X-ray spectral-timing variability of 1A 0535+262 during the 2020 giant outburst

Abstract

Context. The Be/X-ray binary 1A 0535+262 underwent a giant X-ray outburst in November 2020, peaking at ∼ 1 × 10 38 erg s −1 (1-100 keV, 1.8 kpc), the brightest outburst recorded for this source so far. The source was monitored over two orders of magnitude in luminosity with Insight-HXMT, which allowed us to probe the X-ray variability in an unprecented range of accretion rates. Aims. Our goal is to search for patterns of correlated spectral and timing behavior that can be used to characterize the accretion states in hard X-ray transient pulsars.Methods. We have studied the evolution of the spectral continuum emission using hardness-intensity diagrams and the aperiodic variability of the source by analyzing power density spectra. We have used phenomenological models to fit the various broad-band noise components. Results. The hardness-intensity diagram displays three distinct branches that can be identified with different accretion regimes. The characteristic frequency of the noise components correlates with the luminosity. Our observations cover the highest end of this correlation, at luminosities not previously sampled. We have found evidence for a flattening of the correlation at those high luminosities, which might indicate that the accretion disk reached the closest distance from the neutron star surface during the peak of the outburst. We also find evidence for hysteresis in the spectral and timing parameters: at the same luminosity level, the spectrum is harder and the characteristic noise frequency larger during the rise than during the decay of the outburst. Conclusions. As in black hole binaries and low-mass X-ray binaries, the hardness-intensity diagram represents a useful diagnostic tool to define the source state in an accreting pulsar. Our timing analysis confirms previous findings from spectral analysis of a hysteresis pattern of variability where the spectral and timing parameters adopt different values for similar luminosity depending on whether the source is on the rising or decaying phase of the outburst.