The optical fading and X-ray softening of the Galactic X-ray transient AT2019wey

ATel #17190; Sandeep K. Rout (NYU Abu Dhabi), Ronald A. Remillard (MIT), David M. Russell, Kevin Alabarta, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project and The Schools Observatory, LJMU), Jeroen Homan (Eureka Scientific)
on 18 May 2025; 11:50 UT
Credential Certification: Sandeep Rout (sandeep.rout@nyu.edu)

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AT2019wey was discovered on 07 Dec 2019 as an optical transient, ATLAS19bcxp, in the ATLAS survey (Tonry et al 2018). The first X-ray activity of the source was detected on 18 March 2020 by ART-XC and eROSITA onboard SRG (ATel #13571) and later it was classified as a candidate black-hole X-ray binary (Yao et al 2021, Cui et al 2025). The outburst of the source is characterized by a plateau phase that has lasted about five years, interspersed by brief episodes of optical decay and X-ray softening. During this period it has been studied with data from many wavebands, including radio (ATels #13984, #14168, #16218), infrared (ATel #15824), optical (ATels #13576, #14000, #14003), and X-rays (ATels #13932, #13957, #13976, #16418).

Here, we report a dimming of the source in optical wavebands and a corresponding softening in X-rays. We have been regularly monitoring the source since September 2023. The magnitudes remained steady, at V ~ 17.58, g' ~ 17.88, r' ~ 17.31 and i' ~ 16.92, until 17 March 2025 (MJD 60751, after a monitoring gap of about 105 days, when the source appeared fainter by ~ 0.13 mag. We cannot ascertain when, in the gap, the decline started. The optical flux further declined during the next two weeks, with measurements on 31 March 2025 (MJD 60765) yielding V = 17.80 (0.04), r' = 17.58 (0.02), and i' = 17.10 (0.05). The latest observation, on 15 April 2025 (MJD 60780), indicates i' = 17.46 (0.05). The drop in flux seems to be stronger in the redder bands (i' and r') than in the bluer bands (V and g'). The decline in the hard X-ray flux of the source, as measured with Swift/BAT (15-50 keV) and MAXI (2-20 keV), started around MJDs 60700 and 60746, respectively.

NICER has been monitoring AT2019wey since 04 August 2020. The recent data show a strong wave of spectral softening that commenced around MJD 60700. The background-subtracted spectra show an increase in the soft X-ray count rate (0.3-2.0 keV), reaching a peak ~ 940 c/s on MJD 60739. In the most recent observation (MJD 60802), the soft band intensity is above 600 c/s, which is still brighter than the soft X-ray intensity at MJD 60500 by a factor of 4. On the other hand, the NICER hard X-ray count rate (4-12 keV) falls from ~ 19 c/s on MJD 60700 to a level near the detection limit (~ 0.5 c/s) on MJD 60802. These results indicate that the recent drop in count rates for the Swift BAT and MAXI instruments, which have spectral responses higher than that of NICER, are caused by a substantial softening of the X-ray source, rather than an decrease in the accretion rate that might signify a return to quiescence. This recent excursion to values of hard color (4-12 / 2-4 keV) < 0.1, which likely signifies the thermal dominant state of the system, has been seen in AT2019wey during only a few occasions previously, most recently during March 2022.

We will continue monitoring the source as part of a campaign monitoring ~50 low-mass X-ray binaries (Lewis et al. 2008). All LCO photometric analysis and data calibration were performed using the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

LCO and NICER lightcurves of AT2019wey