Low-amplitude optical variations of Swift J1727.8-1613 observed with LCO

ATel #16232; Kevin Alabarta, David M. Russell (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), D. M. Bramich, Payaswini Saikia, Sandeep Rout (NYU Abu Dhabi) and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 7 Sep 2023; 13:15 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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We are observing the recently discovered X-ray transient Swift J1727.8-1613 (GCN #34540), previously known as GRB 230824A (GCN #34536, GCN #34537, ATel #16205 and ATel #16206), with the 1m optical telescopes of the Las Cumbres Observatory (LCO) network, as part of an ongoing monitoring of ~50 LMXBs (Lewis et al. 2008). We analyzed all the images with the real-time data analysis pipeline "X-ray Binary New Early Warning System"(XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details). The source has been identified as a black hole low-mass X-ray binary (BH LMXB) due to its X-ray and optical properties (e.g. ATel #16208).

Here we report the LCO optical timing observations of Swift J1727.8-1613 during the current outburst. The system was observed with a high cadence in three epochs: on August 28, 2023 (MJD 60184.8) and September 1, 2023 (MJD 60188.8) for a duration of ~30 minutes at each epoch using the g' band filter (28 exposures of 30 seconds in each batch), and on September 3, 2023 (MJD 60190.79 and MJD 60190.82) with two batches of 28 exposures of 30 seconds each lasting ~30 minutes using the i' band filter. The time resolution was 60 seconds in each case.

We evaluated the fractional rms amplitude in these three epochs following Vaughan et al. (2003) obtaining a fractional rms of (1.65 +/- 0.02)% and (0.89 +/- 0.03)% on August 28, 2023 and September 1, 2023, respectively in the g' band. In the observations occurring on September 3, 2023, we obtained a fractional rms amplitude of (1.90 +/- 0.03)% and (1.26 +/- 0.03)% in the i' band. This optical variability is fairly low for a BH LMXB with spectral properties typical of a system being in the low/hard state (ATel #16217). Nevertheless, the system is so bright (magnitudes in g', V, r' and i' bands are between 12 and 13) that fast-timing studies are encouraged.

Our optical monitoring indicates the source is now fading at optical wavelengths, from V=12.66; r'=12.60; i'=12.57; Y=12.59 on August 28, 2023 (MJD 60184.8) to V=12.89; r'=12.88; i'=12.85; Y=12.85 on September 2, 2023 (MJD 60189.5) and i'=12.96 on September 3, 2023 (magnitude errors are < 0.01).

We will continue monitoring Swift J1727.8-1613 at optical wavelengths with LCO.

A link to the LCO light curves is below.

LCO optical light curves of Swift J1727.8-1613