The NICER view of Scorpius X-1

¹ Aurora Technology for the European Space Agency, ESAC/ESA, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, Spain
² Departament d’Astronomía i Astrofísica, Universitat de València, Dr. Moliner 50, 46100 Burjassot (València), Spain
³ INAF–Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ Department of Physics, Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, OX1 3RH Oxford, UK
⁵ International Centre for Radio Astronomy Research–Curtin University, GPO Box U1987 Perth, WA 6845, Australia
⁶ Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA 94602, USA
⁷ Serco for the European Space Agency, ESAC/ESA, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, Spain

^⋆ Corresponding author: jose.lopezmiralles@ext.esa.int

Received: 25 April 2025
Accepted: 28 July 2025

Abstract

Context. The neutron star X-ray binary Scorpius X-1 is one of the brightest Z-type sources in our Galaxy, showing frequent periods of flaring activity and different types of relativistic outflows. Observations with RXTE have shown that the strongest X-ray variability appears in the transition to and from the flaring state. During this transition, it has been proposed that the appearance of two particular types of quasi-periodic oscillations (QPOs) might be connected with the ejection of the so-called ultra-relativistic flows.

Aims. In this paper, we present an analysis of the first NICER observations of Scorpius X-1 obtained during a multi-wavelength campaign conducted in February 2019, in order to characterise the properties of QPOs in this source as the system evolves through its various accretion states.

Methods. We computed a light curve and a hardness-intensity diagram to track the evolution of the source spectral properties, while we investigated the X-ray time variability with a dynamical power density spectrum. To trace the temporal evolution of QPOs, we segmented the dataset into shorter, continuous intervals, and computed and fitted the averaged power density spectrum for each interval.

Results. Our analysis shows that the overall behaviour of the source is consistent with the literature; strong QPOs around 6 Hz are detected on the normal branch, while transitions to and from the flaring branch – occurring over timescales of a few hundred seconds – are characterised by rapid, weaker quasi-periodic variability reaching frequencies up to 15 Hz. Despite limited statistical significance, we also identify faint, transient timing features above 20 Hz, which occasionally coexist with the prominent 6 Hz QPOs. Although tentative, the existence of these timing features in the NICER data is crucial for interpreting the simultaneous radio observations from the same multi-wavelength campaign, potentially reinforcing the connection between the ejection of relativistic outflows and the accretion states in Scorpius X-1.