New optical outburst of the black-hole low-mass X-ray binary Swift J1753.5-0127 detected with XB-NEWS; X-ray non-detection with Swift/XRT

ATel #16262; Kevin Alabarta (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), David M. Russell (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), Payaswini Saikia, D. M. Bramich and Sandeep Rout (NYU Abu Dhabi), and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 29 Sep 2023; 16:49 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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The black-hole low-mass X-ray binary Swift J1753.5-0127 began an outburst in June 2005 (ATel #546) and remained active for eleven years until November 2016 (ATel #9735, #9739, #9741, #9758, #9765). Later, in February and April 2017, the system underwent two re-brightenings (ATel #10075, #10081, #10097, #10110, #10114, #10288, #10325, #10664). Here we report on recent optical observations that indicate that Swift J1753.5-0127 has entered a new outburst phase.

The optical observations were performed with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network and light curves can be seen in the figure linked below. Since May 23, 2018 (MJD 58261.45), after the end of the last outburst, the optical flux of Swift J1753.5-0127 has remained at an average magnitude of i' ~ 21.5, varying within an interval of ~0.3 mag. On 2023-09-28 02:30 UTC (MJD 60215.10), we detected a significant increase in the optical flux with a magnitude of 20.14 +/- 0.10 in the i' band. Later that day, two more detections in the V and i' bands confirmed the onset of a new outburst:

2023-09-28 09:57 UTC = MJD 60215.416: V = 20.33 +/- 0.23, i' = 19.82 +/- 0.12. 2023-09-28 18:16 UTC = MJD 60215.76: V = 19.21 +/- 0.07, i' = 18.72 +/- 0.04.

Taking into account the previous values, we detect an optical rise of the outburst of ~3.2 mag/day in both the i' and V bands.

To check for possible outburst activity in X-rays, we observed Swift J1753.5-0127 with the Swift/XRT, starting on September 28 at 16:02 UTC and with a total exposure of 2 ks. No source was detected at the position of Swift J1753.5-0127, with just a single count in the 20-arcsec extraction region. The count rate (~5e-4 cts/s) is consistent with that expected from the background. From the 3-sigma upper limit on the count rate (4.2e-3 cts/s) we estimate an upper limit on the flux, assuming a power-law spectrum with index 1.8 and an absorption column of 2e21 cm^-2 (Plotkin et al. 2017): 1.8e-13 ergs/cm^2/s (0.5-10 keV). The non-detection with Swift could indicate that a possible X-ray outburst is delayed with respect to the optical outburst, as has been seen in a few other X-ray transients.

This is the second outburst of Swift J1753.5-0127 since its discovery. Multi-wavelength observations are encouraged to study the rise of the outburst. 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 team for rapidly executing our ToO observations of Swift J1753.5-0127.

A link to the light curve is below.

LCO optical light curves of Swift J1753.5-0127