Sub-second infrared variability from the archetypal accreting neutron star 4U 1728-34

This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly notices of the Royal Astronomical Society following peer review. The version of record Vincentelli, F., et al. Sub-second infrared variability from the archetypal accreting neutron star 4U 1728-34. Monthly notices of the Royal Astronomical Society, October 2023, vol. 525, n. 2, p. 2509-2518. is available online at: https://doi.org/10.1093/mnras/stad2414
We report on the first simultaneous high-time resolution X-ray and infrared (IR) observations of a neutron star low mass X-ray binary in its hard state. We performed ¿h of simultaneous observations of 4U 1728-34 using HAWK-I@VLT, XMM–Newton, and NuSTAR. The source displayed significant X-ray and IR variability down to sub-second time-scales. By measuring the cross-correlation function between the IR and X-ray lightcurves, we discovered a significant correlation with an IR lead of ¿ms with respect to the X-rays. We analysed the X-ray energy dependence of the lag, finding a marginal increase towards higher energies. Given the sign of the lag, we interpret this as possible evidence of Comptonization from external seed photons. We discuss the origin of the IR seed photons in terms of cyclo-synchrotron radiation from an extended hot flow. Finally, we also observed the IR counterpart of a type-I X-ray burst, with a delay of ¿s. Although some additional effects may be at play, by assuming that this lag is due to light travel time between the central object and the companion star, we find that 4U 1728-34 must have an orbital period longer than ¿h and an inclination higher than 8¿.
This work benefited from the discussions done during the ISSI meeting in Bern "Looking at the disc-jet coupling from different angles". FMV acknowledges support from the grant FJC2020-043334-I financed by MCIN/AEI/10.13039/501100011033 and Next Generation EU/PRTR. This work is supported by the Spanish Ministry of Science under grants PID2020–120323GB–I00, PID2021–124879NB-I00, and EUR2021–122010 TMB acknowledges financial contribution from grant PRIN INAF 2019 n.15. YC acknowledges support from the grant RYC2021-032718-I, financed by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR. LS acknowledges financial contributions from ASI-INAF agreements 2017-14-H.O and I/037/12/0; from “iPeska” research grant (P.I. Andrea Possenti) funded under the INAF call PRIN-SKA/CTA (resolution 70/2016), from PRIN-INAF 2019 no. 15 and from the Italian Ministry of University and Research (MUR), PRIN 2020 (prot. 2020BRP57Z) "GEMS". This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France (DOI : 10.26093/cds/vizier). The original description of the VizieR service was published in 2000, A&AS 143, 23"
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