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

TL;DR

This study presents the first high-time resolution simultaneous X-ray and infrared observations of the neutron star binary 4U 1728-34, revealing sub-second variability, a correlated IR lead over X-rays, and insights into the system's geometry and emission mechanisms.

Contribution

It provides the first simultaneous high-time resolution IR and X-ray data for 4U 1728-34, identifying IR leading X-ray variability and analyzing its implications for emission processes and system parameters.

Findings

Detected IR leading X-ray variability by 30-40 ms.

Observed IR counterpart of a type-I X-ray burst with a 7.2 s delay.

Inferred system parameters suggesting orbital period > 3 hours and inclination > 8°.

Abstract

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 $\approx 2\,$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 timescales. By measuring the cross-correlation function between the infrared and X-ray lightcurves, we discovered a significant correlation with an infrared lead of $\approx 30-40\,$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 $\approx7.2\,$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 $3\,$h and an inclination higher than 8$^\circ$.