XB-NEWS optical monitoring of MAXI J1803-298

ATel #14706; Payaswini Saikia, David M. Russell, Maria Cristina Baglio, D. M. Bramich (NYU Abu Dhabi) and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 13 Jun 2021; 11:51 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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MAXI J1803-298 is a new X-ray transient, discovered by the MAXI/GSC nova alert system on 2021 May 1 (ATels #14587), with follow-up detections at multiple wavelengths by different telescopes like NICER (ATel #14588, ATel #14602, ATel #14606, ATel #14660), Swift (ATel #14591), MITSuME (ATel #14594), SALT (ATel #14597), INTEGRAL (ATel #14598, ATel #14601), MeerKAT (ATel #14607), NuSTAR (ATel #14609), Insight-HXMT (ATel #14613) and AstroSAT (ATel #14629, ATel #14630). From the X-ray spectral and temporal properties of the source (ATel #14602, ATel #14613), and the presence of a large inner disk radius (ATel #14627, ATel #14630), MAXI J1803-298 is thought to be an accreting black hole X-ray binary. The source was recently observed to have a hard-to-soft state transition (ATel #14627).

We report on optical monitoring of MAXI J1803-298 during its discovery outburst, with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, at the position of the optical counterpart candidate identified by Swift/UVOT (ATel #14591). To analyse the LCO data, we use the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details).

We detected the source on 2021 May 2 (MJD 59336.9) in the i'-band with a magnitude of i'=16.01 +/- 0.01. The source has since then gradually faded at optical wavelengths, reaching i'=16.38 +/- 0.01 on May 26 (MJD 59360.5). A similar decline was also observed in the g'-band, with the magnitude gradually changing from g'=16.28 +/- 0.01 on May 18 (MJD 59352.7) to g'=16.67 +/- 0.01 on May 31 (MJD 59365.6). The most recent magnitudes obtained on June 11 (MJD 59375.9) show that the decline continues at optical wavelengths, with the current magnitudes of i'=16.63 +/- 0.01 and g'=16.79 +/- 0.01.

We will continue monitoring the source during its decay as part of an on-going monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical light-curve of MAXI J1803-298