Near-infrared counterpart and optical evolution of the outburst of Swift J1727.8-1613

ATel #16225; M. Cristina Baglio (INAF-OAB), Piergiorgio Casella, Vincenzo Testa, Isabella Mariani (INAF-OAR), David M. Russell (NYU Abu Dhabi), Thomas J. Maccarone (TTU) and Tomaso Belloni (INAF-OAB)
on 2 Sep 2023; 17:46 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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Swift J1727.8-1613 (originally GRB 230824A: GCN #34536, #34537) is a recently discovered candidate black hole X-ray binary. The source is undergoing a bright outburst at all wavebands, from X-rays (ATel #16205, #16206, #16207, #16210, #16215, #16217) up to optical (ATel #16208, #16209) frequencies, and also showed bright detections in the radio thanks to VLA observations (ATel #16211).

We are monitoring the system daily with the Rapid Eye Mount Telescope (REM; La Silla, Chile) at optical (griz) and near-infrared (H) frequencies. The source is well detected; we performed aperture photometry on the target and we calibrated the fluxes against a group of stars in the field of view with magnitudes tabulated in the APASS (optical gri) and 2MASS (NIR) catalogs.

Here we report the results of the first observation, performed on 2023 August 29 (MJD 60185.01). We obtain the following optical-NIR calibrated magnitudes: g=12.92+/-0.03 mag (AB), r=12.71 +/- 0.03 (AB), i=12.69+/-0.03 mag (AB), H=11.35+/-0.05 mag (Vega).
The optical magnitudes show that the source has brightened with respect to the observations reported in ATel #16208; moreover, the brightening is stronger at lower frequencies (delta_i~0.7 mag, delta_r~0.6 mag, delta_g~0.5 mag), suggesting that the source is becoming redder, possibly due to the onset of a compact jet (also suggested by the recent radio detections, ATel #16211).

The monitoring with REM is ongoing.
Further multiwavelength observations are strongly encouraged during the current outburst of the source.

We remember our colleague and friend Tomaso Belloni, who sadly passed away on 2023 August 26, who made invaluable contributions in the past years to the studies of X-ray transients.