Main-on State Research Articles

Polarization perspectives on Hercules X-1: further constraining the geometry

ABSTRACT We conduct a comprehensive analysis of the accreting X-ray pulsar, Hercules X-1, utilizing data from Imaging X-ray Polarimetry Explorer (IXPE) and Nuclear Spectroscopic Telescope Array. IXPE performed five observations of Her X-1, consisting of three in the Main-on state and two in the Short-on state. Our time-resolved analysis uncovers the linear correlations between the flux and polarization degree as well as the pulse fraction and polarization degree. Geometry parameters are rigorously constrained by fitting the phase-resolved modulations of Cyclotron Resonance Scattering Feature and polarization angle with a simple dipole model and Rotating Vector Model, respectively, yielding roughly consistent results. The changes of χp (the position angle of the pulsar’s spin axis on the plane of the sky) between different Main-on observations suggest the possible forced precession of the neutron star crust. Furthermore, a linear association between the energy of Cyclotron Resonance Scattering Feature and polarization angle implies the prevalence of a dominant dipole magnetic field, and their phase-resolved modulations likely arise from viewing angle effects.

Insight-HXMT Views Her X-1: Probing the Dependence of the Cyclotron Line Energy on Flux and Time

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.

Flux-resolved Spectropolarimetric Evolution of the X-Ray Pulsar Hercules X-1 Using IXPE

We conduct a spectropolarimetric study of the accreting X-ray pulsar Hercules X-1 using observations with the Imaging X-ray Polarimetry Explorer (IXPE). IXPE monitored the source in three different epochs, sampling two “Main-on” and one “Short-on” state of the well-known super-orbital period of the source. We find that the 2–7 keV polarization fraction increases significantly from ∼7% to 9% in the Main-on state to ∼15%–19% in the Short-on state, while the polarization angle remains more or less constant or changes slightly, ∼47°–59°, in all three epochs. The polarization degree and polarization angle are consistent with being energy independent for all three epochs. We propose that in the Short-on state, when the neutron star is partially blocked by the disk warp, the increase in the polarization fraction can be explained as a result of the preferential obstruction of one of the magnetic poles of the neutron star.

Evidence for neutron star triaxial free precession in Her X-1 from Fermi/GBM pulse period measurements

ABSTRACT Her X-1/HZ Her is one of the best studied accreting X-ray pulsars. In addition to the pulsating and orbital periods, the X-ray and optical light curves of the source exhibit an almost periodic 35-d variability caused by a precessing accretion disc. The nature of the observed long-term stability of the 35-d cycle has been debatable. The X-ray pulse frequency of Her X-1 measured by the Fermi/GBM demonstrates periodical variations with X-ray flux at the main-on state of the source. We explain the observed periodic sub-microsecond pulse frequency changes by the free precession of a triaxial neutron star (NS) with parameters previously inferred from an independent analysis of the X-ray pulse evolution over the 35-d cycle. In the Fermi/GBM data, we identified several time intervals with a duration of half a year or longer where the NS precession period describing the pulse frequency variations does not change. We found that the NS precession period varies within one per cent in different intervals. Such variations in the free precession period on a year time-scale can be explained by $\lesssim 1{{\ \rm per\ cent}}$ changes in the fractional difference between the triaxial NS’s moments of inertia due to the accreted mass readjustment or variable internal coupling of the NS crust with the core.

LONG-TERM EVOLUTION OF THE MAIN-ON STATES IN HERCULES X-1

Rossi X-ray Timing Explorer All-sky Monitor (RXTE ASM) observations and American Association of Variable Star Observers (AAVSO) optical data were used for a time-series analysis of the long-term activity of Her X-1. The problem of measuring the long-term evolution of the X-ray intensity in the light curve which consists only of separated intense spikes was addressed. For this purpose, the fluence of each Main-On state was determined. The fluences vary generally (not only in the transitions into/from anomalous low states (ALSs)) by tens of percent on timescales of at least several epochs of the cycle length of the superorbital cycle (but without any stable cycle) while irradiation of the donor producing the optical modulation remains considerably more stable. ALS1 and ALS2 are extensions of the tail in the statistical distribution of these fluences. In this interpretation, the variations of the fluences are caused by the changes of the structure of the inner disk region, which produces variable obscuration of the beams (emitting in the ASM band) at the neutron star. A small change of obscuration of these beams by the inner disk region suffices to change the fluences largely. However, the irradiation of the donor is changed significantly less because this inner disk region (which emits beyond the ASM band and acts as the occulting region of the beams) also irradiates the donor.

Broad-band spectroscopy of Hercules X-1 with Suzaku

Abstract Hercules X-1 was observed with Suzaku in the main-on state from 2005 to 2010. The 0.4–100 keV wide-band spectra obtained in four observations showed a broad hump around 4–9 keV in addition to narrow Fe lines at 6.4 and 6.7 keV. The hump was seen in all the four observations regardless of the selection of the continuum models. Thus it is considered a stable and intrinsic spectral feature in Her X-1. The broad hump lacked a sharp structure like an absorption edge. Thus it was represented by two different spectral models: an ionized partial covering or an additional broad line at 6.5 keV. The former required a persistently existing ionized absorber, whose origin was unclear. In the latter case, the Gaussian fitting of the 6.5-keV line needs a large width of σ = 1.0–1.5 keV and a large equivalent width of 400–900 eV. If the broad line originates from Fe fluorescence of accreting matter, its large width may be explained by the Doppler broadening in the accretion flow. However, the large equivalent width may be inconsistent with a simple accretion geometry.

Suzaku Observations of Hercules X-1: Measurements of the Two Cyclotron Harmonics

Abstract The accretion-powered pulsar Her X-1 was observed with Suzaku twice in its main-on state, on 2005 October 5-6 and 2006 March 29-30, for a net exposure of 30.5ks and 34.4ks, respectively. In the 2005 and 2006 observations, the source was detected at an average 10-30keV intensity of 290mCrab and 230mCrab, respectively. The intrinsic pulse period was measured on both occasions at 1.23776s by HXD-PIN, after barycentric and binary corrections. The pulse phase-averaged spectra in the energy range above 10keV were well fitted by the “Negative and Positive power-law times EXponential” (NPEX) model, multiplied by a fundamental cyclotron resonance scattering feature at $\sim$36keV, which appears very significantly in the HXD-PIN data. The resonance profiles were successfully reproduced by a Lorentzian-type scattering cross section, rather than by a Gaussian-type alternative. The pulse phase-averaged HXD-GSO data, covering 50-120keV, are featureless. However, in a differential spectrum between the pulse-decay phase and off-pulse phase, the second-harmonic cyclotron resonance was detected in the GSO data at $\sim$73keV, with a depth of 1.6$_{-0.7}^{+0.9}$. This makes Her X-1 a 6th pulsar with established second-harmonic resonance. The implications of these results are briefly discussed.

INTEGRAL observations of Hercules X-1

Aims. We investigate the X-ray spectral and timing properties of the accreting X-ray pulsar Her X-1 observed with the INTEGRAL satellite in July–August 2005. Methods. The data analyzed in this work cover a substantial part of one main-on state of the source. The short time scale pulse period development is measured. X-ray pulse profiles for different energy ranges and time intervals are constructed. Pulse-averaged and pulse-phase resolved broad band X-ray spectra are studied. Spectral changes during X-ray dips are explored. Results. The X-ray pulse profiles are found to change significantly during the period of observations. For the first time a strong spinup is measured within one 35 d cycle. Spectral characteristics observed during the X-ray dips are consistent with their interpretation as being due to partial covering as has been reported by several authors. The fundamental cyclotron absorption line is firmly observed in both pulse-averaged and pulse-phase resolved X-ray spectra. The energy, width, and the depth of the line are found to vary significantly with pulse phase.

Discovery of a flux-related change of the cyclotron line energy in Hercules X-1

Aims. We present the results of ten years of repeated measurements of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and report the discovery of a positive correlation of the centroid energy of this absorption feature in pulse phase averaged spectra with source luminosity. Methods. Our results are based on a uniform analysis of observations by the RXTE satellite from 1996 to 2005, using sufficiently long observations of 12 individual 35-day Main-On states of the source. Results. The mean centroid energy E c of the CRSF in pulse phase averaged spectra of Her X-1 during this time is around 40 keV, with significant variations from one Main-On state to the next. We find that the centroid energy of the CRSF in Her X-1 changes by ~5% in energy for a factor of 2 in luminosity. The correlation is positive, contrary to what is observed in some high luminosity transient pulsars. Conclusions. Our finding is the first significant measurement of a positive correlation between E c and luminosity in any X-ray pulsar. We suggest that this behaviour is expected in the case of sub-Eddington accretion and present a calculation of a quantitative estimate, which is very consistent with the effect observed in Her X-1. We urge that Her X-1 is regularly monitored further and that other X-ray pulsars are investigated for a similar behaviour.

Vertical MOSFET Devices Fabricated on 3C-SiC with High and Low Material Quality

AbstractVertical DMOSFET devices with sizes from single cell to 3x3 mm2 large devices have been fabricated on cubic 3C-SiC material. The used 3C-SiC substrates had varying material quality. The best quality had a more than 2 orders of magnitude lower density of extended crystal defects than the worst material. The processed and investigated vertical DMOSFET devices were designed for 600 V blocking voltage and had hexagonal unit cells with 2 μm channel length and aluminum implanted termination rings. The p-body was aluminum implanted and the source was phosphorus implanted. As deposited Ti/W contacts were evaluated as source and drain metallization. Single cell vertical DMOSFET devices, processed on the low defect density material quality, gave voltage blocking capabilities of more than 600 V with leakage currents of 0.1 to 10 μA. The same devices could handle currents around 5 mA in on-state at 15 V gate bias. MOSFET devices with 50 to 100 unit cells had blocking voltages of 400 to 500 V with on-state currents of 0.2 to 0.4 A at 15 V gate bias, indicating the potential of 3C-SiC for the fabrication of next generation power MOSFET devices.

XMM-NewtonEPIC and Optical Monitor observations of Her X-1 over the 35-d beat period

We present the results of a series of XMM-Newton European Photon Imaging Camera (EPIC) and Optical Monitor observations of Her X-1, spread over a wide range of the 35-d precession period. We confirm that the spin modulation of the neutron star is weak or absent in the low state - in marked contrast to the main or short-on states. During the states of higher intensity, we observe a substructure in the broad soft X-ray modulation below ∼ 1 ke V, revealing the presence of separate peaks which reflect the structure seen at higher energies. The strong fluorescence emission line at ∼6.4 keV is detected in all observations (apart from one taken in the middle of eclipse), with higher line energy, width and normalization during the main-on state. In addition, we report the detection of a second line near 7 keV in 10 of the 15 observations taken during the low-intensity states of the system. This feature is rather weak and not significantly detected during the main-on state, when the strong continuum emission dominates the X-ray spectrum. Spin-resolved spectroscopy just after the rise to the main-on state shows that the variation of the Fe Kα at 6.4 keV is correlated with the soft X-ray emission. This confirms our past finding based on the XMM-Newton observations made further into the main-on state, and indicates the common origin for the thermal component and the Fe Kα line detected at these phases. We also find that the normalization of the 6.4-keV line during the low state is correlated with the binary orbital phase, having a broad maximum centred near φ oribit ∼ 0.5. We discuss these observations in the context of previous observations, investigate the origin of the soft and hard X-rays and consider the emission site of the 6.4-keV and 7-keV emission lines.

High‐Resolution X‐Ray Spectroscopy of Hercules X‐1 with theXMM‐NewtonReflection Grating Spectrometer: CNO Element Abundance Measurements and Density Diagnostics of a Photoionized Plasma

We analyze the high-resolution X-ray spectrum of Hercules X-1, an intermediate-mass X-ray binary, which was observed with the XMM-Newton Reflection Grating Spectrometer. We measure the elemental abundance ratios by use of spectral models, and we detect material processed through the CNO cycle. The CNO abundances and, in particular, the ratio N/O > 4.0 times solar provide stringent constraints on the evolution of the binary system. The low- and short-on flux states of Her X-1 exhibit narrow-line emission from C VI, N VI, N VII, O VII, O VIII, Ne IX, and Ne X ions. The spectra show signatures of photoionization. We measure the electron temperature, quantify photoexcitation in the Heα lines, and set limits on the location and density of the gas. The recombination lines may originate in the accretion disk atmosphere and corona or on the X-ray-illuminated face of the mass donor (HZ Her). The spectral variation over the course of the 35 day period provides additional evidence for the precession of the disk. During the main-on state, the narrow-line emission is absent, but we detect excesses of emission at ~10-15 A and also near the O VII intercombination line wavelength.

XMM--Newton EPIC observations of Her X-1

We present spin-resolved X-ray data of the neutron star binary Her X-1 taken using the EPIC detectors on XMM‐Newton. The data were taken at three distinct epochs through the 35-d precession period. The energy-dependent light curves of this source vary significantly from epoch to epoch. It is known that the relative phasing of the soft (1 keV) and hard (2 keV) X-rays varies. Here, we find that the phase shift between the soft and hard bands during the main-on state is considerably different from that observed in the past. Further, it continues to change significantly during the other two observations. This suggests that we are observing, for the first time, a substantial and continuous variation in the tilt of the disc, as it is expected if the accretion disc is precessing in the system. Analysis of the spin resolved data confirms that the equivalent width variation of the fluorescence Fe K line at ∼6.4 keV follows that of the soft X-ray emission in the main-on state, thus suggesting a common origin for Fe K line and thermal component. The Fe K line is considerably broader when the source is brightest.

A BeppoSAX observation of Her X-1 during the first main-on after an anomalous low-state: Evidence for rapid spin-down

Results of a BeppoSAX observation of Her X-1 in 2000 October during the first main-on state after the longest recorded anomalous low-state are presented. The 0.1-30 keV spectrum, light curve and pulse profile are all consistent with those measured during previous main on-states, indicating that Her X-1 has resumed its regular 35 day cycle with similar on-state properties as before. However, from a comparison of the measured pulse period with that obtained close to the start of the anomalous low-state, it is evident that Her X-1 continued to spin-down strongly during the anomalous low-state such that the pulse period has returned to a similar value as ~15 years ago. Additionally, the occurrence time of the main-on states after the end of the anomalous low-state indicate that a change in the length, or phasing, of the 35-day cycle occurred during the anomalous low-state.

Analyzing X‐Ray Pulsar Profiles: Geometry and Beam Pattern of Hercules X‐1

We report on our analysis of a large sample of energy-dependent pulse profiles of the X-ray binary pulsar Hercules X-1. We find that all data are compatible with the assumption of a slightly distorted magnetic dipole field as sole cause of the asymmetry of the observed pulse profiles. Further, the analysis provides evidence that the emission from both poles is equal. We determine an angle Θm < 20° between the rotation axis and the local magnetic axis. One pole has an offset δ < 5° from the antipodal position of the other pole. The beam pattern shows structures that can be interpreted as pencil- and fan-beam configurations. Since no assumptions on the polar emission are made, the results can be compared with various emission models. A comparison of results obtained from pulse profiles of different phases of the 35 day cycle indicates different attenuation of the radiation from the poles being responsible for the change of the pulse shape during the main-on state. These results also suggest the resolution of an ambiguity within a previous analysis of pulse profiles of Cen X-3, leading to a unique result for the beam pattern of this pulsar as well. The analysis of pulse profiles of the short-on state indicates that a large fraction of the radiation cannot be attributed to the direct emission from the poles. We give a consistent explanation of both the evolution of the pulse profile and the spectral changes with the 35 day cycle in terms of a warped precessing accretion disk.

RXTE highlights of the 34.85-day cycle of Her X-1

An analysis of the publically available RXTE archive on Her X-1 including data on 23 34.85-day cycles is performed. The turn-on times for these cycles are determined. The number of cycles with a duration of 20.5 orbits has been found exceedingly larger than of shorter (20 orbits) or longer (21 orbits). A correlation between the duration of a cycle and its mean X-ray flux is noted. The mean X-ray light curve shows a very distinct short-on state. The anomalous X-ray absorption dip is found during the first orbit after the turn-on in the main-on state for the cycles starting near the binary phase 0.25, while is present during two successive orbits in the low-on state. The post-eclipse recovery feature have not been found in the main-on state but appears at least for two orbits during the low-on state. The pre-eclipse dips are present both in main-on and low-on state and demonstrate the behaviour as in early observations. The comparison of durations of the main-on and short-on states enabled us to constrain the accretion disk semi-thickness and its inclination to the orbital plane.

RXTE highlights of the 34.85-day cycle of Her X-1

An analysis of the publically available RXTE archive on Her X-1 including data on 23 34.85-day cycles is performed. The turn-on times for these cycles are determined. The number of cycles with a duration of 20.5 orbits has been found exceedingly larger than of shorter (20 orbits) or longer (21 orbits). A correlation between the duration of a cycle and its mean X-ray flux is noted. The mean X-ray light curve shows a very distinct short-on state. The anomalous X-ray absorption dip is found during the first orbit after the turn-on in the main-on state for the cycles starting near the binary phase 0.25, while is present during two successive orbits in the low-on state. The post-eclipse recovery feature have not been found in the main-on state but appears at least for two orbits during the low-on state. The pre-eclipse dips are present both in main-on and low-on state and demonstrate the behaviour as in early observations. The comparison of durations of the main-on and short-on states enabled us to constrain the accretion disk semi-thickness and its inclination to the orbital plane.

An X-ray spectroscopic study of the pre-eclipse dips of Hercules X-1

view Abstract Citations (25) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An X-Ray Spectroscopic Study of the Pre-Eclipse Dips of Hercules X-1 Choi, C. S. ; Nagase, F. ; Makino, F. ; Dotani, T. ; Min, K. W. Abstract The X-ray binary pulsar Her X-1 was observed with the Ginga satellite on 1988 August 28 during the orbital phase of 0.76 to 0.85 in the main-on state of the 35 day cycle. During the observations the X-ray intensity varied by a factor of 5 or more on a time scale as short as 30 s, due mostly to the soft X-ray absorption in the pre-eclipse dip phase. From studies of pulse profiles and energy spectra, we find that there exists in the dip phase an unpulsed component which is approximately 3% of the intensity of the nonabsorbed high level. We suggest that scattering of the source continuum by the optically thin hot corona is responsible for the unpulsed component. In the spectral analysis, we find that the high-state nonabsorbed spectra can be fitted by a power law without absorption, and the spectra observed in the different absorption states can be fitted by two power-law components with the same photon index. An iron-K emission line is required in both cases. The estimated equivalent width of the iron line varies from 0.18 to 0.55 keV in correlation with the change in the absorption column density along the line of sight. We suggest that the fluorescent iron line arises in a cool and relatively small region, like the Alfven surface, and may be partially intercepted by the optically thick gas cloud passing across the line of sight. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/173772 Bibcode: 1994ApJ...422..799C Keywords: Eclipsing Binary Stars; Emission Spectra; Line Spectra; X Ray Absorption; X Ray Astronomy; X Ray Binaries; X Ray Spectra; Astronomical Spectroscopy; Ginga Satellite; Iron; Spaceborne Astronomy; X Ray Spectroscopy; Astronomy; STARS: BINARIES: ECLIPSING; STARS: INDIVIDUAL CONSTELLATION NAME: HERCULES X-1; X-RAYS: STARS full text sources ADS | data products SIMBAD (4)

Spectral behavior of Hercules X-1 - Its long-term variability and pulse phase spectroscopy

Results are presented of the analysis of the broad-energy-band Her X-1 spectra obtained by HEAO 1. Data from 13 pointed observations covered three 35-day cycles in February, August, and September of 1978. The pulse phase-averaged spectrum did not show any apparent variation during the main-on state, except for the overall intensity. The pulse phase-resolved spectrum, on the other hand, showed a moderately strong phase dependence in each of the main-on state pointings. This may imply that the orientation of the emission region is not changing on a monthly time frame, but the geometry of that region may evolve with time.

Deterministic chaos in accreting systems - Analysis of the X-ray variability of Hercules X-1

The irregular X-ray variability of Her X-1 has been investigated in different phases of the 1.7 day orbital period of the binary system Her X-1/HZ Her. The analyzed data have been sampled by the Exosat satellite during two main-on states of the 35 day cycle of the system. Using new system theoretical methods, the attractors of different processes have been reconstructed which are responsible for the temporal evolution of the observed X-ray radiation. For the dynamics of the emission of X-ray radiation from Her X-1, an attractor has been established which is of intriguingly low fractal dimension. This fact provides evidence for a nonstochastic, deterministic chaotic origin of the irregular variability of the emitted X-ray radiation. Higher dimensional deterministic chaos has also been found to govern the dynamical processes in the accretion disk of the system. Corresponding attractors have been derived for those phases when the X-ray source is partially obscured by the accretion disk. 44 references.