ABSTRACT
We present a comprehensive timing analysis on the Be/X-ray binary pulsar RX J0440.9+4431 using observations from NICER and Insight-HXMT during the 2022–2023 outburst. The power density ...spectrum (PDS) of RX J0440.9+4431 exhibits typical aperiodic variability in X-ray flux across a wide frequency range. During a supercritical accretion state, we detect quasi-periodic oscillations (QPOs) at 0.2–0.5 Hz in the light curves of five pulses for RX J0440.9+4431. The observed QPOs manifest during flares, while the flares appear at the peaks of the pulse profiles on a time-scale of seconds and are primarily caused by an increase in hard photons. These flares can be explained by increased material ingestion in the accretion column at a fixed phase, primarily generating hard photons. Alternatively, an increase in accretion rate, independent of phase, may result in highly beamed hard photons within the accretion column, causing the flares. We argue the origin of QPOs to instabilities within the accretion flow. Additionally, we find that the break frequencies in the noise power spectra align well with $\propto L_{\mathrm{x}}^{3 / 7}$ across three orders of magnitude in the luminosity, which points to a relatively strong magnetic field in RX J0440.9+4431, estimated to be ~1013 G.
ABSTRACT
RX J0440.9+4431, a Be/X-ray binary, had its brightest outburst in 2022 since its discovery, with a peak X-ray flux of 2.25 Crab (as recorded by Swift/BAT, 15–50 keV). We analyze the timing ...properties of this giant outburst using data from Insight-Hard X-ray Modulation Telescope and Neutron Star Interior Composition Exlporer, focusing on the evolution of the pulse profile and pulse fraction. We observe that when the luminosity reached around ~$3\times 10^{37}\ {\rm erg\ \rm s^{-1}}$, a transition from double-peaked to single-peaked pulse profiles occurred across the energy range, with the peak of the low-energy profile aligning gradually with the peak of the high-energy profile. This change indicates a transition from subcritical to supercritical accretion. Additionally, we found a concave in the pulse fraction as a function of energy around 20–30 keV throughout the entire outburst period. Compared to the low luminosity, the concave becomes weaker in high luminosities, and overall, the pulse fraction is higher. We propose that this concave could be caused by the scattering of high-energy photons by the atmosphere of a neutron star, leading to a dilution of the pulse fraction. As the accretion reaches the supercritical state, the accretion column height increases, resulting in a larger direct component of strongly beamed X-ray flux, and an elevated pulse fraction.
The black hole X-ray binary source 4U 1543–47 experienced a super-Eddington outburst in 2021, reaching a peak flux of up to ∼1.96 × 10 −7 erg cm −2 s −1 (∼8.2 Crab) in the 2−10 keV band. Soon after ...the outburst began, it rapidly transitioned into the soft state. Our goal is to understand how the accretion disk structure deviates from a standard thin disk when the accretion rate is near Eddington. To do so, we analyzed spectra obtained from quasi-simultaneous observations conducted by the Hard X-ray Modulation Telescope (Insight-HXMT), the Nuclear Spectroscopic Telescope Array ( NuSTAR ), and the Neil Gehrels Swift Observatory ( Swift ). These spectra are well fitted by a model comprising a disk, a weak corona, and a reflection component. We suggest that the reflection component is caused by disk self-irradiation, that is by photons emitted from the inner disk that return to the accretion disk surface as their trajectories are bent by the strong gravity field. In this scenario, the best-fitting parameters imply that the reflected flux represents more than half of the total flux. Using general relativistic ray-tracing simulations, we show that this scenario is viable when the disk becomes geometrically thick, with a funnel-like shape, as the accretion rate is near or above the Eddington limit. In the specific case of 4U 1543–47, an angle ≳45 deg between the disk surface and the equatorial plane can explain the required amount of self-irradiation.
Cygnus X-1, as the first discovered black hole binary, is a key source for understanding the mechanisms of state transitions, and the scenarios of accretion in extreme gravity fields. We present a ...spectral-timing analysis of observations taken with the Insight-HXMT mission, focusing on the spectral-state dependent timing properties in the broad energy range of 1–150 keV, thus extending previous RXTE-based studies to both lower and higher energies. Our main results are the following: a) We successfully use a simple empirical model to fit all spectra, confirming that the reflection component is stronger in the soft state than in the hard state; b) The evolution of the total fractional root mean square (rms) depends on the selected energy band and the spectral shape, which is a direct result of the evolution of the power spectral densities (PSDs); c) In the hard/intermediate state, we see clear short-term variability features and a positive correlation between central frequencies of the variability components and the soft photon index Γ(sub 1), also at energies above 15 keV. The power spectrum is dominated by red noise in the soft state instead. These behaviors can be traced to at least 90 keV; d) The coherence and the phase-lag spectra show different behaviors dependent on different spectral shapes.
Abstract
Recent observations of Her X-1 with NuSTAR (as well as with INTEGRAL, Swift, and Astrosat) have provided evidence that the nearly 20 yr-long decay of the cyclotron line energy since ∼1994 ...has ended and that a stable value has replaced the decay. Using the observations of the Hard X-ray Modulation Telescope (Insight-HXMT) performed between 2017 July and 2020 February, we analyze the spectra of Her X-1 in its main-on state, focusing on tracing the evolution of cyclotron line energy. While our analysis of eight main-on observations with Insight-HXMT (two in coordination with NuSTAR) shows significant differences with the results of NuSTAR, two earlier findings are confirmed: the positive correlation between the cyclotron line energy and the X-ray flux (source luminosity) and the constancy of the flux-normalized cyclotron line energy during 2017–2020—albeit with significant uncertainty about the absolute value.
Abstract
The fast transitions between different types of quasi-periodic oscillations (QPOs) are generally observed in black hole transient sources (BHTs). We present a detailed study of the timing ...and spectral properties of the transitions of type-B QPOs in MAXI J1348–630, observed by Insight-HXMT. The fractional rms variability–energy relationship and energy spectra reveal that type-B QPOs probably originate from jet precession. Compared to a weak power-law dominated power spectrum, when type-B QPOs are present, the corresponding energy spectrum shows an increase in the Comptonization component and the need for the
xillverCp
component, and a slight increase in the height of the corona when using the
relxilllp
model. Therefore, we suggest that a coupled inner disk-jet region is responsible for the observed type-B QPO transitions. The timescale for the appearance/disappearance of type-B QPOs is either long or short (seconds), which may indicate instability of the disk-jet structure. For these phenomena, we hypothesize that the Bardeen–Petterson effect causes the disk-jet structure to align with the BH spin axis or that the disappearance of small-scale jets bound by the magnetic flux tubes leads to the disappearance of type-B QPOs. We observed three events regarding the B/C transitions, one of which occurred over a short time period from ∼9.2 Hz (C) to ∼4.8 Hz (B). The energy spectral analysis for the other two transitions shows that when type-C QPO is present, the Comptonization flux is higher, the spectrum is harder, and the inner radius of the disk changes insignificantly. We suggest that type-C QPOs probably originate from relatively stronger jets or the corona.
Abstract
Using the observations of the high-energy detector of the Hard X-ray Modulation Telescope (Insight-HXMT) for Scorpius X-1 from 2017 to 2020, we search for hard X-ray tails in the X-ray ...spectra in ∼30–200 keV. The hard X-ray tails are found throughout the
Z
-track on the hardness–intensity diagram, and the detected hard X-ray tails become hard and weak from the horizontal branch (HB), through the normal branch (NB), to the flaring branch (FB). Comparing the hard X-ray spectra of Insight-HXMT between Cyg X-1 and Sco X-1, it is concluded that the hard X-ray spectrum of Cyg X-1 shows a high-energy cutoff, implying a hot corona in it, but the high-energy cutoff is not seen in the hard X-ray spectrum of Sco X-1. From fitting the broadband spectrum of Sco X-1 in ∼2–200 keV, it is proposed that the hard X-ray tails in the HB and NB can be explained by the overall Comptonization COMPTB model, suggesting that the hard X-ray tails could have resulted from the Comptonization of the photons from the neutron star (NS) surface by the thermal electrons in the region between the NS and the disk and the energetic electrons in the freefall toward the NS in the converging flow onto the NS. However, this model cannot be responsible for the hard X-ray tails in the FB. Further study on the FB hard X-ray tails is needed.
Abstract
We report the X-ray timing results of the black hole candidate MAXI J1820+070 during its 2018 outburst using the Hard X-ray Modulation Telescope (Insight-HXMT) and Neutron Star Interior ...Composition Explorer Mission (NICER) observations. Low-frequency quasi-periodic oscillations (LFQPOs) are detected in the low/hard state and the hard intermediate state, which lasted for ∼90 days. Thanks to the large effective area of Insight-HXMT at high energies and NICER at low energies, we are able to present the energy dependence of the LFQPO characteristics and phase lags from 0.2 to 200 keV, which has never been explored by previous missions. We find that the centroid frequency of the LFQPOs does not change significantly with energy, while the full width at half maximum and fractional rms show a complex evolution with energy. The LFQPO phase lags at high energies and low energies show consistent energy-dependence relations taking the ∼2 keV as reference. Our results suggest that the LFQPOs from high energy come from the LT precession of the relativistic jet, while the low-energy radiation is mainly from the perpendicular innermost regions of the accretion disk.
ABSTRACT
MAXI J1816–195 is a newly discovered accreting millisecond X-ray pulsar that went outburst in 2022 June. Through timing analysis with Neutron star Interior Composition Explorer (NICER) and ...Nuclear Spectroscopic Telescope Array (NuSTAR) observations, we find a transient modulation at ~2.5 Hz during the decay period of MAXI J1816–195. The modulation is strongly correlated with a spectral hardening, and its fractional rms amplitude increases with energy. These results suggest that the modulation is likely to be produced in an unstable corona. In addition, the presence of the modulation during thermonuclear bursts indicates that it may originate from a disc-corona where the optical depth is likely the main factor affecting the modulation, rather than temperature. Moreover, we find significant reflection features in the spectra observed simultaneously by NICER and NuSTAR, including a relativistically broadened Fe-K line around 6–7 keV, and a Compton hump in the 10–30 keV energy band. The radius of the inner disc is constrained to be Rin = (1.04–1.23) RISCO based on reflection modeling of the broad-band spectra. Assuming that the inner disc is truncated at the magnetosphere radius, we estimate that the magnetic field strength is $\le 4.67 \times 10^{8}\, \rm G$.
ABSTRACT
We present the results of a detailed timing and spectral analysis of the quasi-regular modulation (QRM) phenomenon in the black hole X-ray binary 4U 1630−47 during its 1998 outburst observed ...by the Rossi X-ray Timing Explorer (RXTE). We find that the ∼50–110 mHz QRM is flux dependent, and the QRM is detected with simultaneous low-frequency quasi-periodic oscillations (LFQPOs). According to the behaviour of the power density spectrum, we divide the observations into four groups. In the first group, namely behaviour A, LFQPOs are detected, but no mHz QRM. The second group, namely behaviour B, a QRM with frequency above ∼88 mHz is detected and the ∼5 and ∼7 Hz LFQPOs are almost overlapping. In the third group, namely behaviour C, the QRM frequency below ∼88 mHz is detected and the LFQPOs are significantly separated. In the fourth group, namely behaviour D, neither QRM nor LFQPOs are detected. We study the energy dependence of the fractional rms, centroid frequency, and phase lag of QRM and LFQPOs for behaviour B and C. We then study the evolution of QRM and find that the frequency of QRM increases with hardness, while its rms decreases with hardness. We also analyse the spectra of each observation, and find that the QRM rms of behaviour B has a positive correlation with $F_{\rm power\ law}$/$F_{\rm total}$. Finally, we give our understanding for this mHz QRM phenomenon.
