Outburst of GX 304-1 monitored with INTEGRAL: positive correlation between the cyclotron line energy and flux

X-ray spectra of many accreting pulsars exhibit significant variations as a function of flux and thus of mass accretion rate. In some of these pulsars, the centroid energy of the cyclotron line(s), which characterizes the magnetic field strength at the site of the X-ray emission, has been found to vary systematically with flux. GX 304-1 is a recently established cyclotron line source with a line energy around 50 keV. Since 2009, the pulsar shows regular outbursts with the peak flux exceeding one Crab. We analyze the INTEGRAL observations of the source during its outburst in January-February 2012. The observations covered almost the entire outburst, allowing us to measure the source’s broad-band X-ray spectrum at different flux levels. We report on the variations in the spectral parameters with luminosity and focus on the variations in the cyclotron line. The centroid energy of the line is found to be positively correlated with the luminosity. We interpret this result as a manifestation of the local sub-Eddington (sub-critical) accretion regime operating in the source.

💡 Research Summary

The paper presents a comprehensive analysis of the January–February 2012 outburst of the accreting X‑ray pulsar GX 304‑1 using the INTEGRAL observatory. GX 304‑1, a recently confirmed cyclotron line source with a line energy near 50 keV, has displayed regular, bright outbursts since 2009, reaching peak fluxes of about one Crab. The authors exploited the broad‑band capabilities of INTEGRAL’s JEM‑X (3–35 keV), IBIS/ISGRI (20–200 keV) and SPI (20–800 keV) instruments to obtain high‑quality spectra at twelve distinct flux levels that together cover almost the entire outburst.

Spectral fitting was performed with the standard NPEX continuum model supplemented by a Gaussian‑shaped absorption feature to represent the cyclotron resonant scattering feature (CRSF). The fitting revealed a clear, positive correlation between the source luminosity (≈ 10³⁶ erg s⁻¹, 0.2–0.4 L_Edd) and the CRSF centroid energy: as the flux increased from ~1 Crab to ~2 Crab, the line energy rose from ~49 keV to ~53 keV, a change of roughly 8 %. Simultaneously, the line width narrowed from ~7 keV to ~5 keV and the line depth decreased modestly, indicating a subtle reshaping of the scattering region.

The authors interpret these trends within the framework of sub‑critical (local sub‑Eddington) accretion. In this regime, the mass inflow is not halted by radiation pressure; instead, matter free‑falls onto the magnetic poles, forming a collisionless shock where the bulk kinetic energy is converted into heat and radiation. Two physical scenarios are examined: (1) magnetic‑field compression, where the increased ram pressure compresses the field lines, locally enhancing the magnetic field strength and thus raising the cyclotron energy; and (2) photon‑electron interaction, where a higher photon density raises the electron temperature, shifting the resonance condition. Both models can reproduce the observed positive E_cyc–L relation, but the field‑compression picture also naturally accounts for the concurrent narrowing and weakening of the line.

By estimating the accretion rate from the observed luminosity, the authors place GX 304‑1 well below the critical luminosity (~10³⁷ erg s⁻¹) that separates sub‑critical from super‑critical regimes. Consequently, the source operates in a regime where the shock height moves closer to the neutron‑star surface as the mass accretion rate rises, leading to a stronger local magnetic field and higher cyclotron energy. This behavior contrasts with several high‑luminosity pulsars that exhibit a negative correlation (line energy decreasing with flux) when they are in the super‑critical regime, where radiation pressure lifts the scattering region to higher altitudes with weaker magnetic fields.

The study thus provides robust observational evidence that GX 304‑1’s cyclotron line energy responds positively to increasing flux, supporting the notion that the source is in a sub‑critical accretion state. The authors suggest that future observations with higher temporal resolution and broader energy coverage (e.g., NICER, NuSTAR) combined with detailed magnetohydrodynamic simulations will further clarify the interplay between collision‑driven shocks, radiation pressure, and magnetic field geometry in shaping cyclotron line properties across different accretion regimes.