GRB 250530A

A. Saccardi (CEA/Irfu), S. D. Vergani (LUX-Paris Obs.), D. B. Malesani (DAWN/NBI and Radboud), R. Brivio (INAF/OAB), N. Habeeb (U. Leicester), J.-L. Atteia (IRAP) report on behalf of the Stargate collaboration:

We observed the object visible in the Legacy Survey underlying the position of the optical/NIR afterglow (D'Avanzo et al., GCN 40588
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 ; Rayson et al. GCN 40589
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 ) of the SVOM GRB 250530A (Wang et al., GCN 40576
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 ) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.

A spectrum covering the wavelength range 3000-21000 AA was secured, with a total exposure time of 4x600 s in each of the UVB, VIS, and NIR arms. Our observation started at 00:32:26 UT on 2026 April 22, i.e. about 326.75 days after the SVOM trigger.

Clear emission lines are detected in the visible and near-infrared arms, which we identify as [N II] 6584, H-alpha, the [O III] 4959, 5007 doublet, H-beta, and the [O II] 3726, 3729 doublet, all at a common redshift of z = 1.486. We suggest this to be the redshift of the host galaxy of GRB 250530A.

We acknowledge excellent support from the ESO staff in Paranal, in particular Matias Jones, Felipe Gaete, Luciano Sbordone, and Boris Haeussler.

SVOM/GRM team: Chen-Wei Wang, Chao Zheng, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: M. G. Bernardini (INAF-OAB, LUPM)

Report on behalf of the SVOM team:

SVOM/GRM detected the burst GRB 250530A at 2025-05-30T06:31:54.99 UTC (T0), which is also detected by SVOM/ECLAIRs (Wang et al., GCN #40576
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 ).

With the event-by-event data downloaded through the X-band ground station, we conducted the standard analysis pipeline of GRB 250530A. The GRM light curve shows that this burst consists of a single pulse with a T90 of 15.0 +3.0/-4.0 s in the 15-5000 keV band, which is consistent with the result from VHF data.

With the localization of ECLAIRs (RA=186.71, DEC=-2.5648), the time-averaged spectrum from T0 to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.26 +0.18/-0.13 and the cutoff energy, parameterized as Epeak, is 707 +1140/-360 keV. The event fluence (10-1000 keV) in this time interval is (2.8 +/-0.4)E-06 erg/cm^2. 

Thus GRB 250530A is consistent with Type I GRBs in the 'Amati' relation diagram although the duration is much longer than 2 seconds, as shown at:
https://www.bursthub.cn//admin/static/grb250530A_amati.png
Further follow-up observations are encouraged. 

We note that the calibration of SVOM/GRM is undergoing thus these results are preliminary. Refined analysis will be reported later.

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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM/GRM point of contact for this burst is: Chen-Wei Wang (IHEP) (cwwang@ihep.ac.cn)

B. C. Rayson (Leicester), B. Schneider (LAM), P. D'Avanzo (INAF-OAB), A. Saccardi (CEA/Irfu), G. Corcoran (UCD), B. P. Gompertz (U. Birmingham), N. Habeeb (Leicester), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), G. Pugliese (API), N. R. Tanvir (Leicester) report on behalf of the Stargate collaboration:

We observed the near-infrared source reported by D'Avanzo et al. (GCN 40588
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 ), a candidate afterglow of GRB 250530A detected by SVOM/ECLAIRs (Wang et al., GCN 40576
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 ). We used the ESO VLT UT4 (Yepun) equipped with the HAWK-I near-infrared camera. We obtained a 20 min exposure in the J band, starting at 23:17:32 UT on 2025-05-31, i.e. 1.7 days after the SVOM trigger.

The near-infrared source is well detected in our image and we measured J = 21.65 +/- 0.10 mag (Vega), calibrated against the 2MASS catalogue. Compared with the value reported by D'Avanzo et al. (GCN 40588
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 ), our measurement indicates fading of the counterpart with a power-law decay index of ~0.6. This confirms the source as the NIR afterglow of GRB 250530A. 

As noted by D'Avanzo et al. (GCN 40588
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 ), this source is consistent with a catalogued source reported in the Legacy Survey. This is the likely GRB host, therefore, the red colors and the optical faintness (Li et al., GCN 40582
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 ) are likely due to dust extinction rather than high redshift - as also confirmed by the large column density measured in the X-ray spectrum (Evans et al., GCN 40579
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 ; see also: https://www.swift.ac.uk/SVOM/SVOM_FIELD00017/Source1.php). 


We acknowledge excellent support from the ESO observing staff in Paranal, in particular Thallis Pessi, Israel Blanchard, Miguel Lopez & Aaron Labdon

P. D'Avanzo, (INAF-OAB), A. Melandri (INAF-OAR), L. Izzo (INAF - OACn), M. G. Bernardini (INAF-OAB & LUPM), R. Brivio, M. Ferro (INAF-OAB), M. Pedani (INAF-TNG) on behalf of the CIBO collaboration report:

We observed the field of the GRB 250530A detected by SVOM (Wang et al., GCN Circ. 40576
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 ) with the Italian 3.6m TNG telescope, located in Canary Islands (Spain), equipped with the near-infrared camera NICS in imaging mode. A series of images were obtained with the J filters starting on 2025-05-30T20:57:35 UT (i.e. 14.4 hours post T0). 
Within the refined Swift/XRT error circle (Evans et al., GCN Circ. 40579
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 ; see also: https://www.swift.ac.uk/SVOM/SVOM_FIELD00017/Source1.php) a faint object is detected in the co-added image at the following coordinates:

RA (J2000) = 12:26:50.53
Dec (J2000) = -02:34:32.0
+/- 0.3"

We note that these coordinates are consistent (within 0.4") with those of a catalogue source reported in the DESI Legacy Imaging Survey (Duncan et al., 2022, MNRAS, 512, 366).

From preliminary photometry, we estimate for this source J ~ 21.0 mag (Vega; calibrated against the 2MASS catalogue). 


This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France (DOI : 10.26093/cds/vizier). The original description of the VizieR service was published in 2000, A&AS 143, 23.

D.Y. Li (NAO, CAS), Z. H. Yang, Q. C. Zhao (IHEP, CAS), J. Yang (NJU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250530A (SVOM/sb25053002, Wang et al. GCN #40576
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 ) at 2025-05-30T07:26:04 (UTC), about 1 hour after the SVOM/ECLAIRs trigger, with an exposure time of 2970s. Three uncatalogued sources are detected within the ECLAIRs error circle, among which Source 1 is spatially consistent with the candidate counterpart detected by Swift/XRT (Evans et al. GCN #40579
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 ). Preliminary analysis on these source are automatically conducted, and details are listed as follows. 
Source 1: EPF_J122650.7-023433
RA (J2000): 186.7114
Dec (J2000): -2.5759
Flux: 4.54 x 10^-12 erg/s/cm2 (observed, 0.5-10 kev)
Flux_err: 4.89 x 10^-13 erg/s/cm2 (1 sigma)
Note: This source is spatially consistent with the candidate counterpart detected by Swift/XRT, whose flux is (4.86 +/- 0.27) x 10^-11 erg/s/cm2 (0.3-10 keV).

Source 2: EPF_J122627.9-023807
RA (J2000): 186.6163
Dec (J2000): -2.6354
Flux: 6.31 x 10^-14 erg/s/cm2 (observed, 0.5-10 kev)
Flux_err: 1.94 x 10^-14 erg/s/cm2 (1 sigma)

Source 3: EPF_J122655.8-023705
RA (J2000): 186.7326
Dec (J2000): -2.5759
Flux: 1.33 x 10^-13 erg/s/cm2 (observed, 0.5-10 kev)
Flux_err: 2.74 x 10^-14 erg/s/cm2 (1 sigma)

The position uncertainty of the sources are about 10 arcsec in radius (90% C.L. statistical and systematic).

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). 

P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi  (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-IASFPA), A. D'Ai
(INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara
(PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T.
Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf
of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected source
GRB 250530A (sb25053002), collecting 1.7 ks of Photon Counting (PC) mode data
between T0+994 s and T0+2.8 ks after the trigger. 

We have detected a total of 2 sources. These have been automatically classified as
follows:
  * 0 likely counterparts
  * 1 candidate counterpart (Source 1)
  * 1 uncatalogued X-ray source
  * 0 known X-ray sources

A candidate counterpart has been found. The details of this source are:

  Source 1 (SWIFT J122650.6-023432):
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    RA (J2000.0):  186.7111  =	12h 26m 50.66s
    Dec (J2000.0): -2.5758  =  -02d 34' 32.9"
    Error:	   3.5 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   4.1 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	   0.761 +/- 0.043 ct s^-1
    Mean flux:	   (4.86 +/- 0.27)e-11 erg cm^-2 s^-1
    Peak rate:	   3.73 +/- 0.84 ct s^-1
    Peak flux:	   (2.38 +/- 0.53)e-10 erg cm^-2 s^-1
    ECF:	   6.39e-11 erg cm^-2 ct^-1, assuming NH=2.47e+22 cm^-2,
		   gamma=2.04; determined from a spectral fit.
    XMM UL:	   1.8e-12 erg cm^-2 s^-1, (0.3-10 keV)
	 so the source is 4.4-sigma above this 3-sigma upper limit.
    The source IS FADING at the 4.3-sigma level.
    There is 1 2MASS object within the source's 3-sigma error radius.


Details of the other uncatalogued X-ray source:

  Source 2 (SWIFT J122602.0-023341):
  ==================================
    RA (J2000.0):  186.5087  =	12h 26m 02.09s
    Dec (J2000.0): -2.5614  =  -02d 33' 41.0"
    Error:	   6.3 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   8.1 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	   (8.4 [+3.1, -2.5])e-3 ct s^-1
    Mean flux:	   (1.34 [+0.50, -0.40])e-12 erg cm^-2 s^-1
    Peak rate:	   (8.4 [+3.1, -2.5])e-3 ct s^-1
    Peak flux:	   (1.34 [+0.50, -0.40])e-12 erg cm^-2 s^-1
    ECF:	   1.58e-10 erg cm^-2 ct^-1, assuming NH=1.47e+22 cm^-2,
		   gamma=0.98; determined from a spectral fit.
    XMM UL:	   2.9e-12 erg cm^-2 s^-1, (0.3-10 keV)
	 so the source is 3.4-sigma above this 3-sigma upper limit.
    There is no evidence for fading.
    There is 1 2MASS object within the source's 3-sigma error radius.

All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits  from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.

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/SVOM.

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

Antonio de Ugarte Postigo (LAM), Enrique Moreno Méndez (UNAM), Stéphane Basa (UAR Pytheas), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), and Y. Wang (PMO, CAS):

Further analysis of the data observed by COLIBRÍ (de Ugarte Postigo et al. GCN Circ. 40577
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 ) reveals that the source that we had identified was a known minor planet (2001 VE24).  Further analysis of the field, including a stack of 32x60 s images obtained in the i-band between 2025-05-30 06:40:04 to 07:10:38 UTC do not show any credible counterpart down to a 5-sigma limit of i > 22.0 mag (AB).

We apologize for the confusion generated by our previous GCN.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

Antonio de Ugarte Postigo (LAM), Enrique Moreno Méndez (UNAM), Stéphane Basa (UAR Pytheas), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), and Y. Wang (PMO, CAS):

We imaged the field of the SVOM GRB 250530A (Wang et al., GCN Circ. 40576
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 ) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-05-30 06:40:04 to 06:41:59 UTC (from 489 s after the burst onset) and obtained 3x30 s of exposure in the i-band in our first stack of images.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

We detect an uncatalogued source (after the image subtraction using Legacy Survey/PanSTARRS as template and) consistent with the ECLAIRs error circle (Wang et al., GCN Circ. 40576
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 ) at: 

RA(J2000) = 12:26:56.38 = 186.7349 degrees
Dec(J2000) = -2:41:56.900 = -2.6991 degrees
with an uncertainty of 0.5 arcsec.

The preliminary magnitude derived for that source is:

i = 19.49 +/- 0.07 mag

Compared with subsequents stacks, we observe the fading of the brightness in this candidate, suggesting that this is indeed the afterglow of the GRB. 

Further analysis and observations are ongoing.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

Y. Wang (PMO, CAS), J.X Cao (GXU), D. Turpin (CEA), M. G. Bernardini (INAF-OAB, LUPM), D. Götz (CEA), C. Plasse (CEA), L. Zhang (IHEP) on behalf of the SVOM mission team.

SVOM/ECLAIRs triggered on the gamma-ray burst GRB 250530A (SVOM burst-id sb25053002) starting at 2025-05-30T06:31:54.99 UTC (Tb).

The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was detected by both the Count Rate Trigger (CRT) and  the Image Trigger (IMT). A sequence of 11 alerts was produced. CRT provided the alert with the best signal-to-noise-ratio (SNR) of 9.50 in the 8-50 keV energy band over a time window of 20.40 seconds starting at Tb.

The localization of the best alert is R.A., Dec. 186.71, -2.5648 degrees:
R.A. (J2000) = 12h26m50.37s
Dec. (J2000) = -02d33m53.38s
with a 90% confidence level (C.L.) radius of 8.35 arcmin (including systematic error of 2 arcmin added in quadrature).

The ECLAIRs light curve showed a multiple peak structure with a T90 duration of about 7.57 (-4.98 +2.48).

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250530A.png

SVOM slewed to the burst.

MXT began observing the field after the slew. The onboard software did not detect any x-ray source in the ECLAIRs error region. Further analysis of MXT data will be published in a future circular.

VT began observing the field after the slew. The analysis of the recorded images will be published in a future circular gathering information on the follow-up of the SVOM optical instruments.

The Space-based multi-band astronomical 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.

The Burst Advocate (BA) on shift for this burst is Y. Wang: wangyun@pmo.ac.cn.
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.