Optical and X-ray decay of Swift J1753.5-0127 observed with LCO and Swift/XRT

ATel #16447; Kevin Alabarta, David M. Russell, D. M. Bramich, Payaswini Saikia and Sandeep Rout (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), Jeroen Homan (Eureka Scientific), and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU
on 14 Feb 2024; 14:09 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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The latest outburst of Swift J1753.5-0127 was first detected on September 28, 2023 (MJD 60215.10) with the Las Cumbres Observatory (LCO) network (ATel #16262), when an increase in the optical brightness in the i'-band was observed. Later, on October 1, it was detected in X-rays with Chandra/ACIS-S (ATel #16272) and on October 3, 2023 (MJD 60220.49) with Swift/XRT (ATel #16272) and NICER (ATel #16318), while three days later (ATel #16281), it was detected at radio wavelengths with MeerKAT. After October 4 (MJD 60222.87), the optical flux of Swift J1753.5-0127 remained approximately constant at average magnitudes of V = 16.75 +/- 0.13, R = 16.50 +/- 0.12 and i' = 16.50 +/- 0.08 (the error in these magnitudes is the RMS magnitude deviation) until the system was in Sun constraint from mid-November (ATel #16314 and ATel 16318).

After the Sun constraint, new optical observations with ALFOSC/NOT on February 1, 2024 (MJD 60341.3) showed an optical brightness of Swift J1753.5-0127 of magnitudes V = 19.57+/-0.07, g' = 19.82 +/- 0.10, r'= 19.17 +/- 0.05, i'=18.93 +/- 0.04, z' = 18.61 +/- 0.05 (ATel #16427), suggesting that the outburst is in its decaying phase and heading towards quiescence.

Here, we report new optical and X-ray observations acquired with the LCO network and Swift/XRT. The first LCO observations after the Sun constraint occurred on January 27, 2024 (MJD 60336.67) with magnitudes g' = 18.85 +/- 0.09, r' = 18.33 +/- 0.04 and i' = 18.10 +/- 0.06. Since then, the optical brightness of the system has decreased until January 31 (MJD 60340.67), with magnitudes g' = 20.08 +/- 0.11, r' = 19.42 +/- 0.05 and i' = 19.10 +/- 0.07. During the next LCO observations taken on February 11 (MJD 60351.25), Swift J1753.5-0127 showed a slight increase in the optical brightness with magnitudes g' = 19.88 +/- 0.05 and i' = 18.95 +/- 0.05, suggesting that the source experienced a rebrightening. Since then, Swift J1753.5-0127 faded until the last observations were taken one day later, on February 12 (MJD 60352.24), with magnitudes g' = 20.57 +/- 0.07, V = 20.60 +/- 0.12, r' = 19.65 +/- 0.04 and i' = 19.70 +/- 0.07 (top panel in the figure below).

The first Swift/XRT observation after the Sun constraint was taken on February 4 (MJD 60344.32) and showed a count rate of 0.012 +/- 0.004 c/s in the 0.5-10.0 keV energy range. One week later, on February 11, 2024 (MJD 60351.11), the system was not significantly detected with Swift/XRT, showing an upper limit in its count rate of <0.007 c/s in the 0.5-10.0 keV energy band (black light curve in the bottom panel of the figure below).

Multi-wavelength observations are encouraged to study the end of the outburst and identify any rebrightenings. We will continue to observe the system with LCO.

The LCO observations of Swift J1753.5-0127 are performed as part of an ongoing monitoring program of ~50 low-mass X-ray binaries (Lewis et al. 2008). LCO images are processed and reduced, and magnitudes are extracted and calibrated using a real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

This material is based upon work supported by Tamkeen under the NYU Abu Dhabi Research Institute grant CASS (Center for Astrophysics and Space Science).

We thank the Swift team for rapidly executing our ToO observation of Swift J1753.5-0127.

A link to the light curve is below.

LCO (top) and X-ray (bottom) light curves of Swift J1753.5-0127