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GCN Circular 34397

GRB 230812B: Zwicky Transient Facility Identifies Optical Afterglow Candidate of a Fermi GRB (Trigger 713559497)
2023-08-13T08:02:28Z (6 months ago)
Anirudh Salgundi <>
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Anirudh Salgundi (IITB), Vishwajeet Swain (IITB), Harsh Kumar (IITB), Tomas Ahumada (CIT), Robert Stein (CIT), Igor Andreoni (UMD), Michael Coughlin (UMN), Shreya Anand (CIT), Viraj Karambelkar (CIT), Mansi Kasliwal (CIT), Avery Wold (IPAC), Theophile du Laz (CIT), Simeon Reusch (DESY), Igor Andreoni (UMD),  Eric Bellm (UW), Varun Bhalerao (IITB), Brad Cenko (UMD), Brian Healy (UMN), David Kaplan (UWM), Jannis Necker (DESY), D. Perley (LJMU) report on behalf of the ZTF and GROWTH collaborations:

We observed the localization region of the GRB 230812B (trigger 713559497, GCN 34386) detected by the Gamma-Ray Burst Monitor (GBM) on the Fermi satellite with the 47 square-degree Zwicky Transient Facility (ZTF) camera (Graham et al., 2019; Bellm et al., 2019). We obtained images in the g-, and r- covering 420 square degrees beginning at 2023-08-13 03:34:57 (~8.5 hours after the burst trigger time). This corresponds to ~78% of the probability enclosed in the Earth-occultation corrected GRB localization map. Each exposure was 300 seconds with median depths of 21.9 mag in both g-band and r-band. The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC (Masci et al. 2019).

We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023). We required at least 2 detections separated by at least 15 minutes to select against moving objects. Furthermore, we cross-match our candidates with the Minor Planet Center to flag known asteroids, reject stellar sources (Tachibana and Miller 2018), and apply machine learning algorithms (Mahabal et al. 2019). We require that no spatially coincident ZTF alerts were issued before the detection time of the GBM trigger. Close to 40 sources were time and spatially coincident with the burst, most of them showing g-r ~ 0 mag and a slow evolution. 

We recover the candidate afterglow reported in Zheng et al. (GCN 34395) and Lipunov et al. (GCN 34396), and we highlight its rapid evolution: r-band decay rate ~2 mag/day. We note that this source is ~6" from the source circulated in Page et al. (GCN 34394) detected by the Neil Gehrels Swift Observatory.

We additionally crossmatched the optical candidates to the Swift sources circulated and in Evans et al (GCN 34388, and we find no other coincidences.

The details of the afterglow candidate in the table below: 

ZTF name , AT name , UT first alert , t-t0 (days) , filter , mag (AB) , mag error (AB)
ZTF23aaxeacr , AT 2023pel , 2023-08-13 03:34:56 , 0.35 , r , 18.85 , 0.04
ZTF23aaxeacr , AT 2023pel , 2023-08-13 04:24:05 , 0.39 , g , 19.19 , 0.02 

We encourage further follow-up.

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,
Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan; IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia.
ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341.
GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.
Alert distribution service provided by DIRAC@UW (Patterson et al. 2019).
Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski (Duev et al. 2019). The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at
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