A hidden quasi-periodic oscillation in Cygnus X-1 revealed by NICER

Abstract

Cygnus X-1 is a high-mass black hole binary extensively studied since its discovery in 1964. Its rapid X-ray variability provides insights into accretion physics. Unlike other black hole X-ray binaries, its power spectra are generally featureless and modeled with two broad Lorentzians, without requiring narrow quasi-periodic oscillations. We investigate the possibility that some undetected variability components in power spectra may appear in the imaginary part of the cross spectra and the coherence function. Using NICER observations up to Cycle 6, we study the power, cross, and lag spectra, along with the coherence function, searching for these "imaginary" components. We simultaneously fit the power spectra in two energy bands, 0.3-2 keV and 2-12 keV, and the real and imaginary parts of the cross-spectrum with a multi-Lorentzian model. Assuming each Lorentzian is coherent between the two bands but incoherent with others, we predict intrinsic coherence and phase lags. he intrinsic coherence shows a narrow dip at a frequency increasing from ~1 Hz to ~6 Hz as the power-law index of the Comptonized component increases from ~1.8 to ~2.4. Simultaneously, the phase lags exhibit a steep increase (the "cliff") at the same frequencies. These features vanish when using energy bands similar to RXTE (e.g., 3-5 keV and 5-12 keV). A narrow Lorentzian component with low fractional rms and large phase lag is required to reproduce the coherence drop. Its rms and phase-lag spectra evolve systematically in the hardness-intensity diagram. This "imaginary" QPO behaves like a type-C QPO despite being undetectable in power spectra alone. Similar features in MAXI J1348-630 and MAXI J1820+070 support this interpretation, suggesting this may be the first detection of a type-C QPO in Cygnus X-1.

15 pages, 10 figures, 5 tables. Accepted for publication in Astronomy & Astrophysics

References in corpus (22)

• Cygnus X-1 contains a 21-solar mass black hole -- implications for massive star winds
• Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1
• A public relativistic transfer function model for X-ray reverberation mapping of accreting black holes
• Fast variability from black-hole binaries
• vKompth: A variable Comptonisation model for low-frequency quasi-periodic oscillations in black-hole X-ray binaries
• A two-component Comptonisation model for the type-B QPO in MAXI J1348-630
• A Systematic Analysis of the Phase Lags Associated with the Type-C Quasi-periodic Oscillation in GRS 1915+105
• A variable corona for GRS 1915+105
• The Evolution of the Phase Lags Associated with the Type-C Quasi-periodic Oscillation in GX 339--4 during the 2006/2007 Outburst
• The comptonizing medium of the black-hole X-ray binary MAXI~J1535$-$571 through type-C quasi-periodic oscillations
• A variable corona during the transition from type-C to type-B quasi-periodic oscillations in the black hole X-ray binary MAXI J1820+070
• Variability and phase lags of the type-C quasi-periodic oscillation of MAXI J1348-630 with NICER
• Long term variability of Cygnus X-1. VIII. A spectral-timing look at low energies with NICER
• High hard X-ray polarization in Cygnus X-1 confined to the intermediate hard state: evidence for a variable jet component
• The nature of an imaginary quasi-periodic oscillation in the soft-to-hard transition of MAXI J1820+070
• The evolution of the high-frequency variability in the black hole candidate GRS 1915+105 as seen by RXTE
• Investigating the Theory of Propagating Fluctuations with Numerical Models of Stochastic Accretion Discs
• Disc-corona interaction in the heartbeat state of GRS 1915+105
• The X-ray spectral-timing contribution of the stellar wind in the hard state of Cyg X-1
• Extending the theory of propagating fluctuations: the first fully relativistic treatment and analytical Fourier-Green's functions
• Stellar wind variability in Cygnus X-1 from high-resolution excess variance spectroscopy with Chandra
• Basics of Fourier Analysis for High-Energy Astronomy

Cited by in corpus (6)

• Timing analysis of the black-hole candidate Swift J1727.8-1613: detection of a dip-like feature in the high-energy cross spectrum
• A Hidden Pulse: Uncovering a New Timing Signal in Cygnus X-1 with AstroSat
• Variable QPO lags and reduced coherence between the disc and corona in MAXI J1820+070
• Black-hole X-ray binary Swift J1727.8$-$1613 shows simultaneous Type-B and Type-C quasi-periodic oscillations across the hard-intermediate and soft-intermediate states
• Detection of a Type-C QPO during the soft-to-hard transition in Swift J1727.8-1613
• Characteristics of the High-frequency Humps in the Black hole X-ray Binary Swift J1727.8--1613