[Paper Review] NuSTAR discovery of a luminosity dependent cyclotron line energy in Vela X-1

F. Fuerst, K. Pottschmidt+14 more|Technical University of Denmark, DTU Orbit (Technical University of Denmark, DTU)|Nov 21, 2013

Astrophysical Phenomena and Observations4 citations

TL;DR

This study presents the first direct measurement of a luminosity-dependent cyclotron line energy in Vela X-1 using NuSTAR, revealing a correlation between line energy and X-ray flux over kilo-second timescales. The results support a sub-critical accretion regime with a narrow accretion column and Coulomb-dominated shock, while anti-correlated harmonic and fundamental line strengths suggest photon spawning, resolving long-standing debate about the fundamental line's existence.

ABSTRACT

We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3-79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of Lx ~ 3x10^36 erg/s. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratio of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape.

Motivation & Objective

• To investigate the flux-dependent behavior of cyclotron resonant scattering features (CRSFs) in a persistent high-mass X-ray binary on kilo-second timescales.
• To determine whether the energy of the harmonic CRSF in Vela X-1 varies with luminosity, as predicted by sub-critical accretion models.
• To resolve the long-standing debate about the existence of the fundamental cyclotron line by analyzing its anti-correlation with the harmonic line.
• To constrain the accretion column geometry and neutron star mass using observed line energy and flux correlations.
• To explore the physical conditions in the accretion column, including shock type and magnetic field effects, via high-resolution spectral analysis.

Proposed method

• Performed high-time-resolution spectral analysis on two NuSTAR observations of Vela X-1 separated by nearly one year, with similar orbital phases.
• Fitted the 3–79 keV spectra using an absorbed power-law model with a high-energy cut-off and cyclotron resonant scattering features (CRSFs).
• Tracked temporal variations in continuum flux, absorption column density, and cyclotron line centroid energies on pulse-to-pulse and kilo-second timescales.
• Used the observed line energy–luminosity correlation to test theoretical models of sub-critical accretion with Coulomb-dominated shocks.
• Applied the photon-spawning model to explain the anti-correlation between fundamental and harmonic line strengths.
• Estimated accretion column radius and neutron star mass using the observed line energy evolution and theoretical predictions from Becker et al. (2012).

Experimental results

Research questions

• RQ1 Does the cyclotron line energy in Vela X-1 vary with X-ray flux on kilo-second timescales?
• RQ2 Is the observed line energy–luminosity correlation consistent with sub-critical accretion and a Coulomb-dominated shock?
• RQ3 Why is the fundamental cyclotron line at ~25 keV anti-correlated with the harmonic line at ~55 keV?
• RQ4 What physical conditions in the accretion column explain the variable line energy ratio deviating from 2.0?
• RQ5 What causes the dramatic spectral softening and loss of high-energy cut-off during a low-flux off-state?

Key findings

• The harmonic cyclotron line energy in Vela X-1 increases with luminosity, providing the first direct observational evidence of a luminosity-dependent line energy in a persistent HMXB.
• The observed correlation between line energy and flux is consistent with theoretical predictions for sub-critical accretion with a Coulomb-dominated shock in a narrow accretion column of radius ~0.4 km.
• The fundamental line at ~25 keV is clearly detected, and its strength anti-correlates with the harmonic line strength, supporting the photon-spawning model as the explanation for the long-standing debate on the fundamental line's existence.
• The ratio of the harmonic to fundamental line energy is variable and deviates significantly from 2.0, indicating energy-dependent line broadening or shifting due to photon spawning.
• During a low-flux off-state, the spectrum softened and the high-energy cut-off disappeared, suggesting a transition to a different accretion regime at low mass accretion rates.
• The data favor a neutron star mass exceeding 1.4 M⊙, consistent with independent orbital and optical measurements, and support a narrow accretion column geometry.