Swift J1753.5-0127 reaches a constant optical flux, similar to its previous outburst, while the X-ray count rate slightly decreases

ATel #16318; Kevin Alabarta and David M. Russell (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), M. Cristina Baglio (INAF-OAB), D. M. Bramich, Payaswini Saikia and Sandeep Rout (NYU Abu Dhabi), and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 4 Nov 2023; 12:24 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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A new outburst of the black hole low-mass X-ray binary Swift J1753.5-0127 was 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 3, 2023 (MJD 60220.49), the source was significantly detected for the first time in X-rays with Swift/XRT, showing an unabsorbed X-ray flux of 1.7e-12 ergs/cm^2/s in the 0.5-10.0 keV band (ATel #16272). On October 6, 2023 (ATel #16281), it was first detected at radio wavelengths.

After its first detection with LCO, the optical brightness increased to magnitudes of V = 16.60 +/- 0.02, R = 16.33 +/- 0.02 and i' = 16.48 +/- 0.02 on October 4 (MJD 60222.87). Since then, the optical flux has remained approximately constant at average magnitudes of V = 16.75 +\- 0.13, R = 16.50 +\- 0.12 and i' = 16.50 +\- 0.08 (where the error in these magnitudes is RMS magnitude deviation; see the top panel in the figure below). We note that these magnitudes are consistent with those reported in ATel #16314, acquired with ALFOSC/NOT on October 24 (MJD 60241) and with the typical values during the prolonged decay of the first outburst of the source.

On the same day as the first Swift/XRT detection, posterior analysis of NICER data showed that the source was also detected with a count rate of 0.74 +/- 0.04 c/s in the 0.5-10.0 keV band. Since then, the NICER count rate increased to 43.38 +/- 0.27 c/s on October 8 (MJD 60225.76), the date of the last NICER observation (see the magenta light curve in the bottom panel of the figure below). From October 13 (MJD 60230.30) until October 19 (MJD 60236.67), the Swift/XRT count rate was approximately constant at 5.91 +/- 0.26 c/s (0.5-10.0 keV band). After that, the X-ray rate decreased by ~0.2 c/s per day until October 31 (MJD 60248.15), when the X-ray count rate was 3.59 +/- 0.12 c/s (black light curve in the bottom panel of the figure below).

It is worth noting that the optical flux of Swift J1753.5-0127 became steady just one day after the first detection of the outburst in X-rays (both with Swift/XRT and NICER).

Multi-wavelength observations are encouraged to study the evolution of the outburst while it is still observable. We will continue to observe the system with LCO and Swift/XRT.

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 and NICER teams for rapidly executing our ToO and DDT observations 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