ATel #16427: Is Swift J1753.5-0127 already heading to quiescence?

ATel #16427 (1 Feb 2024): V. V. Neustroev (University of Oulu)

The black hole low-mass X-ray binary Swift J1753.5-0127 entered a new outburst on 2023 September 28 (ATel #16262), triggering several multi-wavelength observing campaigns (ATel #16272, #16283, #16281, #16308, #16314, #16318). After reaching the optical maximum at V = 16.60 +/- 0.02 on 2023 October 4, the optical flux of Swift J1753.5-0127 varied only mildly staying at the level of ~16.75V until the end of October 2023 (ATel #16314, #16318), after which the source was Sun-constrained and could not be observed.

We observed Swift J1753.5-0127 again in the morning of 2024 February 1 (MJD 60341.3), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager and spectrograph. Observations were performed with the Johnson V and SDSS u’, g’, r’, i’, and z’ filters. At the position of the transient, we found a relatively dim source with the following magnitudes (it is not seen in the u’ image):

V=19.57+/-0.07 (Vega system), g’=19.82+/-0.10, r’=19.17+/-0.05, i’=18.93+/-0.04, z’=18.61+/-0.05 (AB system).

While Swift J1753.5-0127 is still more than 2 mag brighter than it has been found in a quiescent state (ATel #10664), our measurements indicate that the transient might already be heading to quiescence. Nevertheless, we note that at the end of the previous outburst, when reached a similarly low optical flux, the transient experienced the following mini-outburst and a few reflares (Zhang et al., 2019, ApJ, 876, 5). As a Sun-constrained period for ground-based optical telescopes is already ending, we encourage multi-wavelength follow-up observations.

The data presented here were obtained with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOT.

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