 | N/A
EP J204334.6+493448  | 310.8943 | 49.5800 | R>19.8
EP J204347.6+493054* | 310.9483 | 49.5149 | N/A
EP J204244.0+494023  | 310.6835 | 49.6730 | R>19.8

calibrated with PanSTARRS stars in the field and not corrected for Galactic extinction. The two sources marked with * are very likely an identical one detected by both Swift/XRT and EP/FXT, but not covered by TRT.

SVOM/ECLAIRs commissioning team: Alexis Coleiro (APC), Stéphane Schanne, Frédéric Chateau, Nicolas Dagoneau, Hervé Le Provost (CEA), Olivier Godet, Laurent Bouchet, Sebastien Guillot, Juliette Alaux (IRAP), Frédéric Piron (LUPM), Karine Mercier, Marie-Claire Charmeau, Stefano Crepaldi (CNES)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Jean-Luc Atteia (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), Bing Zhang (UNLV)

report on behalf of the SVOM team:

GRB 240819A : An X-Ray Transient located on-board by SVOM/ECLAIRs.

During the commissioning phase, the SVOM/ECLAIRs telescope triggered and located an X-Ray Transient (sb24081901), hereafter GRB 240819A, at 2024-08-19T02:35:21.2 UT (T0). 

The following trigger information was received on the ground with low-latency by the SVOM VHF Alert Network: The image SNR is 10.3, measured in the [8-50] keV energy band, over a time window of 40.96 seconds, starting at T0.

The transient location is RA, Dec = 310.91, 49.59  (J2000). This is about 9 degrees from the ECLAIRs optical axis, in the totally coded field of view. The statistical uncertainty on this position is 8.0 arcminutes, to which we recommend to add 10 arcminutes of systematic uncertainty. This location was subsequently observed in X-rays by Swift (GCNC 37203) and Einstein Probe (GCNC 37206), and in the optical by the Thai Robotic Telescope network (GCNC 37207).

We are confident of the reality of this transient, however its location 4° above the galactic plane raises some doubts on its nature: an unknown galactic source or a GRB behind the galactic plane.

While the real-time alert was received within 1 minute of the trigger, we have awaited the availability of the full X-band data to send this GCN, in order to confirm the reality of the source with the full reconstructed sky image.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), French Space Agency (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. ECLAIRs was developed jointly by APC, CEA, CNES and IRAP.

WeiKang Zheng, Alexei V. Filippenko (UCB) and S. Antier (OCA),


SVOM JSWG: Jian-Yan Wei (NAOC, China), Bertrand Cordier (CEA, F),

Shuang-Nan Zhang (IHEP, CN), Stephane Basa (LAM, F), Jean-Luc Attéia

(IRAP, F), Arnaud Claret (CEA, F), Zi-Gao Dai (USTC, CN), Frederic Daigne

(IAP, F), Jin-Song Deng (NAOC, CN), Andrea Goldwurm (APC, F),

Diego  Götz (CEA, F), Xu-Hui Han (NAOC, CN), Cyril Lachaud (APC, F),

En-Wei Liang (GXU, CN), Yu-Lei Qiu (NAOC, CN), Susanna Vergani

(Obs. Paris, F), Jing Wang (NAOC, CN), Chao Wu (NAOC, CN), Li-Ping Xin

(NAOC, CN), Bing Zhang (UNLV, CN)


report on behalf of the KAIT GRB team:


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

Lick Observatory, responded to GRB 240819A from the SVOM/ECLAIRs

trigger sb24081901 (Coleiro et al., GCN 37208
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 ) starting at 03:49

UT, Aug 19th, ~1.23 hours after the trigger. Observations were

performed in 3x3 tiling mode and lasted for about 6 hours, a set

of clear (roughly R) filter images were obtained. The location of

the 4 X-ray sources, two from Swift XRT (Evans et al., GCN 37203
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 )

and two additional ones from EP/FXT (Li et al., GCN 37206
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 ) were all

covered in our images. Preliminary analysis do not reveal any new

optical counterpart candidate within the X-ray error circles,

neither in single image, nor in the co-add images, consistent with

the reports from Jiang et al. (GCN 37207
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 ). The typical limiting

magnitude of our single clear image is about 19.5 mag calibrated to

the Pan-STARRS1 catalog.

S.Q. Jiang, Z.P. Zhu, X. Liu, J. An, S.Y. Fu, D. Xu (NAOC), J.Z. Liu (XAO) report on behalf of a large collaboration:

We observed the Swift/XRT Source 1 (Evans et al., GCN37203
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 ), an X-ray afterglow candidate of GRB 240819A detected by SVOM/Eclairs (Coleiro et al., GCN 37208
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 ), using the 100C telescope of the JinShan project located at Altay, Xinjiang, China. Observations started at 20:16:17.46 UTC on 2024-08-20, i.e., 1.737 day after the T0 of this GRB, and a series of frames were obtained in the Sloan i- and z- bands.

No new optical source is detected within the Swift/XRT Source 1 error circle, down to the following 3-sigma optical upper limits:

-----------------------------------------------------------------
Tmid-T0(hr)  exp(s)   filter      UL(3-sigma)
-----------------------------------------------------------------
41.974        14x120      i           >20.6
42.663        22x120      z           >20.3
-----------------------------------------------------------------

calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction.

We acknowledge the excellent support from S.W. Luo, Z.K. Feng, Q.C. Zhao, M.M. Yang, and L.F. Huo for enabling these observations.

P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), S. Dichiara (PSU),
J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E.
Ambrosi  (INAF-IASFPA)	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 X-ray transient GRB 240819A (GCN 37208
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  Coleiro et
al.), collecting 2.0 ks of Photon Counting (PC) mode data between
T0+19.7 ks and T0+26.6 ks. 

Two uncatalogued X-ray sources have been detected, however none of them
is above the RASS limit or shows definitive signs of fading. Therefore,
at the present time we cannot identify which, if any, is the afterglow.
Details of these sources are given below:

Source 1:
  RA (J2000.0):  311.0531  =  20:44:12.74
  Dec (J2000.0): +49.6821  =  +49:40:55.5
  Error: 6.6 arcsec (radius, 90% conf.)
  Count-rate: (3.4 [+2.0, -1.5])e-3 ct s^-1   
  Distance: 336 arcsec from SVOM/Eclairs position.
  Flux: (9.8 [+5.7, -4.2])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 2:
  RA (J2000.0):  310.9464  =  20:43:47.14
  Dec (J2000.0): +49.5140  =  +49:30:50.3
  Error: 5.8 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: 0.0111 [+0.0035, -0.0029] ct s^-1   
  Distance: 636 arcsec from SVOM/Eclairs position.
  Flux: (4.4 [+1.4, -1.1])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Moreover Source 2, separated by 5.525" from EP J204347.6+493054 (GCN
37206
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  Li et al.), has not been detected from a previous observation
performed by Swift at MJD 54638.495 with a 3 sigma upper limit of
2.9e-03 ct s^-1.

Swift will perform further ToO observations to assess the nature of 
this transient.


The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021702.

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

Antonino D’Ai, Elena Ambrosi (INAF/IASF-PA) report on behalf of the Swift-XRT team:

Swift performed a second epoch follow-up observation for Source2 (D’Avanzo et al., GCN 37218
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 ), the likely counterpart for the SVOM/Eclairs-detected X-ray transient GRB 240819A (Coleiro et al., GCN 37208
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 ). We collected 3.4 ks of Photon Counting (PC) mode data between T0+6.01 days and  T0+7.67 days. 

At the position of Source2 (previously detected at rate 11(+3.5, -2.9) x 10^-3 ct/sec), no source is detected with a 3-sigma upper limit of 3.5 x 10^-3 ct/sec. 

The upper limit is evidence for the lightcurve fading of this source, confirming the afterglow association with GRB 240819A.

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

SVOM/ECLAIRs commissioning team: Sebastien Guillot, Olivier Godet, Laurent Bouchet, Juliette Alaux (IRAP), Tais Maiolino (LUPM), Floriane Cangemi (APC), Stéphane Schanne, Frédéric Chateau, Nicolas Dagoneau, Hervé Le Provost (CEA), Karine Mercier, Marie-Claire Charmeau, Stefano Crepaldi (CNES)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Jean-Luc Atteia (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), Bing Zhang (UNLV)

report on behalf of the SVOM team:


Following the trigger of the X-ray transient, named GRB 240819A, by SVOM/ECLAIRs (Coleiro, et al., GCN 37208
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 ) at 2024-08-19T02:35:21.2 UT (T0), further analyses of the X-band data from ECLAIRs indicate a rather soft event with most photons below 40 keV (signal-to-noise of ~10 for events between 4 to 40 keV; versus S/N < 4 for events above 40 keV).  

In addition, observations by Swift-XRT at T0+0.23 days and T0+0.31 days (D'Avanzo, et al., GCN 37218
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 ) and at T0+6.01 days  (D’ai, et al. GCN 37311
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 ) revealed a fading X-ray source from  11(+3.5, -2.9) x 10^-3 ct/sec to a 3-sigma upper limit of 3.5 x 10^-3 ct/sec. Therefore, the X-ray afterglow of GRB 240819A is likely source 2 (from D’ai, et al. GCN 37311
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 ), localized at RA=310.9464 (20:43:47.14) and Dec.=+49.5140 (+49:30:50.3) with a 90% c.l. error circle of 5.8 arcseconds.

The identification of the fading afterglow and the spectral softness of the event described above, are indicative of a likely X-Ray Flash (XRF, i.e., a soft GRB with lower luminosity and located at a closer distance than typical GRBs), rather than a Galactic transient.

We encourage deep optical follow-up observations to attempt the detection of a possible supernova that could be associated with this event. We caution that, due to the low galactic latitude of the transient, a potential supernova will undergo strong visible extinction (Av ~ 5).

R. Jayaraman (MIT), M.M. Fausnaugh (TTU), R. Vanderspek (MIT), and G. Mo (MIT) report:

The Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015) was observing the entirety of the localization region for the transient GRB 240819A (Coleiro et al., GCN 37208
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 ), which was later identified as an X-ray flash (Guillot et al., GCN 37325
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 ). TESS observed this region at a 200 second cadence continuously from 2.14 days before the trigger to 3.85 days after the trigger. The GRB occurred during TESS observational Sector 82, and the localization fell within Camera 3, CCD 2.

We performed forced difference-imaging photometry at the location of the confirmed X-ray afterglow (D’Ai et al., GCN 37311
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 ) using the full-frame images from the publicly-available TICA data archived at MAST (https://archive.stsci.edu/hlsp/tica). Our analysis routine is described in Fausnaugh et al. 2023 (ApJ 956(2):108). 

There was no source detected at the location from Swift-XRT. We find a 3-sigma upper limit of 17.3 around the time of trigger in the TESS band at the native 200 s cadence. Stacking to 30 minutes yields a deeper limit of 19.4. These limits are consistent with the upper limits reported a few hours after the burst (Zheng et al., GCN 37213
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 ).

This circular includes data collected with the TESS mission, obtained from the MAST data archive at the Space Telescope Science Institute (STScI). Funding for the TESS mission is provided by the NASA Explorer Program. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.