Measurement of the Orbital Parameters, Spin and Spectral Evolution During the Main High State of Her X-1 with Insight-HXMT

Abstract

Based on Insight-HXMT observations, we present a detailed timing analysis and spectral evolution of a complete Main High state for Her X-1 in February 2020. We determine an accurate local ephemeris using the Rømer delay measured from five eclipses. We report the spin period of the neutron star at $P_{\rm spin}=1.23765212 \pm 0.00000026$ s with a spin period derivative of $\dot P_{\rm spin}=-(1.18\pm 0.04)\times 10^{-13}$ s\,s$^{-1}$. By combining the newly measured local values $T_{ecl}$ with those reported in the literature, we refine the orbital ephemeris of Her X-1, obtaining $T_{ecl} = 46359.871956 \pm 0.000010$ MJD and $P_{orb}=1.7001674990 \pm 0.0000000105$ day, then detect a continuous decrease in the orbital period with a rate of $\dot{P}_{\rm orb} = -(1.957 \pm 0.335)\times10^{-11}\,\mathrm{d\,d^{-1}}$. We also investigate the evolution of X-ray spectral parameters during the Main High state. The hydrogen absorption column density $N_{\rm H}$ increased monotonously during the phase, and the photon index kept nearly constant. The cyclotron absorption line was detected with a centroid energy around 38 keV, showing no significant evolution with luminosity. The spectral variations with the superorbital phase are discussed within the accretion disk precession scenario.

Figures (6)

• Figure 1: The top panel shows the background-subtracted X-ray light curves of Hercules X-1 observed by Insight-HXMT between 19 and 29 February 2020, with a time resolution of 10,000 s. Orange points represent the LE (1–10 keV) countrate, and blue points represent the ME (10–20 keV) countrate. The middle panel shows the softness ratio (SR), defined as the LE countrate divided by the ME countrate, with its mean value indicated by a red dashed line. The bottom panel shows the evolution of the ME (10–20 keV) pulse profiles during the Main High State. After MJD 58905, the shoulder peak in the pulse phase range 0.4–0.45 disappears, whereas the centra peak is still present.
• Figure 2: Corner plot of the MCMC posterior distributions of the orbital parameters of Her X-1 assuming an eccentric orbital model. The diagonal panels show the one-dimensional distributions with the median and $1\sigma$ intervals, while the off-diagonal panels show the joint distributions. The small value of $e$ indicates that the orbit is nearly circular.
• Figure 3: The spin period of Her X-1 observed by Insight-HXMT. The red solid curve represents the best-fitting model. The residuals are shown in the lower panel.
• Figure 4: Residuals of $T_{ecl}$ after subtracting the linear component of the best-fitting quadratic ephemeris. The blue pentagrams indicate the results from this work, while the black dashed line represents the best-fitting quadratic ephemeris.
• Figure 5: Spectral fitting results of Her X-1 in the 2–60 keV energy range, obtained by combining five spectra at the Main-On peak phase. The middle panel shows the corresponding fitting residuals, and the bottom panel shows the fit without the cyclotron absorption line component.