Fading of low-mass X-ray binary Swift J1858.6-0814 to quiescence level

ATel #13719; Payaswini Saikia, David M. Russell, Maria Cristina Baglio, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 8 May 2020; 15:33 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The low-mass X-ray binary Swift J1858.6-0814 has been in outburst since 2018 October 25 (MJD 58416; Krimm et al. ATel #12151). The source had a consistently hard X-ray spectrum, with a strong neutral iron K-alpha emission line, prominent X-ray variability and evidence for intrinsic absorption, and has been in this active state for more than a year (ATel #12158, #12220, #12512, #13255). After being sun-constrained for most X-ray telescopes since November 2019, it was again detected with MAXI on 2020 February 6 with enhanced soft X-ray emission, implying renewed activity or a soft state transition (ATel #13455). NICER observed this source on 2020 February 18, 22, 26 and 27, and reported significantly brighter soft X-ray flux and a much weaker K-alpha line, confirming a spectral state change (ATel #13536). Recently, it has been suggested that the compact object in this source is a neutron star, after discovering three X-ray flares consistent with being Type I X-ray bursts with NICER and NuSTAR on 2020 March 6; and with NICER on March 9 and March 20 (ATel #13563).

We have been monitoring the source in optical bands since its discovery, from 2018 November 9 (MJD 58431), with the Las Cumbres Observatory (LCO) 1- and 2-m (Faulkes) telescopes (in i', r', g' Y, V, B bands). Near the start of the outburst we detected strong short-term optical variability (ATel #12180), also reported throughout the long outburst by other groups (ATel #12164, #12186, #12197) as well as a variable accretion disk wind seen as optical P-Cygni profiles (Munoz-Darias et al. 2020).

The analysis of the LCO data are performed with a new real-time data analysis pipeline, the ''X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019 and Pirbhoy et al. 2020, ATel #13451 for details). The magnitude of the source during the outburst varied roughly between i' =16.5 and 18.0 mag for more than a year, with a magnitude of i' = 16.63+/-0.01 just before the onset of sun-constraint on 2019 November 4 (MJD 58791). We resumed our LCO observations on 2020 March 20 (MJD 58928) after the sun-constraint was lifted, and found the source to be comparatively fainter at optical frequencies (i' = 18.47+/-0.01). The flux has been decreasing since then, reaching a magnitude of i'= 19.71+/-0.02 on 2020 April 19 (MJD 58958). The last observed magnitudes are: 2020 May 5 (MJD 58974) : i'= 19.60+/-0.03; g'= 20.80+/-0.12; r'= 19.88+/-0.04; Y= 19.64+/-0.21.

This is the faintest optical flux measured from the source since its discovery, with a difference of ~3 magnitudes with respect to the beginning of the outburst. The archival Pan-STARRS r'-band magnitude of the system is 19.97 (ATel #12186). Hence, it is likely that the source is now close to its quiescent state. The optical monitoring with LCO will continue in order to confirm the onset of quiescence. Multiwavelength observations are encouraged at this time in order to study the source in quiescence, especially optical time-resolved observations to infer an orbital period and mass function.

The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical LCO light curves of Swift J1858.6-0814