NICER / ISS Science Nugget
for January 23, 2025

Linking spectrum and timing features in a neutron-star binary

Neutron star (NS) low-mass X-ray binaries (LMXBs) offer a perfect platform to investigate accretion flow dynamics in an extreme gravitational setting because they are rich in "spectro-temporal" phenomena. First observed in X-rays by a sounding rocket in the 1960s, Cygnus X-2 - the second-brightest source of X-rays in the constellation Cygnus the Swan - was later identified as a binary system in which a NS accretes matter from a low-mass companion star. This persistent and highly luminous X-ray source is a member of the Z-type subclass of LMXBs, because it traces out a 'Z'-shaped track in a hardness-intensity diagram, where the dominance of high vs. low X-ray photon energies is plotted against the overall rate of X-rays. Z sources have characteristic timing features such as quasi-periodic oscillations (QPOs) associated with each branch of their Z tracks. These QPOs act as unique windows that offer insights into the structure and dynamics of the inner accretion flow. Because it is bright, Cyg X-2 has been frequently studied, but mostly in the hard X-ray energy domain, with limited investigations in the < 1 keV photon energy range. NICER, with its high spectro-temporal resolution, provides a unique platform to investigate this source at soft energies. Coupling NICER observations with simultaneous coverage in the hard energy band from NASA's NuSTAR telescope provides a much-needed broadband X-ray view.

In a peer-reviewed study recently published in The Astrophysical Journal, M. Sudha (Wayne State Univ.) and collaborators performed a detailed spectral-timing study of Cyg X-2 using simultaneous NICER and NuSTAR observations. A QPO was detected in the 0.5-3 keV energy band of NICER data, which was absent in the > 3 keV energy band of both observatories. This feature was identified as a ~ 5.4 Hz "normal branch oscillation" (NBO). Such oscillations are generally considered to have a hard spectral origin, but this study reveals a soft spectral origin for the NBO, which challenges our current understanding of the nature of QPOs.

In the corresponding energy spectrum, the source revealed an emission feature at ∼ 1 keV, previously identified as a transition line from iron (Fe) atoms originating in the ionized plasma far from the central source. This feature could be a result of density fluctuations caused by the radiation pressure feedback from the NS. Based on radiation hydrodynamic models, the literature also suggests that NBOs could originate about 300 km away from the NS. The Sudha et al. study proposes that the Fe line and the NBO may originate within the same accretion disk radius range from a common underlying mechanism. A hunt for soft QPOs using NICER and NuSTAR observations of other NS LMXBs is currently underway and is part of a larger project to create a comprehensive spectro-temporal database of NS LMXBs which can enable a systematic study and a unified framework for the different sub-classes of these systems.