EP250315a, GRB 250315A
GCN Circular 39903
Subject
EP250315a: Pan-STARRS r and i-band imaging and photometry
Date
2025-03-28T10:04:31Z (9 months ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
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J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, 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 optical counterpart of EP250315a (Peng et al., GCN 39731; Quirola-Vasquez et al., GCN 39733), using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJD 60755.50 (2025-03-21 12:00 UTC), approximately 6.25 days after the EP-WXT detection (Peng et al., GCN 39731). 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 observation consisted of 6x150s exposures in both the r and i filters with Pan-STARRS1. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked 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 do not detect the optical counterpart discovered by Quirola-Vasquez et al., GCN 39733. At the reported optical counterpart position, we measure 3.5-sigma limiting AB magnitudes of r~21.7 and i~21.7.
GCN Circular 39740
Subject
GRB 250315A/EP250315A: Fermi-GBM Sub-Threshold Detection
Date
2025-03-15T23:01:42Z (10 months ago)
From
mariaedvige.ravasio@ru.nl
Via
Web form
M. E. Ravasio (Radboud Univ.), E. Burns (LSU) and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:
Fermi-GBM had full spatial and temporal coverage of the transient GRB 250315A/EP250315A detected by EP (Peng et al., GCN 39731; Sun et al. GCN 39738), with optical counterpart (Quirola-Vasquez et al. GCN 39733). There was no Fermi-GBM onboard trigger around the refined EP trigger time at T0=2025-03-15T05:58:16 (Sun et al. GCN 39738).
The GBM Targeted Search [1], the most sensitive, coherent search for GRB-like signals was run in the time interval [-50;+500] s from the EP T0, seeking signals between 64 ms and 32.768 s in duration. A transient was significantly detected at T0+254 s on a 32 s timescale. The localization is consistent with the EP one, with a spatial association probability of 99%. The transient was best-fit with a "soft" spectrum (i.e., a Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 39738
Subject
EP250315a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-03-15T15:05:35Z (10 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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H.Sun (NAO, CAS), J. Q. Peng (IHEP, CAS), M. Q. Huang (USTC), W. X. Wang, W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250315a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Peng et al., GCN 39731). An optical counterpart candidate was detected by Gemini North (Quirola-Vasquez et al, GCN 39733). Refined analysis of the WXT data shows that the event started at T0=2025-03-15T05:58:16 (UTC) and lasted for about 280 s before the observation was disrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.97 x 10^20 cm^-2 and a photon index of 0.36 (-/+0.50). The derived average unabsorbed 0.5-4 keV flux is 5.4(-1.7, +2.2) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 6 min after T0, starting at 2025-03-15T06:04:15 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 203.0287, DEC = -9.5301 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is also consistent with the position of the optical counterpart candidate. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 2.97 x 10^20 cm^-2 and a photon index of 1.80(-/+0.05). The derived average unabsorbed 0.5-10 keV flux is 3.7(-/+0.17) x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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).
GCN Circular 39733
Subject
EP250315a: optical counterpart candidate detected by Gemini North
Date
2025-03-15T13:43:51Z (10 months ago)
From
Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>
Via
Web form
J. A. Quirola-Vasquez (Radboud Univ.), P. G. Jonker (Radboud Univ.), D. B. Malesani (DAWN/NBI & Radboud Univ.), J. van Dalen (Radboud Univ.), A. Martin-Carrillo (UCD), A. J. Levan (Radboud Univ. & Warwick Univ.), A. van Hoof (Radboud Univ.), J. Sanchez-Sierras (Radboud Univ.), F. E. Bauer (PUC), M. E. Ravasio (Radboud Univ.), J. Chacon (PUC) report on behalf of a larger collaboration:
We observed the field of EP250315a (Peng et al., GCN 39731), using the GMOS-N instrument mounted on the Gemini North Telescope. We obtained 3 x 40 s frames in the r band at a median time of 3.72 hr after the EP trigger. When compared to the Legacy Survey archive, we find an uncatalogued optical transient (OT) which is detected in the stacked image within the EP/FXT error circle (Peng et al., GCN 39731) at coordinates
R.A. (J2000) = 13:32:06.98
Dec. (J2000) = -09:31:49.0
with an uncertainty of ~0.3 arcsec. The OT has a magnitude r = 21.72+-0.10 AB mag, calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction. We thus conclude the OT is very likely the optical counterpart of EP250315a.
We acknowledge excellent support of the Gemini staff, especially Emanuele Farina.
GCN Circular 39731
Subject
EP250315a: Einstein Probe detection of an X-ray transient
Date
2025-03-15T07:33:28Z (10 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
J. Q. Peng (IHEP,CAS), M. Q. Huang (USTC), H. Sun, W. X. Wang, W. D. Zhang (NAO,CAS) report on behalf of the Einstein Probe team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250315a. The transient triggered EP-WXT (ID: 01709132646) at 2025-03-15T06:02:47 (UTC). The WXT position of the source is R.A. = 203.020 deg, DEC = -9.531 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 203.0307 deg, DEC = -9.5316 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
Further information will be updated when the telemetry data is received.
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).