Optical fading of black hole LMXB XTE J1859+226 during its current mini-outburst

ATel #14415; Eleonora Caruso (NYU Shanghai), Maria Cristina Baglio, Payaswini Saikia, David M. Russell, D. M. Bramich (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific & SRON), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 22 Feb 2021; 07:02 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The black hole low-mass X-ray binary (LMXB) XTE J1859+226 had a sudden optical brightening on Feb. 4th 2021 (MJD 59249) at r ~18.9 mag, as detected by the Zwicky Transient Facility (ATel #14372). This observation was preceded by non-detections 24 hours earlier, with upper limits r ~ 19.5-19.8 mag, and no X-ray activity was reported (ATel #14372). The source continued to brighten in the optical and reached its peak value at a magnitude of R = 18.37 +/- 0.09 on Feb 6th (MJD 59251.56) (source: ANTARES; see ATel #14372). On Feb 7th an X-ray 0.3-10 keV flux detection of 7.9 (+2.0, -1.5) x 10-12 erg cm-2 s-1 was reported using the The Neil Gehrels Swift Observatory (ATel #14375), which represented a significant brightening relative to the source's quiescent 0.3-8 keV flux of 1.5 x 10-15 erg cm-2 s-1 reported by Tomsick et al. (2003). It was concluded that XTE J1859+226 was in the early stages of an outburst and multiwavelength observations were encouraged.

We report on optical monitoring of the source with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes. We have been monitoring the source since 2006. For the analysis of the LCO data, we are making use of a new real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Pirbhoy et al. 2020, ATel #13451 and Goodwin et al. 2020, for details). In our light curve (linked below), we are combining XB-NEWS data with those retrieved from ANTARES. On Feb 8th 2021 (MJD 59253; the first LCO observation since a period of Sun constraint), the source had magnitudes V = 18.84 +/- 0.03 and i' = 18.50 +/- 0.04, which were consistent with a fading. The source has continued to fade, with magnitudes on Feb 16th (MJD 59261) of g' = 20.05 +/- 0.02, r' = 19.63 +/- 0.01, i' = 19.36 +/- 0.02 and z = 19.12 +/- 0.03. The source faded by ~ 1 mag in 9 days in i'-band and ~ 1.1 mag in 6 days in the g' band (ANTARES detection on Feb 11th (MJD 59256.53), g' = 18.91+/- 0.10). The latest ANTARES detection is R = 19.71 +/- 0.15 on Feb 17th (MJD 59262). We also analyzed Swift UVOT observations taken on Feb 6th (MJD 59251), Feb 7th (MJD 59252), and Feb 10th (MJD 59255), which resulted in magnitudes (AB system) of v = 18.19+/-0.25, v = 18.53+/-0.29, and uvw1 = 20.86+/-0.15 respectively and are consistent with the fading trend.

This current outburst is significantly fainter than the first one in 1999, which peaked on Oct 13th 1999 (MJD 51464) with magnitude R = 15.0 +/- 0.1 (IAUC 7279). In the last two LCO detections preceding the outburst, on Oct 13th 2020 (MJD 59135) and Nov 16th 2020 (MJD 59169), the source had magnitudes i' = 21.26 +/- 0.25 and i' = 21.44 +/- 0.22 respectively. The peak of the current outburst is ~3 mag fainter in the R-band than the 1999 outburst, and ~ 3 mag brighter in the i'-band than the quiescence level in 2020. We have detected a clear fading trend of the source, and therefore classify (Zhang et al. 2019) this activity as a mini-outburst. The latest data on Feb 16th show XTE J1859+226 is ~ 2 mag above the quiescence level and we will continue to monitor it. We encourage X-ray and other wavelength observations of the source at these unusual, low luminosity levels. The LCO observations are 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 curves of XTE J1859+226