ATLAS constraints on the optical onset of new X-ray transient Swift J1727.8-1613

ATel #16209; Yuankun David Wang (UW), Eric C. Bellm (UW)
on 25 Aug 2023; 22:51 UT
Credential Certification: Eric Bellm (ecbellm@uw.edu)

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Following the report of a new hard X-ray transient Swift J1727.8-1613 (GRB 230824A, GCN #34537, ATel #16205), we obtained forced photometry measurements at the location reported in GCN #34543 on images taken by the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018).

ATLAS observed unambiguous optical brightening from the source at 2023-08-22 14:02 UTC, when the source was at m_o = 15.99 +/- 0.02 mag. This optical brightening preceded the GRB trigger at 2023-08-24 14:12 UTC (GCN #34537). Three additional o-band observations were taken in the following hour at m_o = 16.1 mag.

ATLAS also observed the location of the source earlier on 2023-08-19 and 2023-08-20. The source is not detected in observations through 2023-08-19 02:42:20. The measurements taken on 2023-08-20 show a steadily rising flux, although only the last measurement of that day provides a signal-to-noise ratio greater than 5.

Time (UTC) mag_o dmag flux (uJy) dflux SNR
2023-08-20 08:17:51 20.01 0.60 36 22 1.6
2023-08-20 08:23:23 19.26 0.31 72 22 3.3
2023-08-20 08:30:11 19.02 0.28 89 25 3.6
2023-08-20 08:30:50 19.38 0.32 64 21 3.0
2023-08-20 08:34:52 19.09 0.23 84 19 4.4
2023-08-20 08:42:25 -23.28 14.38 -2 25 -0.1*
2023-08-20 08:50:30 18.75 0.15 115 18 6.4

*the difference image from this observation has poor subtraction.

These data suggest an optical onset of this outburst near 2023-08-20 08:30, roughly 4.25 days prior to the GRB trigger.

The most recent set of observations from ATLAS show the source at m_o = 14.339 +/- 0.003 on 2023-08-23 07:49 UTC.

ATLAS data are publicly available at https://fallingstar-data.com/.

We acknowledge support from the NSF under grant AST-1812779 and the Heising-Simons Foundation under grant 2018-0908. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.