First X-ray detection of the new outburst of Swift J1753.5-0127 and continuing optical brightening

ATel #16272; J. Homan (Eureka Scientific), K. Alabarta, D. M. Russell, P. Saikia, D. M. Bramich and S. Rout (NYU Abu Dhabi), M. C. Baglio (INAF-OAB), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), P. Charles and P. Gandhi (University of Southampton), M. Diaz Trigo (ESO), R. Fender (University of Oxford), D. Galloway and F. Jiménez-Ibarra (Monash University), A. Goodwin and J. Miller-Jones (Curtin Institute for Radio Astronomy), J. Lasota (Institut Astrophysique de Paris), G. Sivakoff (University of Alberta), Y. J. Yang (University of Hong Kong)
on 7 Oct 2023; 19:35 UT
Credential Certification: Jeroen Homan (jeroenhoman@icloud.com)

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The black hole low-mass X-ray binary Swift J1753.5-0127 began a new optical outburst on September 28, when a significant increase in the optical brightness in the i' band was detected (magnitude of 20.14 +/- 0.10) using the Las Cumbres Observatory (LCO) network (ATel #16262). Here we report on the onset of the new outburst in X-rays. The latest optical magnitudes are provided as well.

As reported in ATel #16262, on September 28, the same day of the first detection in optical bands, we observed Swift J1753.5-0127 with Swift/XRT, obtaining an upper limit on the unabsorbed X-ray flux of 2.1e-13 ergs/cm^2/s in the 0.5-10 keV band. Subsequently, the source has been observed with Chandra/ACIS-S (5.7 ks exposure) on October 1 and with Swift/XRT on October 2, 3, 4 and 5 (~0.95 ks exposures each). The Chandra observation revealed a weak X-ray source at the position of Swift J1753.5-0127, with a 0.5-10 keV count rate of 2.6e-3 cts/s. Using PIMMS, and assuming an absorbed power law spectrum (photon index = 1.8, nH = 2e21 cm^-2; Plotkin et al. 2017, ApJ, 848, 92), we converted the count rate to an unabsorbed 0.5-10 keV flux of ~6e-14 ergs/cm^2/s. This corresponds to a luminosity of ~4.5e32 erg/s for an assumed Galactic Bulge distance of 8 kpc, suggesting that the source was still in or close to quiescence at X-ray energies, 3 days after the start of the outburst in the optical.

After a non-detection with Swift on October 2 (<3.4e-13 ergs/cm^2/s), the first Swift detection was made on October 3, 11:30 UTC, when the source had an unabsorbed 0.5-10 keV X-ray flux of 1.7e-12 ergs/cm^2/s (using the same PIMMS conversion as above). This was followed by detections with X-ray fluxes of 5.2e-12 ergs/cm^2/s and 1.7e-11 ergs/cm^2/s on October 4 and 5, respectively. The detection in X-rays on October 3 indicates an optical/X-ray outburst delay of ~5 days.

Since September 28, the optical brightness (as measured with the LCO 1m and 2m telescopes) has been increasing to magnitudes of V = 16.64 +/- 0.02, R = 16.35 +/- 0.01 and i' = 16.46 +/- 0.02 on October 7. The increase in brightness, since the first detection in the V-band, is in the V and i’ bands: Delta_V = 2.57 +/- 0.07 mag and Delta_i' = 2.26 +/- 0.04 mag, respectively, while the rising rate has been decreasing from ~3.2 mag/day on September 28 to ~0.2 mag/day during the last week. The increase in brightness is higher at shorter wavelengths, suggesting an important contribution of the accretion disc to the brightening of the system and the beginning of the new outburst.

Multi-wavelength observations are encouraged to study the rise of the outburst. We will continue to observe the system in the optical and X-rays.

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 Chandra teams for rapidly executing our ToO and DDT observations of Swift J1753.5-0127.