GRB 250521C
GCN Circular 40608
Subject
GRB 250521C: NOT spectroscopic observations exclude the afterglow candidate AT2025mgj
Date
2025-06-02T16:07:14Z (5 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
A. Martin-Carrillo (UCD), B. Schneider (LAM), L. Izzo (INAF/OACN and DARK/NBI), A. de Ugarte Postigo (LAM), G. Corcoran (UCD), B. Gompertz (U. Birmingham), D. B. Malesani (DAWN/NBI and Radboud), R. H. Rasmussen (NOT and Aarhus Univ.), report on behalf of a larger collaboration.
We carried out spectroscopic observations of AT2025mgj (Andreoni et al., GCN 40566), a candidate optical counterpart to GRB 250521C (Fermi GBM team, GCN 40515), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 4x900 s exposures starting at 21:16 UT on 2025-05-31 (10.3 days after the Fermi trigger) with a wavelength coverage of 3800-9500 AA.
The spectrum shows a blue, featureless continuum, peaking at around 4500 AA. Redward of the peak, the continuum is consistent with a power law F_lambda propto lambda^-2.7. The overall shape does not resemble a power-law spectrum, which would be typical of GRB afterglows, nor a SN spectrum, given the lack of any features. Although the spectrum does not fully resemble those of dwarf novae in outbursts, the spectral energy distribution is overall consistent with previous examples (e.g. Aviles et al. 2018, doi:10.14482/INDES.30.1.303.661).
In summary, our spectroscopic observations confirm that AT2025mgj is not associated with GRB250521C, as already suggested by Busmann et al. (GCN 40597) and Gillanders et al. (GCN 40605) based on photometric observations.
GCN Circular 40605
Subject
GRB 250521C / AT2025mgj: Pan-STARRS multi-band and multi-epoch imaging and photometry
Date
2025-06-02T15:01:40Z (5 months ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, (Oxford/QUB), S. Srivastav (Oxford), M. Nicholl, D. Young, M. Fulton (QUB), T.-W. Chen (NCU, Taiwan) A. S. B. Schultz, T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).
We observed the fast-fading transient AT2025mgj (Andreoni et al., GCN 40566), tentatively associated with the short-GRB 250521C (Fermi GBM Team, GCN 40515) using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJDs 60825.26 and 60827.26 (approximately 8.7 and 10.7 days post-GRB, respectively). The Pan-STARRS system consists of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters.
Our observations consisted of 120s (MJD 60825.26) and 200s exposures (MJD 60827.26) in grizy-bands. All images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4).
From these difference images, we recover the following photometry measurements:
MJD Filter AB mag
60825.26 g 18.97 +/- 0.06
60825.26 r 19.18 +/- 0.08
60825.26 i 19.29 +/- 0.08
60825.26 z 19.5 +/- 0.2
60827.26 g 19.29 +/- 0.05
60827.26 r 19.39 +/- 0.06
60827.26 i 19.62 +/- 0.06
60827.26 z 19.57 +/- 0.08
From our multi-band Pan-STARRS photometry, we estimate a fade rate of ~0.1 mag/day in r-band. Our rate of fading across our photometric bands match those reported previously by Martin-Carrillo et al. (GCN 40584), Becerra et al. (GCN 40590), and Busmann et al. (GCN 40597).
The rate of fading and persistent blue colour, with some indication of cooling, are all reminiscent of typical CV behaviour. While the connection to GRB 250521C cannot be definitively ruled out, the data match the lightcurve behaviour of CVs. Late-time, deep imaging is required to unambiguously uncover the nature of AT2025mgj. A point source would confirm AT2025mgj is indeed a CV, whereas an extended source in its vicinity would indicate that it was the afterglow of GRB 250521C.
GCN Circular 40597
Subject
GRB 250521C/AT2025mgj: FTW optical and NIR observations
Date
2025-06-01T18:58:47Z (5 months ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:
We observed the candidate counterpart AT2025mgj (Andreoni et al., GCN 40566; Martin-Carrillo et al., GCN 40584; Becerra et al., GCN 40590) of GRB 250521C (Fermi GBM team, GCN 40515) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 180 s starting at 2025-05-30T20:31:19 UT (9.3 days after the GRB trigger). We detect the source at
r = (19.28 +/- 0.01) mag
i = (19.45 +/- 0.02) mag
J = (20.03 +/- 0.07) mag.
We note that the blue color is inconsistent with an afterglow origin, despite the similar power law temporal decay (Martin-Carrillo et al., GCN 40584; Becerra et al., GCN 40590). The source is therefore unlikely to be associated to GRB 250521C. The most likely origin in a Cataclysmic Variable (CV) as noted by Andreoni et al. (GCN 40566). However, given the coincidence with GRB 250521C, we encourage additional observations to conclusively determine the nature of this transient.
The r and i band magnitudes are calibrated against the PS1 catalog and the J band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.
We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.
GCN Circular 40590
Subject
GRB 250521C/ZTF25aarhkyn/AT2025mgj: COLIBRÍ optical observations
Date
2025-05-31T15:39:50Z (5 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Francis Fortin (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of ZTF25aarhkyn/AT2025mgj (Andreoni et al., GCN Circ. 40566) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-05-31 03:33 to 04:32 UTC, after 9.6 days after the trigger of GRB 250521C (Fermi GBM team, GCN 40515) and obtained 16 minutes of exposure in each of the g, r and i filters.
The data were reduced and stacked using the COLIBRÍ pipeline. 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 detected the candidate ZTF25aarhkyn/AT2025mgj (Andreoni et al., GCN Circ. 40566), at preliminary magnitudes of:
g = 19.34 +/- 0.03
r = 19.46 +/- 0.03
i = 19.50 +/- 0.05
Compared with the NOT observations reported by Martin-Carrillo et al. (GCN Circ. 40584), we estimate a steeper temporal decay index of ~1.5. The measured colours are consistent with those typically observed in GRBs, suggesting a similar nature. Additionally, the temporal evolution may indicate that AT2025mgj is a possible off-axis GRB; however, the absence of a nearby galaxy makes this scenario unlikely.
Further observations are planned in order to unravel the nature of this object.
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.
GCN Circular 40584
Subject
GRB 250521C/AT2025mgj: NOT optical observations of the afterglow candidate
Date
2025-05-30T16:12:30Z (5 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
A. Martin-Carrillo (UCD), A. de Ugarte Postigo (LAM), G. Corcoran (UCD), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Acebron Munoz (UNICAN), report on behalf of a larger collaboration.
We observed the position of AT2025mgj (Andreoni et al., GCN 40566), a candidate optical counterpart to GRB 250521C (Fermi GBM team, GCN 40515), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained exposures in the SDSS r band (3x300 s) and SDSS z band (5x200 s) starting at 21:06 UT on 2025-05-29 (8.32 days after the Fermi trigger).
The proposed candidate is well detected in both bands with an AB magnitudes of
r = 19.22 +/- 0.06
z = 19.67 +/- 0.08.
The photometric calibration was performed using nearby stars from the Pan-STARRS catalog and the magnitudes are not corrected for Galactic extinction.
Assuming the explosion time of GRB 250521C, when compared to the last magnitude reported by Andreoni et al., (GCN 40566), our observation shows a power-law decay index of ~1.3, consistent with those typically seen in GRB afterglows. However, it should be noted that the object is much brighter than a typical GRB afterglow at T0+8 days.
Further observations are planned (and encouraged) to establish the nature of this object and its association to GRB 250521C.
GCN Circular 40566
Subject
GRB 250521C (short): Zwicky Transient Facility identification of a possible fast optical transient counterpart
Date
2025-05-29T02:45:22Z (5 months ago)
From
Igor Andreoni at JSI/UMD/NASA <igor.andreoni@gmail.com>
Via
Web form
Igor Andreoni (UNC), Anna Ho (Cornell), Vishwajeet Swain (IITB), Michael Coughlin (UMN), on behalf of the ZTF Collaboration
The optical transient ZTF25aarhkyn / AT2025mgj was first detected by the Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al. 2019) on 2025-05-23 03:42 UT at a magnitude r = 17.61 +- 0.07 (AB) during the regular survey. The previous non-detection at the transient location was on 2025-05-09 05:23.
AT2025mgj was identified as a rapidly evolving transient by the ''ZTF Realtime Search and Triggering'' project (ZTFReST; Andreoni & Coughlin et al., 2021) and by a custom filter for fast transient discovery (Ho et al., 2022) within the ZTF Collaboration. A fading rate of ~0.4 mag/day was observed in r-band in the first two days after the initial detection. This was followed by a slower evolution. The latest photometric data point, r~18.7 mag, was taken on 2025-05-27 04:07 UT. Only r-band data points are available, so we do not have color information.
The rapidly fading transient AT2025mgj is spatially and temporally consistent with the “likely short” gamma-ray burst GRB 250521C detected by the Fermi GBM instrument on 2025-05-21 13:31:01.20 UT (Fermi GBM team, GCN Circular 40515). The GRB location was reported with a statistical uncertainty of 16.05 deg, the angular separation between AT2025mgj and the center of the GBM localization is 11.05 deg.
Archival ZTF and Legacy Survey images do not reveal high S/N detections. A faint (r=24.21 mag) source may be present near the transient location in Legacy Survey DR10, which is classified as a likely galaxy according to Tractor modeling, although classification is challenging at such faint magnitudes.
We caution that, with the data in hand, we cannot exclude that AT2025mgj is a Galactic source. The transient was found at a relatively low Galactic latitude (l, b = 161.329994, 21.4 deg) and the light curve behavior may resemble a CV.
Follow-up observations are strongly encouraged to determine the nature of this optical transient and its possible association with GRB 250521C.
Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.
GCN Circular 40515
Subject
GRB 250521C: Fermi GBM Final Localization
Date
2025-05-21T18:36:09Z (5 months ago)
From
Lorenzo Scotton at UAH <lscottongcn@outlook.com>
Via
Web form
The Fermi GBM team reports the detection of a likely SHORT GRB
"At 13:31:01.20 UT on 21 May 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250521C (trigger 769527066/250521563).
The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 105.00, Dec = 65.63 (J2000 degrees, equivalent to J2000 7h 0m, +65d 37'),
with a statistical uncertainty of 16.05 degrees.
The angle from the Fermi LAT boresight is 91 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250521563/quicklook/glg_skymap_all_bn250521563.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250521563/quicklook/glg_healpix_all_bn250521563.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250521563/quicklook/glg_lc_medres34_bn250521563.gif"