Probing the accreting millisecond X-ray pulsar SAX J1808.4-3658 using the evolution of its spectral and aperiodic timing properties

TL;DR

The study analyzes SAX J1808.4-3658 during its 2022 outburst with NICER and contemporaneous AstroSat data to decompose its X-ray emission into disk and Comptonizing corona components, while tracking an evolving intrinsic absorber. Spectral modeling with disk and a centrally located corona, plus a Gaussian near ~1 keV, reveals a disk seed photon population and a compact corona whose properties change as the outburst decays; an evolving $n_H$ indicates an intrinsic absorber in addition to the Galactic column. A two-component aperiodic timing spectrum shows low-frequency (∼0.004–2 Hz) and high-frequency (∼10–100 Hz) Lorentzians, both contributing to disk and coronal variability, with a pronounced hard lag of ≈11 ms for the low-frequency band and a smaller lag for the high-frequency band, implying propagation of disk fluctuations and corona-related processing. The results constrain the accretion geometry, including a disk that remains partially covered by a compact corona, seed photons from the disk driving Comptonization, and an intrinsic absorber that evolves with accretion rate, providing insights into AMXP disk–corona coupling and enabling an upper bound on the neutron star radius from the inferred disk radius.

Abstract

Understanding accretion components in neutron star (NS) low mass X-ray binary (LMXB) systems is important to probe fundamental aspects of accretion mechanism and evolution of the system constraining its physical properties. Here, we present spectral and aperiodic timing analyses of the NICER and AstroSat data from the accretion powered millisecond X-ray pulsar (AMXP) SAX J1808.4-3658 during its 2022 outburst. We find that emissions from a softer accretion disk and a harder, centrally located, compact, partially covering, Comptonizing corona explains the continuum spectra from the source throughout the outburst. The disk inner edge temperature, the coronal electron temperature and photon index are found to be around ~ 0.5-0.9 keV, a few keV and ~ 1.1-1.8, respectively, during the entire outburst. We also find an intrinsic atomic hydrogen medium in the system, which substantially and systematically evolved throughout the outburst. We detect two broadband aperiodic features (~ 0.004-2 Hz; ~ 10-100 Hz), with the former having a significant hard lag of ~ 11 ms between 1.5-10.0 keV and 0.5-1.5 keV photons. We conclude that both the disk photons and the photons up-scattered by the corona contributed to each aperiodic feature, with the disk and the corona contributing more to the low and high frequency ones, respectively.

Figures (7)

• Figure 1: Left panel: The 0.5--10.0 keV NICER light curve for the observation IDs 5050260101 to 5050260106 and from 5574010101 to 5574010111 of the source SAX J1808. The green band shows the time range of AstroSat observations. Right panel: The hardness-intensity diagram (HID) of NICER observations, with hardness defined in the ranges 3--10 keV/0.5--3 keV. The total photon counts are taken from the energy range 0.5--10 keV. The color scheme in both the panels are same and 1$\sigma$ error bar has been reported (see sections \ref{['subsec:nicer']}, \ref{['subsec:hid']}).
• Figure 2: Spectral fitting of the NICER data from the source SAX J1808 (see section \ref{['subsec:preliminary']}). Left plot: The unabsorbed best-fit spectral model (solid line) for the observations 5050260102 (red: as depicted in the lightcurve) and 5574010107 (green: as depicted in the lightcurve) using the XSPEC model tbabs*(gaussian+nthcomp+diskbb) with a $\chi^{2}$/dof of 107/142 for the former. For both the observations, the dash-dot curve shows the disk spectrum, the dashed line shows the Comptonized spectrum, and the dotted curve is for a Gaussian line. Right plot: The spectral fitting of the observation 5050260102 using the XSPEC model tbabs*(gaussian+nthcomp+bbodyrad) with a $\chi^{2}$/dof of 238/144. Data and model spectrum are shown in this plot. Here, the solid (red), dashed and dotted lines show the blackbody, Comptonized, and Gaussian components of the spectrum, respectively. The two bottom panels in each plot display the fit residuals without the Gaussian line (middle panel) and with the Gaussian line (bottom panel). This figure shows that a disk component fits the lower energy part of the continuum spectrum much better than a blackbody component. Moreover, the intersection points of the dash-dot and dashed curves of the left plot show that the spectrum is dominated by the disk emission below $\sim3$ keV and by the Comptonized emission above $\sim3$ keV. 1$\sigma$ error bar has been reported in the middle and bottom residual panels in both the plots (section \ref{['subsec:preliminary']}).
• Figure 3: This figure shows the spectral fitting (top panel) for the NICER (blue) and AstroSat/LAXPC (green) contemporaneous data from the 2022 outburst of the source SAX J1808 using the XSPEC model tbabs*(gaussian+thcomp$\otimes$diskbb) in the energy range 0.5--20.0 keV ($\chi^{2}$/dof = 113/151). The middle panel shows the residual plot ($\chi$ = (Data$-$Model)/error) without a Gaussian component. The bottom panel shows the residual plot after including a Gaussian component at $\sim 1$ keV. 1$\sigma$ error bar has been reported in all three panels (section \ref{['subsec: spectral evolution']}).
• Figure 4: The above figures display the light curve and the $\chi^{2}$ contour curves for all our analyzed NICER observations of the source SAX J1808 during its 2022 outburst. The contour plots show the degeneracy between covering fraction (f), $\Gamma$ and k$T_\mathrm{e}$. The contour curves correspond to the 3$\sigma$ confidence level, and each symbol shows the best-fit values for the corresponding observation. The color scheme in each contour plot is the same as in the light curve. The legend shows the Obs. no. corresponding to each observation ID, see Table \ref{['tab:NICER_TABLE']}.
• Figure 5: Evolution of best-fit spectral parameter values throughout the 2022 outburst of SAX J1808 using the NICER data (see section \ref{['subsec: spectral evolution']} for discussion). The XSPEC model tbabs*(gaussian+thcomp$\otimes$diskbb) is applied to the first 13 ObsIDs (5050260101$-$5050260106 and 5574010101$-$5574010107) and tbabs*(thcomp$\otimes$diskbb) is applied to the last four ObsIDs (5574010108$-$5574010111). Each panel (except (a) and (g)) shows five plots using symbols in different colors, each of which is for a frozen covering fraction ($f$) value (considered values: 0.1, 0.15, 0.2, 0.25, 0.3). The black curve in the panels (except (a)) shows the evolution of the parameter when $f$ is kept free during the spectral fitting. Panel (a): The evolution of the unabsorbed flux ($0.5-10$ keV) across the observations. Panel (b): The evolution of the absorption column density ($n_\mathrm{H}$). Panel (c), (d): The evolution of disk normalization ($N_\mathrm{disk}$) and disk inner edge temperature ($\mathrm{k}T_\mathrm{in}$), respectively. Panel (e), (f): The evolution of Comptonization photon index ($\Gamma$) and electron temperature (k$T_\mathrm{e}$; y-axis is limited to $\sim 12$ keV), respectively. Panel (g): The evolution of the best-fit covering fraction ($f$) values. Panel (h): The evolution of the Gaussian normalization ($N_\mathrm{Gauss}$) values. 1$\sigma$ error bar has been reported in all the above panels.