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

Vincentelli, F M; Casella, P; Borghese, A; Cavecchi, Y; Mastroserio, G; Stella, L; Altamirano, D; Padilla, M Armas; Baglio, M C; Belloni, T M; Casares, J; Cúneo, V A; Degenaar, N; Trigo, M Díaz; Fender, R; Maccarone, T; Malzac, J; Sánchez, D Mata; Middleton, M; Migliari, S; Muñoz-Darias, T; O’Brien, K; Panizo-Espinar, G; Sánchez-Sierras, J; Russell, D M; Uttley, P

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F M Vincentelli

P Casella

A Borghese

Y Cavecchi

G Mastroserio

L Stella

D Altamirano

M Armas Padilla

M C Baglio

T M Belloni

J Casares

V A Cúneo

N Degenaar

M Díaz Trigo

R Fender

T Maccarone

J Malzac

D Mata Sánchez

M Middleton

S Migliari

T Muñoz-Darias

G Panizo-Espinar

J Sánchez-Sierras

D M Russell

P Uttley


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 ≈ 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 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 ≈ 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 ≈ 7.2 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◦.


Vincentelli, F. M., Casella, P., Borghese, A., Cavecchi, Y., Mastroserio, G., Stella, L., …Uttley, P. (2023). Sub-second infrared variability from the archetypal accreting neutron star 4U 1728−34. Monthly Notices of the Royal Astronomical Society, 525(2), 2509-2518.

Journal Article Type Article
Acceptance Date Aug 2, 2023
Online Publication Date Aug 25, 2023
Publication Date 2023-10
Deposit Date Jan 25, 2024
Publicly Available Date Jan 25, 2024
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 525
Issue 2
Pages 2509-2518
Public URL


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